Evapotranspiration Measurements Research Articles

Evaluating four gap-filling methods for eddy covariance measurements of evapotranspiration over hilly crop fields

Abstract. Estimating evapotranspiration in hilly watersheds is paramount for managing water resources, especially in semiarid/subhumid regions. The eddy covariance (EC) technique allows continuous measurements of latent heat flux (LE). However, time series of EC measurements often experience large portions of missing data because of instrumental malfunctions or quality filtering. Existing gap-filling methods are questionable over hilly crop fields because of changes in airflow inclination and subsequent aerodynamic properties. We evaluated the performances of different gap-filling methods before and after tailoring to conditions of hilly crop fields. The tailoring consisted of splitting the LE time series beforehand on the basis of upslope and downslope winds. The experiment was setup within an agricultural hilly watershed in northeastern Tunisia. EC measurements were collected throughout the growth cycle of three wheat crops, two of them located in adjacent fields on opposite hillslopes, and the third one located in a flat field. We considered four gap-filling methods: the REddyProc method, the linear regression between LE and net radiation (Rn), the multi-linear regression of LE against the other energy fluxes, and the use of evaporative fraction (EF). Regardless of the method, the splitting of the LE time series did not impact the gap-filling rate, and it might improve the accuracies on LE retrievals in some cases. Regardless of the method, the obtained accuracies on LE estimates after gap filling were close to instrumental accuracies, and they were comparable to those reported in previous studies over flat and mountainous terrains. Overall, REddyProc was the most appropriate method, for both gap-filling rate and retrieval accuracy. Thus, it seems possible to conduct gap filling for LE time series collected over hilly crop fields, provided the LE time series are split beforehand on the basis of upslope–downslope winds. Future works should address consecutive vegetation growth cycles for a larger panel of conditions in terms of climate, vegetation, and water status.

Comparison of Micrometeorological Methods used for the Determination of Actual Evapotranspiration

Comparison of Micrometeorological Methods used for the Determination of Actual Evapotranspiration Levent ŞAYLAN1, Yunus Özkoca2*, Barış Çaldağ1, Fatih Bakanoğulları3 1Istanbul Technical University, Faculty of Aeronautics and Astronautics, Department of Meteorological Engineering, İstanbul *Antalya Provincial Disaster and Emergency Directorate, Antalya 3Atatürk Soil Water and Agricultural Meteorology Research Institute, Kırklareli * * Corresponding Author: yunus.ozkoca@afad.gov.tr Abstract: Among upmost important effects of climatic variability and change are the related variations in type, amount and timing of precipitation at global or regional scales. This aspect causes different effects from the agricultural point of view. Among these effects the drought phenomenon comes to the forefront. Efforts on eliminating the agricultural drought problem are mainly made by irrigation. That’s why the upmost important portion of available water is consumed by agricultural irrigation, globally. Concordantly, the evapotranspiration plays a crucial role on determination of the irrigation water requirement for agriculture and planning the related systems. Usage of the methods on direct measurement of actual evapotranspiration by global means, however; is quite scarce. In this study, the Bowen Ratio Energy Balance and micrometeorological Eddy Covariance Methods, which are widely in use for the measurement of actual evapotranspiration, have been used within the 2009-2010 wheat growing season in Kırklareli. In conclusion, a comparative evaluation of these approaches was done for the advantages and disadvantages by means of their determination abilities of actual evapotranspiration. Key words: Evaporation, Evapotranspiration, Bowen Ratio, Energy Balance, Eddy Covariance

Validation of reference evapotranspiration from Meteosat Second Generation (MSG) observations

This article presents validation results of the grass Reference Evapotranspiration (ETo) product, which is provided operationally by the Satellite Applications Facility on Land Surface Analysis (LSA SAF). ETo is considered to be the evapotranspiration from a hypothetical extensive well-watered field covered with green grass (12 cm high and with 0.23 albedo), under the given down-welling short-wave radiation. The LSA SAF product is estimated from daily solar radiation at the surface via a methodology designed to be applicable to that well-defined reference surface. In line with its definition, the LSA SAF ETo product is based on estimates of the radiative energy available at the surface.Validation of ETo estimates is challenging, given the difficulties of finding reliable in situ measurements that meet the ETo definition. We show that for the site that matches closely the reference surface (Cabauw, The Netherlands), the LSA SAF product outperforms other commonly used methodologies, even when these use in situ observations as input. However, observations taken at other stations, located in areas that do not deviate greatly from the reference surface, put into evidence the high uncertainty in local measurements. It is shown that the LSA SAF ETo product follows well in situ values, with average differences of 0.3 mm/day or less for mid-latitude sites and of the order of 1 mm/day for the Spanish sites near Cordoba and Albacete. Local advection effects cannot be ignored in measurements performed in the latter stations, where summer conditions are mostly warm and dry. In situ ETo measurements performed in those cases with lysimeters within limited fields are higher than satellite estimates due to advection of warm dry air from the vicinity, which acts as an extra source of energy. It is shown that this effect can be parameterized as a function of near surface air temperature. However, it is argued that those local advection effects should not occur in the idealized surface referred above.In contrast to other ETo estimates, the LSA SAF product is not influenced by local aridity or advection effects, and therefore it is particularly appropriate for large scale climate assessments, including drought monitoring (e.g. by considering the ratio of actual and reference evapotranspiration). Additionally, it provides suitable estimates of irrigation requirements in support of water management.

Enhancement of a Parsimonious Water Balance Model to Simulate Surface Hydrology in a Glacierized Watershed

AbstractThe U.S. Geological Survey monthly water balance model (MWBM) was enhanced with the capability to simulate glaciers in order to make it more suitable for simulating cold region hydrology. The new model, MWBMglacier, is demonstrated in the heavily glacierized and ecologically important Copper River watershed in Southcentral Alaska. Simulated water budget components compared well to satellite‐based observations and ground measurements of streamflow, evapotranspiration, snow extent, and total water storage, with differences ranging from 0.2% to 7% of the precipitation flux. Nash Sutcliffe efficiency for simulated and observed streamflow was greater than 0.8 for six of eight stream gages. Snow extent matched satellite‐based observations with Nash Sutcliffe efficiency values of greater than 0.89 in the four Copper River ecoregions represented. During the simulation period 1949 to 2009, glacier ice melt contributed 25% of total runoff, ranging from 12% to 45% in different tributaries, and glacierized area was reduced by 6%. Statistically significant (p < 0.05) decreasing and increasing trends in annual glacier mass balance occurred during the multidecade cool and warm phases of the Pacific Decadal Oscillation, respectively, reinforcing the link between climate perturbations and glacier mass balance change. The simulations of glaciers and total runoff for a large, remote region of Alaska provide useful data to evaluate hydrologic, cryospheric, ecologic, and climatic trends. MWBM glacier is a valuable tool to understand when, and to what extent, streamflow may increase or decrease as glaciers respond to a changing climate.

Making sense of cosmic-ray soil moisture measurements and eddy covariance data with regard to crop water use and field water balance

Changes in soil moisture influence the water availability to crop plants and soil ecological processes like carbon and nutrient cycling, impacting on crop productivity and environmental performance (greenhouse gas emissions, leaching) of agricultural systems. While traditional soil moisture measurements are done using point-based methods, the recent development of the cosmic-ray soil moisture neutron sensor (CRNS) offers the opportunity to measure soil water at the field scale. However, due to its shallow (<300 mm) and variable measurement depth, the relevance of the measurements to crop water use has been questioned. In this paper, we combine point-based soil moisture measurements (soil cores, TDR), areal-based soil moisture and evapotranspiration measurements (CRNS, eddy covariance), and soil-plant systems modelling together to investigate the consistency in measured soil moisture and crop water use with these different methods We also quantify how relevant the CRNS soil moisture measurements are in understanding the water use of cereal crops (wheat and barley). Our results show that crop water uptake from CRNS layers accounted for 50–90% of the total water uptake in dry environments (location, year) with annual rainfall <300 mm, but only 30–50% of the total crop water uptake in wetter environments (locations, years). This demonstrates a higher relevance of CRNS measurements in semi-arid and arid regions where water is a limiting factor for crop growth and other ecological processes. The high temporal resolution of soil moisture data from CRNS can be assimilated with eddy covariance measurements and point measurements in field to better calibrate soil-plant models and to more accurately simulate field water balance.

Surface Renewal Application for Estimating Evapotranspiration: A Review

The estimation of evapotranspiration (ET) is essential for meteorological modeling of surface exchange processes, as well as for the agricultural practice of irrigation management. Hitherto, a number of methods for estimation of ET at different temporal scales and climatic conditions are constantly under investigation and improvement. One of these methods is surface renewal (SR). Therefore, the premise of this review is to present recent developments and applications of SR for ET measurements. The SR method is based on estimating the turbulent exchange of sensible heat flux between plant canopy and atmosphere caused by the instantaneous replacement of air parcels in contact with the surface. Additional measurements of net radiation and soil heat flux facilitate extracting ET using the shortened energy balance equation. The challenge, however, is the calibration of SR results against direct sensible heat flux measurements. For the classical SR method, only air temperature measured at high frequency is required. In addition, a new model suggests that the SR method could be exempted from calibration by measuring additional micrometeorological variables. However, further improvement of the SR method is required to provide improved results in the future.

Rain-fed and irrigated cropland-atmosphere water fluxes and their implications for agricultural production in Southern Amazonia

Cropland has increasingly occupied large areas in Southern Amazonia since the 1990s, yet few direct field-based evapotranspiration (ET) measurements are available. We used the eddy covariance method to measure ET on a farm with multiple cropping cycles per year over September 2015 through February 2017. This period covered two soybean crops and associated rotations in each of two adjacent fields: a rain-fed field planted to soybean, maize, brachiara, and soybean, and an irrigated field planted to soybean, rice, bean, and soybean. Total ET during these crop cycles was 1265 ± 294 mm and 1414 ± 181 mm respectively for rain-fed and irrigated fields compared to 3099 mm of precipitation. Soybean ET was similar in both rain-fed and irrigated fields and ranged from 332 ± 82 mm (October 2015–February 2016) to 423 ± 99 mm (October 2016–February 2017), while rain-fed maize ET was 313 ± 68 mm (February–July 2016). The main difference between the fields was observed in the dry season (June–August 2016) when irrigation was applied for bean production. A crop model validated with the field data, showed that (1) crop transpiration represented 48–52% of ET for both soybean and maize, and (2) that irrigation for soybean planted at the end of the dry season resulted in marginal improvements to crop water productivity. Results provide insight into the use of irrigation to increase cropping frequency rather than yield with regional land and water implications at the Brazilian agricultural frontier.

The effect of land cover type and structure on evapotranspiration from agricultural and wetland sites in the Sacramento–San Joaquin River Delta, California

Water is a limited and valuable resource in California. A large proportion of the fresh water for southern California is supplied by the Sacramento and San Joaquin rivers. With recent efforts to restore large areas of land in the Sacramento–San Joaquin Delta region from farmland to managed wetlands, it is important to investigate the effect of wetland restoration on the local water cycle. We measured evapotranspiration using the eddy covariance method over six different sites in the Sacramento–San Joaquin Delta from 2013 to 2016. The three restored wetlands sites differed in time since restoration and wetland structure, i.e. the area of open water compared to closed vegetation. The three agricultural sites were a flooded rice field, an alfalfa field, and a grazed cattle pasture. In most years, annual evapotranspiration was significantly lower at the drained agricultural sites, with 652 ± 131 mm (2 year average ± standard deviation, SD) for the pasture and 901 ± 24 mm (3 year average ± SD) for the alfalfa field, compared to the two open water wetlands, with 1091 ± 144 mm (4 year average ± SD) for the Mayberry wetland and 1140 ± 67 mm (3 year average ± SD) at the East End wetland. Annual evapotranspiration at the flooded rice site (975 ± 50 mm, 4 year average ± SD) or the closed vegetation wetland (West Pond wetland, 996 ± 63 mm, 4 year average ± SD) was not significantly different from the drained alfalfa site across individual years. Our analysis showed that the structural difference between the wetlands, specifically the fraction of open water compared to closed vegetation, has a large impact on evapotranspiration dynamics. Through analysis of normalized equilibrium evaporation and nighttime evapotranspiration measurements we deduced that evapotranspiration at the wetland with a low fraction of open water surfaces was almost entirely dominated by plant transpiration with very little contribution from evaporation, despite the fact that the site was flooded and water was readily available. Both evaporation and transpiration contributed substantially to evapotranspiration at the two wetlands with larger fraction of open water surfaces. At the closed canopy site evaporation from subcanopy water seemed to be inhibited through two mechanisms: first, the closed canopy prevented heating of the water column and led to significantly cooler water temperatures, which reduced surface vapor pressure. Second, the closed canopy decoupled the water surface from the atmosphere and inhibited turbulent transport of water vapor away from the water surface. Our results provide valuable insights into the water use in California wetlands and can inform decisions on how to maximize water conservation during wetland restoration.

Dampak Perubahan Tataguna Lahan Terhadap Keseimbangan Air Wilayah Pulau Seram. Studi Kasus : Das Way Pia Di Kabupaten Maluku Tengah, Provinsi Maluku

This research was conducted to estimate the values of parameters which described the physical characteristics of catchments area and monthly water balance components, and understand the sensitivity of the water balance components to change in the parameter value due to the physical changes occurring in Way Pia catchments area, Ceram Island. The method used involved calculation of the regional water balance in the current time (normal) and during the time of land use change, using an evapoclimatonomy model. The main model inputs were monthly rainfall, global radiation and run off. Analyses of data were conducted with five steps as follows : (1) mathematical formulation of the evapoclimatonomy model, (2) algorithm formation and transfer to computer code, (3) establishment of parameters and calibration, (4) validation of model, and (5) experimentation of model. The current physical conditions of Way Pia catchments area were characterized by: average of parameter value of albedo, a = 0,16; rainfall threshold, Pn = 100 mm; surface run off ratio, np = 0,23; evapority, ep = 0,42; measure of soil water loss from sub surface, vN = 0,12; and measure of evapotranspiration of soil water from sub surface, vE = 0,20. Model output of the current water balance consisted of annual value of soil moisture, m = 272 mm; total evapotranspiration, E = 1393 mm, and total run off, N = 920 mm. The land use change in the form of land clearing will increase the parameter values of a and np, which affect on increasing of direct run off (N') of 13% as compared to current conditions.

Evapotranspiration is resilient in the face of land cover and climate change in a humid temperate catchment

AbstractIn temperate humid catchments, evapotranspiration returns more than half of the annual precipitation to the atmosphere, thereby determining the balance available to recharge groundwaters and support stream flow and lake levels. Changes in evapotranspiration rates and, therefore, catchment hydrology could be driven by changes in land use or climate. Here, we examine the catchment water balance over the past 50 years for a catchment in southwest Michigan covered by cropland, grassland, forest, and wetlands. Over the study period, about 27% of the catchment has been abandoned from row‐crop agriculture to perennial vegetation and about 20% of the catchment has reverted to deciduous forest, and the climate has warmed by 1.14 °C. Despite these changes in land use, the precipitation and stream discharge, and by inference catchment‐scale evapotranspiration, have been stable over the study period. The remarkably stable rates of evapotranspirative water loss from the catchment across a period of significant land cover change suggest that rainfed annual crops and perennial vegetation do not differ greatly in evapotranspiration rates, and this is supported by measurements of evapotranspiration from various vegetation types based on soil water monitoring in the same catchment. Compensating changes in the other meteorological drivers of evaporative water demand besides air temperature—wind speed, atmospheric humidity, and net radiation—are also possible but cannot be evaluated due to insufficient local data across the 50‐year period. Regardless of the explanation, this study shows that the water balance of this landscape has been resilient in the face of both land cover and climate change over the past 50 years.

Water budget, performance of evapotranspiration formulations, and their impact on hydrological modeling of a small boreal peatland-dominated watershed

Peatlands occupy around 13% of the land cover of Canada, and thus they play a key role in the water balance at high latitudes. They are well known for having substantial water loss due to evapotranspiration. Since measurements of evapotranspiration are scarce over these environments, hydrologists generally rely on models of varying complexity to evaluate these water exchanges in the global watershed balance. This study quantifies the water budget of a small boreal peatland-dominated watershed. We assess the performance of three evapotranspiration models in comparison with in situ observations and the impact of using these models in the hydrological modeling of the watershed. The study site (∼1 km2) is located in the eastern James Bay lowlands, Québec, Canada. During summer 2012, an eddy flux tower measured evapotranspiration continuously, while a trapezoidal flume monitored streamflow at the watershed outlet. We estimated evapotranspiration with a combinational model (Penman), a radiation-based model (Priestley–Taylor), and a temperature-based model (Hydro-Québec), and performed the hydrological modeling of the watershed with HYDROTEL, a physically based semi-distributed model. Our results show that the Penman and Priestley–Taylor models reproduce the observations with the highest precision, while a substantial drop in performance occurs with the Hydro-Québec model. However, these discrepancies did not appear to reduce the hydrological model efficiency, at least from what can be concluded from a 3-month modeling period. HYDROTEL appears sensitive to evapotranspiration inputs, but calibration of model parameters can compensate for the differences. These findings still need to be confirmed with longer modeling periods.

Hydro-ecological controls on dissolved carbon dynamics in groundwater and export to streams in a temperate pine forest

Abstract. We studied the export of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) from forested shallow groundwater to first-order streams, based on groundwater and surface water sampling and hydrological data. The selected watershed was particularly convenient for such study, with a very low slope, with pine forest growing on sandy permeable podzol and with hydrology occurring exclusively through drainage of shallow groundwater (no surface runoff). A forest plot was instrumented for continuous eddy covariance measurements of precipitation, evapotranspiration, and net ecosystem exchanges of sensible and latent heat fluxes as well as CO2 fluxes. Shallow groundwater was sampled with three piezometers located in different plots, and surface waters were sampled in six first-order streams; river discharge and drainage were modeled based on four gauging stations. On a monthly basis and on the plot scale, we found a good consistency between precipitation on the one hand and the sum of evapotranspiration, shallow groundwater storage and drainage on the other hand. DOC and DIC stocks in groundwater and exports to first-order streams varied drastically during the hydrological cycle, in relation with water table depth and amplitude. In the groundwater, DOC concentrations were maximal in winter when the water table reached the superficial organic-rich layer of the soil. In contrast, DIC (in majority excess CO2) in groundwater showed maximum concentrations at low water table during late summer, concomitant with heterotrophic conditions of the forest plot. Our data also suggest that a large part of the DOC mobilized at high water table was mineralized to DIC during the following months within the groundwater itself. In first-order streams, DOC and DIC followed an opposed seasonal trend similar to groundwater but with lower concentrations. On an annual basis, leaching of carbon to streams occurred as DIC and DOC in similar proportion, but DOC export occurred in majority during short periods of the highest water table, whereas DIC export was more constant throughout the year. Leaching of forest carbon to first-order streams represented a small portion (approximately 2 %) of the net land CO2 sink at the plot. In addition, approximately 75 % of the DIC exported from groundwater was not found in streams, as it returned very fast to the atmosphere through CO2 degassing.

A Subsurface Drip Irrigation System for Weighing Lysimetry

Abstract. Large, precision weighing lysimeters can have accuracies as good as 0.04 mm equivalent depth of water, adequate for hourly and even half-hourly determinations of evapotranspiration (ET) rate from crops. Such data are important for testing and improving simulation models of the complex interactions of surface water and energy balances, soil physics, plant growth, and biophysics that determine crop ET in response to rapid microclimate dynamics. When crops are irrigated with sprinkler systems or other rapid additions of water, the irrigation event is typically short enough that not much ET data are compromised by the lysimeter mass change due to irrigation. In contrast, subsurface drip irrigation (SDI) systems may take many hours to apply an irrigation, during which time the lysimeter mass change is affected by both ET rate and irrigation application rate. Given that irrigation application rate can be affected by pressure dynamics of the irrigation system, emitter clogging and water viscosity changes with temperature over several-hour periods, it can be difficult to impossible to separate the ET signal from the interference of the irrigation application. The inaccuracies in the data can be important, particularly for comparisons of sprinkler and SDI systems, since they are of the order of 8 to 10% of daily ET. We developed an SDI irrigation system to apply irrigations of up to 50 mm to large weighing lysimeters while limiting the period of lysimeter mass change due to irrigation delivery to approximately ten minutes by storing the water needed for irrigation in tanks suspended from the lysimeter weighing system. The system applied water at the same rate as the SDI system in the surrounding field, allowed irrigation over periods of any duration, but often exceeding 12 h, without directly affecting lysimeter mass change and the accuracy of ET rate determinations, and allowed irrigation overnight without compromising lysimeter daily ET measurements. Errors in lysimeter ET measurements using the previous SDI system, which was directly connected to the field irrigation system, were up to 10% of daily ET compared with negligible error using the new system. Errors using the previous, directly connected, SDI system varied over time due to variable system pressure, and possibly due to water temperature (viscosity) changes and emitter clogging. With the new system, all of the water transferred to the lysimeter weighing system was eventually applied by the SDI system regardless of temperature, pressure, or emitter clogging. Differences between planned and applied irrigation depth were less than 2% over the irrigation season. Keywords: Evapotranspiration, ET, Subsurface drip irrigation, SDI, Weighing lysimeter.

Lysimeters in Vadose Zone Research

Core Ideas Lysimeters bridge the gap between laboratory and field experiments. Lysimeters allow the measurement of a complete water balance. Lysimeters provide a complete mass balance for a representative ecosystem segment. Lysimeters are methodological experimental tools to study the water and matter fluxes in the vadose zone, as well as the environmental fate of chemicals. Lysimeters are available in various types. The most sophisticated lysimeters are filled monolithically, are equipped with a pressure‐controlled lower boundary, and are weighable, allowing the measurement of hydraulic fluxes, i.e., rainfall, drainage, evapotranspiration, dew, and hoar frost with high precision. This special section of Vadose Zone Journal reports on current lysimeter research.

Evaporation dominates evapotranspiration on Alaska’s Arctic Coastal Plain

ABSTRACT The dynamics of evapotranspiration (ET), such as the partitioning to evaporation and transpiration, of polygonal ground on the Arctic Coastal Plain are not well understood. We assessed ET dynamics, including evaporation and transpiration partitioning, created by microtopographic features associated with high- and low-centered polygons. Chamber ET and leaf-level transpiration measurements were conducted in one-week field campaigns in two growing seasons with contrasting weather conditions. We found that ET was greater in the drier and warmer sampling period (2013) compared to the colder and wetter one (2014). Evaporation dominated ET, particularly in the wetter and colder sampling period (>90% in 2014 vs. 80% in 2013). In the 2013 sampling period, wetter and warmer conditions increased ET and the contribution of transpiration to ET. If the soils warm with degrading permafrost, ET and the fraction contributed by transpiration may increase to a certain threshold, when moisture must increase with rising temperatures to further increase these fluxes. While the fraction of transpiration may rise with warmer soils, it is unlikely that transpiration will completely dominate ET. This work highlights the complexities of understanding ET in this dynamic environment and the importance of understanding differences across polygonal ground.

Actual Evapotranspiration of Unirrigated Grass in a Smart Field Lysimeter

Core Ideas Small smart lysimeters make high‐quality research of low and dense crops easy. FAO 56 can estimate well the reference crop ET on non‐ or mildly stressed sites. Weather station grass (not water stressed) has lower ET than the reference crop. Lysimeters are basic instruments for evapotranspiration measurement. This study characterized the actual evapotranspiration of unirrigated and unfertilized grass in a warm region of the Czech Republic on a Chernozem loamy soil. An SFL‐300 weighing lysimeter (diameter 0.3 m, depth 0.3 m) was used for this purpose. The suction at its bottom was maintained at the same level as in the native soil nearby. We selected 585 rainless days with regular records for the analysis of daily differences. On most days, the lysimeter‐measured actual evapotranspiration, ETa, was smaller than the Penman–Monteith FAO 56 reference crop evapotranspiration, ET0. The FAO 56 procedure was found to be a reasonable estimator of the unstressed evapotranspiration in a moderately stressed environment. The ETa/ET0 ratio and the canopy surface resistance, rs, depend on the soil water content and suction measured at 5 cm. These graphs break down into horizontal unstressed parts and declining (for ETa/ET0) or inclining (for rs) water‐stressed parts. The ratio ETa/ET0 is about 85% and rs is about 250 s m−1 when the grass is not under water stress. The annual curve of the unstressed crop coefficient has a sine shape. An energy balance criterion suggests that advection of heat is important in winter but not so much in summer. The study provides parameters of evapotranspiration for a canopy that can be found on many standard weather stations and demonstrates that high‐quality research into evapotranspiration of low, dense, and shallow‐rooting crops is possible with small lysimeters of this type.

Water requirements of short rotation poplar coppice: Experimental and modelling analyses across Europe

Poplars are among the most widely used short rotation woody coppice (SRWC) species but due to their assumed high water use, concerns have been raised with respect to large-scale exploitation and potentially detrimental effects on water resources. Here we present a quantitative analysis of the water requirements of poplar SRWC using experimental data and a soil water balance modelling approach at three different sites across Europe. We used (i) eddy covariance (EC) measurements (2004–2006) at an irrigated SRWC grown on a previous rice paddy in northern Italy, (ii) Bowen ratio and energy balance (BREB) measurements (2008–2015) and EC (2011–2015) at a SRWC in rain-fed uplands in the Czech Republic, and (iii) EC measurements (2010–2013) at a SRWC on a previous agricultural land with a shallow water table in Belgium. Without any calibration against water balance component measurements, simulations by the newly developed soil water balance model R-4ET were compared with evapotranspiration (ET) measurements by EC and BREB with a resulting mean root mean square error (RMSE) of 0.75 mm day−1. In general, there was better agreement between EC and the model (RMSE = 0.66 mm day−1) when EC data were adjusted for lack of energy balance closure. A comparison of the simulated and measured soil water content yielded a mean RMSE of 0.03 m3 m−3. The mean annual crop coefficient, i.e. the ratio between actual and reference ET, was 0.82 (ranging from 0.65 to 0.95) while the monthly maxima reached 1.16. These values indicated that ET of poplar SRWC was significantly lower than ET of a well-watered grass cover at the annual time scale, but exceeded ET of the reference cover at shorter time scales during the growing season. We show that the model R-4ET is a simple, yet reliable tool for the assessment of water requirements of existing or planned SRWC. For very simple assessments on an annual basis, using a crop coefficient of 0.86 (adjusted to a sub-humid climate), representing an average value across the three sites in years with no evident drought stress, is supported by this analysis.

Portable canopy chamber measurements of evapotranspiration in corn, soybean, and reconstructed prairie

Evapotranspiration (ET) is a vital component of a field water balance. Canopy chambers are a promising method for determining crop ET because they are portable and applicable at a relatively small plot (m2) scale. Although a variety of canopy chamber designs have been proposed, field tests are still necessary to evaluate chamber performance for measuring crop ET. The objectives of this study are (1) to construct and use an improved canopy chamber to measure ET of three crops [corn (Zea mays, L.), soybean (Glycine max), and reconstructed mixed prairie] and (2) to compare the canopy chamber measurements with flux tower results and field water balance measurements (i.e., rainfall, soil water storage, ET and drainage). Three cropping systems including corn/soybean in a corn-soybean rotation, and reconstructed mixed prairie were studied in central Iowa. Canopy chamber daytime measurements were performed on 18days in 2013 (a relatively dry growing season) and on 15days in 2014 (a relatively wet growing season). Based on the results, the differences in daily ET and seasonal cumulative ET between canopy chambers and an eddy covariance flux tower over the measurement periods were within 5%, providing evidence that the portable canopy chamber can accurately measured ET. The chamber ET values and field water balance ET values had similar patterns over the 2013 and 2014 measurement periods, and the differences of cumulative results were less than 10%. In conclusion, the canopy chamber was proven to be an effective method for measuring small plot ET.

Measurement and Partitioning of Evapotranspiration for Application to Vadose Zone Studies

Core Ideas Partitioned evaporation and transpiration is important for validating vadose zone models. New partitioning approaches overcome spatiotemporal limitations of previous methods. Some techniques can be applied to existing data to increase E and T observations. Intercomparisons of approaches at a variety of field sites are needed to better assess each approach. Partitioning evapotranspiration (ET) into its constituent components, evaporation (E) and transpiration (T), is important for numerous hydrological purposes including assessing impacts of management practices on water use efficiency and improved validation of vadose zone models that parameterize E and T separately. However, most long‐established observational techniques have short observational timescales and spatial footprints, raising questions about the representativeness of these measurements. In the past 15 yr, new approaches have allowed ET partitioning at spatial scales ranging from the pedon to the globe and at long timescales. In this update, we review some recent methodological developments for partitioning ET. These include micrometeorological approaches involving the flux variance partitioning of high‐frequency eddy covariance observations and proxies for photosynthesis and transpiration such as measurements of isotopic fractionation and carbonyl sulfide uptake. We discuss advances in partitioning the energy balance between canopy and soil using remote sensing. We conclude that the flux variance partitioning with raw eddy covariance data and the two‐source energy balance approaches with remote sensing platforms may have the greatest potential for partitioning ET, in part because large public repositories of eddy covariance and satellite data could be readily reprocessed to partition ET.

Assessing Crop Coefficients for Natural Vegetated Areas Using Satellite Data and Eddy Covariance Stations.

The Food and Agricultural Organization (FAO) method for potential evapotranspiration assessment is based on the crop coefficient, which allows one to relate the reference evapotranspiration of well irrigated grass to the potential evapotranspiration of specific crops. The method was originally developed for cultivated species based on lysimeter measurements of potential evapotranspiration. Not many applications to natural vegetated areas exist due to the lack of available data for these species. In this paper we investigate the potential of using evapotranspiration measurements acquired by micrometeorological stations for the definition of crop coefficient functions of natural vegetated areas and extrapolation to ungauged sites through remotely sensed data. Pastures, deciduous and evergreen forests have been considered and lower crop coefficient values are found with respect to FAO data.