HS10.4

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.

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.

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.

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.

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.

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 R_n. 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 (PE_fine), coarse sand (PE_coarse) and gravel sand (PE_gravel) 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.

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.