Evapotranspiration (ET) is an essential component of the water balance. Remote sensing based agrometeorological models are presently most suited for estimating crop water use at both field and regional scales. Numerous ET algorithms have been developed to make use of remote sensing data acquired by sensors on airborne and satellite platforms. In this paper, a literature review was done to evaluate numerous commonly used remote sensing based algorithms for their ability to estimate regional ET accurately. The reported estimation accuracy varied from 67 to 97% for daily ET and above 94% for seasonal ET indicating that they have the potential to estimate regional ET accurately. However, there are opportunities to further improving these models for accurately estimating all energy balance components. The spatial and temporal remote sensing data from the existing set of earth observing satellite platforms are not sufficient enough to be used in the estimation of spatially distributed ET for on-farm irrigation management purposes, especially at a field scale level (*10 to 200 ha). This will be constrained further if the thermal sensors on future Landsat satellites are abandoned. However, research opportunities exist to improve the spatial and temporal resolution of ET by developing algorithms to increase the spatial resolution of reflectance and surface temperature data derived from Landsat/ ASTER/MODIS images using same/other-sensor high resolution multi-spectral images.

The two source energy balance model (TSEB) can estimate evaporation (E), transpiration (T), and evapotranspiration (ET) of vegetated surfaces, which has important applications in water resources management for irrigated crops. The TSEB requires soil (T S) and canopy (T C) surface temperatures to solve the energy budgets of these layers separately. Operationally, usually only composite surface temperature (T R) measurements are available at a single view angle. For surfaces with nonrandom spatial distribution of vegetation such as row crops, T R often includes both soil and vegetation, which may have vastly different temperatures. Therefore, T S and T C must be derived from a single T R measurement using simple linear mixing, where an initial estimate of T C is calculated, and the temperature-resistance network is solved iteratively until energy balance closure is reached. Two versions of the TSEB were evaluated, where a single T R measurement was used (TSEB-T R) and separate measurements of T S and T C were used (TSEB-T C-T S). All surface temperatures (T S , T C , and T R) were measured by stationary infrared thermometers that viewed an irrigated cotton (Gossypium hirsutum L.) crop. The TSEB-T R version used a Penman-Monteith approximation for T C , rather than the Priestley-Taylor-based formulation used in the original TSEB version, because this has been found to result in more accurate partitioning of E and T under conditions of strong advection. Calculations of E, T, and ET by both model versions were compared with measurements using microlysimeters, sap flow gauges, and large monolythic weighing lysimeters, respectively. The TSEB-T R version resulted in similar overall agreement with the TSEB-T C-T S version for calculated and measured E (RMSE = 0.7 mm d À1) and better overall agreement for T (RMSE = 0.9 vs. 1.9 mm d À1), and ET (RMSE = 0.6 vs. 1.1 mm d À1). The TSEB-T C-T S version calculated daily ET up to 1.6 mm d À1 (15%) less early in the season and up to 2.0 mm d À1 (44%) greater later in the season compared with lysimeter measurements. The TSEB-T R also calculated larger ET later in the season but only up to 1.4 mm d À1 (20%). ET underestimates by the TSEB-T C-T S version may have been related to limitations in measuring T C early in the season when the canopy was sparse. ET overestimates later in the season by both versions may have been related to a greater proportion of non-transpiring canopy elements (flowers, bolls, and senesced leaves) being out of the T C and T R measurement view.

Irrigation for crop production in the semi-arid Texas High Plains is dependent on groundwater withdrawals from the Ogallala Aquifer, which is declining because withdrawals exceed natural recharge. Irrigation development in the region accelerated during the 1950s. Both irrigated area and volume pumped peaked in 1974 and steadily declined during [1974][1975][1976][1977][1978][1979][1980][1981][1982][1983][1984][1985][1986][1987][1988][1989]. By 2004, however, irrigated area was nearly the same as it was in 1958, and volume pumped had increased slightly. Several strategies to reduce groundwater withdrawals were reviewed without any reductions in irrigated land area or crop productivity. The most promising evaluated were: (1) increasing weather-based irrigation scheduling using the Texas High Plains Evapotranspiration Network (TXHPET); (2) converting gravity-irrigated land (27% of total) to centre pivot irrigation; and (3) replacing high-water to lower-water demand crops (i.e., corn to cotton). If the land area using the TXHPET network was doubled, and if gravity-irrigated lands were reduced to 10%, groundwater withdrawals could be reduced by 14%. An additional reduction of 8% may be possible by converting half of the irrigated corn area to cotton. Copyright # 2008 John Wiley & Sons, Ltd. Plusieurs stratégies pour réduire les prélèvements d'eaux souterraines sans réduire la surface irriguée ou diminuer la productivité ont été analysées. Les plus intéressantes sont: (1) augmenter le pilotage de l'irrigation par le réseau de mesure de l'évapotranspiration des hautes plaines du Texas (TXHPET); (2) convertir l'irrigation gravitaire (27% du total) en irrigation par pivot; et (3) remplacer les cultures à forte demande en eau par des cultures à demande plus faible (soit le maïs par le coton). Si la surface utilisant le réseau TXHPET doublait, et si l'irrigation gravitaire était réduite de 10%, les prélèvements d'eaux souterraines pourraient être réduits de 14%. Une réduction complémentaire de 8% serait possible en convertissant la moitié de la surface irriguée de maïs en coton.

The Priestley-Taylor (PT) approximation for computing evapotranspiration was initially developed for conditions of a horizontally uniform saturated surface sufficiently extended to obviate any significant advection of energy. Nevertheless, the PT approach has been effectively implemented within the framework of a thermal-based two-source model (TSM) of the surface energy balance, yielding reasonable latent heat flux estimates over a range in vegetative cover and climate conditions. In the TSM, however, the PT approach is applied only to the canopy component of the latent heat flux, which may behave more conservatively than the bulk (soil 1 canopy) system. The objective of this research is to investigate the response of the canopy and bulk PT parameters to varying leaf area index (LAI) and vapor pressure deficit (VPD) in both natural and agricultural vegetated systems, to better understand the utility and limitations of this approximation within the context of the TSM. Micrometeorological flux measurements collected at multiple sites under a wide range of atmospheric conditions were used to implement an optimization scheme, assessing the value of the PT parameter for best performance of the TSM. Overall, the findings suggest that within the context of the TSM, the optimal canopy PT coefficient for agricultural crops appears to have a fairly conservative value of ;1.2 except when under very high vapor pressure deficit (VPD) conditions, when its value increases. For natural vegetation (primarily grasslands), the optimal canopy PT coefficient assumed lower values on average (;0.9) and dropped even further at high values of VPD. This analysis provides some insight as to why the PT approach, initially developed for regional estimates of potential evapotranspiration, can be used successfully in the TSM scheme to yield reliable heat flux estimates over a variety of land cover types.

Robust spatial information about environmental water use at field scales and daily to seasonal timesteps will benefit many applications in agriculture and water resource management. This information is particularly critical in arid climates where freshwater resources are limited or expensive, and groundwater supplies are being depleted at unsustainable rates to support irrigated agriculture as well as municipal and industrial uses. Gridded evapotranspiration (ET) information at field scales can be obtained periodically using land-surface temperature-based surface energy balance algorithms applied to moderate resolution satellite data from systems like Landsat, which collects thermal-band imagery every 16 days at a resolution of approximately 100 m. The challenge is in finding methods for interpolating between ET snapshots developed at the time of a clear-sky Landsat overpass to provide complete daily time-series over a growing season. This study examines the efficacy of a simple gap-filling algorithm designed for applications in data-sparse regions, which does not require local ground measurements of weather or rainfall, or estimates of soil texture. The algorithm relies on general conservation of the ratio between actual ET and a reference ET, generated from satellite insolation data and standard meteorological fields from a mesoscale model. The algorithm was tested with ET retrievals from the Atmosphere-Land Exchange Inverse (ALEXI) surface energy balance model and associated DisALEXI flux disaggregation technique, which uses Landsat-scale thermal imagery to reduce regional ALEXI maps to a finer spatial resolution. Daily ET at the Landsat scale was compared with lysimeter and eddy covariance flux measurements collected during the Bushland Evapotranspiration and Agricultural Remote sensing EXperiment of 2008 (BEAREX08), conducted in an irrigated agricultural area in the Texas Panhandle under highly advective conditions. The simple gap-filling algorithm performed reasonably at most sites, reproducing observed cumulative ET to within 5-10% over the growing period from emergence to peak biomass in both rainfed and irrigated fields.

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Discrepancies can arise among surface flux measurements collected using disparate techniques due to differences in both the instrumentation and theoretical underpinnings of the different measurement methods. Using data collected primarily within a pair of irrigated cotton fields as a part of the 2008 Bushland Evapotranspiration and Remote Sensing Experiment (BEAREX08), flux measurements collected with two commonly-used methods, eddy covariance (EC) and lysimetry (LY), were compared and substantial differences were found. Daytime mean differences in the flux measurements from the two techniques could be in excess of 200 W m À2 under strongly advective conditions. Three causes for this disparity were found: (i) the failure of the eddy covariance systems to fully balance the surface energy budget, (ii) flux divergence due to the local advection of warm, dry air over the irrigated cotton fields, and (iii) the failure of lysimeters to accurately represent the surface properties of the cotton fields as a whole. Regardless of the underlying cause, the discrepancy among the flux measurements underscores the difficulty in collecting these measurements under strongly advective conditions. It also raises awareness of the uncertainty associated with in situ micrometeorological measurements and the need for caution when using such data for model validation or as observational evidence to definitively support or refute scientific hypotheses.

Drought is an important factor limiting corn (Zea mays L.) yields in the Texas High Plains, and adoption of drought‐tolerant (DT) hybrids could be a management tool under water shortage. We conducted a 3‐yr field study to investigate yield, evapotranspiration (ET), and water use efficiency (WUE) in DT hybrids. One conventional (33D49) and 4 DT hybrids (P1151HR, P1324HR, P1498HR, and P1564HR) were grown at three water regimes (I100, I75, and I50, referring to 100, 75, and 50% ET requirement) and three planting densities (PD) (5.9, 7.4, and 8.4 plants m−2). Yield (13.56 Mg ha−1) and ET (719 mm) were the greatest at I100 but WUE (2.1 kg m−3) was the greatest at I75. Although DT hybrids did not always have greater yield and WUE than 33D49 at I100, hybrids P1151HR and P1564HR consistently had greater yield and WUE than 33D49 at I75 and I50. Compared to 33D49, P1151HR and P1564HR had 8.6 to 12.1% and 19.1% greater yield at I75 and I50, respectively. Correspondingly, WUE was 9.8 to 11.7% and 20.0% greater at I75 and I50, respectively. Greater PD resulted in greater yield and WUE at I100 and I75 but PD did not affect yield and WUE at I50. Yield and WUE in greater PD (8.4 plants m−2) were 6.3 to 8.3% greater than those in smaller PD (5.9 plants m−2). The results of this study demonstrated that proper selection of DT hybrids can increase corn yield and WUE under water‐limited conditions.