STANDARDIZED ASCE PENMAN-MONTEITH: IMPACT OF SUM-OF-HOURLY VS. 24-HOUR TIMESTEP COMPUTATIONS AT REFERENCE WEATHER STATION SITES

Abstract

The standardized ASCE Penman-Monteith (ASCE-PM) model was used to estimate grass-reference evapotranspiration(ETo) over a range of climates at seven locations based on hourly and 24 h weather data. Hourly ETo computationswere summed over 24 h periods and reported as sum-of-hourly (SOH). The SOH ASCE-PM ETo values (ETo,h,ASCE) werecompared with the 24 h timestep ASCE-PM ETo values (ETo,d) and SOH ETo values using the FAO Paper 56 Penman-Monteith(FAO56-PM) method (ETo,h,FAO). The ETo,h,ASCE values were used as the basis for comparison. The ETo,d estimated higherthan ETo,h,ASCE at all locations except one, and agreement between the computational timesteps was best in humid regions.The greatest differences between ETo,d and ETo,h,ASCE were in locations where strong, dry, hot winds cause advective increasesin ETo. Three locations showed considerable signs of advection. Some of the differences between the timesteps was attributedto uncertainties in predicting soil heat flux and to the difficulty of ETo,d to effectively account for abrupt diurnal changes inwind speed, air temperature, and vapor pressure deficit. The ETo,h,FAO values correlated well with ETo,h,ASCE values (r2 >0.997), but estimated lower than ETo,h,ASCE at all locations by 5% to 8%. This was due to the impact of higher surfaceresistance during daytime periods. Summing the ETo values over a weekly, monthly, or annual basis generally reduced thedifferences between ETo,d and ETo,h,ASCE. The differences suggest that using ETo,d rather than ETo,h,ASCE would result inunderestimation or overestimation of ETo. Summing the ETo,d values over multiple days and longer periods for peak ETomonths resulted in inconsistent differences between the two timesteps. The results suggest a potential improvement in accuracywhen using the standardized ASCE-PM procedure applied hourly rather than daily. The hourly application helps to accountfor abrupt changes in atmospheric conditions on ETo estimation in advective and other environments when hourly climatedata are available.