a comparison of asce and fao-56 reference evapotranspiration for a hourly time step in fariman weather station

abstract potential evapotranspiration is one of basic parameter in hydrologic cycle that should be estimate in irrigation design and scheduling, watershed hydrology studies. 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 hourly et0 (et0,hourly,asce and et0, hourly,fao) estimations for semiarid climate conditions and to compare the 24 h sum of asce (et0,24 h,asce) and fao-56 hourly et0 (et0,24 h,fao) with the daily et0 (et0,d,fao) computed from the daily fao-56 equation, which is identical to asce daily et0 equation. 278-days, i.e., 2008–2009 continuous hourly and daily weather data from the automated internet weather station where placed in private farm in fariman khorasan razavi province were used. it was evident that during the day, et0,hourly,asce was higher than et0, hourly ,fao due to a lower surface resistance parameter value, while at night et0, hourly,asce was lower than et0, hourly,fao due to a higher surface resistance parameter value. the et0, hourly,fao was about 18% less than et0, hourly,asce and et0,24 h,fao was about 14% lower than et0,24 h,asce. the difference between et0, hourly,asce and et0, hourly,fao during the day and night was highly dependent on wind speed. for the entire year, et0,24 h,fao was 2.6% higher than et0,d,fao while et0,24 h,asce was 17% higher than et0,d,fao. these results demonstrated that for applications that require hourly time steps or daily et0 for the entire year, the use of et0, hourly ,fao and et0,24 h,fao, respectively, will yield more consistent outcomes.