Estimated Evapotranspiration Rates Research Articles

Performance of potential evapotranspiration models across different climatic stations in New South Wales, Australia

Study regionNew South Wales, southeast Australia Study focusEstimating potential evapotranspiration (ETp) rates, detecting its temporal trends and analysing its interannual oscillation are critical for long-term assessment of water availability and regional drought. This study aimed to evaluate the comprehensive performance of 12 simplified models in characterising ETp against the benchmark Penman model across different climate sites in southeast Australia. This study used Taylor skill score (S), normalised root mean square error (nRMSE) and relative mean bias error (rMBE) to estimate models’ capability in estimating ETp rates. Then, this study adopted Mann-Kendall test and continuous wavelet transform (CWT) to test temporal trends and periodicity of ETp estimated by all models. New hydrology insights for the regionJensen-Haise model was capable to produce fair (nRMSE ≤ 30%) estimates of daily ETp across all stations. Models except Mak1 were generally able to produce reasonable estimates of ETp at larger time scale. In addition, we found that the 12 alternative ETp models generally agreed with the Penman model on the primary (9.6–12.4-year) and quasi (2.6–3.9-year) periods of ETp, but they did not show matchable ability in detecting ETp temporal trends. The comprehensive investigation on models’ performance will shed light on models’ selection in estimation of drought and hydrological cycle.

Prediction of Evapotranspiration in the Pampean Plain from CERES Satellite Products and Machine Learning Techniques

A key aspect in agricultural zones, such as the Pampean Plain of Argentina, is to accurately estimate evapotranspiration rates to optimize crops and irrigation requirements and the floods and droughts prediction. In this sense, we evaluate six machine learning approaches to estimate the reference and actual evapotranspiration (ET0 and ETa) through CERES satellite products data. The results obtained applying machine learning techniques were compared with values obtained from ground-based information. After training and validating the algorithms, we observed that Support Vector machine-based Regressor (SVR) showed the best accuracy. Then, with an independent dataset, the calibrated SVR were tested. For predicting the reference evapotranspiration, we observed statistical errors of MAE = 0.437 mm d−1, and RMSE = 0.616 mm d−1, with a determination coefficient, R2, of 0.893. Regarding actual evapotranspiration modelling, we observed statistical errors of MAE = 0.422 mm d−1, and RMSE =0.599 mm d−1, with a R2 of 0.614. Comparing the results obtained with the machine learning models developed another studies in the same field, we understand that the results are promising and represent a baseline for future studies. Combining CERES data with information from other sources may generate more specific evapotranspiration products, considering the different land covers.

Prediction of Evapotranspiration in the Pampean Plain from CERES Satellite Products and Machine Learning Techniques

A key aspect in agricultural zones, such as the Pampean Plain of Argentina, is to accurately estimate evapotranspiration rates to optimize crops and irrigation requirements and the floods and droughts prediction. In this sense, we evaluate six machine learning approaches to estimate the reference and actual evapotranspiration (ET0 and ETa) through CERES satellite products data. The results obtained applying machine learning techniques were compared with values obtained from ground-based information. After training and validating the algorithms, we observed that Support Vector machine-based Regressor (SVR) showed the best accuracy. Then, with an independent dataset, the calibrated SVR were tested. For predicting the reference evapotranspiration, we observed statistical errors of MAE = 0.437 mm d−1, and RMSE = 0.616 mm d−1, with a determination coefficient, R2, of 0.893. Regarding actual evapotranspiration modelling, we observed statistical errors of MAE = 0.422 mm d−1, and RMSE =0.599 mm d−1, with a R2 of 0.614. Comparing the results obtained with the machine learning models developed another studies in the same field, we understand that the results are promising and represent a baseline for future studies. Combining CERES data with information from other sources may generate more specific evapotranspiration products, considering the different land covers.

Estimating Evapotranspiration Using Chloride Mass Balance in a New Mexico Paired Basin Study 2009-2019

A paired basin study in the upper Santa Fe River watershed following forest thinning and prescribed burns successfully measured water budget components in a treated and an untreated (control) basin. The paired basin study was established to investigate questions that have arisen regarding changes in water yield from forest treatments. Precipitation, stream flow, soil moisture, and chloride concentrations in precipitation and stream flow were measured to quantify the water budget components. The results from eleven years of data collection and analysis have a high degree of confidence with respect to measuring the water budget components based on the mass balance of water and chloride. The differences in the geologic structure and topography between the two paired basins appeared to impact the water budgets more than the forest treatments, except during periods when winter precipitation and snowmelt represented a significant component of inflow. Although this paired basin study was not able to portray a simple relationship between forest thinning and water yield, the chloride concentration methodology used to estimate evapotranspiration (ET) was successful. These detailed observations of chloride deposition and transport characteristics may be relevant for other researchers working in forested basins with substantial ET. ET rates were estimated by examining the cycle of chloride entering and exiting each basin over six integration periods. ET was estimated to be about 90% to 94% of precipitation in the treated basin and 77% to 86% in the control basin. The higher ET in the treated basin both before and after forest treatments may be due to the much greater area of west-facing hillslopes in the treated basin, which receive warm afternoon sun, and the greater area of rock cover in the control basin. Variation in the chloride concentration of collected precipitation samples from different sites indicates that horizontal precipitation of chloride in the tree canopy is an important consideration when using the chloride mass balance approach to calculate water budget components.

Estimating Evapotranspiration from Commonly Occurring Urban Plant Species Using Porometry and Canopy Stomatal Conductance

Evapotranspiration (ET) is a key moisture flux in both the urban stormwater management and the urban energy budgets. While there are established methods for estimating ET for agricultural crops, relatively little is known about ET rates associated with plants in urban Green Infrastructure settings. The aim of this study was to evaluate the feasibility of using porometry to estimate ET rates. Porometry provides an instantaneous measurement of leaf stomatal conductance. There are two challenges when estimating ET from porometry: converting from leaf stomatal conductance to leaf ET and scaling from leaf ET to canopy ET. Novel approaches to both challenges are proposed here. ET was measured from three commonly occurring urban plant species (Sedum spectabile, Bergenia cordifolia and Primula vulgaris) using a direct mass loss method. This data was used to evaluate the estimates made from porometry in a preliminary study (Sheffield, UK). The Porometry data captured expected trends in ET, with clear differences between the plant species and the reproducible decreasing rates of ET in response to reductions in soil moisture content.

Using multiple methods to investigate the effects of land-use changes on groundwater recharge in a semi-arid area

Abstract. Understanding the applicability and uncertainties of methods for documenting recharge rates in semi-arid areas is important for assessing the successive effects of land-use changes and understanding groundwater systems. This study focuses on estimating groundwater recharge rates and understanding the impacts of land-use changes on recharge rates in a semi-arid area in southeast Australia. Two adjacent catchments were cleared ∼180 years ago following European settlement, and a eucalypt plantation forest was subsequently established ∼15 years ago in one of the catchments. Chloride mass balance analysis yields recharge rates of 0.2 to 61.6 mm yr−1 (typically up to 11.2 mm yr−1). The lower of these values probably represents recharge rates prior to land clearing, whereas the higher likely reflects recharge rates following the initial land clearing. The low pre-land-clearing recharge rates are consistent with the presence of old groundwater (residence times up to 24 700 years) and the moderate-to-low hydraulic conductivities (0.31 to 0.002 m d−1) of the aquifers. Recharge rates estimated from tritium activities and water table fluctuations reflect those following the initial land clearing. Recharge rates estimated using water table fluctuations (15 to 500 mm yr−1) are significantly higher than those estimated using tritium renewal rates (0.01 to 89 mm yr−1; typically <14.0 mm yr−1) and approach the long-term average annual rainfall (∼640 mm yr−1). These recharge rates are unrealistic given the estimated evapotranspiration rates of 500 to 600 mm yr−1 and the preservation of old groundwater in the catchments. It is likely that uncertainties in the specific yield results in the water table fluctuation method significantly overestimating recharge rates, and despite the land-use changes, the present-day recharge rates are relatively modest. These results are ultimately important for assessing the impacts of land-use changes and management of groundwater resources in semi-arid regions in Australia and elsewhere.

Models and measurements of seven years of evapotranspiration on a high elevation site on the Central Tibetan Plateau

Evapotrantaspiration is a crucial part of the hydrological cycle but few ground observatories for the Tibetan Plateau exist. In this study, we present lysimeter measurements from the growing season during seven years at a remote field location on the Tibetan Plateau. The measurements show rates between 2.5 and 3 mm·d−1 during the warmer months from June to August, dropping to 2 to 2.5 mm·d−1 in September. This results in a total volume of evapotranspiration of approximately 300 mm·yr−1 for the months from June to September. The inter-daily variability is however large, and comparison to meteorological variables suggest that this is largely driven by radiation and humidity. Data for a single season from a nearby flux tower allows us to compare the two common measurement methods for evapotranspiration in the field, showing an overall good agreement between the approaches. We also tested commonly applied models used to estimate evapotranspiration rates, namely the FAO-Penman-Monteith (PM) and the Priestly-Taylor (PT) model, which both make use of radiation data as well as the simpler Hargreaves-Samani (HS) and Rohwer (R) models which only need air temperature and wind speed as input. The most data intensive model (PM) has the lowest root mean square error (RMSE) (1.36 mm·d−1) and the mean bias error (MBE) (−0.05 mm·d−1) and reproduces the daily variability generally well. The much simpler HS model performs slightly worse (1.38 and 0.35 mm-d−1), but fails to reproduce the variability, due to its lack of information of local radiation and humidity data. Our results are in line with large scale estimates of evapotranspiration for the cold and arid region, provide a first long time series of in-situ measurements from a high elevation site and suggest that both the PM and HS models are appropriate when no direct measurements are available.

Using GRACE to quantify the depletion of terrestrial water storage in Northeastern Brazil: The Urucuia Aquifer System

Covering a plateau area of approximately 125,000 km2, the Urucuia Aquifer System (UAS) represents a national strategic water resource in the drought-stricken Northeastern part of Brazil. Variations in terrestrial water storage (TWS) extracted using a three-model-ensemble from the Gravity Recovery and Climate Experiment (GRACE) mission showed a negative balance equal to water stress. Monthly GRACE-derived water storage changes from 2002 to 2014 were compared with those derived from an independent hydrologic water balance of the region using in situ measurements and estimated evapotranspiration rates. Trend analyses revealed a TWS depletion rate of 6.5 ± 2.6 mm yr−1, but no significant decline in precipitation as observed from available data records. Water storage depletion was found to be driven by anthropogenic impacts rather than by natural climatic variability. The obtained results demonstrate that GRACE is able to adequately capture water storage changes at the subregional scale, particularly during dry seasons.

Evaluating closed chamber evapotranspiration estimates against eddy covariance measurements in an arctic wetland

Evapotranspiration (ET) is an important hydrological flux with a strong influence on greenhouse gas emissions from thawing permafrost. This study examines the suitability of the closed chamber method in characterising the spatial heterogeneity of ET fluxes in a typical polygonal tundra landscape in the Lena River Delta, northern Siberia. Actual evapotranspiration is compared across scales using: (1) ground-based chamber measurements to observe ET at the microsite scale; and (2) tower-based eddy covariance (EC) measurements which provide spatially averaged ET observations at the ecosystem level. Adopting an upscaling approach using EC estimates as a benchmark, the authors assess the suitability of the closed chamber method in an arctic wetland environment. A short closure time (40 s) and a linear model to describe the change in chamber headspace water vapour concentration were employed to estimate ET rates from the chamber measurements. Using a correction factor chamber fluxes were successfully scaled to the EC data. Yet, findings suggest that the performance of the closed chamber method is highly sensitive to the prevailing hydrometeorological conditions, and it is likely that the sorption and desorption of water molecules to the inside of the chamber and tubing has a strong impact on results. A number of methodological issues are presented in this paper which question the use of closed chamber measurements as a stand-alone tool for measuring ET in arctic wetland environments. However, when paired alongside a trusted benchmark, chambers can provide valuable information on ET at the microsite scale.

Complementary relationship estimation of actual evapotranspiration in extreme cold and arid areas: a case study of the Hotan River Basin, northwest China

AbstractApplication of complementary relationship (CR) approaches using only routine meteorological data is a very convenient method of estimating actual evapotranspiration (ETa). Reanalysis datasets and remote sensing data provide good tools to overcome the difficulties in obtaining observation data. This study of the Hotan River Basin (HRB) in northwest China serves as a prime example for estimation of ETa during 2006–2014 by using the modified generalized CR. Based on comparison and analysis, the maximum potential evaporation calculated by the Penman-based equation was adopted. The estimated ETa rates were verified using a regional water balance method at annual time scales because of the limited available data. The calibration parameter was calibrated based on the elevation and underlying surface types. The mean annual ETa ranged from 2.3 mm to 800 mm during 2006–2014. ETa rates in the plains regions were higher than those in the mountainous regions. Most of ETa was concentrated in the months of May to September. A water deficit occurred in the middle and lower regions, while a water surplus occurred in the upper regions. This study not only provided a new concept for calibration, but also a potential solution for different underlying surfaces and time scales.

Dry Season Evapotranspiration Dynamics over Human-Impacted Landscapes in the Southern Amazon Using the Landsat-Based METRIC Model

Although seasonal and temporal variations in evapotranspiration (ET) in Amazonia have been studied based upon flux-tower data and coarse resolution satellite-based models, ET dynamics over human-impacted landscapes are highly uncertain in this region. In this study, we estimate ET rates from critical land cover types over highly fragmented landscapes in the southern Amazon and characterize the ET dynamics during the dry season using the METRIC (Mapping Evapotranspiration at high Resolution with Internalized Calibration) model. METRIC, a Landsat-based ET model, that generates spatially continuous ET estimates at a 30 m spatial resolution widely used for agricultural applications, was adapted to the southern Amazon by using the NDVI indexed reference ET fraction (ETrF) approach. Compared to flux tower-based ET rates, this approach showed an improved performance on the forest ET estimation over the standard METRIC approach, with R2 = 0.73 from R2 = 0.70 and RMSE reduced from 0.77 mm/day to 0.35 mm/day. We used this approach integrated into the METRIC procedure to estimate ET rates from primary, regenerated, and degraded forests and pasture in Acre, Rondônia, and Mato Grosso, all located in the southern Amazon, during the dry season in 2009. The lowest ET rates occurred in Mato Grosso, the driest region. Acre and Rondônia, both located in the southwestern Amazon, had similar ET rates for all land cover types. Dry season ET rates between primary forest and regenerated forest were similar (p > 0.05) in all sites, ranging between 2.5 and 3.4 mm/day for both forest cover types in the three sites. ET rates from degraded forest in Mato Grosso were significantly lower (p < 0.05) compared to the other forest cover types, with a value of 2.03 mm/day on average. Pasture showed the lowest ET rates during the dry season at all study sites, with the dry season average ET varying from 1.7 mm/day in Mato Grosso to 2.8 mm/day in Acre.

Using evapotranspiration to assess drought sensitivity on a subfield scale with HRMET, a high resolution surface energy balance model

Evapotranspiration (ET) rates provide a valuable within-season indicator of plant productivity, as well as data on fluxes of water in a landscape. Applying remote sensing for ET estimation has potential to improve the sustainable management of water resources in agricultural settings. Most current ET models, however, rely on ‘dry’ and ‘wet’ pixels within a given scene to partition turbulent fluxes between latent and sensible heat, thus limiting their ability to map ET throughout the growing season at extremely high (meter scale) spatial resolutions. Here, we develop a field-validated surface energy balance model, High Resolution Mapping of EvapoTranspiration (HRMET), which requires only basic meteorological data, spatial surface temperature and canopy structure data. We use HRMET to estimate ET rates over two commercial cornfields in south-central Wisconsin during the 2012 growing season, which was characterized by severe drought. HRMET results indicate that the magnitude of within-field variability in ET rates was primarily driven by water availability. The application of remotely sensed data to precision agriculture has also been hampered by turnaround time between image acquisition and availability. We introduce relative ET (ETR), which enables comparison of ET rates between image dates by normalizing for variability caused by weather and crop stage. ETR also provides an intuitive, index-like metric for evaluating spatial variability in ET on a subfield scale. ETR maps illuminate persistent patterns in ET across measurement dates that may be driven by soil water availability and topography. ETR is used to develop a novel paired-image technique that can map subfield sensitivity classes to stressors such as drought. Sensitivity class mapping can be used to circumvent issues related to turnaround time to facilitate the incorporation of remotely sensed data into precision agriculture.

Further evaluation of the Sim-ReSET model for ET estimation driven by only satellite inputs

A simple remote sensing evapotranspiration (ET) model (Sim-ReSET) has been proposed but only tested using field measurements at a site with a semi-arid climate. Its performance for mapping ET using only satellite data remained unknown. In this study, the Sim-ReSET model was further evaluated for ET estimation driven by only MODIS data products. The estimated ET rates were compared with ground-based observational data from a variety of ecosystems and climates across China. The results show that MODIS-based ET estimates are consistent with both the ET measurements from eddy covariance flux towers and those from the Penman-Monteith method combined with micrometeorological data. Evaporation fraction (EF) is indicative of land surface moisture. The derivative EF maps demonstrate that the proposed ET data set obtained from the Sim-ReSET model and MODIS data is capable of capturing the spatio-temporal pattern of land surface moisture for different land covers with different climates. Editor Z.W. Kundzewicz Citation Sun, Z.G., Wang, Q.X., Matsushita, B., Fukushima, T., Ouyang, Z., Watanabe, M., and Gebremichael, M., 2013. Further evaluation of the Sim-ReSET model for ET estimation driven by only satellite inputs. Hydrological Sciences Journal, 58 (5), 994–1012.

Evapotranspiration from pilot-scale constructed wetlands planted with Phragmites australis in a Mediterranean environment

This article reports the results of evapotranspiration (ET) experiments carried out in Southern Italy (Sicily) in a pilot-scale constructed wetland (CW) made of a combination of vegetated (Phragmites australis) and unvegetated sub-surface flow beds. Domestic wastewater from a conventional wastewater treatment plant was used to fill the beds. Microclimate data was gathered from an automatic weather station close to the experimental plant. From June to November 2009 and from April to November 2010, ET values were measured as the amount of water needed to restore the initial volume in the beds after a certain period. Cumulative reference evapotranspiration (ET0) was similar to the cumulative ET measured in the beds without vegetation (ETcon), while the Phragmites ET (ET phr ) was significantly higher underlining the effect of the vegetation. The plant coefficient of P. australis (Kp) was very high (up to 8.5 in August 2009) compared to the typical Kc for agricultural crops suggesting that the wetland environment was subjected to strong “clothesline” and “oasis” effects. According to the FAO 56 approach, Kp shows different patterns and values in relation to growth stages correlating significantly to stem density, plant height and total leaves. The mean Water Use Efficiency (WUE) value of P. australis was quite low, about 2.27 g L−1, probably due to the unlimited water availability and the lack of the plant's physiological adaptations to water conservation. The results provide useful and valid information for estimating ET rates in small-scale constructed wetlands since ET is a relevant issue in arid and semiarid regions. In these areas CW feasibility for wastewater treatment and reuse should also be carefully evaluated for macrophytes in relation to their WUE values.

Genotypic Differences in Water Use Efficiency of Common Bean under Drought Stress

Common bean (Phaseolus vulgaris L.) is extensively grown in production zones where water is limiting. Crop water use efficiency is the ratio of biomass or seed yield produced per unit of water evapotranspired in a particular environment. Transpiration efficiency (TE) is the ratio of yield per unit of water transpired. The objectives of this study were to: (i) determine the water use efficiency (WUE) for six common bean genotypes (BAT 477, Morales, SEN 3, SEN 21, SER 16, and SER 21) in the greenhouse and for two genotypes in the field (Morales and SER 16) and (ii) determine TE for two common bean genotypes using estimated evapotranspiration rates in the field. Three greenhouse trials and two field trials were conducted during 3 yr in Puerto Rico. Three water levels in the greenhouse and two in the field were applied. Actual evapotranspiration was estimated using the generalized Penman–Monteith model based on aerodynamic and surface resistance, and with drainage type lysimeters in the field. Differences among genotypes for WUE were found in the greenhouse experiments, with SEN 3 and SER 21 showing superior WUE in several treatments. In the field, TE and WUE were affected by water level, and TE was consistent with previously reported coefficients for common bean.

ERRATUM “Estimating Evapotranspiration and Seepage for a Sinkhole Wetland from Diurnal Surface‐Water Cycles”1,2

: This study used measured diurnal surface-water cycles to estimate daily evapotranspiration (ET) and seepage for a seasonally flooded sinkhole wetland. Diurnal surface-water cycles were classified into five categories based on the relationship between the surface-water body and the surrounding ground-water system (i.e., recharge/discharge). Only one class of diurnal cycles was found to be suitable for application of this method. This subset of diurnal cycles was used to estimate ET and seepage and the relative importance of each transfer process to the overall water budget. The method has limited utility for wetlands with erratic hydrologic regimes (e.g., wetlands in urban environments). This is due to violation of the critical assumption that the inflow/outflow rate remains constant throughout the day. For application to surface-water systems, the method is typically applied with an assumed specific yield of 1.0. This assumption was found to be invalid for application to surface-water systems with a noncylindrical pond geometry. An overestimation of ET by as much as 60% was found to occur under conditions of low pond stage and high water loss. The results demonstrate the high ET rates that can occur in isolated wetlands due to contrasting roughness and moisture conditions (oasis and clothesline effects). Estimated ET rates ranged from 4.1 to 18.7 mm/day during the growing season. Despite these large ET rates, seepage (recharge) was found to be the dominant water loss mechanism for the wetland.

Estimating Evapotranspiration and Seepage for a Sinkhole Wetland From Diurnal Surface‐Water Cycles1

Abstract: This study used measured diurnal surface‐water cycles to estimate daily evapotranspiration (ET) and seepage for a seasonally flooded sinkhole wetland. Diurnal surface‐water cycles were classified into five categories based on the relationship between the surface‐water body and the surrounding ground‐water system (i.e., recharge/discharge). Only one class of diurnal cycles was found to be suitable for application of this method. This subset of diurnal cycles was used to estimate ET and seepage and the relative importance of each transfer process to the overall water budget. The method has limited utility for wetlands with erratic hydrologic regimes (e.g., wetlands in urban environments). This is due to violation of the critical assumption that the inflow/outflow rate remains constant throughout the day. For application to surface‐water systems, the method is typically applied with an assumed specific yield of 1.0. This assumption was found to be invalid for application to surface‐water systems with a noncylindrical pond geometry. An overestimation of ET by as much as 60% was found to occur under conditions of low pond stage and high water loss. The results demonstrate the high ET rates that can occur in isolated wetlands due to contrasting roughness and moisture conditions (oasis and clothesline effects). Estimated ET rates ranged from 4.1 to 18.7 mm/day during the growing season. Despite these large ET rates, seepage (recharge) was found to be the dominant water loss mechanism for the wetland.

Evapotranspiration rates and crop coefficients for a restored marsh in the Sacramento–San Joaquin Delta, California, USA

AbstractThe surface renewal method was used to estimate evapotranspiration (ET) for a restored marsh on Twitchell Island in the Sacramento–San Joaquin Delta, California, USA. ET estimates for the marsh, together with reference ET measurements from a nearby climate station, were used to determine crop coefficients over a 3‐year period during the growing season. The mean ET rate for the study period was 6 mm day−1, which is high compared with other marshes with similar vegetation. High ET rates at the marsh may be due to the windy, semi‐arid Mediterranean climate of the region, and the permanently flooded nature of the marsh, which results in very low surface resistance of the vegetation. Crop coefficient (Kc) values for the marsh ranged from 0·73 to 1·18. The mean Kc value over the entire study period was 0·95. The daily Kc values for any given month varied from year to year, and the standard deviation of daily Kc values varied between months. Although several climate variables were undoubtedly responsible for this variation, our analysis revealed that wind direction and the temperature of standing water in the wetland were of particular importance in determining ET rates and Kc values. Published in 2007 by John Wiley & Sons, Ltd.

Comparison of different methods to measure and model actual evapotranspiration rates for a wet sloping grassland

Accurate measurements of the turbulent exchanges of mass and energy at the land surface are necessary for a good understanding of the various components of the hydrological cycle. The two most commonly used methods to measure evapotranspiration rates are the Bowen ratio energy balance (BREB) and the Eddy correlation (EC) methods. These methods are applicable when a number of requirements, mostly with respect to terrain topography and homogeneous fetch extension, are fulfilled. On the other hand, meteorological variables can be used to calculate evapotranspiration rates. The two most frequently used methods for this purpose are the Penman-Monteith (PM) combination equation and the Priestley-Taylor (PT) approximation. The objective of this paper is to compare these different methods under non-ideal conditions, more specifically for a wet sloping grassland. The BREB-based and EC-based latent heat fluxes are intercompared, and a good agreement between the estimates from both methods is found. A comparison between the results of the PM and PT methods and the measured latent heat fluxes is then done. A strong annual cycle in the calculated values for the PT alfa factor ( α), with a mean annual average value of 1.21 ± 0.79, has been observed. This annual cycle is related to the annual cycle of the humidity of the soil, which can be evaluated by either the soil moisture or the vapor deficit of the air. A strong annual cycle in the inverted surface resistances has also been observed. A relationship between the inverted surface resistances and α has been found, in which the highest values for α coincide with low surface resistances, and vice versa. The results indicate that the methods to measure and calculate latent heat fluxes are in a good agreement, and that imposing an annual cycle in the surface resistance and α leads to an improvement in the estimated evapotranspiration rates. The results suggest that it is possible to measure or model evapotranspiration rates in situations where the theoretical requirements (more specifically a non-sloping surface) are not met.

Evapotranspiration on western U.S. rivers estimated using the Enhanced Vegetation Index from MODIS and data from eddy covariance and Bowen ratio flux towers

We combined remote sensing and in-situ measurements to estimate evapotranspiration (ET) from riparian vegetation over large reaches of western U.S. rivers and ET by individual plant types. ET measured from nine flux towers (eddy covariance and Bowen ratio) established in plant communities dominated by five major plant types on the Middle Rio Grande, Upper San Pedro River, and Lower Colorado River was strongly correlated with Enhanced Vegetation Index (EVI) values from the Moderate Resolution Imaging Spectrometer (MODIS) sensor on the NASA Terra satellite. The inclusion of maximum daily air temperatures ( T a) measured at the tower sites further improved this relationship. Sixteen-day composite values of EVI and T a were combined to predict ET across species and tower sites ( r 2 = 0.74); the regression equation was used to scale ET for 2000–2004 over large river reaches with T a from meteorological stations. Measured and estimated ET values for these river segments were moderate when compared to historical, and often indirect, estimates and ranged from 851–874 mm yr − 1 . ET of individual plant communities ranged more widely. Cottonwood ( Populus spp.) and willow ( Salix spp.) stands generally had the highest annual ET rates (1100–1300 mm yr − 1 ), while mesquite ( Prosopis velutina) (400–1100 mm yr − 1 ) and saltcedar ( Tamarix ramosissima) (300–1300 mm yr − 1 ) were intermediate, and giant sacaton ( Sporobolus wrightii) (500–800 mm yr − 1 ) and arrowweed ( Pluchea sericea) (300–700 mm yr − 1 ) were the lowest. ET rates estimated from the flux towers and by remote sensing in this study were much lower than values estimated for riparian water budgets using crop coefficient methods for the Middle Rio Grande and Lower Colorado River.