A comparison of ASCE and FAO-56 reference evapotranspiration for a 15-min time step in humid climate conditions

Abstract

Many applications in diverse disciplines require estimates of evapotranspiration (ET) at hourly or smaller time steps. The primary objectives of this study were to compare the American Society of Civil Engineers (ASCE) and FAO-56 Penman-Monteith equations for 15-min ET 0 0 (ET 0,15 - min,ASCE and ET 0 . 15 - min,FAO ) estimations for humid climate conditions and to compare the 24 h sum of ASCE (ET 0,24 h,ASCE ) and FAO-56 15-min ET 0 (ET 0,24 h,FAO ) with the daily ET 0 (ET 0,d,FAO ) computed from the daily FAO-56 equation, which is identical to ASCE daily ET 0 equation. Ten-year, i.e., 1997-2006 continuous 15-min and daily weather data for 11 representative and well-distributed sites throughout Georgia, USA were used. It was evident that during the day, ET 0,75-min,ASCE was higher than ET 0,15-min,FAO due to a lower surface resistance parameter value, while at night ET 0,15-min,ASCE was lower than 0ET 0,15-min,FAO due to a higher surface resistance paramete r valu e. The ET 0,15-min,FAO was about 5% less than ET 0,15-min,ASCE and ET O . 24 h,FAO was about 5% lower than ET 0,24 h,ASCE .The difference between ET O . 15-min,ASCE and ET 0,15-min,FAO during the day and night was highly dependent on wind speed. During the three summer months, i.e., June, July and August, on average, ET 0,24 h,FAO was only 1% higher than ET O,D,FAO while ET O,24 h,ASCE was 5% higher than ET 0,d,FAO . For the entire year, ET 0,24 h,FAO was 8% higher than ET o,d,FAO while ET 0,24 h,ASCE was 13% higher than ET 0,d,FA O .The ET 0,24 h,FAO and ET o,d,FAO had a better agreement than ET 0,24 h,ASCE and ET o,d,FAO throughout the year and during the summer months. It is also worth noting that the daily calculations for FAO-56 and ASCE were identical. These results demonstrated that for applications that require 15-min time steps or daily ET 0 for the entire year, the use ofET 0,15-min,FAO andET 0,24 h,FAO , respectively, will yield more consistent outcomes. The use of ET o,d,FAO during the summer months can be as accurate as the use of ET o,24 h,FAO for applications that require daily time steps, such as irrigation scheduling.