Estimation of Growing Season ET using Wyoming ET Calculator

Accurate estimations of Evapotranspiration (ET) and Consumptive Irrigation Requirement (CIR) are essential 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, and interpolates them into daily reference ET using either linear or cubic functions. The purpose of this project is to replace the current Excel model with a GIS-based ET calculator. Our approach uses daily weather data to calculate daily reference and actual ET, and then aggregate actual ET into monthly and seasonal ET. Among 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. Wyoming ET Calculator, a GIS-based ET tool, was developed to calculate daily potential ET, CIR, and actual ET, using daily reference ET, crop coefficients, effective precipitation ratios, and water stress factors. Total monthly and growing season ET and CIR were determined over the Upper Green River Basin in Wyoming. The long term trends of these totals from 1960-2009 were analyzed and compared to trends in weather data (minimum and maximum temperatures, wind speed, and dew point temperature). We also evaluated the total monthly and growing season ET from Wyoming ET Calculator against satellite-based ET (METRIC ET) estimations for June, July, and August of 2009 around an irrigated area near the Wind River Mountain Range in Wyoming. The total monthly ET from Wyoming ET Calculator agrees very well with total monthly ET from METRIC for well-watered crop areas. For other areas, the Wyoming ET Calculator tends to overestimate total monthly ET values than METRIC, because the tool assumes all NLCD crop area are being irrigated.