Estimates of evapotranspiration from in-situ measurements: bridging scales and addressing uncertainties

Evapotranspiration (ET) is the key water flux at the interface of soil, vegetation and atmosphere. In-situ measurements to estimate ET (and its individual components evaporation and transpiration) have been developed in different research disciplines and cover a range of scales, from the point scale of individual sap flow sensors in trees over pedon-scale lysimeters to eddy covariance footprints. Each estimate and each scaling step includes a method-specific set of uncertainties which are rarely communicated. This is problematic for connecting different methods and the effort to scale up to remote sensing products from satellites or to model resolutions.

This session will mainly focus on the variety of ET estimates from different in-situ devices such as lysimeters, sap flow sensors, eddy covariance stations, scintillometers, approaches like the Bowen ratio method and others, including reporting and comparing the respective uncertainties of the methods. Additionally, we want to address the scale dependency of the various approaches and the scale gap between in-situ ET data, remote sensing products and catchment- or landscape-scale modelled ET. We welcome contributions that (1) assess and compare established and new in-situ ET measurements, (2) address uncertainty in the respective methods, (3) analyse trends as well as spatial and temporal patterns in in-situ measured ET data, (4) include cross-scale comparisons and scaling approaches and (5) incorporate in-situ measurements into modeling approaches.

Co-organized by AS2/BG3
Convener: Sibylle K. Hassler | Co-conveners: Harrie-Jan Hendricks Franssen, Corinna Rebmann
vPICO presentations
| Thu, 29 Apr, 11:45–12:30 (CEST)

vPICO presentations: Thu, 29 Apr

Chairpersons: Sibylle K. Hassler, Corinna Rebmann, Harrie-Jan Hendricks Franssen
Rafael Poyatos, Víctor Granda, Víctor Flo, Maurizio Mencuccini, and Jordi Martínez-Vilalta

Transpiration from forests and woodlands is the main component of terrestrial evapotranspiration. Ecosystem-scale transpiration estimates are needed to inform models and remote sensing products so that they can improve their quantification of the magnitudes, spatiotemporal patterns, and environmental sensitivity of transpiration at regional to global scales. Tree-level sap flow measurements can be used to estimate ecosystem transpiration in forests and woodlands and these data are now globally available in the SAPFLUXNET database (Poyatos et al. 2020). However, observational errors, sampling assumptions, and missing data propagate uncertainties in the upscaling process to the ecosystem level. Here we quantify ecosystem transpiration and its uncertainty, from hourly to annual scales, across SAPFLUXNET sites using two different approaches. In a first approach, we estimated hourly sap flow per unit basal area at the species level, which was then aggregated to the stand level using species-specific basal areas available in SAPFLUXNET metadata. In this approach, uncertainty was quantified from the observed variability in tree-level sap flow within a species. In a second approach, we used empirical relationships between tree diameter and sap flow to obtain stand-level transpiration and propagated the uncertainty in this relationship to the stand-level values. For both approaches, sap flow data obtained with uncalibrated heat dissipation probes were also adjusted using a recently published correction based on sap flow calibrations. The different upscaling methods, implemented in R code, will allow reproducible upscaling and uncertainty quantification from SAPFLUXNET datasets, paving the way towards a better understanding of ecosystem transpiration and its controls across the globe.

Poyatos, R., Granda, V., Flo, V., […], Steppe, K., Mencuccini, M., Martínez-Vilalta, J. (2020). Global transpiration data from sap flow measurements: the SAPFLUXNET database, Earth System Science Data Discussions, 1–57, .

How to cite: Poyatos, R., Granda, V., Flo, V., Mencuccini, M., and Martínez-Vilalta, J.: Towards a consistent quantification of ecosystem transpiration and its uncertainty from the SAPFLUXNET database, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13326, https://doi.org/10.5194/egusphere-egu21-13326, 2021.

Paulo Bittencourt, Lucy Rowland, Stephen Sitch, Rafael Poyatos, Diego Miralles, and Maurizio Mencuccini

Transpiration (T) is a key driver of ecosystem energy, water and carbon flows and is tightly linked to climate and land-use change. While global models rely extensively on remotely sensed transpiration products to evaluate land-surface processes, ground-truth validation for these products does not exist. At best, data from eddy-covariance evapotranspiration is used, but the T component is partitioned based-on a set of complex assumptions, which are in themselves poorly validated for many parts of the world. Sapflow (SF) measurements allow direct quantification of tree-level T which can be used as ground-truth for T-products in forested areas. A recent global network of sapflux data, (SapFluxNet – SFN) has provided the first quality-controlled sapflow dataset at a global scale, opening up new opportunities to evaluate global T products.  Using the SFN-SF and Global Land Evaporation Amsterdam Model (GLEAM) T product, we address i) how the time course of the two products scale with one another, and ii) whether this scaling is different between days with low, median or high T/ SF within months; in addition, iii) we evaluate errors patterns of GLEAM-T in relation to SFN-SF and test whether these errors are biased by site climate or by model inputs. Our results shows GLEAM-T scales with SFN-SF, especially for days with median transpiration, but this scaling, rather than 1:1, has a slope of 0.9, which causes underestimation of SFN-SF at high GLEAM-T values. The scaling is shallower for low and high transpiration days leading to a higher bias in those days. In addition, GLEAM-T scales from SFN-SF with an offset, which compensate the shallower scaling at median values at the expense of increasing bias at extremes. Our results also show errors of GLEAM-T in relation to SFN-SF are not random but depend on the location`s climate and on the soil moisture stress factor used within GLEAM transpiration model. Our work bridges, for the first time, the scale difference between trees and pixels and shows the potential of using ground-truth SF measurements for evaluating biases and patterns in global products.

How to cite: Bittencourt, P., Rowland, L., Sitch, S., Poyatos, R., Miralles, D., and Mencuccini, M.: Using a global tree sap flow database as ground-truth for transpiration products validation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11096, https://doi.org/10.5194/egusphere-egu21-11096, 2021.

Rémy Schoppach, Kwok Pan Chun, Quing He, Fabiani Ginevra, and Julian Klaus

Multi-species forests display a substantial tree-to-tree variability in transpiration induced by various vegetation and landscape characteristics. However, how to model transpiration accounting for tree-to-tree variability still needs to be developed. Diameter at breast height (DBH) is a representative variable of tree characteristics related to age, size and social position in the canopy. Landscape characteristics affecting transpiration are usually defined by topographical factors including slope, aspect, curvature, flow accumulation and topographic position index. Among all transpiration drivers, DBH and topographical factors represent the most stable controls over a growing season. Both play a key-role in defining the accessibility and the availability of the water sustaining transpiration flux and consequently in determining tree-to-tree variability in transpiration. However, previous studies showed that DBH and topographical factors can exhibit contrasting effects on sap velocity (a proxy of transpiration) depending on species and the hydro-meteorological conditions. So far, we are still lacking a detailed understanding of the species-specific influence of DBH and topographical factors on sap velocity, which hampers our ability to predict future forest water-use by impeding our capability to build robust procedures for upscaling sap-flow that accounts for tree-to-tree variability. In this study, we used a relative importance analysis to investigate the specie-specific and dynamic role of DBH and topographical factors on sap velocity. We monitored sap velocity in 28 beech (Fagus sylvatica) and oak (Quercus robur/petraea) trees in a 0.45 km² forested catchment. We found that the relative importance of DBH and topographical factors depended on species-specific water-use strategies. Based on these results, we developed a novel and robust procedure for upscaling sap velocity using a species-specific non-linear model of sap velocity response to temperature. This new procedure accounts for tree DBH and terrain’s slope for providing modelled time series of sap velocity. Finally, we compared our new procedure with other available upscaling procedure. In both cases, we used the measured sap velocity data to build models for each approach; then, we compared the modelled sap velocity to the real corresponding measured values of individual 28 trees in order to evaluate the differences between sap velocity estimations resulting from the two approaches. Over the whole year, the common procedure overestimated oak sap velocity by 39% ± 5.0 SE and 5% ± 2.3 SE for beech, while our new procedure led to an underestimation of only 4.8% ± 2.0 SE for oak and 12% ± 1.4 SE for beech. Our novel procedure also reduced the standard error of the estimation in both species and therefore the uncertainty on sap velocity of each tree. Moreover, our new procedure appeared to particularly outperform the common procedure during dry summer months when the estimation of forest transpiration is critical. During this period, our procedure slightly underestimated sap velocity by 5.8 ± 1.7% and 1.1 ± 1.9% while the common one overestimated sap velocity by 32.3 ± 4.8% and 8.5 ± 2.6% for oak and beech trees, respectively.

How to cite: Schoppach, R., Chun, K. P., He, Q., Ginevra, F., and Klaus, J.: A novel and robust procedure for upscaling sap velocity data based on the species-specific role of DBH and slope for explaining tree-to-tree variability , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7403, https://doi.org/10.5194/egusphere-egu21-7403, 2021.

Yogi Suardiwerianto, Sofyan Kurnianto, Adibtya Asyhari, Tubagus Muhamad Risky, Muhammad Fikky Hidayat, Rendy Kurnia, and Chandra Shekhar Deshmukh

Transpiration is a key process in the terrestrial ecosystems linking water, carbon, and energy exchanges between the vegetation and the atmosphere. However, the understanding of transpiration rate, its spatiotemporal dynamics, and the controlling factors in tropical peatlands are still constrained by limited measurements. This study aims to investigate the transpiration rates at the stand level of Acacia plantation under different groundwater levels. The measurements were performed at two large-scale lysimeter plots with groundwater level of 40 and 80 cm below the ground surface. The transpiration rate was quantified based on sap flow measurements from 16 trees with different diameters at breast height using heat ratio method. The initial results indicate that the transpiration rate was closely correlated to the meteorological parameters, including atmospheric vapor pressure deficit and solar radiation. The two plots with different groundwater level regimes exhibit the same diurnal pattern of transpiration rate yet shows differences in their magnitude. The findings from this study will improve the understanding about relative contribution of transpiration to the total water balance under different groundwater levels. Further, an ongoing measurement of above and below-ground biomass growth and hydrological modeling work will advance the knowledge on plant-water interaction from this ecosystem.

How to cite: Suardiwerianto, Y., Kurnianto, S., Asyhari, A., Muhamad Risky, T., Fikky Hidayat, M., Kurnia, R., and Shekhar Deshmukh, C.: Transpiration of Acacia plantation under managed tropical peatland in Riau, Sumatra, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8251, https://doi.org/10.5194/egusphere-egu21-8251, 2021.

Sibylle K. Hassler, Peter Dietrich, Ralf Kiese, Mirko Mälicke, Matthias Mauder, Jörg Meyer, Corinna Rebmann, Marcus Strobl, and Erwin Zehe

Comparing estimates of evapotranspiration (ET) from different in-situ measurements – or between in-situ measurements and remote sensing products or modelling outputs – always entails the challenge of different scales and method-specific uncertainties. Especially when the estimates originate in different research disciplines, addressing and quantifying the various sources of uncertainty of the scaled ET values becomes a difficult task for individual researchers who are not familiar with all the methodological details.

The BRIDGET toolbox – developed within the Digital Earth project – wants to support the integration and scaling of diverse in-situ ET measurements by providing tools for storage, merging and visualisation of multi-scale and multi-sensor ET data. This requires an appropriate metadata description for the various measurements as well as an assessment of method-specific uncertainties which need to be supported by domain experts. We combine these tools in a standalone python package and also implement them in an existing virtual research environment (V-FOR-WaTer).

Our first use case defines and quantifies the various sources of uncertainty when scaling sap flow values from individual sensor measurements in a tree up to the transpiration estimate of a stand. Comparison estimates come from eddy covariance measurements, lysimeters and remote sensing products.

How to cite: Hassler, S. K., Dietrich, P., Kiese, R., Mälicke, M., Mauder, M., Meyer, J., Rebmann, C., Strobl, M., and Zehe, E.: Integration of evapotranspiration estimates from scaled sap flow values and eddy covariance measurements in the BRIDGET toolbox, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3889, https://doi.org/10.5194/egusphere-egu21-3889, 2021.

Uta Moderow, Stefanie Fischer, Thomas Grünwald, Philipp Körner, Uwe Spank, Ronald Queck, and Christian Bernhofer

Evapotranspiration changes with landuse, soil conditions and meteorological conditions. Landscapes in middle Europe are typically of mosaic pattern at micro to local scale with different landuses adjacent to one another and we therefore have areas of different ET. This work shall address how ET changes with landuse based on 10 years of Eddy-Covariance data for different landuses. In a first step, it will be focussed on two landuses (coniferous forest and grassland). ET obtained via measurements will be compared to ET obtained via modelling by a using a one dimensional soil-vegetation transfer scheme and a machine learning approach using gradient boosting. Results will be analysed whether typical characteristic properties of the respective landscape are preserved (e.g. Bowen ratio) as well as differences between land uses (e.g. differences in yearly ET estimates).

How to cite: Moderow, U., Fischer, S., Grünwald, T., Körner, P., Spank, U., Queck, R., and Bernhofer, C.: ET of a mosaic landscape – measurements and modelling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13524, https://doi.org/10.5194/egusphere-egu21-13524, 2021.

Qiong Han, Qin Liu, Tiejun Wang, Lichun Wang, Chongli Di, Xi Chen, Keith Smettem, and Shailesh Singh

Relative contributions from environmental factors to daily actual evapotranspiration (ETa) across a variety of climate zones and ecosystem types is a widely open research question, especially regarding the roles played by soil water content (SWC; water supply) and net radiation (Rn; energy supply) in controlling ETa. Here, the boosted regression tree method was employed to quantify environmental controls on daily ETa using the global FLUXNET dataset. Overall, the SWC impact on daily ETa increased with increasing aridity index (Φ). However, unlike the traditional Budyko theory that is applied at annual and mean annual scales, the daily FLUXNET data revealed that Rn still played a pivotal role at most sites (roughly Φ < 4), indicating that Rn could be a leading control on daily ETa even at water-limited sites. The variations in the relative controls of SWC and Rn also partly depended on factors affecting water availability for daily ETa (e.g., vegetation characteristics, soil texture, and groundwater depth). Especially, leaf area index exerted a stronger influence on ETa at drier sites than at relatively humid sites, suggesting that near-surface hydrological processes are more sensitive to vegetation variations due to their ability to extract deep soil water and enhance ETa, particularly under arid and semi-arid climatic conditions. As a result, the net effect of environmental controls other than SWC and Rn on ETa was more important at drier sites. Our findings illustrate how environmental controls on daily ETa change as climate and ecosystem vary, which has important implications for many scientific disciplines including hydrological, climatic, and agricultural studies.

How to cite: Han, Q., Liu, Q., Wang, T., Wang, L., Di, C., Chen, X., Smettem, K., and Singh, S.: Diagnosis of environmental controls on daily actual evapotranspiration across a global flux tower network: the roles of water and energy, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-453, https://doi.org/10.5194/egusphere-egu21-453, 2021.

Jun Jie Gao, Han Chen, Jinhui Jeanne Huang, Edward McBean, Han Li, Jiawei Zhang, and Zhiqing Lan

Quantification of the contribution of transpiration (T) to evapotranspiration (ET) is important to understand the impact of climate change on the hydrological cycle and guide precision irrigation. So far, few studies have examined seasonal variability of T/ET and its drivers under urban area. In this study, we applied a modified Shuttleworth-Wallace (S-W) model to partition ET for a locust tree forest in jinnan district of Tianjin city. The new model considers the impact of carbon dioxide emissions on vegetation transpiration and significantly improves the performance of the original S-W model. The Eddy Covariance (EC) and stable water isotope method was used to monitor and partition ET in locust tree forest. Isotope composition of ET (δET), soil evaporation (δE) and vegetation transpiration (δT) were determined using the Keeling-plot method, Craig-Gordon model and Steady-state assumption model (SSA), respectively. The verification result suggest the modified S-W model could provide reliable prediction for ET and its components. The modified S-W model estimated T/ET ranges from 0 to 1, with a near continuous increase over time in the early growing season when leaf area index (LAI) is small and then convergence towards a stable value when LAI is larger. The results show seasonal change in T/ET can be described well as a function of LAI, implying that LAI is a first order factor affecting ET partitioning, and soil moisture also influence the ET partitioning. This study reveals the change in T/ET patterns and its controlling factors in urban woodland areas. Understanding the impact of urbanization and human activities on the urban water cycle will allow more effective water use in urban environments.


How to cite: Gao, J. J., Chen, H., Huang, J. J., McBean, E., Li, H., Zhang, J., and Lan, Z.: Partition of daily evapotranspiration using stable water isotope method and a modified Shuttleworth-Wallace Model for urban forest area, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8138, https://doi.org/10.5194/egusphere-egu21-8138, 2021.

Fyodor Tatarinov, Jonathan Muller, Eyal Rotenberg, and Dan Yakir

Infrared gas analyzers (IRGAs) are commonly used in Eddy Covariance (EC) system and are used for, in particular, the ecosystem water cycle. However, they suffer from a measurement drift of absolute concentrations with time, leading to the increasing bias of readings. It is recommended in the manuals to do a factory calibration once every 1-2 years (e.g., LI-6262) or user calibration when considerable drift occurs (e.g., LI-7000). However, our experience shows that a significant drift can occur within a few days already. At our semi-arid EC site of Yatir Forest (31˚20'N, 35˚03'E, Israel), we are measuring a vertical air humidity profile (absolute humidity, Cw in mmol×mol-1, and relative, RH, %),  to study the VPD regime within the canopy and to analyze dew formation events, which requires highly accurate RH measurements, however accurate RH measurements are difficult to achieve.

Air humidity in Yatir is measured by three different instruments: (1) LI-7000 close-pass IRGA above the canopy for EC flux calculations; (2) LI-6262 close-pass IRGA with inlets in 4 different heights from above the ground up to the sonic height, used for humidity profile measurements; (3) Rotronic HC2S3 air humidity (RH) and temperature (T) sensor above the canopy. Both IRGAs are placed within a temperature-controlled box, and calibrated for zero and span with N2, dew point generator and laboratory standard gases every 1-2 weeks. The Rotronic sensor has very low drift and does not require calibration, but is assumed to be less accurate, especially under high and low RH.

To achieve highly accurate measurements on daily time scale we propose a correction routine that rely on the stability of the RH probe, and the accuracy of the IRGAs after calibration. Every time the IRGA is calibrated, a correction-1 to the RH probe is produced. Between calibrations, the trends in the drifting IRGAs data are corrected (correction-2) to the interpolated stable RH probe data.

For the flux measurements, the mean absolute Cw error before correction was 1.0 mmol×mol-1, which translates under average temperature of 25°C and RH of 50% to errors of RH, VPD and dew point of 3.0%, 93.5 Pa and 0.9°C, respectively. Following our correction procedure, reduced the error to 0.5 mmol×mol-1, which decreased the errors in RH, VPD and dew point under the same conditions to 1.5%, 47 Pa and 0.4°C, respectively. For the humidity profile, Cw error after correction decreased from 1.9 mmol×mol-1 to 0.5 mmol×mol-1, which decreased the errors in RH, VPD and dew point under the same conditions by 4.1%, 131 Pa and 1.2°C, respectively.

We will describe the method in more detail and demonstrate its application to our field measurements.

How to cite: Tatarinov, F., Muller, J., Rotenberg, E., and Yakir, D.: Field calibration and correction of air water concentration measurements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6513, https://doi.org/10.5194/egusphere-egu21-6513, 2021.

Femke A. Jansen, Adriaan J. Teuling, Remko Uijlenhoet, Cor M.J. Jacobs, and Pieter Hazenberg

Distinct differences in surface characteristics between a water body and a land surface result in different drivers of evaporation and therefore its dynamics. It is essential to include and represent this difference in the parameterization of open water evaporation (Ewater) to improve operational hydrological models. Additionally, more accurate parameterization becomes even more crucial to predict potential changes in quantity and dynamics of Ewater in a changing climate in support of optimal water management now and in the future.

For this purpose, we performed a long-term measurement campaign to measure Ewater and related meteorological variables over a large lowland reservoir in the Netherlands. During the summer seasons of 2019 and 2020 eddy-covariance systems were applied at two locations at the border of lake IJsselmeer in the Netherlands. These high temporal resolution measurements gave us the opportunity to explore the dynamics and identify the underlying driving mechanisms of Ewater. Using the data collected during the summer of 2019 we were able to develop a simple regression model for both measurement locations. Combinations, both sums and products, of the following independent variables were considered: global radiation, wind speed, water skin temperature, vapour pressure deficit, and vertical vapour pressure gradient. The product of wind speed and vertical vapour pressure gradient best explained the observed hourly Ewater rates, which is consistent with the commonly used aerodynamic approach. The model was validated using the data of 2020. Additionally, we compared measured Ewater to Ewater computed with Makkink’s equation, which is currently used in the Dutch operational hydrological models to estimate Ewater. Although a correction factor is applied to account for the difference between land evaporation and Ewater, Makkink is not able to capture the dynamics of Ewater. This was reflected in the timing and shape of the evaporation peak at both daily and monthly scales. The disagreement of Ewater dynamics found between the measured and simulated Ewater even more demonstrates the value and need of a correct parameterization of Ewater.

How to cite: Jansen, F. A., Teuling, A. J., Uijlenhoet, R., Jacobs, C. M. J., and Hazenberg, P.: Evaporation from a large lowland reservoir – observed dynamics during a warm summer, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4577, https://doi.org/10.5194/egusphere-egu21-4577, 2021.

Adrien Pierre, Daniel Nadeau, Pierre-Érik Isabelle, Antoine Thiboult, Alain Rousseau, and François Anctil

Observations of sensible and latent heat fluxes over inland water bodies are unfortunately scarce and, yet, critical to the development of adequate lake parameterization for numerical weather forecast and climate models. When available, they usually consist of eddy covariance (EC) or lysimeter measurements, both representative of a relatively small footprint area, typically of a few hectares in the case of the EC approach. Over the past decades, we have seen the emergence of bichromatic scintillometry (SC), which allows for a ‘regional’ (~km2) estimation of turbulent heat fluxes. In brief, two beams travelling from a set of transmitters to a set of receivers scintillate in the turbulent air above the surface of interest and enable, using the Monin-Obukhov Similarity Theory, the computation of sensible and latent heat fluxes at the land-atmosphere interface. While a handful of studies have looked at the performance of this approach over land surfaces, very few have assessed it over water bodies. This study presents an evaluation of scintillometry-derived turbulent heat fluxes over an 85-km2 boreal hydropower reservoir of eastern Canada (50.69°N, 63.24°W) with respect to those obtained with EC measurements collected on a nearby floating platform. The scintillometer beam path travelled for 1.7 km over a surface of the reservoir that reached depths of ~100m, from 14 August to 9 October 2019. Results indicate positive, day-and-night, latent heat fluxes throughout the whole period; highlighting that the reservoir steadily released heat over the second half of the open water period, from mid-august until freeze-up. Sensible heat fluxes peaked at night due to the near-surface air temperature vertical gradient reaching its daily maximum. For sensible heat fluxes, the SC approach corroborates well with the EC approach, while for latent heat fluxes, the agreement between EC and SC decreases. This suggests that the larger footprint of the SC system might be affected by heterogeneous surface flux characteristics in the reservoir, which encapsulates the need for large-scale measurements. Grouping results by atmospheric stability regimes does not improve comparison results. These results provide an opportunity to validate an innovative approach for measuring turbulent fluxes at a regional scale and, hence, improving our understanding of turbulent fluxes over large reservoirs and lakes.

How to cite: Pierre, A., Nadeau, D., Isabelle, P.-É., Thiboult, A., Rousseau, A., and Anctil, F.: Scintillometry Observations of Sensible and Latent Heat Fluxes over a Boreal Reservoir, Quebec, Canada, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6213, https://doi.org/10.5194/egusphere-egu21-6213, 2021.

Rouhin Mitra, Mekonnen Gebremichael, Isabel Franco Trigo, and Henk A.R. de Bruin

Reference evapotranspiration (ETo), a hypothetical concept to estimate evapotranspiration from irrigated and large grass fields is crucial in finding the irrigation water demand in places with extensive agricultural practice. In general, the FAO method (based on the Penman-Monteith equation) is used to estimate ETo from stations that are placed in locations that violate the requirements for reference evapotranspiration. In this study we compare radiation-based methods used to estimate reference evapotranspiration such as ETo De Bruin and ETo Makkink with more conventional ETo approaches in FAO PM method and Priestley Taylor method using in-situ measurements from stations placed in two different settings: (1) Areas that are well-irrigated but surrounded by dry land, (2) Areas that are dry but extensive. We use two spatially dense networks of stations: 1) CIMIS stations of California located in irrigated and in-extensive fields, (2) MESONET stations of Oklahoma located on dry surfaces.  We analyze the differences in the ETo estimates and hypothesize that the radiation-based estimates give more accurate results in the conditions given above for irrigation advisory. We also assess the spatial variability of the different ETo estimates and attempt to investigate the reason behind the differences in these estimates due to the climatic factors.

How to cite: Mitra, R., Gebremichael, M., Trigo, I. F., and de Bruin, H. A. R.: Comparison of FAO Crop Reference Estimates and Radiation based Estimates for Daily Reference Evapotranspiration Estimation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14622, https://doi.org/10.5194/egusphere-egu21-14622, 2021.

Wanxin Li, Harrie-Jan Hendricks Franssen, Philip Brunner, Zhi Li, and Wenke Wang

The potential evaporation rate (PE) depends on the available energy at the land-atmosphere interface and soil properties. The application of the full-form Penman-Monteith equation (PM) is often simplified. For example, the ground heat flux G is often assumed to be zero for calculating daily evaporation as the value of G is relatively small compared to the net radiation Rn. This and other simplifications consider that the PE value is mainly determined by meteorological variables and independent from soil properties. As the influence of soil textures on PE have so far received little attention, we analyzed data from a lysimeter experiment in the Guanzhong Basin, China. The potential evaporation rate was measured over saturated fine sand (PEfine), coarse sand (PEcoarse) and gravel sand (PEgravel) at a high temporal resolution. Meteorological variables, ground heat flux and soil temperature at different depths were observed from July 2018 to August 2019. The measured PE showed clear differences between the three saturated bare soils especially during spring and summer. Our previous research showed that these PE differences are controlled by differences in the available energy, related to differences in the total ground heat flux G for the three materials and different albedos. Both a detailed energy balance and the full-form PM equation can explain the PE differences between the different soil textures on the basis of hourly data. On the other hand, if the full-form PM equation is applied on daily data only minor differences in PE between the three textures are calculated. Our research suggests that the available energy should be calculated as precisely as possible, considering the soil porosity, thermal conductivity as well as albedo for the different soil textures, in order to estimate evaporation. 

How to cite: Li, W., Hendricks Franssen, H.-J., Brunner, P., Li, Z., and Wang, W.: The role of available energy in estimating potential evaporation over different soil textures, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10755, https://doi.org/10.5194/egusphere-egu21-10755, 2021.

Basem Aljoumani, Anne Timm, Josep Sanchez, Björn Kluge, Gerd Wessolek, and Birgit Kleinschmit

Paved surfaces are a necessary infrastructure of cities, traditionally they are designed to carry vehicular, pedestrian traffic and transport products, and they provide public spaces for social communication. These paved surfaces also function as channels for waste matter, sewage, gas and electrical and as transport processes of water, matter, and energy between the soil and atmosphere in urban areas. In other hand, their characteristics lead to an altered hydrological balance compared to rural counterparts.

This study aimed to gain new insights into urban hydrological balance, in particular, the evaporation from paved surfaces. Hourly data of evaporation obtained from two high-resolution weighable lysimeters, these lysimeters are covered in two pavement sealing types commonly used for sidewalks in Berlin: cobblestones and concrete slabs. Soil volumetric water content and soil temperature of sandy soil was measured in the lysimeters with capacitance soil moisture sensors at 5cm depth. Moreover, time series consisted of hourly measurements climatology observations was obtained by climate station located near to the lysimeters. The measurements started in June 2016 and have been carried out for one year.

The data could be paired to estimate the variation of evaporation and how it was affected by cobblestones and concrete slabs and environmental factors.  In this case, a generalized additive model (GAM) for each sealing type was built, where the model response was the difference between the paired samples of evaporation from cobblestones and concrete slabs and the explanatory variables were the observations from the climate station and lysimeter data according to each sealing type. The statistical model tries to explain how the explanatory variables are related to a higher or lesser difference in evaporation between the two surfaces. As the result, the modelling approach showed that the evaporation from cobblestones tends to be higher than concrete slabs when the air temperate and soil temperature at 5 cm depth increases. The evaporation from cobblestone was also higher when the relative humidity was low, while the evaporation from concrete slabs was higher than cobblestones when the relative humidity was between 50 - 75%. When the relative humidity was higher than 75% the model showed that there was no difference in evaporating between the two sealing types.  The model showed also that the evaporation from concrete slabs tends to be higher than cobblestones when the solar radiation increases. Moreover, when the cumulative precipitation data in 9-hour intervals was higher than 10mm the cobblestone evaporates more than concrete slabs.

How to cite: Aljoumani, B., Timm, A., Sanchez, J., Kluge, B., Wessolek, G., and Kleinschmit, B.: Measurements of Evaporation in Urban area: A Comparison of two soil sealing types, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16298, https://doi.org/10.5194/egusphere-egu21-16298, 2021.