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A long-term water use planning model for California

Snyder, R.L., Orang, M., Matyac, S.
Acta horticulturae 2002 no.584 pp. 115-121
computer software, cover crops, crop coefficient, evaporation, evapotranspiration, irrigation water, models, planning, rain, soil water deficit, water requirement, California
Long-term planning for consumptive use (evapotranspiration) by crops is an important activity by the State of California. The information is needed to determine evapotranspiration of applied water (ETaw), which is equal to the seasonal cumulative evapotranspiration minus the initial soil water deficit and the total effective rainfall. This provides an estimate of the net water requirement to irrigate a crop. If the water requirement is divided by the weighted mean application efficiency, the result is a site-specific total irrigation water requirement to produce a crop. For many years, published crop coefficient (Kc) values were used to estimate crop evapotranspiration (ETc) from reference evapotranspiration (ETo); however, because the equations for ETo have changed, since the Kc values were developed, they are frequently in error. Also, the published Kc values do not adequately account for the influence of rainfall frequency on early or late season ETc nor do they properly account for cover crops. In fact, it is difficult to find references as to how the published Kc values were determined. Based on newly published information on midseason Kc values and on bare soil evaporation, a user-friendly Excel computer program (CUP) was developed to improve long-term estimates and to account for rainfall, cover crop, and immaturity effects on Kc values and ETc. In this paper, the advantages of the new model over traditional long-term ETc estimates and errors in the old estimates will be discussed.