seasonal sensitivity analysis for climatic variables of asce-penman-monteith model in a semi-arid climate

seasonal variations of climatic parameters are significant in arid and semi-arid regions and sensitivity of each parameter may differ in different seasons. no work has been done in this regard in iran. therefore, in this study, sensitivity analysis of the asce-penman-monteith grass reference evapotranspiration (eto) equation was investigated on the basis of variation of mean air temperature (tmean), vapor pressure deficit (vpd), wind speed at 2 meter height (u2), and short wave solar radiation (rs) in the semi-arid climate of kerman, southeast of iran. the sensitivity coefficients were derived for each variable on a daily basis. the results showed that the computed eto was sensitive to vpd in all months, to u2 during march to november, and to rs during the summer months. the change in eto was linearly related to the change in the climatic variables, with in most cases. the sensitivity coefficient for rs was higher during the summer months and lower during the winter months. increase in eto with respect to the increase in the aforementioned climate variable changed by month. on an annual average, 1 c increase in tmean, 1 ms-1 increase in u2, and one mj m-2d-1 increase in rs resulted in, respectively, 0.11, 0.37, and 0.09 mm d-1increases in eto. a 0.4 kpa increase in vpd resulted in 0.85 mm d-1 increase in eto. generally, various meteorological parameters should be measured with high accuracy in order to use the combination model.