Monthly evapotranspiration (ET) rates for 1979–2015 were estimated by the latest, calibration‐free version of the complementary relationship (CR) of evaporation over the conterminous United States. ...The results were compared to similar estimates of three land surface models (Noah, VIC, Mosaic), two reanalysis products (National Centers of Environmental Protection Reanalysis II, ERA‐Interim), two remote‐sensing‐based (Global Land Evaporation Amsterdam Model, Penman‐Monteith‐Leuning) algorithms, and the spatially upscaled eddy‐covariance ET measurements of FLUXNET‐MTE. Model validations were performed via simplified water‐balance derived ET rates employing Parameter‐Elevation Regressions on Independent Slopes Model precipitation, United States Geological Survey two‐ and six‐digit Hydrologic Unit Code (HUC2 and HUC6) discharge, and terrestrial water storage anomalies from Gravity Recovery and Climate Experiment, the latter for 2003–2015. The CR outperforms all other multiyear mean annual HUC2‐averaged ET estimates with root‐mean‐square error = 51 mm/year, R = 0.98, relative bias of −1%, and Nash‐Sutcliffe efficiency = 0.94, respectively. Inclusion of the Gravity Recovery and Climate Experiment data into the annual water balances for the shorter 2003–2015 period does not have much effect on model performance. Similarly, the CR outperforms all other models for the linear trend of the 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 measurements, a benefit that clearly diminishes at the HUC2 scale. As the employed CR is calibration‐free and requires only very few meteorological inputs, yet it yields superior ET performance at the regional scale, it may serve as a diagnostic and benchmarking tool for more complex and data intensive models of terrestrial evapotranspiration rates.
Key Points
Nine gridded ET products were evaluated against water‐balance estimates of the mean—and linear trend in—annual ET rates over HUC2 and HUC6 basins of the conterminous United States
The calibration‐free CR with only minimal meteorological data requirement performed the best overall
Due to its performance, data efficiency, and simple structure the calibration‐free CR could serve as a benchmarking and diagnostic tool for more complex models
Drought, a slow‐growing natural disaster that affects large areas worldwide, has serious social and economic consequences. There is compelling evidence to suggest that drought will be more prevalent ...in various regions in the future, due to the impact of climate change. Evapotranspiration is an important drought evolution control factor, and various evapotranspiration models show different drought evolution trends. In the study described here, meteorological data from 840 Chinese weather stations, for 1960–2018, were used to estimate potential evapotranspiration (PET), using the Thomthwaite (TH), Hargreaves (HG), Priestley–Taylor (PT), Penman and Penman–Monteith (PM) models. We then compared various drought indices (the standardized precipitation index SPI, the standardized precipitation evapotranspiration index SPEI, and the Palmer drought severity index PDSI), calculated using inputs from the five evapotranspiration models. The results showed that (a) evapotranspiration means and trends, as estimated by the five PET models, were significantly different, with Penman giving the highest estimate while TH the lowest. Differences between the PET model outcomes reduced under climate warming scenarios; (b) significantly different evapotranspiration models and drought indices can give rise to varied results, with the climate becoming wetter under the Penman and PM models (especially in western northwest China and eastern south China), and drier under the TH model; (c) the different drought forecasts obtained using the evapotranspiration models decreased with increasing precipitation, for all regions; (d) the SPEI is more suitable for drought monitoring compared with PDSI due to the higher sensitive to the choice of evapotranspiration model of SPEI; and (e) SPEI‐PM and SPEI‐Penman is the best drought indices in all regions. SPEI‐PT and SPEI‐HG can be applied in regions with relatively abundant precipitation (northern north China, southern north China, eastern south China, western south China, and Tibetan Plateau), and SPEI‐PM and SPEI‐Penman can be suitable for drought monitoring in arid and semi‐arid regions (western northwest China and eastern northwest China). The results presented herein are expected to aid in the selection of a proper drought index for disaster assessment.
SPEI results covering 1960–2018 for China and its subregions. Drought evolution detected using different drought indices and PET models showed different trends. It was also apparent in the regions that trend differences between different models were reduced with increasing precipitation. Additionally, PDSI result differences from using different PET models were smaller than those achieved by the SPEI, implying that the SPEI is more sensitive to the choice of evapotranspiration model.
Evapotranspiration is a major component of the interaction between land‐surface processes and the atmosphere. Climate Forecast System Reanalysis (CFSR) data offer a promising database for overcoming ...the limitations in availability and reliability of climatological data and, hence, for understanding the evapotranspiration process. Using these data on grid‐by‐grid daily, seasonal and yearly scales, the present study attempts to advance the spatio‐temporal evaluation of two radiation‐based and three temperature‐based methods for estimating potential evapotranspiration (PET) against estimates of grass reference evapotranspiration (ETo) by FAO Penman–Monteith method (FAO‐PM). The analysis was performed for the period 1979–2013, considering the second largest (79 000 km2) river system in Ethiopia, that is, Omo‐Gibe basin, which accommodates national parks and vast hydropower, cultivation and afforestation developments and discharges its flow to Lake Turkana in Kenya. Despite the large regional variations in climate and elevation, the results in overall emphasize the outperformance of the simple temperature method, viz. Hargreaves–Samani method, in capturing both the annual and seasonal FAO‐PM estimates. Calibration of the Hargreaves–Samani equation is, however, a requisite for spectacular improvement of its performance. Accordingly, new coefficients of the equation are proposed. The annual trends in the basin's ETo increased with rising temperature and decreasing relative humidity, wind speed, and solar radiation, but with decreasing (increasing) rainfall in the upper region (the middle and lower regions). It is deduced that trends in simple methods do not necessarily reflect the true trends in ETo. Annual ETo decreases with increasing elevation and annual rainfall. The present findings are discussed in the context of a worldwide literature, thereby improving the understanding of the best performing PET methods in similar data‐scarce national or transboundary rivers basin in Ethiopia, the region or worldwide. The wider implications regarding water loss from reservoirs and the rain‐fed food and sugar production in the basin under study are also highlighted.
Using Climate Forecast System Reanalysis (CFSR) data in ecologically and climatically varying Omo‐Gibe River Basin in Ethiopia, commonly used potential evapotranspiration formulae were spatio‐temporally evaluated versus physically‐sound FAO Penman–Monteith methods.
Despite its simplicity, Hargreaves–Samani method outperformed and, with calibrated coefficients, it even greatly improved the estimates on daily, seasonal and annual scales.
Beside giving insights into the outstanding method in similar data‐scarce basins, the present study highlights implications of the observed climatic trends for water availability and food security in the basin.
The occurrence of water shortages ascribed to projected climate change, especially in the Mediterranean region, fosters the interest in remote sensing (RS) applications to optimize water use in ...agriculture. Remote sensing evapotranspiration and water demand estimation over large cultivated areas were used to manage irrigation to minimize losses during the crop growing cycle. The research aimed to explore the potential of the MultiSpectral Instrument (MSI) sensor on board Sentinel-2A to estimate crop parameters, mainly surface albedo (α) and Leaf Area Index (LAI) that influence the dynamics of potential evapotranspiration (ETp) and Irrigation Water Requirements (IWR) of processing tomato crop (Solanum lycopersicum L.). Maximum tomato ETp was calculated according to the FAO Penman-Monteith equation (FAO-56 PM) using appropriate values of canopy parameters derived by processing Sentinel-2A data in combination with daily weather information. For comparison, we used the actual crop evapotranspiration (ETa) derived from the soil water balance (SWB) module in the Environmental Policy Integrated Climate (EPIC) model and calibrated with in-situ Root Zone Soil Moisture (RZSM). The experiment was set up in a privately-owned farm located in the Tarquinia irrigation district (Central Italy) during two growing seasons, within the framework of the EU Project FATIMA (FArming Tools for external nutrient Inputs and water Management). The results showed that canopy growth, maximum evapotranspiration (ETp) and IWR were accurately inferred from satellite observations following seasonal rainfall and air temperature patterns. The net estimated IWR from satellite observations for the two-growing seasons was about 272 and 338 mm in 2016 and 2017, respectively. Such estimated requirement was lower compared with the actual amount supplied by the farmer with sprinkler and drip micro-irrigation system in both growing seasons resulting in 364 (276 mm drip micro-irrigation, and 88 mm sprinkler) and 662 (574 mm drip micro-irrigation, and 88 mm sprinkler) mm, respectively. Our findings indicated the suitability of Sentinel-2A to predict tomato water demand at field level, providing useful information for optimizing the irrigation over extended farmland.
•Remote sensing (RS) for crop water requirements calculation is proposed.•Validations of the canopy variables estimated by RS are reported.•Potential ET by RS was compared with actual ET modelled by EPIC.•Results confirm the usefulness of RS data in supporting irrigation scheduling.
Various empirical methods that use meteorological data have been developed for estimating evapotranspiration. However, there are currently no online tools available for the estimation of daily ...evapotranspiration based on user-provided daily data. Here, we introduce ETCalc (
https://etcalc.hydrotools.tech
), a free, unique online tool that integrates eight methods (i.e. Penman-Monteith, Thornthwaite, Blaney - Criddle, Turc, Priestley - Taylor, Hargreaves, Jensen - Haise and Abtew) for estimation of daily potential evapotranspiration, reference evapotranspiration and, by employing user-defined crop (or cover) coefficients, daily actual evapotranspiration, based on user-provided daily meteorological data. ETCalc has been developed in response to the effort of the Canadian federal government to encourage easier and open access to science and is applicable to any area for which basic meteorological data are available and hence, its suitability is not restricted to particular geographical areas. Through a streamlined interface, ETCalc allows for uploading of user-provided data, tabular and graphical inspection of the input and output data, as well as export of the output data and of the associated metadata. The use of ETCalc is exemplified using 10-year daily meteorological data from Charlottetown, Prince Edward Island, Canada for comparing the output from each of the ETCalc methods and for the calculation of the precipitation deficit.
•FAO56 advances in ET computation are retrospectively reviewed and analyzed.•Definition, procedures and advances in computing reference ET are reviewed.•Estimation, use and transferability of Kc's ...and their adjustment to local conditions are discussed.•The estimation of evaporation and transpiration with the dual Kc is analyzed.•ET estimation under non-standard conditions and with remote sensing are reviewed.
The FAO Irrigation and Drainage Paper No 56 on Crop Evapotranspiration has been in publication for more than 15 years. The paper advanced the accuracy and consistency of operational computation of evapotranspiration (ET) for agricultural and other land use types. The paper included updated definition and procedures for computing reference ET, an update on estimating crop coefficients (Kc), the adoption of the dual Kc for separate estimation of crop transpiration and soil evaporation, and an upgraded estimation of crop ET under water and salt stress and other non-standard conditions. These advances are retrospectively reviewed in this paper. The advances in computing reference ET were primarily through the adoption of specific and consistent characteristics for the grass reference crop using the Penman–Monteith equation parameterized to represent a living reference surface. That standardization made the Kc more visual and understandable as a factor that relates the ET characteristics of a specific crop to the defined reference crop. Methodologies were introduced to estimate reference ET under conditions of limited weather data while retaining the use of the PM equation. Advances in adopted Kc research included techniques to estimate Kc based on the architecture of crops, notably height and fraction of ground cover. Other advances included consistent and straight-forward techniques for applying the dual Kc method via soil and evaporation process modeling on a daily timestep. New techniques were introduced for using yield response and salinity threshold values to estimate reductions in ET caused by elevated soil salinity. In addition, recommendations were given for adjusting ET for impacts of surface mulching, intercropping, and sparse vegetation. The successful adoption of the FAO–PM reference ET and Kc approaches owes primarily to the simplicity, yet relatively high level of robustness of the procedures, and to transferability and repeatability of the Kc method. Future development needs are discussed.
Accurate simulation of evapotranspiration is of substantial importance to hydrology, ecology, agriculture, and water resources management. Evapotranspiration is equal to the fraction of potential ...evapotranspiration (PET) constrained by soil water. PET can be calculated from meteorological observations with a wide global distribution and high density. However, it is necessary to determine how to accurately simulate daily evapotranspiration through PET. We have developed a non-linear function for simulating evapotranspiration through PET constrained by soil water at daily scale. The evaluation results show that the accuracy of the evapotranspiration simulation using the non-linear function was higher than that of linear relations and complementary relationship (CR) methods. In the temperature-based PET equations, the Hargreaves-Samani equation was the closest to the Penman-Monteith calculation values. The simulation accuracy of the CR methods obviously improved after parameter calibration. The accuracy has a large variability at the global scale. Daily evapotranspiration can be simulated with PET data in some regions with a high accuracy (Nash and Sutcliffe efficiency coefficient > 0.60), including most regions of Eurasia, eastern and southern North America, and northern South America. However, other regions showed a poor performance (Nash and Sutcliffe efficiency coefficient < 0.20), including western North America, the Mediterranean region, and the eastern and western coastal regions of Australia. Our results indicate that the accurate simulation of daily evapotranspiration can be achieved based on meteorological data in most regions of the world. Owing to the wide distribution of global meteorological observations, the accurate simulation of the daily evapotranspiration method proposed in this study can be applied in other regions across the globe.
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•A non-linear function for simulating daily evapotranspiration is proposed.•The accuracy of the simulation has a large variability at the global scale.•The accurate simulation of evapotranspiration can be achieved in most regions.
Numerous models had been developed to predict the annual evapotranspiration (ET) in vegetated lands across various spatial scales. Fu's (Scientia Atmospherica Sinica, 5, 23–31) and Zhang's (Water ...Resources Research, 37, 701–708) ET simulation models have emerged as highly effective and have been widely used. However, both formulas have the non‐quantitative parameters (m in Fu's model and w in Zhang's model). Based on the collected 1789 samples from global long‐term hydrological studies, this study discovered significant relations between m (or w) and vegetation coverage or greenness in collected catchments. Then, we used these relations to qualify the parameters in both Zhang's and Fu's models. Results show that the ET estimation accuracies of Fu's (or Zhang's) model are significantly improved by about 13.49 mm (or 6.74 mm) for grassland and cropland, 38.52 mm (or 29.84 mm) for forest and shrub land (coverage<40%), 19.74 mm (or 16.17 mm) for mixed land (coverage<40%), respectively. However, Zhang's model shows higher errors compared with Fu's model, especially in regions with high m (or w) values, such as those with dense vegetations or P/E0 (annual precipitation to annual potential ET) smaller than 1.0. Additionally, this study also reveals that for regions with vegetation cover less than 40%, the annual ET is not only determined by vegetation types, but also relates to the sizes of vegetation‐covered areas. Conversely, for regions with vegetation cover more than 40%, the annual ET is mainly determined by the vegetation density rather than vegetation types or vegetation coverage. Thus, linking m (or w) parameters with vegetation greenness allows leveraging remote sensing for forest management in data‐scarce areas, safeguarding regional water resources. This study pioneers integrating vegetation‐related indices with basin parameters, advocating for their crucial role in more effective hydrological modelling.
Zhang's model could be seen as another format of Fu's model for regions where the value is smaller than 3.0. But Zhang's model is obviously not suitable to be applied in regions where the m value is bigger than 3.0. In summary, we would recommend using Fu's model to simulate the annual ET of vegetated lands, especially for regions with dense vegetation covers or P/E0 < 1.0. P is the annual precipitation (mm); E0 is the annual potential evapotranspiration (mm).
Quantifying the impact of climate change on evapotranspiration is necessary for devising accurate water and energy budgets in light of global warming. Nevertheless, in the Middle East and North ...Africa (MENA), little has been done to bridge this gap. This study, then, implements Penman and Budyko approaches to climatic data retrieved from the sixth phase of the Coupled Model Intercomparison Project (CMIP6) to assess evapotranspiration and water availability evolutions through the twenty-first century. Outcomes reveal that the MENA region is indeed vulnerable to a surge in temperature, which can increase evapotranspiration losses and decrease water availability. Under the shared socioeconomic pathway (SSP2-4.5), the potential evapotranspiration (PET) has been projected to increase throughout the MENA region by up to 0.37 mm per year during the middle of the twenty-first century (2021–2050) and by up to 0.51 mm per year during the end of the twenty-first century (2071–2100). Meanwhile, the actual evapotranspiration (AET) has been projected to increase by up to 0.3 (~0.2) mm per year before 2050 (2100). The trends in both projections (PET and AET) are exaggerated under SSP5-8.5. The analysis predicted a shortage of water availability (precipitation—AET), which is alarming for most MENA regions. Relative to the reference period (1981–2010), the decline in annual water availability would reach 26 (62) mm by 2100 under SSP2-4.5 (SSP5-8.5). The rise in temperatures appears to be the principal reason for MENA and water availability responses. This study’s outcomes can facilitate accurate and realistic predictions related to evapotranspiration and water availability, which are key elements in not only managing water resources but also in devising effective climate change mitigation and adaptation plans.
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