Displays

Worldwide 55-80% of the rainfall evaporates from the surface, making it a major water drain for the earth's water resources and a major supply of moisture to the atmosphere. Evaporation is relevant for crop growth and has a high impact on the severity of drought and floods. Nonetheless, this key process is still a highly uncertain, insufficiently quantified process. Also effecting weather forecasts as the available water is used as their boundary condition in atmospheric models. The persistent problem herein is our restricted understanding of the key processes of the land-atmosphere interface, as well as their interplay with hydrological and atmospheric processes. The major bottleneck is the difficulty to properly measure the land-atmosphere interface at the right spatial and temporal scale.

In this talk I will propose an experimental approach that enables data collection for the full surface energy balance at the land-atmosphere interface. This will be achieved by developing and exploiting a 'spider web' - like measurement approach with temperature measuring fibre optic cables (Distributed Temperature Sensing). This will enable simultaneously and continuously measurements of high-resolution temperature, humidity, wind, and soil moisture gradients. Which allows derivaiton of the sensible, latent, and ground heat flux and storage. The spider web offers a better representation of the land-atmosphere interface for the purpose to provide a knowledge base for improving flood and drought predictions and weather forecasts.

How to cite: Coenders-Gerrits, M. and Schilperoort, B.: Spider webbing the land-atmosphere interface, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2918, https://doi.org/10.5194/egusphere-egu2020-2918, 2020.

Evaporation is the main water outflow and a key component of the water and surface energy balance in the endorheic basins of the Atacama Desert. This is very localized to confined environments such as saline lakes, wetlands and crop fields. In these environments, the understanding of evaporation is challenging due to the interaction between the large-scale forcing and local scale turbulence over heterogeneous surfaces. Here, the advection of momentum, heat or moisture plays an important role in the enhancement of evaporation. To understand the evaporation dynamics over such environments, we performed a comprehensive 10-days experiment: the E-DATA (Evaporation caused by Dry Air Transport over the Atacama Desert), localized under extreme conditions in the Salar del Huasco saline lake (22,3°S - 68,8°W - 3790 m a.s.l.), Chile. The measurement strategy was based on spatially distributed high-resolution surface and airborne observations in combination with WRF (Weather Research Forecasting) modeling. The main findings of the experiment show that evaporation is mainly controlled by the lack of turbulence in the morning and by regional-scale forcing in the afternoon, which leads to a sudden increase in mechanical turbulence, therefore in the evaporation flux.  

This work compares two in-situ independent measurements of surface heat fluxes over the saline lake, by using an Eddy Covariance (EC) system and an Optical-Microwave Scintillometer (OMS). Our results show in general a good agreement between EC and OMS measurements of latent (L_vE) and sensible (H) heat fluxes over the water surface (R²: 0,90-0,96). During the morning, slight differences are observed between the EC and OMS measurements. However, differences up to 200 W m^-2 are observed in the afternoon for L_vE and up to 20 Wm^-2 for H. The first analysis shows that these differences given during the afternoon are likely attributed to Monin-Obukhov stability (MOST) functions, which need to be developed yet for open water surfaces. Moreover, differences in the footprint of both measurement systems together with dramatic wind changes between the morning and afternoon may play a role. Finally, inaccurate bandpass filtering of the raw scintillometer signal may be a factor in the differences between EC and OMS, where we are currently working to refine our results. Our findings highlight the advantages and disadvantages of each measurement method over an open water body and provide a discussion about its performance.    

How to cite: Lobos Roco, F., Hartogensis, O., Vila, J., de la Fuente, A., and Suarez, F.: Evaporation measurements with an Optical-Microwave Scintillometer system over a Saline lake in the Atacama Desert, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6064, https://doi.org/10.5194/egusphere-egu2020-6064, 2020.

In the past, most field studies on evaporation have focussed on land-atmosphere interactions, while the turbulent exchange above inland water surfaces have remained underexposed. However, due to the differences in characteristics of a land surface and a water body there are other driving mechanisms underlying the process of evaporation. This results in a difference in dynamics of surface evaporation between the land use types and consequently should lead to a different parameterization in hydrological models. Especially in a changing climate the importance of having an understanding of the driving mechanisms of open water evaporation (E­_water) becomes more crucial to better predict to what extent the quantity and dynamics of E_water could change in the future. This is essential to improve the parameterization of E_water in operational hydrological models and therefore to optimize water management now and in the future. For this purpose, we set-up a long-term measurement campaign to measure E_water and related meteorological variables over a large lowland reservoir in the Netherlands.

During the hot summer of 2019 two eddy-covariance systems were operational around lake IJsselmeer in the Netherlands. These high-temporal measurements are used to study the dynamics and to identify the forcing mechanisms of E_water. We present the turbulent heat flux dynamics at several temporal scales over the summer season of 2019 and show how they are related to potential drivers and parameters. From this we develop a simple data based model for estimating hourly E_water rates. Additionally, we compare E_water resulting from the direct measurements to E_water derived from commonly used evaporation models. Furthermore, we investigate and discuss the effect of including spatial variability on the total water loss of the IJsselmeer through E_water. We achieve this by using the skin water temperature, which is considered an important predictor in the estimation of E_water. Therefore, we use satellite products containing this information to extrapolate the in-situ observations towards spatially distributed rates of E_water.

How to cite: Jansen, F. A., Teuling, A. J., Jacobs, C. M. J., and Hazenberg, P.: Observed evaporation dynamics from a large lowland reservoir during a hot summer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8130, https://doi.org/10.5194/egusphere-egu2020-8130, 2020.

The determination of the water balance parameters precipitation (P), leachate (L), evapotranspiration (ET) and storage change (ΔS) plays an important role for understanding the processes within the interface atmosphere, vegetation, soil and groundwater. Furthermore, these parameters are also required for the calibration of environmental models (e.g., vadose zone models), which can be applied at larger areas for managing water resources at the aquifer scale.

Weighable lysimeters are qualified tools to measure the water balance parameters in-situ in high temporal resolution. However, there exist different methods to derive evapotranspiration from lysimeter measurements. A simple approach uses precipitation measurements by external gauges and determine ET = P – L – ΔS for certain time steps. This method implicates precipitation gauge errors (e.g., due to wind loss, wetting loss, evaporation loss and due to in- and out-splashing water drops), which are transferred to ET calculation. Measuring errors can be reduced by a larger area of the measuring gauge´s surface and positioning the collecting vessel at ground level. Large weighing lysimeters are integrated into their typical surrounding and avoid oasis effects. Thus, lysimeter provide a perfect situated measuring tool for quantifying precipitation by measuring the positive mass changes as well as evapotranspiration by measuring the negative mass changes of the upper boundary fluxes. Though, this method implicates external effects (background noise, influence of vegetation and wind) which affect the mass time series. While the background noise of the weighing is rather well known and can be filtered out of the mass time series, the influence of wind, which blows through the vegetation and affects measured lysimeter mass, cannot be corrected easily since there is no clear relation between wind speed and the measured outliers of lysimeter mass. Moreover, the influence of random noise is dependent on the evaluation interval, lysimeter design, load cells etc. The “averaging method”, where measured lysimeter masses are averaged over a certain period of time (e.g., 1 min lysimeter mass measurements are averaged to a 10 min mean) would minimize the problem of random noise, but is not able to consider short mass change events. Another method uses threshold values to separate random noise from real mass changes (mass changes smaller than the threshold are not counted as P or ET). This “threshold method” does still have limitations, because an adequate threshold is dependent on the occurring event (smooth evaporation, heavy precipitation or strong wind) and, therefore, need to be adjusted over the time. The most sophisticated method (“AWAT”) combines a moving average with a variable window width and a variable threshold value (Peters et al., 2014). The presented work shows a comparison between the above mentioned methods for a lysimeter from Wagna test site (Austria).

Peters, A., T. Nehls, H. Schonsky, G. Wessolek (2014): Separating precipitation and evapotranspiration from noise – a new filter routine for high-resolution lysimeter data. Hydrology and Earth System Sciences 18, 1189-1198.

How to cite: Klammler, G. and Fank, J.: Different methods to derive evapotranspiration from lysimeter measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5381, https://doi.org/10.5194/egusphere-egu2020-5381, 2020.

Soil water availability is a key factor in determining tree’s transpiration and sap flow rates, and varies with topography and soil depth. Reclaimed landscapes provide us with the unique opportunity to address the effects of those two variables independently on trees’ water uptake, and their relationship to climatic variation. We explored the relationship between individual tree water uptake and atmospheric variables for trembling aspen (Populus tremuloides) and white spruce (Picea glauca), and assessed how this relationship changed across different hillslope positions and rooting space. Growing season (May to September) sap and transpiration fluxes were monitored using heat ratio method sap flow sensors on trembling aspen and white spruce trees in 2014 and 2015 on a reclaimed hillslope in northern Alberta, Canada, with two different soil cover depths providing different rooting spaces. Both species’ sap flow rates and transpiration rates were strongly correlated to climatic variables such as vapor pressure deficit, precipitation events, air temperature, with slight differences in the relationship between topographical positions and soil depths. Site-level atmospheric water fluxes were obtained through eddy covariance measurements at the top of the hillslope. This allowed for a direct linkage of individual tree water uptake measurement to water flux measurements taken at the landscape-level. Understanding how distinct rooting and physiological characteristics of tree species and their growing conditions can be extrapolated to different scales, is crucial to our understanding of both atmospheric and edaphic water fluxes.

How to cite: Merlin, M., Landhäusser, S. M., and Carey, S. K.: From tree to stand: Scaling relationships of water uptake and climate in a reclaimed boreal mixedwood stand., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22489, https://doi.org/10.5194/egusphere-egu2020-22489, 2020.

The evapotranspiration (ET) process is a key term of soil water balance. In the Mediterranean climates ET represents the main loss term, that could return up to 70% of annual precipitation to the atmosphere. Due to the high seasonal and annual variability of precipitation typical of this this ecosystems, ETmay be 90% of annual precipitation. Considering that in the Mediterranean areas most of the available water for drinking purpose and for agriculture depends on the water stored in the artificial basins during the rainy period, the quantification of ET and its dynamics is of great importance.

ET exhibits a temporal pattern that varies from seconds to decades, and it is mainly dependent as well as by precipitation, also by its guiding factors (e.g. soil water moisture, solar radiation and vapor pressure deficit). Hence, identify the main factors that influence ET  becomes fundamental to understand its temporal variability, and is needed when modeling ET over different timescales.

The case study is the Orroli site in Sardinia (Italy), a typical semi-arid Mediterranean ecosystem, for which are available eddy covariance measurements of sensible heat (H), latent heat (LE) fluxes, and soil moisture, radiation, air temperature and air humidity measurements, over 15 years. The Mediterranean site is typically characterized by strong interannual variability of meteorological conditions, which can drastically impacts water resources variability during spring and summers, the key seasons for the water resources planning and management of the region.

Based on the half-hour time series, the meteorological measurements were considered into investigation, and their variability has been detected at different time scale, from seconds to year. The conventional Pearson correlation coefficient between ET and its guiding factors has been estimated, and showed the main influence of soil moisture and vapor pressure deficit on ET process, and suggested that the their control on ET vary with timescale.

Furthermore, the orthonormal wavelet transformation (a spectral analysis methodology), was used to investigate the time scale variability of ET in the frequency domain, and identify the role of its guiding factors for different time scales. The ET spectral density has significant peaks at the daily, seasonal and annual time-scales. In particular, the variability of the ET spectral density exhibits two order magnitude more than the daily variability. The wavelet cospectra of ET and its guiding factors showed that the interaction is strongest for the seasonal and the annual time scales.

How to cite: Corona, R., Montaldo, N., and Katul, G. G.: Multiscale analysis of evapotranspiration and carbon assimilation for a long time series of micrometeorological observation in a typical semi-arid Mediterranean ecosystem, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19244, https://doi.org/10.5194/egusphere-egu2020-19244, 2020.

Evapotranspiration is one of the most important components of the terrestrial hydrological cycle, which depicts atmospheric water demand and accounts for loss of more than 60% land-surface precipitation globally. Decrease in potential/reference evapotranspiration (ET_p), despite significant increase in near-surface air temperature is reported at many locations across the world in the recent decades. This counter-intuitive phenomenon known as evaporation-paradox could be attributed to decrease in net solar radiation and/or wind speed and/or increase in terrestrial evapotranspiration (ET_a). Gaining insight into evaporation-paradox requires understanding complex interaction between land-plant-atmosphere systems. Bouchet–Morton complementary relationship (CR) hypothesizes that at regional scale there exists a feedback mechanism between ET_a and ET_p for homogeneous surfaces having low advection of heat and moisture. It postulates that increase in regional ET_a consumes energy thereby cooling and humidifying the overpassing air, which would result in reduction of regional ET_p. Similarly, available excess energy which is not used for evapotranspiration (due to decrease in regional ET_a) would result in an increase of regional ET_p through warming and drying of the atmosphere. Recent improvements in remote sensing technology provide scope to quantify ET_a and use it for evaluating validity of CR at regional scale to discern the possible cause for evaporation-paradox. If the CR is valid for a region, models could be developed to estimate regional ET_a using ET_p estimated using regional values of its predictor hydro-climate variables. Prior studies on Indian subcontinent found evidence of evaporation-paradox at various sites scattered widely in space. But there is lack of attempts to establish existence of the paradox at regional scale and discern possible cause(s) for the same. In this backdrop, research is envisaged to (i) form homogeneous ET_a and ET_p regions in India using a novel dynamic fuzzy clustering approach, (ii) investigate existence of evaporation-paradox in each of those regions, and (iii) identify validity of CR and discern possible cause(s) for the paradox, if evident. ET_a is typically estimated from eddy covariance flux towers, remote sensing techniques, or computed from land surface models which often suffer from limitations of scale and data. Uncertainty arising due to the use of (i) two different hydro-climate re-analysis datasets for ET_p estimation, and (ii) one remote sensing based and three land surface model derived ET_a products is assessed. The dynamic clustering approach yielded 18 homogeneous ET_p regions and 30 homogeneous ET_a regions in India. The role of CR on evaporation-paradox was evident in eight regions. The effect of vegetation and climate on CR is studied at regional scale using NDVI (normalized difference vegetation index). In addition, existence of CR hypotheses is verified in 405 major river basins of different sizes located in diverse climate regions across the globe using combination of several model derived and remotely sensed ET_a and ET_p datasets. This study is of significance, as evidence of the effect of location, vegetation and climate on CR at regional scale gives scope for developing region-specific models to arrive at ET_a estimates directly from ET_p which could be estimated/forecasted from hydro-climate variables.

How to cite: Masanta, S. K. and Venkata Vemavarapu, S.: Evaluating Validity of Bouchet-Morton Complementary Relationship at Regional Scale through Terrestrial Evapotranspiration derived using Remote Sensing Platform and Land Surface Models , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-910, https://doi.org/10.5194/egusphere-egu2020-910, 2020.

Remote sensing-based RS observations can provide evapotranspiration ET estimations across temporal and spatial scales. In this study, two MODIS-based global ET, namely MODIS16 and two-source energy balance model TSEB are compared and evaluated using the surface water-balance WB ET method at monthly time-scale with 1 km spatial resolution for the entire land phase of Denmark (42,087 km²). Then, the drivers and underlying dependence structures of ET datasets against land-atmosphere parameters are appropriately quantified using a linear-based multivariate principal component analysis PCA –and nonlinear-based bivariate empirical Copula analysis. For calculation of the surface WB ET method, in addition to the standard WB ET procedure (ET = precipitation P – discharge Q), we introduce a novel modification of standard WB method, which considers a groundwater exchange term. Here, modelled net intercatchment groundwater flow (GW_net) is also included in the ET calculation (ET = P – Q + GW_net); where the simulations are done by the national water resources model of Denmark (the DK-model) executed in the physically-based distributed MIKE-SHE hydrologic modelling code. The differences between the two WB methods are presented and discussed in detail to highlight the importance of considering GW data when investigating water-budget of small catchments. Our analysis will also be extended to compare ET datasets at different spatial scales (catchment size), aiming at further exploring the performance and ET uncertainties of remote sensing-based models. Our results indicate that the novel approach of adding GW-data in WB ET calculation results in a more trustworthy WB ET spatial pattern. This is especially relevant for smaller catchments where GW-exchange can be significant. Large discrepancy is observed in TSEB/MODIS16 ET compared to WB ET spatial pattern at the national scale; however, ∆ET values are regionally small for most watersheds (~60% of all). Also, catchment-based analysis highlights that RS/WB ∆ET decreases from <100km² to >200km² watersheds, and about 56% (67%) of all catchments have ∆ET ±50 mm/year for TSEB (MODIS16). PCA-based analysis revealed that each ET dataset is largely driven by different parameters. However, land surface temperature LST and solar radiation Rs are found as most relevant driving variables. In addition, Copula-based analysis captures a nonlinear structure of the joint relationship with multiple densities amongst ET products and the parameters, showing a complex underlying dependence structure. Overall, both PCA and Copula analyses indicate that WB and MODIS16 ET products represent a closer spatial pattern compared to TSEB. This study will help improve standard WB ET estimate method and contribute to deeper understanding the inter-correlations and real complex relationships between ET datasets and the nature of land-atmosphere parameters.

How to cite: Soltani, M., Stisen, S., and Koch, J.: Spatial pattern evaluation of remote-sensing evapotranspiration products using surface water-balance approach: application of geostatistical functions for quantifying drivers and dependence structures of ET data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8687, https://doi.org/10.5194/egusphere-egu2020-8687, 2020.

Evapotranspiration (ET) is a key flux in hydrological cycles; it is affected by both climate and land-use change. A recent study across 42 study sites in four land-use types in lowland Sumatra (Indonesia) reported that local and regional transpiration are on the rebound due to the high water use and continuing expansion of oil palm plantations. Conventional ET assessment methods such as satellite-based thermography or the eddy covariance (EC) technique lack the high spatial resolution and spatial replicability, respectively, that are required for ET assessments in dynamic and heterogeneous, mosaic-like landscapes. For such assessments of ET, near-surface airborne thermography offers new opportunities for studies with high numbers of spatial replicates and in a fine spatial resolution. In our study, we tested drone-based thermography and the subsequent application of three energy balance models (DATTUTDUT, TSEB-PT, DTD) using the widely accepted EC technique as a reference method. The study site was a mature oil palm plantation in lowland Sumatra. For 61 flight missions, latent heat flux estimates of the DATTUTDUT model with measured net radiation agreed well with eddy covariance measurements (r²=0.85; MAE=47; RMSE=60) across variable weather conditions and daytimes. Confidence intervals for slope and intercept of a Deming regression suggest no difference between drone-based and eddy covariance method, thus indicating interchangeability. TSEB-PT and DTD yielded agreeable results, but all three models are highly sensitive to the configuration in which net radiation is assessed. Overall, we conclude that drone-based thermography with energy-balance modeling is a reliable method complementing available methods for ET studies. It offers promising, additional opportunities for fine grain and spatially explicit studies. Further steps in the near future will include the testing and if necessary calibrating of the method across different biomes as well as ecological applications.

How to cite: Röll, A., Ellsäßer, F., Stiegler, C., June, T., Hendrayanto, H., Knohl, A., and Hölscher, D.: Evapotranspiration assessments from drone-based thermography - a method comparison in an oil palm plantation and a look ahead, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20725, https://doi.org/10.5194/egusphere-egu2020-20725, 2020.

Evapotranspiration (ET) is a key component of terrestrial water cycle that plays an important role in the Earth system. Aaccurate estimation of ET is crucial in various hydrological, meteorological, and agricultural applications. In situ measurements of ET are costly and cannot be readily scaled to regional scales relevant to weather and climate studies. Therefore, there is a need for techniques to make quantitative estimates of ET using land surface state observations that are widely available from remote sensing across a range of spatial scales.

In this work, A variational data (VDA) assimilation framework is developed to estimate ET by assimilating Soil Moisture Active Passive (SMAP) soil moisture and Geostationary Operational Environmental Satellite (GOES) land surface temperature data into a coupled dual-source energy and water balance model.

The VDA framework estimates the key parameters of the coupled model, which regulate the partitioning of available energy (i.e., neutral bulk heat transfer coefficient (CH_N) and evaporative fraction from soil (EF_S) and canopy (EF_C)). The uncertainties of the retrieved unknown parameters are estimated through the inverse of Hessian of cost function, obtained using the Lagrangian methodology. Analysis of the second-order information provides a tool to identify the optimum parameter estimates and guides towards a well-posed estimation problem.

The VDA framework is implemented over an area of 21780 km² in the U.S. Southern Great Plains (with computational grid size of 0.05 degree) during a nine-month period. The maps of retrieved evaporation and transpiration are used to study a number of dynamic feedback mechanisms between the land and atmosphere, such as the dependence of evapotranspiration on vegetation and soil moisture.

How to cite: farhadi, L. and Abdolghafoorian, A.: An Integrated Variational Framework for Mapping Evapotranspiration by Assimilating GOES LST and SMAP data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2026, https://doi.org/10.5194/egusphere-egu2020-2026, 2020.

Thermal infrared (TIR) remote sensing has a wide array of applications in the environmental sciences, but such applications often require absolute temperature estimates with a high degree of accuracy. Low cost microbolometer-based imaging sensors present a possible alternative for such applications, being lightweight enough for deployment on small Unmanned Aerial Systems (UASs), and thus potentially opening up a new range of applications requiring high spatial or temporal resolution and flexible flight planning. These sensors however lack temperature stabilization of the imaging focal plane array (FPA), prohibiting the reliable retrieval of absolute temperature. Here we present a radiometric calibration methodology developed in laboratory settings using a temperature-controlled chamber and programmable blackbody, allowing for independent control of sensor and target temperatures. These laboratory data provided the basis for linear calibration equations that account for both mean and non-uniformity corrections of the FPA raw radiance counts, as a function of ambient sensor operating temperature. Multiple independent experimental trials were used to extensively validate the algorithm in the laboratory, demonstrating a retrieval error of less than 1 degree Celsius. The calibration methodology was tested under realistic field conditions during a two-day field campaign that utilized ground-based observations of land surface temperature (LST) for both a collection of ground targets with a range of reflectance / emissivity properties, and agricultural plots in Northern California. These field experiments included the deployment of the uncooled microbolometer imaging sensor on a UAS, with acquisitions made throughout a highly variable diurnal period. These UAS experiments demonstrated the effectiveness of the pre-flight calibration methodology under field conditions with excellent agreement between retrieved LST and ground-based infrared thermometers for both homogeneous tarps (R^2 = 0.95) and heterogeneous vegetation plots (R^2 = 0.69 across all crop types), with the full range of target temperatures spanning approximately 15-60 degrees Celsius throughout the campaign. The prediction error for absolute temperature estimates of field targets was found to be within 1 degree Celsius, within the range considered acceptable for many vegetation monitoring applications. We further present results of the application of these UAS-based remote measurements of LST to quantify evapotranspiration (ET) for multiple crop systems. UAS flights were conducted over wheat, soybean and maize fields throughout diurnal periods during the growing season of each crop. LST observations were integrated into the Surface Temperature Initiated Closure (STIC) biophysical evapotranspiration model to estimate ET. Validation against eddy covariance system estimates of evapotranspiration (latent energy flux) shows high predictive accuracy (R^2 > 0.95).

How to cite: Drewry, D., Dutta, D., Mallick, K., Johnson, W., and Brockers, R.: Remote Quantification of Land Surface Temperature and Evapotranspiration Using Thermal Infrared Observations from Unmanned Aerial Systems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11804, https://doi.org/10.5194/egusphere-egu2020-11804, 2020.

The Satellite Application Facility on Analysis on Land Surface Analysis (LSA-SAF) has been set up by the European Organization of the Exploitation of Meteorological Satellite (EUMETSAT, see http://lsa-saf.eumetsat.int/). Its major goal is the development of products characterizing the condition of the Earth's continental surfaces on the basis of meteorological satellite observations.

The exchange of energy and water fluxes between the Earth's surface and the atmosphere is a major phenomenon driving a series of processes that impact human life. Noteworthy examples are: agriculture yields, local weather conditions, water availability, intensity and extent of droughts, the ability of ecosystems to provide services to society, etc. The relevance of these processes has motivated the exploitation of satellite observations from the Meteosat Second Generation (MSG) to develop algorithms for the estimation of evapotranspiration (ET) and both latent and sensible heat fluxes in an operational framework functioning in near-real time.

The LSA-SAF ET product comprises half-hourly and daily estimates across Europe, Africa and the east side of South America. The quality of the ET product has been assessed by contrasting the estimates to in-situ measurements in flux measurement stations scattered across diverse climatic regions and plant cover types. The validation exercises -conducted by the development team as well as by independent studies- have corroborated the good quality of the product.

This contribution is intended to share details of the main principles of the algorithm (with insight to latest developments), the forcing variables (including several products derived from the SEVIRI instrument on-board MSG) and the ways of accessing and using the data.

How to cite: Barrios, J. M., Arboleda, A., and Gellens-Meulenberghs, F.: The LSA-SAF ET product: an operational service of sub-daily estimation of evapotranspiration in near-real time across Europe, Africa and Eastern South America, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18108, https://doi.org/10.5194/egusphere-egu2020-18108, 2020.

The main objective of this study is to exploit thermal remote sensing data for evapotranspiration (ET) modelling in the European Alps. This geographic region has been noted as a hot spot of climate change triggered by increasing number of drought events in recent years, with impacts on natural and agricultural vegetation. Evapotranspiration is considered as one of the major indicators for examining water anomalies in plants. The state-of-art ET models exploiting thermal remote sensing data have shown a large potential in water cycle monitoring. However, existing satellite-derived products do not provide adequate spatial resolution for mountain ecosystems affected by complex orography, common overcast and land-cover heterogeneity. Even though fine resolution imagery fills the gap regarding non-homogenous areas, its long revisit time and frequent cloud contamination hamper spatially continuous ET modelling. In this context, our aim is to overcome these limitations by downscaling and gap-filling 1-km MODIS LST (MOD11A1) to retrieve daily LST maps at 250 m spatial resolution, which can be considered a reasonable scale in the selected area. Firstly, we downscale MODIS LST images with the Random Forest (RF) algorithm by exploiting the relationship between coarse resolution MODIS LST and 250-m explanatory variables, including digital elevation model and normalized difference vegetation index. The 1-km MODIS LST and the downscaled product were compared with fine resolution Landsat LST images. The random forest results show an improvement of about 20% in the agreement between Landsat and 250-m MODIS LST compared to statistics obtained for MOD11A1. Secondly, we propose to recover missing values of LST pixels beneath the clouds. Considering local-scale climate variability of the study area, we present a novel approach based on investigating the relationships between LST and meteorological data under clear- and cloudy-sky conditions. The abovementioned improvements are planned to be used for energy balance modelling of ET with relevant implications on water availability assessment in the Alpine region.

How to cite: Bartkowiak, P., Castelli, M., Colombo, R., and Notarnicola, C.: Thermal remote sensing data enhancement over Alpine Vegetated Areas for evapotranspiration modelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3484, https://doi.org/10.5194/egusphere-egu2020-3484, 2020.

Having recognized the limitations in spatial representativeness and/or temporal coverage of (i) current ground evapotranspiration (ET_a) observations, and; (ii) land surface model (LSM) and remote sensing (RS) based ET_a estimates due to uncertainties in soil and vegetation parameters, a calibration-free nonlinear complementary relationship (CR) model is employed with inputs of air and dew-point temperature, wind speed, and net radiation to estimate monthly ET_a over conterminous United States during 1979–2015. Similar estimates of three land surface models (Noah, VIC, Mosaic), two reanalysis products (NCEP-II, ERA-Interim), two remote-sensing-based (GLEAM, PML) algorithms, and the spatially upscaled eddy-covariance ET_a measurements of FLUXNET-MTE plus this new result from CR were validated against water-balance-derived results. We found that the CR outperforms all other methods in the multiyear mean annual HUC2-averaged ET_a estimates with RMSE = 51 mm yr⁻¹, R = 0.98, relative bias of −1 %, and NSE = 0.94, respectively. Inclusion of the GRACE data into the annual water balances for the considerably shorter 2003–2015 period does not have much effect on model performance. Besides, the CR outperforms all other models for the linear trends in annual ET rates over the HUC2 basins. Over the significantly smaller HUC6 basins where the water-balance validation is more uncertain, the CR still outperforms all other models except FLUXNET-MTE, which has the advantage of possible local ET_a measurements, a benefit that clearly diminishes at the HUC2 scale.

   Because the employed CR method is calibration-free and requires only very few meteorological inputs, yet it yields superior ET performance at the regional scale, we further employed this method to develop a new 34-year (1982-2015) ET_a product for China. The new Chinese ET_a product was first validated against 13 eddy-covariance measurements in China, producing NSE values in the range of 0.72–0.95. On the basin scale, the modeled ET_a values yielded a relative bias of 6%, and an NSE value of 0.80 in comparison with water-balance-derived evapotranspiration rates across ten major river basins in China, indicating the CR-simulated ET_a rates reliable over China. Further evaluations suggest that the CR-based ET_a product is more accurate than seven other mainstream ET_a products. During last three decades, our new ET_a product showed that annual ET_a increased significantly over most parts of western and northeastern China, but decreased significantly in many regions of the North China Plain as well as in the eastern and southern coastal regions of China. This new CR-derived ET_a product would benefit the community for long-term large-scale hydroclimatological studies.

How to cite: Ma, N., Szilagyi, J., and Zhang, Y.: The calibration-free complementary relationship (CR) approach aids large-scale ET estimation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8217, https://doi.org/10.5194/egusphere-egu2020-8217, 2020.

Evapotranspiration (ET) is a major water flux of ecosystems and represents globally 60-80% of the incoming precipitation lost by terrestrial environments. In forested lands, tree transpiration (TR) is the dominant component of ET, yet remains challenging to measure. Over the years, sap-flow sensors have become the standard tool for quantifying tree TR and different methods based on thermal approaches have been developed. Heat ratio methods (HRM) are considered as the most reliable and accurate method to quantify absolute flows. Leading commercial brands ensure an accurate measurement of positive flows up to 100 cm hr^-1 but different studies have highlighted a saturation effect at high flows with threshold for accuracy remaining elusive[RS1] . Due to climate change, the occurrence, the severity and the duration of extreme events like heat waves and dry periods are expected to increase in future, so the potential for high TR rate periods will also increase. Therefore, it is crucial to determine the species-specific environmental conditions allowing a reliable measurement of TR in order to improve or understanding of eco-hydrological and physiological processes during high potential TR periods that can be crucial for vegetation survival. In this study, we tested the accuracy of HRM sap-flow sensors for beech (Fagus sylvatica) and oak (Quercus robur) tree species under extreme vapor pressure deficit (VPD) conditions in order to determine threshold for reliable measurements. In greenhouse conditions, we collected a complete and dense series of TR response to VPD between 0.7 to 8.3 kPa for potted beech and oak trees using three different methods: infrared gas analyser, gravimetric method, and HRM sap-flow sensors. Responses shown a linear trend at the low-canopy leaf level (41.5 and 45.1 mg H₂O m^-2 s^-1 kPa^-1 respectively for beech and oak) but a bi-linear conformation at the whole plant level (1^st slope = 12.04 ± 0.7 mg H₂O m^-2 s^-1 kPa^-1 and break-point at 3.9 ± 0.07 kPa for beech trees). Sap-flow sensors using the HRM method displayed a clear inability to reliably measure flows under high VPD conditions. Thresholds of 2.25 ± 0.04 and 2.87 ± 0.14 kPa were identified as the maximum limit of method reliability for beech and oak respectively. In highly demanding environments, we suggest a bi-linear extrapolation beyond VPD threshold for better quantifying tree TR. Further experiments aiming at characterizing TR responses to VPD for a broad range of species and in different water deficit conditions are certainly needed for better understanding tree transpiration at the whole stand level.

How to cite: Schoppach, R., Ekwalla Hangue, D., and Klaus, J.: Assessing reliability of HRM sap-flow sensors under large range of vapor pressure deficit, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3617, https://doi.org/10.5194/egusphere-egu2020-3617, 2020.

In forested regions, transpiration as the main component of evaporation fluxes is important for evaporation partitioning. Physiological behaviors among various vegetation species are quite different. Thus, an accurate estimation of the transpiration rate from a certain tree species needs specific parameterization of stomatal response to multiple environmental conditions. In this study, we chose a 300-m² beech forest plot located in Vydra basin, the Czech Republic, to investigate the transpiration of beech (Fagus sylvatica) from the middle of the vegetative period to the beginning of the deciduous period, covering 100 days. The study area experienced bark beetle infestation, and the trees are newly formed, and mixed forest stands (spruce and beech) have transformed into beech stands. From the differences in the rooting depth of each kind of tree, an impact on the long-term water regime is expected. Furthermore, trees can change soil moisture distribution or water storage in aquifers by transpiration. Therefore, the sap flow equipment was installed in six trees with varying ages among 32 beech trees in the plot, and the measurements were used to infer the stomatal conductance for the beech forest. The diurnal pattern of stomatal conductance and the response of stomatal conductance under the multiple environmental conditions were analyzed. The results showed that the stomatal conductance inferred from sap flow reached the highest at midday but, on some days, there was a significant drop at midday, which might be attributed to the limits of the hydraulic potential of leaves (trees). The response of stomatal conductance showed no pattern with solar radiation and soil moisture, but it did show a clear correlation with the vapor deficit, in particular when explaining the midday drop. The relation to temperature was rather scattered as the measured period was in the moderate climate. The findings highlighted that the parametrization of stress functions based on the typical deciduous forest does not perfectly represent the measured stomatal response of beech. Therefore, measurements of sap flow can assist in better understanding transpiration in newly formed beech stands after bark beetle outbreaks in Central Europe.

Keywords: Transpiration; beech forest; stomatal conductance; sap flow measurement

How to cite: Su, Y., shao, W., Vlček, L., and Langhammer, J.: Ecohydrological Behaviour of Mountain Beech Forest: Quantification of Stomatal Conductance Using Sap Flow Measurements , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4631, https://doi.org/10.5194/egusphere-egu2020-4631, 2020.

Cross-compartment fluxes of mass and energy play a key role in the functioning of the earth system. Yet their understanding is largely hampered by the fact that related observations occur on multiple scales, involve multiple sensors, and data are collected across different research disciplines. Evapotranspiration (ET) is one of these fluxes and of key importance in the Earth’s water and energy cycle, and comparisons and scaling of in-situ ET measurements face the same challenges.

BRIDGET aims to provide tools that will allow storage, merging and visualisation of multi-scale and multi-sensor ET data and ultimately facilitate their scientific analysis. Approaches to estimate ET are manifold with respect to the underlying observations, scales, footprints and associated uncertainties, and measurements are gathered within various research disciplines. For this toolbox we therefore need to incorporate the appropriate metadata catalogue that describes the data comprehensively across disciplines. Special emphasis is placed on providing uncertainty estimates for the data, particularly when scaling functions are applied. Finally, we develop tools for visualisation including the different support of the measurements and (geo-)statistical analysis of the various ET data.

The BRIDGET toolbox is envisioned as a standalone python package but will also be implemented in an already existing virtual research environment (V-FOR-WaTer), facilitating the merging of different ET estimates across sensors and scales.

How to cite: Hassler, S. K., Dietrich, P., Kiese, R., Mauder, M., Meyer, J., Rebmann, C., and Zehe, E.: BRIDGET: a toolbox for the integration and scaling of diverse in-situ evapotranspiration measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17412, https://doi.org/10.5194/egusphere-egu2020-17412, 2020.

The project MOMENT (Model Monitoring EveNTs) investigates the interplay between carbon and water cycles with special focus on the impacts of drought and heatwaves as well as their long-term trends. This project aims to investigate new monitoring and modeling methods to explain the interplay between carbon and water cycles of ecosystems on different time and spatial scales.

To achieve this goal, we need reliable information about the ecosystem and its drivers. We measure, for example, mass and energy exchange between the ecosystem and the lower atmosphere with the eddy covariance method, which allows us to obtain data on a half-hourly scale. Nevertheless, unfavorable weather conditions, as well as malfunctions of the instruments, can lead to a serious amount of data gaps. Different gap-filling methods are available, with the Marginal Distribution Sampling (MDS) by Reichstein et al. (2005) being the most common one. Here, we investigate, how different filling approaches influence the uncertainty of evapotranspiration (ET) data for a German forest. We especially focus on the imputation of evaporation from intercepted canopy water, because open-path EC systems rarely work correctly during and after rain events.

Even though the EC technique is a well-established method to measure ET at the ecosystem level, many approaches require rather the share of transpiration, such as the validation of some ecosystem models. Partitioning ET into its components is difficult due to the manifold drivers involved, and measuring ecosystem transpiration is challenging due to measurement limitations and assumptions, that have to be made. Therefore, we examine the possibility to retrieve information about the share of transpiration by using EC data only without additional measurement campaigns.

Reference:
Reichstein, M. et al.: On the separation of net ecosystem exchange into assimilation and ecosystem respiration: Review and improved algorithm, Glob. Chang. Biol., 11(9), 1424–1439, doi:10.1111/j.1365-2486.2005.001002.x, 2005.

How to cite: Pohl, F., Hildebrandt, A., Werban, U., and Rebmann, C.: Uncertainty of evapotranspiration fluxes measured with eddy covariance , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14822, https://doi.org/10.5194/egusphere-egu2020-14822, 2020.

In this study, the real evapotranspiration (ET) obtained using the eddy covariance (EC) technique from field crops located in the central Peruvian Andes (Huancayo Observatory, 12.04° S, 75.32°, 3350 msnm) is analyzed. Data from a sonic anemometer and a krypton hygrometer are used to estimate daily and monthly ET variability and to explore relationships with meteorological and surface variables. The results show that the mean value of daily evapotranspiration is estimated to be 3.45 mm/day during the wet season (January to March) while in the dry season (June to August) the value is 0.95 mm/day. In addition, linear regressions were used in order to evaluate the relationship of meteorological variables with evapotranspiration. As a result, solar radiation is the meteorological variable that has a strong relationship with evapotranspiration during the wet season (r2=0.76, p-value <0.005) and soil moisture during the dry season (r2=0.77, p-value <0.005). These results indicate a clear water-energy limitation depending on the season. Besides, the empirical evapotranspiration equations of FAO Penman-Monteith, Priestley-Taylor and Hargreaves were validated. Where the Priestley-Taylor equation is the empirical equation that best fits the observed data of evapotranspiration by EC (r2=0.70, p-value< 0.005).

How to cite: Callañaupa Gutierrez, S. M., Segura Cajachagua, H., Saavedra, M., Flores, J., Cuxart, J., and Silva, Y.: Seasonal variability of evapotranspiration in the central Andes of Peru using eddy covariance techniques and empirical methods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6158, https://doi.org/10.5194/egusphere-egu2020-6158, 2020.

In this study we compare turbulent energy fluxes obtained from eddy covariance (EC) (LI-7500A, LI-COR + Windmaster, Gill Instruments) and large aperture scintillometer (BLS900, Scintec) over an agricultural field (wheat field, straw and bare soil). As the EC method provides direct measurements of sensible heat (H_EC) and latent heat (LE_EC) fluxes we use it as a reference method. The EC method enables to determine fluxes within a footprint centered around the point of measurement in the middle of the field. The scintillometer provides an estimation of sensible heat flux (H_SC), derived from air refractive index fluctuation integrated over the measurement path length, in this case 570 m diagonally across whole field. The reference measurements of the radiation balance components consist of 4-component net radiometer for net radiation (Rn) (NR01, Hukseflux), three soil heat flux plates for soil heat flux (G) monitoring (HFP01, Hukseflux), including thermocouples for quantification of the heat storage above the soil heat flux plates. The scintillometer-based latent heat (LE_SC) is calculated as a residuum from available energy (Rn-G) and H_SC, provided by scintillometer. The measurement of radiation balance components was located at the top of 3.5 m mast with the EC system, while the soil heat flux plates were collocated around in 5 cm depth. The site is a flat, rectangular agricultural field (app. 16.5 ha), in the north-eastern Austria, Danube river lowland (48.21N, 16.622E), sown with winter wheat during growing season 2019. The measurement campaign was established in February 2019 with aim for multi-seasonal monitoring. The EC measurement height is 2.7 m, the scintillometer transmitter and receiver are fixed on 4 m masts, facing towards each other from NW and SE corners of the field.

Comparison of the EC-based turbulent fluxes (H_EC+LE_EC) and the available energy (Rn-G) during the period March to Mid-June showed a very good agreement, resulting in the energy balance closure of 0.96 (R² = 0.93). This suggest high accuracy and robustness of the measurement setup together with the ability of the EC method to capture all scales of eddies responsible for energy transport at this site. The comparison of methods indicates that H_SC overestimated H_EC by 10 % (R² = 0.74) and LE_SC underestimated LE_EC by 13 % (R² = 0.81). Related to Rn, the H_EC, LE_EC and G fluxes accounted for 22 % (R² = 0.53), 59 % (R² = 0.70) and 15% (R² = 0.62) of the Rn flux, respectively. We assume that the combination of EC and scintillometer method has a potential to bring deeper insight into the analysis of the energy balance closure problem.

How to cite: Orság, M., Fischer, M., Eitzinger, J., and Trnka, M.: Comparison of energy fluxes from eddy covariance and scintillometer over an agricultural field, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10168, https://doi.org/10.5194/egusphere-egu2020-10168, 2020.

Evapotranspiration is an important parameter for grassland ecosystems because the (actual) evapotranspiration explains the exchange of water and energy between soil, land surface and atmosphere. Understanding the effects of changing grassland yields on evapotranspiration rates is essential for the assessment of the water- and plant water balance of grassland sites under climate change. However, evapotranspiration is difficult to measure, and the suitability of the various methods strongly depends on the time and spatial scale considered. Thus, the aim of this work is to compare different measurements of actual evapotranspiration (ETa) at a managed alpine grassland site. The study area is located in the northern alps of Austria, at the Agricultural Research and Education Centre Raumberg-Gumpenstein (Styria). Here, the ETa data of a high resolution weighable lysimeters, are compared with ETa data measured by a scintillometer system BLS900 (Scintec, Germany). The system measures sensible heat flux integrated along the near-infrared beam of 880 nm, length of 356 m and height of 6.3 m above grassy surface. The ETa is calculated as a residual from the energy balance equation. Another independent source of ETa data is the Bowen ratio energy balance system (BREB), which is placed roughly in the middle of the scintillometer path and adjacently (few meters) to the lysimeter.

During the observation period (vegetation period 2018; March-November), ETa calculated from the weighable lysimeter was 573 mm in total and showed the highest absolute value compared to the other measurements. The calculated ETa from the BREB system is 505 mm (including condensation) and 526 mm (excluding condensation).

At the beginning of the vegetation growth, the scintillometer system measured lower values of ETa than the lysimeter, but higher values than the BREB system. Contrary, at the end of May, the lysimeter ETa showed the lowest values compared to the other two systems. This can be related to the fact that the grass on the lysimeter was cut three times per year, whereas the management of other areas on the experimental site was different. The same effects were observed at the second and third cut, always with the fact that the scintillometer system showed higher values than the BREB system. After two weeks of the first and second cut, the vegetation on the lysimeters was established faster than on the surrounding grassland. As a consequence, the lysimeter ETa showed again the highest values. Only after the third cut at the end of September, the vegetation was slowly growing and the scintillometer as well the BREB system showed higher ETa values till the end of the observation month in November. These results suggest that the evapotranspiration rates are strongly dependent on the management of the grassland, which needs to be considered in the selection and design of evapotranspiration measurements.

How to cite: Forstner, V., Vremec, M., Orság, M., Pozníková, G., Birk, S., Herndl, M., and Schaumberger, A.: Comparison of actual evapotranspiration based on lysimeter, scintillometer and bowen ratio energy balance method, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11054, https://doi.org/10.5194/egusphere-egu2020-11054, 2020.

The challenges induced by the continuous urbanization and the climate change effects, such as extreme events (e.g. flooding or heat waves) or the intense increase of the urban temperatures (Urban Heat Island), encourage the implementation of Blue and Green Solutions (BGS). These solutions are inspired by the nature, favouring natural process in the cities like water infiltration or evapotranspiration (ET), reducing air temperature during heatwaves events.

Characterize the thermal behavior governing a BGS is necessary to promote their implementation. Consequently, this research studies the energy fluxes –and particularly the evapotranspiration one- of a 1 ha wavy-shape green roof located in Champs-Sur-Marne (France), called Blue Green Wave (BGW). Therefore, a Large Aperture Scintillometer MKI, a CNR4 radiometer and 4 Type K thermocouples were installed on the BGW to measure the sensible heat flux of convection, the net radiation and the heat conduction into the soil substrate. The latent heat flux of ET was deduced from the energy balance.

Each LAS unit was placed on the highest locations of the roof with about 100 m of distance between them. Diaphragms for short-range applications were placed in front of the units. The measurements were conducted on sunny and randomly days during the 2019 summer over an average time period of 7 hours.

It appears that LAS sensible heat flux measurements on completely sunny days follow the net radiation flux trend. However, on cloudy days important flux fluctuations are noticed. Therefore, a sensitivity analysis was carried out to illustrate the significant correlation between the wind and the sensible heat flux during short time periods. In parallel, the heat conduction was analysed through a thermal gradient of temperature and a Fourier analysis demonstrating a poor conduction rate mainly on drier conditions of the BGW.

Finally, the deduced latent heat was compared with the measurements of a dynamic evaporation chamber, confirming a significant over estimation of the latent heat computed from the energy balance. This can be explained by the sum of uncertainties related to each energy flux component, in addition to the restraint conditions of LAS measurement operation on the BGW (application over the limits of MOST theory). A multifractal analysis to determinate the temporal and spatial scaling behaviour of latent heat flux is ongoing.

How to cite: Castellanos Diaz, L. A., Versini, P. A., Tchiguirinskaia, I., Bonin, O., and Ramier, D.: Large Aperture Scintillometer measurements above a large green roof to assess the evapotranspiration flux, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17901, https://doi.org/10.5194/egusphere-egu2020-17901, 2020.

In semiarid and arid regions, irrigated agriculture consumes most of water resources via evapotranspiration (ET) that mainly consists of evaporation (E) from bare soil and transpiration (T) from plant tissue. Generally, T is regarded as beneficial water use that contribute to plant production but E is considered as water waste. Therefore, daily ET and ET components E and T at filed scale are often required for improving water resource management strategy in semiarid and arid regions. Recently, time-continuous daily ET at filed scale have been achieved based on remote sensing-based ET model and multi-satellite data fusion, but few study focus on estimating of daily field-scale ET component of E and T. In this study, a daily filed-scale ET partitioning method based on the two source energy balance (TSEB) model and the spatial and temporal adaptive reflectance fusion model (STARFM) was applied and verified in a typical arid area dominated by irrigated cropland and natural desert. The comparisons of instantaneous land surface fluxes and daily ET modeled from proposed method and that derived from eddy covariance (EC) systems and automated weather stations (AWS) set up in irrigated cropland and desert indicate that reasonable surface fluxes partitioning and daily ET can be estimated by using this method. The root mean square error (RMSE) for cropland and desert are 0.87 mm and 0.84 mm, respectively. Evaluations of E and T partitioning capabilities of this proposed method based on E/ET and T/ET derived from isotopic technology at the irrigated cropland site show that the modeled E/ET and T/ET agree well with observations in terms of both magnitude and dynamics. Finally, the multi-year spatiotemporal patterns of modeled ET, E and T at filed scale with reasonable seasonal variation and spatial diversity were produced using the ET partitioning method to provide reasonable information for monitoring water use in study area.

How to cite: Li, Y. and Huang, C.: Estimating daily evaporation and transpiration at field scale (100 m) based on TSEB and data fusion using MODIS and Landsat data in irrigated agriculture area, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12368, https://doi.org/10.5194/egusphere-egu2020-12368, 2020.

Reference crop evapotranspiration (ET₀) forms an essential forcing variable in hydrological, agricultural, irrigation and climate models. Among several available methods for ET₀ estimation using regularly recorded climate data, the Food and Agriculture Organization (FAO) Penman-Monteith (PM) equation is popular among researchers due to its accuracy across different environments. However, routine use of the FAO-PM equation is hampered in data-scarce situations because of the requirement of input data pertaining to a large number of climate variables. Therefore, simpler alternative methods for ET₀ estimation such as the Blaney-Criddle (BC) and Hargreaves (HG) have been proposed by previous researchers. However, for routine use of these empirical equations, local calibration of the model parameters may be desirable. Also, a few previous attempts have been made to replace the daily mean temperature with an effective temperature calculated as a weighted average of daily maximum and minimum temperatures.  Therefore, the present study was taken up to evaluate the effect of two aspects on the accuracies of the BC and HG models 1) replacing mean temperature with effective temperature defined using different parameterizations 2) local calibration of parameters. For this purpose, climate records for the historical period 2006-2016 of 67 stations located across ten agro-climatic zones of Karnataka State, India were used and the analysis was carried out using a monthly time step. Since measured ET₀ data was unavailable, calibration was performed using PM ET₀ estimates and performance was evaluated using various statistical measures. Overall results showed that the BC equation with mean temperature yielded better results than the ones with effective temperature with calibrated parameters. However, the HG method showed an improvement with the use of effective temperature. Information on the spatial distribution of calibrated parameters was derived which will prove useful to practitioners who wish to derive ET₀ estimates with only temperature inputs.

How to cite: Siddalingamurthy, N. and Nandagiri, L.: Performance of Modified Temperature-Based Reference Crop Evapotranspiration Models Across Different Agro-Climatic Zones in Karnataka State, India, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1101, https://doi.org/10.5194/egusphere-egu2020-1101, 2020.

Understandings the processes and estimating the amount of wet surface evaporation across various scales are crucial to the evaporation research. The Penman (1948) and Priestley-Taylor (1972) equations are derived for a wet patches and an extensive wet surface respectively, with an obviously different effects of advection. However, the evaporation for a wet surface between these two scales is difficult to estimate because of the changing advections. The sigmoid generalized complementary (SGC) equation, which expresses the ratio of actual evaporation (E) to Penman potential evaporation (E_Pen) as a function of the proportion of the radiation term (E_rad) in E_Pen, is used to model the wet surface evaporation process by setting the symmetric parameter to be infinity, and was validated by data from flux sites over a lake site (CN-MLW) from China, a wetland site (US-WPT) from the United State, and a paddy site (JP-MSE) from Japan. The SGC equation robustly describes the growth of E/E_Pen upon E_rad/E_Pen with upper flatness part over the wet surface with significant changing advection effects, and could account for the variation of the Priestley-Taylor coefficient directly. Thus, the SGC equation outperforms the Priestley-Taylor equation with a constant coefficient for estimating wet surface evaporation at the scale with changing advections.

How to cite: Han, S. and Tian, F.: Sigmoid generalized complementary equation for wet surface evaporation at the scale with changing advections , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6718, https://doi.org/10.5194/egusphere-egu2020-6718, 2020.

Our study assessed the effects of ecological environmental flows from one nation to another, using remote sensing.  Remote sensing approaches to plant water use quantification can inform binational, integrated water resources management.  We provide plant water use estimates to plan for allocation of water in the Colorado River in USA and Mexico. Our study examined multi-year effects of a 2014 historically important binational experiment (the Minute 319 agreement of a water treaty between the U.S. and Mexico) on vegetative response along the riparian corridor for the years following the pulse flow which began in 2014.  We divided our study area into seven reaches and used remotely sensed imagery to exam vegetation greenness and plant water use or evapotranspiration (ET, the loss of water through evaporation from the instruments, the 250 m Moderate Resolution Imaging Spectroradiometer (MODIS) and 30 m Landsat 8 OLI satellite imagery to track ET and several vegetation indices to estimate the greenness of vegetation (e.g., NDVI, scaled NDVI, EVI, EVI2).  The Minute 319 environmental flow produced a 17% increase in VI (“Greenness”) as detected with Landsat throughout the riparian corridor in 2014.  The significant greening up was observed across reaches within the riparian zone, as well as in the non-inundated outer parts of the riparian floodplain, where groundwater supported existing vegetation.  However, after just two years (by the end of 2016) there was a 22% decrease in VI throughout the riparian corridor.  In 2017, an annual overall increase of 2% in greenness was calculated, before falling again, by 8%, over the year 2018.  From 2015-2018, the initial post-pulse greenup and ET as measured by Landsat (30m) & MODIS (250m) steadily declined, falling below pre-pulse levels in all reaches.  The VI response becomes bimodal and disintegrates after 2016 in all reaches except for in Reach 4, the restoration zone.  Our longer time-series analysis from 2000-2019 showed an overall increase in VIs (higher Greenness) and ET (more water loss) in the year of the 2014 pulse and in the year, 2015.  The higher VI and ET indicate that there was enough water in the riparian zone to generate a positive response from plants.  These results reversed a decline in VI and ET since the last major flood in 2000, but the effect did not last after the first couple of years after the pulse flow.  Our longer-term data results from 2000 through 2019 (approximately the last 20 years), showed that Landsat EVI (Greenness) declined 34% and ET (mm/day) declined 38% and since the 2014 Pulse Flow through 2019, Landsat EVI (Greenness) declined 20% and ET (mm/day) declined 23%.  The pulse flow in 2014 contributed enough water to slow the declines by almost two-thirds.  Added in-stream water helped native and invasive riparian species in terms of stand structure, extent and greenness but only for the very short-term.  Our results support the conclusion that the Minute 319 environmental flows from the U.S. to Mexico had a positive, but short-lived (one or two year), impact on vegetation growth in the delta. 

How to cite: Nagler, P., Barreto-Muñoz, A., Didan, K., Jarchow, C., Chavoshi Borujeni, S., Nouri, H., Herrmann, S., and Gómez-Sapiens, M.: Short and long-term analyses of plant greenness and evapotranspiration dynamics in the riparian zone of the Colorado River Delta before and after the 2014 Minute 319 environmental pulse flow , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5932, https://doi.org/10.5194/egusphere-egu2020-5932, 2020.

The world’s population residing in urban areas grew from 30% in 1950 to about 60% in 2020 and is expected to reach 68% by 2050. As urban areas continue to grow, green spaces in cities are getting ever more treasured. Most cities have adopted strategies to be greener to improve their resilience and livability. To make the best of the benefits offered by urban green spaces, healthy greenness is essential and this means additional water consumption. Water limitation usually results in drying out of green areas in summer, when benefits and services by green spaces are highly demanded (e.g. cooling effect). In the 21^st century, vulnerability to water shortage is not restricted to dry regions anymore;  water scarcity in the time of need is threatening the livability of cities even in wet regions (i.e. extreme summers in Europe). In this study, we estimate for the first time, to our knowledge, the blue water consumption of urban green spaces. We measure the evapotranspiration of an urban green space using three approaches of in-situ, observational-based and remote sensing, and employ principles of water footprint. We assess the blue and green water footprint of urban greenery to maintain green areas of a city based on their water demand, not the abstracted water or irrigated water. In the case of Adelaide Parklands in Australia, the annual total water footprint is 1114mm, of which 17% consumes in spring, 42% in summer, 27% in autumn, and 14% in winter. The average blue water footprint of the Parklands calculates 0.66 m³ per square meter per annum. The hot and dry summer causes a high total water footprint compared to the cold and wet winter. This study is transferable to other cities for quantification of blue water consumption of urban green spaces and their water footprint. These findings may help to guide urbanisation priorities to move toward greening cities with no extra pressure on scarce water resources.

How to cite: Nouri, H., Chavoshi Borujeni, S., and Hoekstra, A.: Evapotranspiration, water demand and water footprint of urban green spaces , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5512, https://doi.org/10.5194/egusphere-egu2020-5512, 2020.

With the acceleration of urbanization, the canopy evapotranspiration (ET) of vegetation plays an increasingly important role in urban surface energy and water budget [1-2]. A reasonable assessment of urban vegetation ET requires not only the estimation of high-resolution ET but long-term monitoring due to high heterogeneity of city, especially that of metropolis, and changeable land management policy. The study takes advantages of Google Earth Engine (GEE) platform to investigate how canopy ET of green patches in Chinese metropolis, represented by Beijing, changes from 1984 to 2018. Typical green patches including city park, community green belt, large area lawns etc. in study area were manually vectored on GEE based on a thorough examination of historic high-resolution google earth images and thermal images. Using a simple Taylor skill fusion method by Yao et al. [3], 853 cloudless Landsat 5/8 surface reflectance images were used to retrieve long-time series ET for each green patch identified with 30 m spatial resolution. Time series analysis combined with robust regression were employed for trend detection. Results indicated that the ET of green patches in Beijing significantly increased from 1984 at a mean rate of 18.05 ± 4.21 W/m²/10 year (r² = 0.42, p < 0.001). However, such enhancement varied in different green patch type. This talk will graphically depict the spatial pattern of enhanced green patch ET, explore their changes over long-term urbanization and the potential cooling capacity for urban heat island alleviation.

References:

[1] Grimmond, C. S. B. and Oke, T. R., 1991. An evapotranspiration‐interception model for urban areas. Water Resources Research, 27(7): 1739-1755.

[2] Chen, X., et al., 2019. Canopy transpiration and its cooling effect of three urban tree species in a subtropical city- Guangzhou, China. Urban Forestry & Urban Greening (43): 126368.

[3] Yao, Y. J. et al., 2017. Estimation of high-resolution terrestrial evapotranspiration from Landsat data using a simple Taylor skill fusion method. Journal of Hydrology (553): 508-526.

How to cite: Zhao, J. and Zhao, X.: Observed canopy evapotranspiration enhancement of green patches in Chinese metropolis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4494, https://doi.org/10.5194/egusphere-egu2020-4494, 2020.

Evapotranspiration (ET) is a key variable to terrestrial climate system, transferring water from the surface to the atmosphere, regulating air temperature and carbon exchanges, thus, linking the water, carbon and water cycles. Despite its great importance, ET patterns in tropical biomes are not fully understood yet. Studies with eddy covariance (EC) ET measurements and remote sensing models demonstrated a huge importance over ET drivers and limiting factors. In this context, this study aimed to assess the ET process in the tropics, from local to basin scale, using EC measurements (from the LBA project) and remote sensing models (MOD16 and GLEAM). At local scale, measurements and estimates were evaluated against net radiation, precipitation and vegetation index (EVI), in order to assess how these drivers control ET patterns. Then, a Budyko approach was applied at basin scale to calculate how water and energy constrain ET in large basins, including Amazon, Solimões, Purus, Medeira, Tapajós, and Xingu rivers. Our results demonstrated disagreements between models to represent maximum and minimum ET rates at tropical forest vegetation (at K43, K67 and K83 sites), with ET measurements peaking during the dry season, in a pattern coincident with annual net radiation cycle. Moreover, deep rooting of well-established rainforests, available soil moisture and increased solar radiation allow ET processes to be maintained during the dry season. ET estimates from MOD16 algorithm agree with these patterns, however, estimates from GLEAM indicates maximum ET rates during the rainy season. At cropland/pasture vegetation (at K77 site), also located in central Amazon, EC measurements showed moderate negative agreement with net radiation (R² = -0.48) and positive with precipitation (R² = 0.53), with decreasing ET rates during the dry season. GLEAM showed ET rates reduction in dry months, but also showed a peak in during wet season, while increasing ET estimates are observed for MOD16, both presented similar behavior as in tropical forest sites. Furthermore, measurements in the southwest part (RJA and FNS sites) did not show clear seasonal patterns, and both MOD16 and GLEAM algorithms, agree with decreasing ET rates during the dry season, showing a significant relationship with precipitation and vegetation indices. Results based on the Budyko approach indicated agreement between the models, indicating a predominant energy-limited condition when evaluated whole basin (at Óbidos station), or basins located in the northern and western parts of Amazon (in Amazon, Purus, and Negro basins), which corroborates with other studies, where ET has limited energy availability. However, our results also demonstrated disagreements in basins located in the southern and eastern parts (in Madeira, Tapajós and Xingu basins), where MOD16 showed some water-limited conditions, whilst it was not observed for GLEAM algorithm. Whether the models agree in terms of seasonality and water and energy limitations, they also disagree between them and ground measurements. This study highlighted the importance to understand limitations of multi-models and multi-scale ET processes for hydroclimatological studies in the tropics.

How to cite: Moreira, A. A. and Ruhoff, A. L.: Assessing evapotranspiration drivers and patterns at multiple scale in the Amazon basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1112, https://doi.org/10.5194/egusphere-egu2020-1112, 2020.

In the arid and semi-arid regions, the bare soil evaporation dominates the total evapotranspiration (ET). To date, in most of the process-based ET models, the constraint on the actual evaporation from bare soil due to water stress is either related to an empirical function of near-surface humidity or represented by a water stress factor linked with surface soil moisture. However, the relative humidity (RH) shows a hysteretic effect on the ET event, and the relationship between soil water stress and soil moisture is nonlinear, usually leading to the overestimation of ET in arid and semi-arid regions. In this study, we plan to improve the ET estimates on dry land by implementing a physically-based water stress constraint method, which is developed by parameterizing the Buckingham-Darcy’s law and yielded an excellent performance with laboratory data. The physically-based water stress constraint scheme is further incorporated into two different ET models (i.e. PT-JPL, MOD16) to generate the global ET estimates, whereby the latest ERA5-land reanalysis data and MODIS NDVI\LAI is adopted as model inputs. We not only validate the simulated ET with available flux observations but also intercompare the performances of different schemes in estimating ET in the arid and semi-arid regions. This study will provide a new way to characterize the regional soil water stress on the ET estimates especially in the arid and semi-arid conditions.

How to cite: Zhang, K., Zheng, D., Wang, Y., and Zhu, G.: Improvement of evapotranspiration estimates over arid and semi-arid regions with a physically based water stress constraint scheme, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12757, https://doi.org/10.5194/egusphere-egu2020-12757, 2020.

The latent heat flux (LE) governs the associated heat flux from the interactions between the land surface and its atmosphere and plays an important role in the surface water and energy balance. The Qilian Mountains is the largest marginal mountain system in the northeast of the Qinghai-Tibet Plateau. An accurate representation of spatio-temporal patterns of LE over Qilian Mountains is essential in many terrestrial biosphere, hydrosphere, and atmosphere applications. Various satellite-derived LE products have been widely used to estimate terrestrial LE, yet each individual LE product exhibits large discrepancies. To reduce uncertainties from individual product and improve terrestrial LE estimation over Qilian Mountains, we produced five satellite-derived LE products using traditional algorithms based on Moderate Resolution Imaging Spectroradiometer (MODIS) NDVI, LAI products and China Meteorological Forcing Dataset (CMFD), and implemented the fusion of these five LE products using Extremely Randomized Trees (Extra-Trees) combining information from ground-based measurements. A validation using ground-based measurements was applied at different plant function types and the validation results demonstrate that the fusion product using Extra-Trees outperformed all the LE products used in the fusion method. Comparing with three other machine learning fusion models such as Gradient Boosting Regression Tree (GBRT), Random Forest (RF) and Gaussian Process Regression (GPR), Extra-Trees exhibits the best performance in terms of both training and validation accuracy. This fusion LE product also outperformed when compared against two state-of-the-art global LE products such as Global Land Surface Satellite (GLASS) and Moderate Resolution Imaging Spectroradiometer (MODIS). The fusion LE product showed improvements in the linear correlation, bias and RMSE at different plant function types. Our results suggest that the fusion method using Extra-Trees could be successfully applied to other region and to enhance the estimation of other hydrometeorological variables.

How to cite: Shang, K., Yao, Y., Yang, J., Chen, X., Bei, X., and Guo, X.: Estimation of terrestrial latent heat flux over Qilian Mountains by the fusion of five satellite-derived products using Extremely Randomized Trees, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2817, https://doi.org/10.5194/egusphere-egu2020-2817, 2020.

In water-controlled systems, the evapotranspiration (ET) is a key indicator of the ecosystem health and the water status of the vegetation. Continuous monitoring of this variable over Mediterranean savannas (landscape consisting of widely-spaced oak trees combined with pasture, crops and shrubs) provides the baseline required to evaluate actual threats (e.g. vulnerable areas, land-use changes, invasive species, over-grazing, bush encroachment, etc.) and design management actions leading to reduce the economic and environmental vulnerability. However, the patched nature of these agropastoral ecosystems, with different uses (agricultural, farming, hunting), and their complex canopy structure, with various layers of vegetation and bare soil, pose additional difficulties. The combination of satellite mission with high/medium spatial/temporal resolutions provides appropriate information to characterize the variability of the Mediterranean savanna, assessing resource availability at local scales.

The aim of this work is to quantify ET and water stress at field-scale over a dehesa ecosystem located in Southern Spain, coupling remote sensing-based water and energy balance models. A soil water balance has been applied for five consecutive hydrological years (between 2012 and 2017) using the vegetation index (VI) based approach (VI-ETo model), on a daily scale and 30 m of spatial resolution. It combines FAO56 guidelines with the spectral response in the visible and near-infrared regions to compute more accurately the canopy transpiration. Landsat-8 and Sentinel-2 images, meteorological, and soil data have been used. This approach has been adapted to dehesa ecosystem, taking into account the double strata of annual grasses and tree canopies. However, the lack of available information about the spatial distribution of soil properties and the presence of multiple vegetation layers with very different root depths increase the uncertainty of water balance calculations. The combination with energy balance-based models may overcome these issues. In this case, the two-source energy balance model (TSEB) has been applied to explore the possibilities of integrating both approaches.  ET was estimated using TSEB in the days with available thermal data, more accurately assessing the reduction on ET due to soil water deficit, and allowing the adjustment of water stress coefficient in the VI-ETo model.

The modeled ET results have been validated with field observations (Santa Clotilde; 38º12’N, 4º17’ W; 736 m a.s.l.), measuring the energy balance components with an eddy covariance system and complementary instruments. The VI-ETo model has proven to be robust to monitor the vegetation water use of this complex ecosystem. However, the integration of the energy balance modelling has improved the estimations during the dry periods, with highly stressed vegetation, enabling a continuous monitoring of ET and water stress over this landscape.

How to cite: Carpintero, E., Andreu, A., Gómez-Giráldez, P. J., and González-Dugo, M. P.: Monitoring evapotranspiration and water stress of Mediterranean oak savannas using optical and thermal remote sensing-based approaches , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21742, https://doi.org/10.5194/egusphere-egu2020-21742, 2020.

Land surface temperature (LST) from remote sensing has been widely used to estimate regional and local scale evapotranspiration (ET). However, remotely sensed LST viewed by the same sensor from different angles would lead to different LST retrievals and this would lead to the deviation in ET estimations with LST as input. The terrestrial surface bidirectional reflectance distribution function (BRDF) are commonly inverted against multiple cloud-free, atmospherically-corrected directional reflectance values that sufficiently sample the anisotropy caused by different view angles. The MODerate-resolution Imaging Spectroradiometer (MODIS) product MCD43A1 contains three-dimensional (3D) data sets and can provide weighting parameters for the models used to derive the Albedo. The MODIS MCD43A4 is reflectance product providing reflectance data adjusted using a bidirectional reflectance distribution function (BRDF) to model the values as if they were taken from nadir view and solar zenith. Here we intend to operate the correction of the angle effect in LST with these two MODIS BRDF related products in ET estimation. The two products are used to provide reflectance with consistent view angle and with solar zenith of satellite sensor and 0° solar zenith, respectively, and then corresponding fractional vegetation cover (FVC) are calculated with two kinds of corrected reflectance, respectively. Combining the soil temperature (T_s) and vegetation temperature (T_v) components which are separated from MODIS LST and have no directional effects with the corrected FVC, the nadir LST (with solar zenith of satellite sensor and 0°solar zenith, respectively) were obtained. Finally, ET were estimated with the surface energy balance system (SEBS) model using the remote sensed LST and the two kinds of corrected LST as input, respectively. The results showed that compared to ET measurements, the ET estimations with two kinds of corrected LST as input performed much better than that with uncorrected LST as input, and ET estimation with corrected LST in which FVC are calculated from MCD43A1 had highest accuracy.

How to cite: Jiang, Y., Tang, R., Jiang, X., and Li, Z.: Evapotranspiration estimation with MODIS land surface temperature after correction of the angle effect with two MODIS BRDF related products, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12983, https://doi.org/10.5194/egusphere-egu2020-12983, 2020.

Olive trees are one of the most important crops in the Mediterranean basin (10.5 Mha), accounting for 97.5% of the world’s olive cultivation area with relevant social and economic benefits and ecological consequences. Concretely, it takes up 2.7 Mha in Spain, of which more than 1.6 are in Andalusia. Olive cultivation demands climate-smart management to facilitate crop adaptation to climate scenario and predictable development. A more efficient water use and management optimization is an especially important issue and, therefore, quantifying and modeling evapotranspiration (ET) is essential.
However, given the lack of a satellite thermal mission with both high spatial resolution and frequent revisit time, we have evaluated in this work a data fusion methodology (Gao et al., 2012) that combines Sentinel-2 and Sentinel-3 images with the two-source energy balance model (Norman et al.,1995) proposed by Guzinski & Nieto et al. (2019). Estimates of actual ET were produced at 20 m resolution from January 2016 to December 2019 in an irrigated olive grove in Southern Iberian Peninsula. Preliminary results have been validated (every 5-10 days depending on Sentinel images availability and cloud cover) by ground-based in situ data using Eddy Covariance (EC) technique, showing mean absolute errors between estimated values and those obtained by EC: 156 Wm^-2 (net radiation), 76 Wm^-2 (soil heat flux), 36 Wm^-2 (sensible heat flux), 210 Wm^-2 (latent heat flux).

Gao, F., Kustas, W. P., & Anderson, M. C. (2012). A data mining approach for sharpening thermal satellite imagery over land. Remote Sensing, 4(11), 3287–3319. https://doi.org/10.3390/rs4113287

Guzinski, R., & Nieto, H. (2019). Evaluating the feasibility of using Sentinel-2 and Sentinel-3 satellites for high resolution evapotranspiration estimations. Remote Sensing of Environment, 221, 157–172. https://doi.org/10.1016/j.rse.2018.11.019

Norman, J. M., Kustas, W. P., & Humes, K. S. (1995). Source approach for estimating soil and vegetation energy fluxes in observations of directional radiometric surface temperature. Agricultural and Forest Meteorology, 77(3–4), 263–293. https://doi.org/10.1016/01681923(95)02265Y

How to cite: Aguirre García, S. D., Aranda-Barranco, S., Nieto, H., Serrano-Ortiz, P., Sánchez-Cañete, E. P., and Guerrero-Rascado, J. L.: Evaluation of Sentinel-2 SMI and Sentinel-3 SLSTR data for estimating evapotranspiration in an irrigated olive orchard in Southern Iberian Peninsula., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19331, https://doi.org/10.5194/egusphere-egu2020-19331, 2020.

Approximately two-thirds of continental precipitation is evaporated back into the atmosphere. This highlights the influence of terrestrial evaporation for the distribution of hydrological resources, from catchment to planetary scales. The ability to monitor terrestrial evaporation dynamics is critical for climatological applications, since evaporation directly affects air temperature, influences air humidity and cloud formation, and is intrinsically connected to photosynthesis. To date, terrestrial evaporation cannot be observed directly from space, and in situ networks remain too sparse for both research and practical activities, making terrestrial evaporation one of the most uncertain components of Earth’s energy and water balance. However, a range of approaches have been proposed over the last decade to indirectly derive evaporation by applying models that combine the satellite-observed environmental and climatic drivers of the flux. One of these pioneering methods is the Global Land Evaporation Amsterdam Model (GLEAM; Miralles et al. 2011). 

GLEAM combines global satellite observations of meteorological variables – (e.g.) precipitation, surface net radiation and air temperature – and surface characteristics – (e.g.) soil and vegetation water content and snow depth. Since its publication almost 10 years ago, the model has been widely used to analyse trends in the water cycle, study land–atmospheric feedbacks or benchmark climate models. Advantages of GLEAM over analogous methods are the estimation of evaporation under cloud conditions due to the exploitation of microwave data, the explicit estimation of root-zone soil moisture data, and the detailed calculation of rainfall interception. Current model development efforts concentrate on (a) the increase in spatial resolution for its application to water management and agricultural applications and (b) the assimilation of novel satellite observations. This presentation provides a general overview of the framework and concentrates on ongoing efforts that strive in the direction of assimilating Gravity Recovery and Climate Experiment (GRACE) and Soil Moisture Active–Passive (SMAP) observations to improve the root-zone soil moisture estimates.

References Miralles, D. G., Holmes, T. R. H., De Jeu, R. A. M., Gash, J. H., Meesters, A. G. C. A. and Dolman, A. J.: Global land-surface evaporation estimated from satellite-based observations, Hydrol. Earth Syst. Sci., 15(2), 453–469, doi:10.5194/hess-15-453-2011, 2011.

How to cite: Martin-Gonzalez, R., Martens, B., De Lannoy, G., Lievens, H., R. Pagán, B., Rains, D., Zhong, F., and G. Miralles, D.: GLEAM evaporation and soil moisture: current state and ongoing efforts, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14905, https://doi.org/10.5194/egusphere-egu2020-14905, 2020.

The evaporation of rainfall intercepted by canopies back into the atmosphere – often referred to as rainfall interception loss – is a significant component of terrestrial evaporation in many ecosystems. The physical process of rainfall interception loss can usually be broken down into three phases: (1) wetting up of the canopy, (2) saturated canopy conditions, and (3) drying out after rainfall has ceased. During each of these phases, the process is affected by many factors, including rainfall characteristics, such as gross rainfall, rainfall intensity and rainfall duration, vegetation characteristics such as canopy height, leaf area and the orientation of branches and leaves, and meteorological conditions such as temperature, wind speed and relative humidity. The Global Land Evaporation Amsterdam Model (GLEAM; Miralles et al. 2011) estimates terrestrial evaporation, including forest rainfall interception loss, at the global scale mostly from satellite data. However, the model estimation of interception loss has not been updated since its release almost 10 years ago (Miralles et al. 2010).

In this regard, improving the estimation of interception loss in the model remains a priority. In GLEAM, rainfall interception is estimated using the revised Gash analytical model by Valente et al. (1997), in which the canopy storage and mean wet canopy evaporation rate are both considered constants in both space and time. In addition, only tall-canopy interception is considered. Here we explore the potential of the modified Gash's model by Van Dijk and Bruijnzeel (2001), which uses time variant canopy storage and evaporation functions dependent on leaf area index, for its application at global scales. In addition, due to its dependency on leaf area index, the model is applied to the estimation of rainfall interception loss of low vegetation types such as shrubs and grasses. An extensive meta-analysis of previous interception loss field campaigns provides an extensive archive of data to parameterize and/or validate model estimates over multiple ecosystem types. This presentation provides a general overview of the challenges in rainfall interception loss modelling at global scales and the first results of the global benchmarking of the Valente et al. (1997) and the Van Dijk and Bruijnzeel (2001) formulations against in situ data.

References

Miralles D G, Gash J H, Holmes T R H, et al. Global canopy interception from satellite observations[J]. Journal of Geophysical Research: Atmospheres, 2010, 115(D16).

Miralles D G, Holmes T R H, De Jeu R A M, et al. Global land-surface evaporation estimated from satellite-based observations[J]. Earth Syst. Sci., 2011, 15(2): 453–469.

Valente F, David J S, Gash J H C. Modelling interception loss for two sparse eucalypt and pine forests in central Portugal using reformulated Rutter and Gash analytical models[J]. Journal of Hydrology, 1997, 190(1-2): 141-162.

Van Dijk A, Bruijnzeel L A. Modelling rainfall interception by vegetation of variable density using an adapted analytical model. Part 1. Model description[J]. Journal of Hydrology, 2001, 247(3-4): 230-238.

How to cite: zhong, F., Martens, B., van Dijk, A., Ren, L., Jiang, S., and Miralles, D. G.: Global estimates of rainfall interception loss from satellite observations: recent advances in GLEAM, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13975, https://doi.org/10.5194/egusphere-egu2020-13975, 2020.

Understory vegetation has the important effect that cannot be ignored on Evapotranspiration. In previous studies, laser scanner was used to measure small-scale biomass and airborne LiDAR was used to assess light availability to understory vegetation, which in turn was converted to understory biomass production. However, it is difficult to measure watershed-scale understory biomass with high resolution. In this study, Structure from Motion (SfM) was used to reconstruct understory vegetation structure by a manual low-flying drone under the canopy with radial paths in a line thinning plantation and a spot thinning plantation made by Japanese cedar and cypress. By generating Orthomosaic image and dense point cloud data, we then extracted Excess Green Index (ExG) and Canopy Height Model (CHM), combining with understory biomass data from field harvesting to establish a quantitative relationship between the CHM and biomass, which was then used to map biomass and vegetation coverage in the study area. The results indicated that (1) a flight height of 7-10 meters is more conducive to understory vegetation reconstruction, with a photo quality greater than 0.8 and a point cloud density of more than 20 points/cm². (2) a regression cubic model based on the CHM has acceptable accuracy and biomass estimate capability (P<0.01), with a coefficient of determination of 0.75. (3) compared with the spot thinning, the understory biomass under the line thinning scenario was higher(average biomass 3.03kg/m²). (4) vegetation coverage based on the ExG index of visible light analysis was affected by ambient light(strong sunlight on a sunny day), and it cannot reflect the seasonal changes of understory vegetation biomass. These results disclosed the potential of the dense point cloud from drone SfM for estimating understory biomass. With this method, we will measure more than 5000m² of headwater catchment and output a understory biomass map.

How to cite: Zhang, Y., Onda, Y., Kato, H., Sun, X., and Gomi, T.: Understory biomass measurement based on SfM data by a manual low-flying drone under the canopy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14009, https://doi.org/10.5194/egusphere-egu2020-14009, 2020.

Calculating actual bare soil evaporation (ET_a) on the basis of potential bare soil evaporation (PE) is a widely followed approach in many disciplines including hydrogeology, hydrology and agricultural sciences. This approach considers that PE is independent from soil properties, and only ET_a is affected by soil properties. In this work, in a unique experiment, seasonal and diurnal cycles of PE over saturated bare soil were assessed for lysimeters installed in the Guanzhong Basin, China. The assessment was made for different soil textures including gravel (PE_gravel), coarse sand (PE_coarse) and fine sand (PE_fine) and also open water (PE_water). Meteorological variables, ground heat flux and soil temperature were captured at a high temporal resolution (5 min.) for more than 15 consecutive months. The daily evaporation rates over saturated bare soil (PE_soil) showed clear differences between gravel, coarse sand and fine sand, with higher PE for fine sand, smaller PE for coarse sand and smallest PE for gravel, during spring and summer. In addition, PE_water was smaller than PE for the saturated bare soil lysimeters. In autumn and winter, the measured PE rates over different surfaces showed only minor differences. The measurement data also revealed that during spring and summer night-time PE was considerable with ~1.0 mm ET per night. These results can be quantitatively explained with detailed calculations of the energy balance, considering the different porosities for gravel, coarse sand and fine sand, as well as the thermal conductivities of the phases which constitute the porous media. 

How to cite: Li, W., Wang, W., Brunner, P., Wang, Z., Li, Z., Wang, Y., Lu, Y., and Hendricks Franssen, H.-J.: Evaporation over saturated bare soil: the role of soil texture, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-412, https://doi.org/10.5194/egusphere-egu2020-412, 2020.