Field Testing the ASCE Standardized Reference ET Equation and Recommended Crop Coefficients for Irrigation Scheduling in Northeast Colorado.

Knowledge of crop evapotranspiration (ETr) rates and corresponding soil water deficits is essential in deciding when and how much to irrigate. The American Society of Civil Engineers (ASCE) standardized reference ET equation is increasingly being adapted in Colorado for calculating reference crop ET. However, the ASCE equation along, with literature-recommended crop coefficients (Kc), needs to be tested under Colorado conditions. The objectives of this study were: 1) field test and evaluate the ASCE standardized reference ET equation and recommended crop coefficients for estimating corn (ETc) at two sites Northeast Colorado, and 2) field test an ET-based irrigation scheduling spreadsheet tool. The study was conducted in one center pivot sprinkler irrigated corn field and one furrow irrigated corn field in Greeley, Colorado. Profile soil water content, soil water deficit, crop height, and leaf area index (LAI) were measured weekly for both locations in 2010 and 2011. Daily weather data from an automated weather station near the two sites were input in the Ref-ET program to calculate alfalfa reference ET (ETr). The calculated ETr values were multiplied with literature recommended Kc values to estimate daily ETc. The daily calculated ETc values were then used in a simple water balance spreadsheet to estimate the soil water deficit. Calculated daily soil water deficits were compared with actual deficits determined from gravimetric samples. Comparisons of calculated and observed soil water deficits from the 2010 and 2011 growing seasons will be shown. Recommendations for improving the accuracy of the simple water balance spreadsheet for irrigation scheduling will be given.