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Standardized ASCE Penman-Monteith: impact of sum-of-hourly vs. 24-hour timestep computations at reference weather station sites

Irmak, S., Howell, T.A., Allen, R.G., Payero, J.O., Martin, D.L.
Transactions of the ASAE 2005 v.48 no.3 pp. 1063
evapotranspiration, equations, grasses, simulation models, mathematical models, meteorological parameters, diurnal variation, climatic zones, Florida, Texas, Nebraska, California
The standardized ASCE Penman-Monteith (ASCE-PM) model was used to estimate grass-reference evapotranspiration (ET(o)) over a range of climates at seven locations based on hourly and 24 h weather data. Hourly ET(o) computations were summed over 24 h periods and reported as sum-of-hourly (SOH). The SOH ASCE-PM ET(o) values (ET(o,h,ASCE)) were compared with the 24 h timestep ASCE-PM ET(o) values (ET(o,d)) and SOH ET(o) values using the FAO Paper 56 Penman-Monteith (FAO56-PM) method (ET(o,h,FAO)). The ET(o,h,ASCE) values were used as the basis for comparison. The ET(o,d) estimated higher than ET(o,h,ASCE) at all locations except one, and agreement between the computational timesteps was best in humid regions. The greatest differences between ET(o,d) and ET(o,h,ASCE) were in locations where strong, dry, hot winds cause advective increases in ET(o). Three locations showed considerable signs of advection. Some of the differences between the timesteps was attributed to uncertainties in predicting soil heat flux and to the difficulty of ET(o,d) to effectively account for abrupt diurnal changes in wind speed, air temperature, and vapor pressure deficit. The ET(o,h,FAO) values correlated well with ET(o,h,ASCE) values (r2 greater than or equal to 0.997), but estimated lower than ET(o,h,ASCE) at all locations by 5% to 8%. This was due to the impact of higher surface resistance during daytime periods. Summing the ET(o) values over a weekly, monthly, or annual basis generally reduced the differences between ET(o,d) and ET(o,h,ASCE). Summing the ET(o,d) values over multiple days and longer periods for peak ET(o) months resulted in inconsistent differences between the two timesteps. The results suggest a potential improvement in accuracy when using the standardized ASCE-PM procedure applied hourly rather than daily. The hourly application helps to account for abrupt changes in atmospheric conditions on ET(o) estimation in advective and other environments when hourly climate data are available.