High Resolution Mapping of Reference ET for the State of Wyoming

Accurate estimation of Consumptive Use (CU) and Consumptive Irrigation Requirement (CIR) are essential components for water resources planning and management. The Wyoming State Engineer’s Office currently determines monthly reference evapotranspiration (ET) with an Excel Spreadsheet ET model using average monthly data from a nearby weather station (usually an airport weather station) for the irrigated area of interest. The monthly reference ET is then interpolated into daily reference ET using either linear or cubic functions. The purpose of this project is to enhance the current Excel model with a GIS-based ET calculator. Our approach will use daily weather data to calculate reference ET and actual ET, and then aggregate them into monthly and seasonal ET. Among the many reference ET equations available, the ASCE Standardized Reference Evapotranspiration Equation (ASCE-ET) and the Hargreaves-Samani equations were selected to calculate daily reference ET. Data needed for the equations (minimum and maximum temperatures, wind speed, and dew point temperature) was gathered at various resolutions from several sources, such as weather stations in Wyoming and from the National Centers for Environmental Prediction (NCEP). High resolution (1km x 1km) gridded reference ET maps were produced from interpolated weather data for the three major river basins in southern Wyoming (North Platte River, Green River, and Bear River basins). Monthly and seasonal reference ET values will be determined using daily reference ET. Gridded reference ET values from the two methods will be compared with the reference ET using data from a single weather station. These reference ET maps will then be used to develop a GIS-based ET calculator.