Comparison of Reference Evapotranspiration From the American Society of Civil Engineers (ASCE) Standardized Penman Monteith and Kimberly-Penman Equations in Northeast Colorado.

Abstract Reference evapotranspiration (ET) has been used to represent atmospheric demand for water. The Colorado Agricultural Meteorological Network (CoAgMet) has been calculating alfalfa reference crop ET (ETr) using the Kimberly-Penman (K-P) equation. Recently, CoAgMet has started to also provide alfalfa reference ET from the ASCE standardized Penman Monteith equation (ASCE-PM). The objectives of this study were 1) to compare the alfalfa reference crop ET from the ACSE-PM and the K-P equations at three locations in northeast Colorado, and (2) evaluate the effects of weather factors on the reference ET values from the two equations in Northeast Colorado. Hourly weather data was collected and obtained from CoAgMet from 3 weather stations located in Fort Collins, Greeley and Iliff during the years 2008 to 2011. This data was then input into the Ref- ET program to calculate hourly reference ET from the two equations. Comparisons of the two reference ET equations were examined using R2, Root-Mean-Square Error (RMSE), Relative Error (RE), and index of agreement (d). The R2 values ranged from 0.93 to 0.99; d ranged from 0.98 to 0.99, RMSE ranged from 0.29 to 0.75 mmd-1 and RE ranged from -1.03% to -8.12%. The seasonal cumulative ETr calculated using the K-P equation were 6 % lower than the ASCE-PM values. During the months May through October the ETr for the two equations had a smaller difference than the months November through April. Based on the most complete year of record (2011) the highest mean value of daily ETr was observed at the western-most location (Fort Collins, mean ETr = 4.35 mmd-1) followed by the middle location (Greeley, mean ETr = 4.13 mmd-1), and the lowest ETr values from the eastern-most site (Iliff, mean ETr = 3.90 mmd-1). For 2011, the highest mean value of relative humidity and lowest mean value of solar radiation and wind gusts were observed at the eastern-most site (Iliff) that resulted in the lowest mean ETr value among the three sites.