Annual Evapotranspiration Estimates Research Articles

Evapotranspiration estimation using Surface Energy Balance Model and medium resolution satellite data: An operational approach for continuous monitoring

Monitoring spatial and temporal trends of water use is of utmost importance to ensure water and food security in river basins that are challenged by water scarcity and climate change induced abnormal weather patterns. To quantify water consumption by the agriculture sector, continuous monitoring is required over different spatial scales ranging from field (< 1 ha) to basin. The demand driven requirement of covering large areas yet providing spatially distributed information makes the use of in-situ measurement devices unfeasible. Earth observation satellites and remote sensing techniques offer an effective alternative in estimating the consumptive use of water (Actual EvapoTranspiration (ETa) fluxes) by using periodic observations from the visible and infrared spectral region. Optical satellite data, however, is often hindered by noises due to cloud cover, cloud shadow, aerosols and other satellite related issues such as Scan Line Corrector (SLC) failure in Landsat 7 breaking the continuity of temporal observations. These gaps have to be statistically filled in order to compute aggregated seasonal and annual estimates of ETa. In this paper, we introduce an approach to develop a gap-filled multi-year monthly ETa maps at medium spatial resolution of 30 m. The method includes two major steps: (i) estimation of ETa using the python based implementation of surface energy balance model called PySEBAL and (ii) temporal interpolation using Locally Weighted Regression (LWR) model followed by spline based spatial interpolation to fill the gaps over time and space. The approach is applied to a large endorheic Lake Urmia Basin (LUB) basin with a surface area of ~ 52,970 km2 in Iran for the years 2013–2015 using Landsat 7 and 8 satellite data. The results show that the implemented gap filling approach could reconstruct the monthly ETa dynamics over different agriculture land use types, while retaining the high spatial variability. A comparison with a similar dataset from FAO WaPOR reported a very high correlation with R2 of 0.93. The study demonstrates the applicability of this approach to a larger basin which is extendible and reproducible to other geographical areas.

A decadal (2008–2017) daily evapotranspiration data set of 1 km spatial resolution and spatial completeness across the North China Plain using TSEB and data fusion

Daily continuous evapotranspiration (ET) estimates of 1 km spatial resolution can benefit agricultural water resources management at regional scales. Thermal infrared remote sensing-derived land surface temperature (LST) is a critical variable for ET estimation using energy balance-based models. However, missing LST information under cloudy conditions remains a long-standing barrier for spatiotemporally continuous monitoring of daily ET at regional scales. In this study, LST data of 1 km spatial resolution at 11:00 local solar time under all-weather conditions across the North China Plain (NCP) were first generated using a data fusion approach developed previously. Second, combined with the generated LST data, MODIS products, and meteorological forcing from the China Land Data Assimilation System, the Two-Source Energy Balance model (TSEB) and a temporal upscaling method were jointly used to estimate daily ET at 1 km spatial resolution across the NCP for a decade from 2008 to 2017. In particular, to better incorporate the impact of crop phenology on ET and improve the ET estimation, the fraction of greenness in TSEB was determined in terms of a leaf area index threshold during the crop growth period. Compared with observed instantaneous latent heat flux (LE) corrected for energy balance closure, the estimated LE reasonably captures inter- and intra-annual variations in LE measured at the Huailai, Daxing, Weishan, and Guantao flux towers, with R2 of 0.63–0.79. Estimated daily ET against in situ ET measurements with energy balance closure at the Huailai, Daxing, and Guantao sites showed good performance in terms of R2 greater than 0.70 and RMSE below 0.91 mm/d. These accuracies are comparable with published results, with our ET data set validated by many more observations than previous studies and featuring spatiotemporal continuity and high spatial resolution across the entire NCP for a decade. Furthermore, seasonal ET variations reflected by our product outperform two widely used global products in capturing water consumption characteristics in the winter wheat-summer maize rotation system. In terms of temporal trends, annual ET estimates across the NCP show a decreasing and then increasing trend over the past decade, which is attributed to the increased cropping intensity over the recent years reflected by an increase in leaf area index.

Intercomparison and evaluation of three global high-resolution evapotranspiration products across China

Accurate quantification of large-scale evapotranspiration (ET) has an important scientific and practical significance. In this study, three global high-resolution ET products were intercompared and evaluated across China; the evaluated ET products were the Global Land Evaporation Amsterdam Model (GLEAM) version 3.2, the Global Land Data Assimilation System (GLDAS) version 2.0 Catchment Land Surface Model (CLSM) dataset, and the Numerical Terradynamic Simulation Group (NTSG) dataset. The evaluations were performed at the site scale with eddy covariance (EC) based observations from eight flux stations, and at the basin scale with water balance-based ET estimates from 22 river basins in China. The intercomparison results indicated that the three products consistently presented an increasing trend over a large proportion of China but large differences in the trend of the annual ET and the mean annual ET estimates. The trends of the annual ET estimates derived from the GLEAM, GLDAS, and NTSG products are 0.449, 0.904 and 1.261 mm/yr2, respectively; the countrywide mean annual ET derived from the three products are 390.2, 443.4 and 419.9 mm/yr, respectively. The site-scale evaluation results indicated that the GLEAM and NTSG products achieved comparable consistencies with the monthly gauge observations and outperformed the GLDAS product; GLEAM was more consistent with the daily gauge observations than GLDAS. At the basin scale, all three products were unable to reasonably reproduce the water balance-based annual ET time series in most basins; the three products systematically overestimated ET in the wet basins compared with the water balance-based ET estimates. The differences in the ET estimates among the ET products may be largely attributed to the discrepancies in the forcing data and model algorithms.

Evapotranspiration from mine-affected riparian sites along the Vaal River in central South Africa

Long-term gold mining within the Witwatersrand Basin Goldfields has led to the creation of over 200 tailings storage facilities (TSFs) releasing acid mine drainage. Plumes of contaminated shallow groundwater occur downslope of these TSFs. One such area lies adjacent to the Vaal River east of the town of Orkney. Natural vegetation in the riparian soils comprises a mixture of grasslands, reeds and woodlands dominated by Vachellia karroo. The Mine Woodlands Project initiated by Witwatersrand University and AngloGold Ashanti is exploring the use of engineered woodlands for hydraulic control of plumes and immobilization of contaminants to protect the wider hydrological resource. To estimate the net increase in evapotranspiration (ET) resulting from various remedial woodlands, ET from existing vegetation types requires quantification. Growing season patterns of ET were measured and modelled in riparian grassland and two V. karroo woodland sites close to the Vaal River. Two independent estimates of annual ET from these woodland sites (674 and 787 mm) were similar to that from grassland (697 mm) and much less than the 1100–1200 mm reported from some forms of engineered evergreen woodlands established in the vicinity. We conclude that the Vachellia woodlands have low potential for hydraulic control of contaminant plumes.

A worldwide evaluation of basin-scale evapotranspiration estimates against the water balance method

Summary Evapotranspiration (ET) plays a critical role in linking the water and energy cycles but is difficult to estimate at regional and basin scales. In this study, we present a worldwide evaluation of nine ET products (three diagnostic products, three land surface model (LSM) simulations and three reanalysis-based products) against reference ET ( ET wb ) calculated using the water balance method corrected for the water storage change at an annual time scale over the period 1983–2006 for 35 global river basins. The results indicated that there was no significant intra-category discrepancy in the annual ET estimates for the 35 basins calculated using the different products in 35 basins, but some products performed better than others, such as the Global Land surface Evaporation estimated using the Amsterdam Methodology (GLEAM_E) in the diagnostic products, ET obtained from the Global Land Data Assimilation System version 1 (GLDAS 1) with the Community Land Model scheme (GCLM_E) in LSM simulations, and ET from the National Aeronautics and Space Administration (NASA) Modern Era Retrospective-analysis for Research and Applications reanalysis dataset (MERRA_E) in the reanalysis-based products. Almost all ET products (except MERRA_E) reasonably estimated the annual means (especially in the dry basins) but systematically underestimated the inter-annual variability (except for MERRA_E, GCLM_E and ET simulation from the GLDAS 1 with the MOSAIC scheme – GMOS_E) and could not adequately estimate the trends (e.g. GCLM_E and MERRA_E) of ET wb (especially in the energy-limited wet basins). The uncertainties in nine ET products may be primarily attributed to the discrepancies in the forcing datasets and model structural limitations. The enhancements of global forcing data (meteorological data, solar radiation, soil moisture stress and water storage changes) and model physics (reasonable consideration of the water and energy balance and vegetation processes such as canopy interception loss) will undoubtedly improve the estimation of global ET in the future.

Evaluating Landsat 8 evapotranspiration for water use mapping in the Colorado River Basin

Abstract Evapotranspiration (ET) mapping at the Landsat spatial resolution (100 m) is essential to fully understand water use and water availability at the field scale. Water use estimates in the Colorado River Basin (CRB), which has diverse ecosystems and complex hydro-climatic regions, will be helpful to water planners and managers. Availability of Landsat 8 images, starting in 2013, provides the opportunity to map ET in the CRB to assess spatial distribution and patterns of water use. The Operational Simplified Surface Energy Balance (SSEBop) model was used with 528 Landsat 8 images to create seamless monthly and annual ET estimates at the inherent 100 m thermal band resolution. Annual ET values were summarized by land use/land cover classes. Croplands were the largest consumer of “blue” water while shrublands consumed the most “green” water. Validation using eddy covariance (EC) flux towers and water balance approaches showed good accuracy levels with R2 ranging from 0.74 to 0.95 and the Nash–Sutcliffe model efficiency coefficient ranging from 0.66 to 0.91. The root mean square error (and percent bias) ranged from 0.48 mm (13%) to 0.60 mm (22%) for daily (days of satellite overpass) ET and from 7.75 mm (2%) to 13.04 mm (35%) for monthly ET. The spatial and temporal distribution of ET indicates the utility of Landsat 8 for providing important information about ET dynamics across the landscape. Annual crop water use was estimated for five selected irrigation districts in the Lower CRB where annual ET per district ranged between 681 mm to 772 mm. Annual ET by crop type over the Maricopa Stanfield irrigation district ranged from a low of 384 mm for durum wheat to a high of 990 mm for alfalfa fields. A rainfall analysis over the five districts suggested that, on average, 69% of the annual ET was met by irrigation. Although the enhanced cloud-masking capability of Landsat 8 based on the cirrus band and utilization of the Fmask algorithm improved the removal of contaminated pixels, the ability to reliably estimate ET over clouded areas remains an important challenge. Overall, the performance of Landsat 8 based ET compared to available EC datasets and water balance estimates for a complex basin such as the CRB demonstrates the potential of using Landsat 8 for annual water use estimation at a national scale. Future efforts will focus on (a) use of consistent methodology across years, (b) integration of multiple sensors to maximize images used, and (c) employing cloud-computing platforms for large scale processing capabilities.

Partitioning evapotranspiration in an intact forested watershed in southern China

AbstractAccurate estimates and partition of evapotranspiration (ET) are difficult but critical for understanding terrestrial ecosystem processes and primary productivity. A multi‐year, multi‐technique study including the catchment water balance (12 years), a semi‐empirical ET model (9 years), eddy covariance (9 years), canopy water balance (1 year) and sap flow (1 year) techniques was conducted to measure ET and its components within an intact forested watershed in the subtropical region of southern China. Over a 9‐year period, the estimates of annual ET using the catchment water balance, eddy covariance and semi‐empirical ET model techniques were in close agreement, averaging 809.9 ± 62.8, 803.8 ± 36.6 and 801.6 ± 46.9 mm per year, respectively, amounting to an average of approximately 50.2% of the mean annual precipitation. Qualitative similarities in seasonal and diurnal variation were observed between the sap flow and eddy covariance estimates of water flux. Sap flow estimates of transpiration were approximately 60.2% of the annual ET estimated with the eddy covariance technique. Interception evaporation was 31.7% of the total ET and was demonstrated to be a main contributor of total evaporation. Soil evaporation was calculated to be approximately 8.1% of the total annual ET and 4.7% of the annual precipitation. Furthermore, daytime transpiration exhibited a logarithmic increase with increases in the vapour pressure deficit (VPD) on daily scale in both the wet and dry seasons, and tended to level off when the VPD was &gt;1 kPa because of the stomatal regulation of transpiration. Correspondingly, nighttime sap flow amounted to an average of approximately 6% of the total daily sap flow, and no significant correlations with the VPD, air temperature or relative humidity were found. Essential differences among the mechanisms of evaporation and transpiration were demonstrated, suggesting that soil evaporation is primarily a climate‐driven process, with air temperature and wind speed as the predominant driving forces. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Satellite-based analysis of evapotranspiration and water balance in the grassland ecosystems of Dryland East Asia.

The regression tree method is used to upscale evapotranspiration (ET) measurements at eddy-covariance (EC) towers to the grassland ecosystems over the Dryland East Asia (DEA). The regression tree model was driven by satellite and meteorology datasets, and explained 82% and 76% of the variations of ET observations in the calibration and validation datasets, respectively. The annual ET estimates ranged from 222.6 to 269.1 mm yr−1 over the DEA region with an average of 245.8 mm yr−1 from 1982 through 2009. Ecosystem ET showed decreased trends over 61% of the DEA region during this period, especially in most regions of Mongolia and eastern Inner Mongolia due to decreased precipitation. The increased ET occurred primarily in the western and southern DEA region. Over the entire study area, water balance (the difference between precipitation and ecosystem ET) decreased substantially during the summer and growing season. Precipitation reduction was an important cause for the severe water deficits. The drying trend occurring in the grassland ecosystems of the DEA region can exert profound impacts on a variety of terrestrial ecosystem processes and functions.

Estimation of Actual Evapotranspiration along the Middle Rio Grande of New Mexico Using MODIS and Landsat Imagery with the METRIC Model

Estimation of actual evapotranspiration (ET) for the Middle Rio Grande valley in central New Mexico via the METRIC surface energy balance model using MODIS and Landsat imagery is described. MODIS images are a useful resource for estimating ET at large scales when high spatial resolution is not required. One advantage of MODIS satellites is that images having a view angle &lt; ~15° are potentially available about every four to five days. The main challenge of applying METRIC using MODIS is the selection of the two calibration conditions due to the low spatial resolution of MODIS. A calibration procedure specific to MODIS is described that utilizes the higher vegetation index areas of the image along with a consistently low ET location to develop the estimation function for sensible heat flux. This paper compares ET images for the Rio Grande region as produced by both MODIS and by Landsat. Application of METRIC energy balance processes along the Middle Rio Grande using MODIS imagery indicates that one can successfully produce monthly and annual ET estimates that are similar in value to those obtained using Landsat imagery if a cross-calibration scheme is considered. However, spatial fidelity is degraded.

Evapotranspiration and water balance of an anthropogenic coastal desert wetland: Responses to fire, inflows and salinities

Evapotranspiration (ET) and other water balance components were estimated for Cienega de Santa Clara, an anthropogenic brackish wetland in the delta of the Colorado River in Mexico. The marsh is in the Biosphere Reserve of the Upper Gulf of California and Delta of the Colorado River, and supports a high abundance and diversity of wildlife. Over 95% of its water supply originates as agricultural drain water from the USA, sent for disposal in Mexico. This study was conducted from 2009 to 2011, before, during and after a trial run of the Yuma Desalting Plant in the USA, which will divert water from the wetland and replace it with brine from the desalting operation. The goal was to estimate the main components in the water budget to be used in creating management scenarios for this marsh. We used a remote sensing algorithm to estimate ET from meteorological data and Enhanced Vegetation Index values from the Moderate Resolution Imaging Spectrometer (MODIS) sensors on the Terra satellite. ET estimates from the MODIS method were then compared to results from a mass balance of water and salt inflows and outflows over the study period. By both methods, mean annual ET estimates ranged from 2.6 to 3.0mmd−1, or 50 to 60% of reference ET (ETo). Water entered at a mean salinity of 2.6gL−1 TDS and mean salinity in the wetland was 3.73gL−1 TDS over the 33 month study period. Over an annual cycle, 54% of inflows supported ET while the rest exited the marsh as outflows; however, in winter when ET was low, up to 90% of the inflows exited the marsh. An analysis of ET estimates over the years 2000–2011 showed that annual ET was proportional to the volume of inflows, but was also markedly stimulated by fires. Spring fires in 2006 and 2011 burned off accumulated thatch, resulting in vigorous growth of new leaves and a 30% increase in peak summer ET compared to non-fire years. Following fires, peak summer ET estimates were equal to ETo, while in non-fire years peak ET was equal to only one-half to two-thirds of ETo. Over annual cycles, estimated ET was always lower than ETo, because T. domingensis is dormant in winter and shades the water surface, reducing direct evaporation. Thus, ET of a Typha marsh is likely to be less than an open water surface under most conditions.

Actual evapotranspiration estimation by ground and remote sensing methods: the Australian experience

AbstractOn average, Australia is a dry continent with many competing uses for water. Hence, there is an urgent need to know actual evapotranspiration (ETa) patterns across wide areas of agricultural and natural ecosystems, as opposed to just point measurements of ETa. The Australian Government has tasked the science agencies with operationally developing monthly and annual estimates of ETa and other hydrological variables, and with forecasting water availability over periods of days to decades, as part of its national water assessment programme. To meet these needs, Australian researchers have become leaders in developing large‐area methods for estimating ETa at regional and continental scales. Ground methods include meteorological models, eddy covariance towers, sap flow sensors and catchment water balance models. Remote sensing methods use thermal infrared, mid infrared and/or vegetation indices usually combined with meteorological data to estimate ETa. Ground and remote sensing ETa estimates are assimilated into the Australian Water Resource Assessment, which issues annual estimates of the state of the continental water balance for policy and planning purposes. The best ETa models are estimated to have an error or uncertainty of 10% to 20% in Australia. Developments in Australian ETa research over the past 20 years are reviewed, and sources of error and uncertainty in current methods and models are discussed. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Integration of the GG model with SEBAL to produce time series of evapotranspiration of high spatial resolution at watershed scales

Lack of good quality satellite images because of cloud contamination or long revisit time severely degrades predictions of evapotranspiration (ET) time series at watershed/regional scales from satellite‐based surface flux models. We integrate the feedback model developed by Granger and Gray (the GG model) with the Surface Energy Balance Algorithm for Land (SEBAL), with the objective to generate ET time series of high spatial resolution and reliable temporal distribution at watershed scales. First, SEBAL is employed to yield estimates of ET for the Baiyangdian watershed in a semihumid climatic zone in north China on cloud‐free days, where there exists the complementary relationship (CR) between actual ET and pan ET. These estimates constitute input to the GG model to inversely derive the relationship between the relative evaporation and the relative drying power of the air. Second, the modified GG model is used to yield ET time series on a daily basis simply by using routine meteorological data and Moderate Resolution Imaging Spectroradiometer (MODIS) albedo and leaf area index products. Results suggest that the modified GG model that has incorporated remotely sensed ET can effectively extend remote sensing based ET to days without images and improve spatial representation of ET at watershed scales. Utility of the evaporative fraction method and the crop coefficients approaches to extrapolate ET time series depends largely on the number and interval of good quality satellite images. Comparison of ET time series from the two techniques and the proposed integration method for days with daily net radiation larger than 100 W m−2 and corresponding pan ET clearly shows that only the integration method can exhibit an asymmetric CR at the watershed scale and daily time scale. Validation performed using hydrologic budget calculations indicate that the proposed method has the highest accuracy in terms of annual estimates of ET for both watersheds in north China.

Estimating Evapotranspiration from a Small Area on a Grass-Covered Slope Using the BBH Model of Soil Hydrology

Evapotranspiration (ET) from a small area on a grass-covered slope with an inclination of 8.7 degrees was estimated using the bucket-with-a-bottom-hole (BBH) model of soil hydrology. The BBH model has seven parameters, two of which were determined by experiment. The other five parameters were identified by trial and error, based on the decision criterion that the root mean square (RMS) of the difference between the predicted and measured values of soil moisture contained in the active soil surface layer (ASSL) takes a minimum value. Estimates of ET made using the BBH model were compared to the measurements taken using the Bowen ratio method. If the measurements can be regarded as the true values, the results are summarized as follows: - The estimates of annual ET made using the BBH model are reliable, although they may be somewhat smaller than the true values. - The estimates of monthly ET made using the BBH model can be used in analyses of water balance if an accuracy of better than ±10% is not required. - The accuracy of estimating daily ET using the BBH model was about ±60%. The main characteristics of water balance on the grass-covered slope obtained using the BBH model were summarized as follows: - Only less than one quarter of precipitation was allotted to ET on the slope. - Spatial variation in ET within an area several meters in size on the grass-covered slope was shown to be rather large, especially in the summer months. During the winter months, however, it was insignificant. The variation in annual ET among the three observation points within the area was about 20% as great as their mean.

A Re-Evaluation of the Average Annual Global Water Balance

Legates and Mather (1992) used the Thornthwaite/Mather water balance and mean monthly air temperature and precipitation data on a 0.5° of latitude by 0.5° of longitude grid to refine spatial estimates of annual evapotranspiration and runoff on a global scale. Because Legates and Mather used the Thornthwaite potential evapotranspiration equation, they were unable to apply bias-adjustments to their precipitation estimates because, as Legates and Mather discovered, the Thornthwaite equation implicitly accounts for gage measurement biases by underestimating potential evapotranspiration. Here, we re-evaluate the results of Legates and Mather using bias-adjusted precipitation estimates and an alternative method of estimating potential evapotranspiration (i.e., the Hamon, 1963, method). These new results are compared with those from previous studies and are considered to be an improvement over the Legates and Mather values, as they are based on bias-adjusted precipitation estimates.

Long-term water budget estimation with the modified distributed model – LISFLOOD-WB over the Lushi basin, China

In a modification of the distributed hydrological model, LISFLOOD-WB, a two-source canopy scheme is used to predict both the canopy transpiration and soil evaporation. A revised soil storage capacity curve from the Xinanjiang model is applied to take into account the sub-grid heterogeneity. The modified model is used to estimate the long-term (1980–1997) water budget of the Lushi basin (4423 km2), China. All the input data fields are integrated in a four-dimensional GIS data structure with a raster grid spacing of 1-km. The basin channel network is determined from digital elevation data, and the spatial pattern of canopy leaf area index (LAI) is retrieved from NOAA/AVHRR NDVI images. Generally, the model efficiency for discharge prediction is acceptable, but the discharges are overestimated in the driest years and underestimated in the wettest years. The results indicated that the influence of inter-annual variation of spatial patterns of LAI detected by NOAA/AVHRR NDVI on the estimates of annual evapotranspiration is negligible. Annually averaged ratios of overland flow, infiltration and canopy interception to precipitation are 24±7%, 56±10% and 20±2%, respectively. The inter-annual variations of precipitation and predicted evapotranspiration are relatively high with standard deviations of 5.1 mm day−1 and 2.4 mm day−1, respectively, whereas the inter-annual variation of the net radiation is much less. Monthly temporal patterns of soil moisture follow precipitation strongly. Spatially precipitation and LAI are both significantly correlated with evapotranspiration, although precipitation has a slightly more dominant control. The linear relationship between water yield and LAI is weak on a grid by grid basis.

Energy exchange across a chronosequence of slash pine forests in Florida

We measured net atmospheric exchanges of energy and water vapor using eddy covariance along a chronosequence of Pinus elliottii plantations in north Florida: a recent clear-cut, a mid-rotation stand, and a 24-year-old, rotation-aged stand. Reflected energy averaged 0.26 of incoming solar radiation at the clear-cut and 0.18 at the closed-canopy stands. The sum of sensible ( S), latent (LE) and soil heat fluxes accounted for 89 and 85% of net radiation ( R net) at the clear-cut and mid-rotation age sites. Both S and LE were linearly related to R net at all sites. Seasonal differences occurred in the proportion of R net attributable to S and LE. S was a much smaller proportion of R net when the clear-cut and the mid-rotation age stands were flooded in the summer. LE was a greater proportion of R net during the summer/fall at all sites when LAI was greatest. Bowen ratios ( S/LE) were 0.34, 0.50 and 0.59 in the summer/fall and 0.71, 0.77 and 1.00 in the winter/spring at the clear-cut, mid-rotation and rotation-aged stands, respectively. Maximum rates of evapotranspiration (ET) in the summer were 0.6 mm h −1 at all sites. Mean daily rates averaged 3.3 mm per day in the summer/fall and 2.0 mm per day in the winter/spring. Although, changes in LAI and canopy structure were large, annual ET estimates were similar and averaged 959, 951 and 1110 mm per year along the chronosequence. Results suggest that energy partitioning and annual ET in these pine forests are more sensitive to environmental fluctuations than to management activities.

A comparison of methods for determining forest evapotranspiration and its components: sap-flow, soil water budget, eddy covariance and catchment water balance

A multi-year, multi-technique study was conducted to measure evapotranspiration and its components within an uneven-aged mixed deciduous forest in the Southeastern United States. Four different measurement techniques were used, including soil water budget (1 year), sap flow (2 years), eddy covariance (5 years), and catchment water budget (31 years). Annual estimates of evapotranspiration were similar for the eddy covariance and catchment water balance techniques, averaging 571±16 mm (eddy covariance) and 582±28 mm (catchment water balance) per year over a 5-year period. There were qualitative similarities between sap flow and eddy covariance estimates on a daily basis, and sap flow estimates of transpiration were about 50% of annual evapotranspiration estimated from eddy covariance and catchment studies. Soil evaporation was estimated using a second eddy covariance system below the canopy, and these measurements suggest that soil evaporation explains only a small portion of the difference between sap flow estimates of transpiration and eddy covariance and catchment water budget estimates of evapotranspiration. Convergence of the catchment water balance and eddy covariance methods and moderately good energy balance closure suggests that the sap flow estimates could be low, unless evaporation of canopy-intercepted water was especially large. The large species diversity and presence of ring-porous trees at our site may explain the difficulty in extrapolating sap flow measurements to the spatial scales representative of the eddy covariance and catchment water balance methods. Soil water budget estimates were positively correlated with eddy covariance and sap flow measurements, but the data were highly variable and in error under conditions of severe surface dryness and after rainfall events.

Evapotranspiration of Melaleuca Forest in South Florida

Melaleuca forests cover extensive areas of south Florida, and are widely assumed to have extraordinarily high evapotranspiration (ET) rates. This investigation indicates that the transpiration of the melaleuca forest is dominated by vapor deficits, with net radiation playing a secondary role. Evaporation of intercepted rainfall contributes between 12 and 23% to the monthly ET, with lower contributions during the dry season and higher contributions during the wet season. Annual ET from the melaleuca forest is estimated to be approximately 1,600 mm, with 300 mm contributed by intercepted rainfall. Comparison of these results with predictions of other forest-ET models indicate that the Shuttleworth model overestimates the ET of melaleuca forest by about 5% in a typical month as a result of neglecting the canopy wetness effect. The Clader-Newson model, which equates forest transpiration to short-vegetation transpiration, produces annual ET estimates that are within 6% of the annual ET estimated in this study. However, the discrepancies in monthly ET range between -8 and +17%.