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ARS Home » Plains Area » Bushland, Texas » Conservation and Production Research Laboratory » Soil and Water Management Research » Research » Publications at this Location » Publication #181789
Title: STANDARDIZED ASCE PENMAN-MONTEITH: IMPACT OF SUM-OF-HOURLY VS. 24-HOUR TIMESTEP COMPUTATIONS AT REFERENCE WEATHER STATION SITES

Author
item IRMAK, S - UNIVERSITY OF NEBRASKA
item Howell, Terry
item ALLEN, R - UNIVERSITY OF IDAHO
item PAYERO, J - UNIVERSITY OF NEBRASKA
item MARTIN, D - UNIVERSITY OF NEBRASKA

Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/28/2005
Publication Date: 5/1/2005
Citation: Irmak, S., Howell, T.A., Allen, R.G., Payero, J.O., Martin, D.L. 2005. Standardized ASCE Penman-Monteith: Impact of sum-of-hourly vs. 24-hour timestep computations at reference weather station sites. Transactions of the ASAE. 48(3):1063-1077.

Interpretive Summary: The calculation of water use from vegetative surfaces is often referenced to a short, well-water grass or a taller, well-watered alfalfa crop. These computed water use rates are called the reference evapotranspiration (ET) and used in various crop simulation models and in irrigation scheduling models. The recent availability of hourly weather data from automated water stations has permitted the use of hourly reference evapotranspiration equations that can be summed over the day to be more widely used. Few comparisons of the hourly summed daily ET rates have been made for various climates with ET computations using 24-hr daily weather data. This work compared the summed hourly ET data for a day with computed ET using daily weather data for seven sites for multiple years at those sites. In most cases, the summed hourly computed ET for a day was a more consistent representation of the reference ET of grass compared with the daily computed reference ET, particularly in windy, dry environments. The results indicated an improvement in reference ET computations when hourly weather data were used compared with daily weather data. These results support the ASCE (American Society of Civil Engineers) standardized procedures for computing hourly reference ET rates.

Technical 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 computations were summed over 24 h periods and reported as sum-of-hourly (SOH). The SOH ASCE-PM ETo values (ETo,h,ASCE) were compared 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 higher than 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 increases in ETo. 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 ETo,d to effectively account for abrupt diurnal changes in wind 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 surface resistance during daytime periods. Summing the ETo values over a weekly, monthly, or annual basis generally reduced the differences between ETo,d and ETo,h,ASCE. The differences suggest that using ETo,d rather than ETo,h,ASCE would result in underestimation or overestimation of ETo. Summing the ETo,d values over multiple days and longer periods for peak ETo 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 ETo estimation in advective and other environments when hourly climate data are available.