comparison of daily with sum-of- hourly reference evapotranspiration in kerman reference weather station

abstract in recent years, automatic weather stations have been widely used for recording meteorological data in different time scales. therefore the accurate estimation of eto by combination equations can be evaluated using these set of short time scales data. daily eto can also be calculated by summation of hourly eto values. the purpose of this study is to compare the eto values estimated by hourly and daily data. totally, 7270 hourly meteorological data obtained from the automated weather reference station where placed in shahid bahonar university of kerman, iran during april to december 2005 and january to march 2006. the penman-monteith equations proposed by the food and agriculture organization (fao-56) and american society of civil engineers (asce) were used for hourly and daily (24 hours) eto estimation. the paired t- student test was used for comparison of estimated eto values by two methods (daily and hourly summation) in each month. the results of this study showed significant differences between eto values estimated by daily and hourly summation data in both equations at 5 percent level. the hourly summation method overestimated eto values from 5.8 to 44.6 percent in different months using fao-56 penman-monteith equation and from 7.4 to 47.6 percent using asce penman-monteith equation. the regression coefficients of correlation equations between the daily and hourly summation method in both combination models were strongly significant.