Agricultural Water Use Models

The following computer application programs have been cooperatively developed and released by DWR and the University of California, Davis.

Consumptive Use Program PLUS (CUP+) is a Microsoft Excel application that estimates crop evapotranspiration (ETc) and evapotranspiration of applied water (ETaw). ETaw is a seasonal estimate of the irrigation water requirement for evapotranspiration of a crop minus any water supplied by effective rainfall and effective seepage.


ETaw information is needed to determine the demand side of water requirements. In addition to using monthly climate data, the program uses daily measured weather data to estimate daily soil-water balances for surfaces that account for evapotranspiration losses and water contributions from rainfall, seepage, and irrigation. Soil water-holding characteristics, effective rooting depths, and irrigation frequency are used with rainfall and ETc data to calculate a daily water balance and determine effective rainfall and ETaw. ETaw is equal to the seasonal cumulative ETc minus the effective rainfall.


The application generates a wide range of tables and charts useful for irrigation planning. It also can be used to study the impact of climate change on evapotranspiration and irrigation water needs.

The Simulation of Evapotranspiration of Applied Water (SIMETAW) program was developed to help water planners and researchers improve their long-term estimates of net crop-water requirements. This program can simulate many years of weather data from monthly climate data and uses this simulated data to estimate reference evapotranspiration (ETo) and ETc.


In addition, simulated daily rainfall and soil-crop information are used to determine effective rainfall and ETaw, where ETaw is an estimate of the ETc minus any water supplied by effective rainfall. The simulation program allows for investigation of how climate change might affect water demand. In addition, the use of the widely adopted Penman-Monteith equation for ETo and improved methodology for applying crop coefficients for estimating ETc are both used to improve ETaw accuracy.

California Simulation of Evapotranspiration of Applied Water (Cal-SIMETAW) was designed to estimate daily soil-water balance to determine ETc and ETaw for California Water Plan updates. This model requires weather data, soils, crop coefficients, rooting depths, seepage, etc., that influence crop-water balance.


The model uses daily weather data, derived from the monthly Parameter-elevation Relationships on Independent Slopes Model (PRISM) climate data, and daily U.S. National Climatic Data Center climate station data to cover California with a 4×4 kilometer (km) grid. From the PRISM data, ETo is then estimated by using a Hargreaves-Samani equation calibrated to estimate regional Penman-Monteith equation-derived ETo to account for spatial climate differences.


In addition to using historical data, Cal-SIMETAW can use near-real-time data from Spatial California Irrigation Management Information System (CIMIS), which is a model that combines weather station data and remote sensing to provide a statewide grid of ETo information.


Cal-SIMETAW uses the Soil Survey Geographic Database (SSURGO) soil-characteristic data and crop information with precipitation and ETc data to generate hypothetical water-balance irrigation schedules to determine ETaw. (ETaw is an estimate of the seasonal irrigation requirement that assumes 100 percent application efficiency.) Cal-SIMETAW can also generate daily weather data from monthly mean values to study climate change scenarios and possible impacts on water demand.

Publications And Reports


California Weather Data (xls: 3.52 MB – 635 KB ZIP). The California Weather Data program is an Excel database that provides daily mean weather and ETo data from CIMIS to input into the CUP and SIMETAW models.

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