CONSUMPTIVE USE PROGRAM (CUP) MODEL

Abstract

Introduction A user-friendly Microsoft Excel application program “Consumptive Use Program +” or “CUP+” was developed to help growers and water agencies determine reference evapotranspiration (ETo), crop coefficient (Kc) values, crop evapotranspiration (ETc), and evapotranspiration of applied water (ETaw), which provides an estimate of the net irrigation water diversion needed to produce a crop. The application also can be used to study the impact of climate change on evapotranspiration and irrigation water needs. CUP+ computes reference evapotranspiration (ETo) from daily solar radiation, maximum and minimum temperature, dew point temperature, and wind speed using the daily Penman-Monteith equation. In addition, the program uses a curve fitting technique to derive one year of daily weather data from the monthly data and to estimate daily ETo. It also uses daily rainfall data to estimate bare soil evaporation as a function of mean of ETo and wetting frequency in days. A bare soil Kc value is calculated to estimate the offseason evapotranspiration and as a baseline for in-season Kc calculations. CUP+ accounts for the influence of orchard cover crops on Kc values and it accounts for immaturity effects on Kc values for tree and vine crops. Further, the program computes and applies all ETo and Kc values on a daily basis to determine crop water requirements by month, by season, by year. The water balance model is similar to that used in the Simulation of ET of Applied Water (SIMETAW) application program, which was also developed as a cooperative effort between the University of California (UC) and the Department of Water Resources (DWR). The application outputs a wide range of tables and charts that are useful for irrigation planning.