ET of applied water and the cup+ model

In 2011, a user-friendly Excel application program (CUP) was developed to improve the computation of evapotranspiration (ET), dissemination of crop coefficient (K_c) data, and crop evapotranspiration (ET_c) information for use in California. In addition to other methods, CUP computes reference evapotranspiration (ET_o) from either monthly mean or daily values of solar radiation, maximum and minimum temperature, dew point temperature (or maximum and minimum relative humidity), and wind speed using the standardized reference ET equation. A cubic spline curve fitting technique is used to derive one year of daily weather and ET_o data from the monthly data. The daily ET_o and rainfall data are used to estimate a crop coefficient (K_e) for bare soil evaporation (E) as a function of mean of ET_o and rainfall wetting frequency for the entire year. The daily bare soil K_e values are employed as a baseline for in- and off-season soil evaporation estimates. For orchard and vine crops, CUP accounts for canopy size and cover crop contributions to ET_c. Crop and soil information are input and used to determine seasonal K_c curves and to characterize soil water characteristics within the rooting zone. In 2014, the application name was changed to CUP+, and the ability to adjust ET_o for climate change and to estimate ET of applied water (ET_aw) were added to the model when using input daily climate data. The program generates several useful tables and charts including crop water requirements by month, by season, and by year and seasonal ET_aw for most crops that are irrigated to minimize water or other stresses. It can also estimate the ET for deficit irrigated crops by including the stress effects in the K_c values. This paper describes “CUP plus” and how ET_aw is used for water resources planning.