Prediction Accuracy for Projectwide Evapotranspiration Using Crop Coefficients and Reference Evapotranspiration

Abstract

Abstract:The Imperial Irrigation District is a large irrigation project in the western United States having a unique hydrogeologic structure such that only small amounts of deep percolation leave the project directly as subsurface flows. This structure is conducive to relatively accurate application of a surface water balance to the district, enabling the determination of crop evapotranspiration (ETJ as a residual of inflows and outflows. The ability to calculate ETc from discharge measurements provides the opportunity to assess the accuracy and consistency of an independently applied crop coefficient-reference evapotranspiration (K ET o ) procedure integrated over c the project. The accuracy of the annual crop evapotranspiration via water balance estimates was ±6% at the 95% confidence level. Calculations using K and ET o were based on the FAa-56 dual crop coefficient approach and included separate calculation of evaporation c from precipitation and irrigation events. Grass reference ET o was computed using the CIMIS Penman equation and ETc was computed for over 30 crop types. On average, Kc-based ET computations exceeded ETc determined by water balance (referred to as ETc WB) by 8% on an annual basis over a 7 year period. The 8% overprediction was concluded to stem primarily from use of K c that represents potential and ideal growing conditions, whereas crops in the study area were not always in full pristine condition due to various water and agronomic stresses. A 6% reduction to calculated Kc-based ET was applied to all crops, and a further 2% reduction was applied to lower value crops to bring the project-wide ET predicted by Kc-based ET into agreement with ETc WB' The standard error of estimate (SEE) for annual ETc for the entire project based on K c , following the reduction adjustment, was 3.4% of total annual ETc, which is considered to be quite good. The SEE for the average monthly ETc was 15% of average monthly ETc. A sensitivity analysis of the computational procedure for K c showed that relaxation from using the FAa-56 dual K method to the more simple mean (i.e., single) K curve and relaxation of specificity c c of planting and harvest dates did not substantially increase the projectwide prediction error The use of the mean K curves, where effects c of evaporation from wet soil are included as general averages, predicted 5% lower than the dual method for monthly estimates and 8% lower on an annual basis, so that no adjustment was required to match annual ET derived from water balance. About one half of the reduction in estimates when applying the single (or mean) K c method rather than the dual K c method was caused by the lack of accounting for evaporation from special irrigations during the off season (i.e., in between crops).

Evaluation of Landsat-Based METRIC Modeling to Provide High-Spatial Resolution Evapotranspiration Estimates for Amazonian Forests

“…In this study, utmost care and our best effort was put into selecting appropriate anchor pixels using the process outlined in Section 2.2.1. Irrigated crop land is usually an ideal target for cold pixel selection and the assumption of cold pixel ET with a fully vegetated and "wet" area is equal to 1.05 × alfalfa reference ET r [19]. In some cases, the NDVI values from the cold pixels selected over pasture were less than 0.75, which may indicate less than full cover conditions.…”

Bare soil evaporation under high evaporation demand: a proposed modification to the FAO-56 model

“…The FAO-56 model is based on FAO-24 (Doorembos & Pruitt, 1975), which calculates the evapotranspiration (ET) based on the crop coefficient (K c ) and the atmospheric demand (ET p at that time, but currently termed ET 0 ). The FAO-56 incorporates the dual crop coefficient (Allen, 2000;Allen et al, 1998Allen et al, , 2005a, which divides K c into parts: crop and bare soil, equation (3). One of the advantages of the dual coefficient proposed by FAO-56, compared to FAO-24, is that it allows the calculation of both evaporation and transpiration, and it permits their calculation beyond standard conditions (well-watered plants).…”

Determinación del consumo de agua del duraznero por lisimetría

“…Esta metodología demostró un buen desempeño al ser validada con datos observados de campo por Allen et al (2005a) y Godinho et al (2007). La fracción húmeda y expuesta se calculó como: siendo:…”

“…Los valores menores de profundidad corresponden a suelos arenosos y los valores mayores a suelos de francos a pesados. Pereira (2004) y Allen et al (2005a) presentan criterios para seleccionar el método de cálculo de los coeficientes de cultivo. Proponen que se utilicen los coeficientes culturales de base (K cb + K e ) en lugar de los coeficientes culturales medios (K c ) cuando se trata de trabajos con fines de investigación, así como en la conducción del riego en tiempo real con sistemas de alta frecuencia, sobre todo en regiones con precipitaciones frecuentes.…”