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ARS Home » Pacific West Area » Maricopa, Arizona » U.S. Arid Land Agricultural Research Center » Water Management and Conservation Research » Research » Publications at this Location » Publication #156629
Title: PREDICTION ACCURACY FOR PROJECT-WIDE EVAPOTRANSPIRATION USING CROP COEFFICIENTS AND REFERENCE EVAPOTRANSPIRATION

Author
item ALLEN, RICHARD - UNIV OF ID, KIMBERLY
item Clemmens, Albert
item BURT, CHARLES - CAL POLY, SLO, CA
item SOLOMON, KEN - CAL POLY, SLO, CA
item O'HALLORAN, TIM - KRWA, FRESNO, CA

Submitted to: Journal of Irrigation and Drainage Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/10/2004
Publication Date: 2/1/2005
Citation: Allen, R.G., Clemmens, A.J., Burt, C.M., Solomon, K., O'Halloran, T. 2005. Prediction accuracy for project-wide evapotranspiration using crop coefficients and reference evapotranspiration. Journal of Irrigation and Drainage Engineering. 131(1):24-36.

Interpretive Summary: Estimates of water use have often been based on diversions from the water source. However, for many applications only a fraction of the diverted water is actually consumed. The rest of the water either flows downhill or percolates to groundwater and can be used downstream. In irrigated agriculture, crop evapotranspiration is the dominate consumptive use of water. In the past, it has been difficult to estimate crop water use in large irrigation projects because of the variety of crops, planting and harvest dates, cultural practices, etc. In this paper, a detailed analysis was made of crop consumptive use within the Imperial Irrigation District. Detailed records of crop acreage, weather data, planting and harvest information, etc. were used to estimate the evapotranspiration for crops grown under nonwater limiting conditions. Measurement of inflow and outflow were used to develop a water balance of this same district. The weather-based crop ET was roughly 8% higher than that predicted with the water balance, suggesting that some areas were not using water at the nonwater limiting rate, likely due to high levels of soil salinity. These results are useful to water resources professionals interested in methods for estimating crop water use.

Technical 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 (ETc) 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 (Kc ETo) procedure integrated over the project. The accuracy of the annual ETc WB estimates was + or - 6% at the 95% confidence level. Calculations using Kc and ETo were based on the FAO-56 dual crop coefficient approach and included separate calculation of evaporation from precipitation and irrigation events. Grass reference ETo 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 seven-year period. The 8% overprediction was concluded to stem primarily from use of Kc 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 for the entire project based on Kc, 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 Kc showed that relaxation from using the FAO-56 dual Kc method to the more simple mean (i.e., single) Kc curve and relaxation of specificity of planting and harvest dates did not substantially increase the project-wide prediction error. The use of the mean Kc curves, where effects 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) Kc method rather than the dual Kc method was caused by the lack of accounting for evaporation from special irrigations during the off-season (i.e., in between crops).