Sensitivity analysis of the reference crop evapotranspiration in a humid region

This study examined the sensitivity of reference crop evapotranspiration (ET0) to climatic variables in a humid region in Iran. ET0 was estimated using the FAO-56 Penman-Monteith (PMF-56), Blaney-Criddle (BC), and Hargreaves-Samani (HG) methods. Sensitivity analysis was performed by two distinct methods which were (i) changing the value of a certain climatic parameter in a range between +/- 20% of its long-term mean with an increment of 5%, and calculating the percentage of change in ET0, while the other parameter values were kept constant; and (ii) calculating the sensitivity coefficients (SCs) for each of the climatic variables. For each of the climatic parameters, the Iso-SC maps were plotted using the Arc-GIS software. Results indicated that the most sensitive parameter for ET0 was the maximum air temperature (T-max) by PMF-56 and HG methods. Increasing T-max up to 20% led to an increase in ET0 between 8.5 and 15%, at the selected stations by PMF-56. In contrast, the less sensitive parameter for ET0 was the minimum air temperature (T-min) for PMF-56 and T-mean for HG. For PMF-56, increasing the minimum relative humidity (RHmin) to 20% led to a decrease in ET0 in the range between 0.5 and 5%. The highest values of SC in the cases of T-max and T-min were found to be equal to 0.8 and 0.53, respectively. Similarly, the SC in the case of RHmin varied between - 0.29 and - 0.0038. This range for wind speed was between 0.06 and 0.22 and in the case of sunshine hours it was between 0.272 and 0.385. These findings would be useful in the scientific management of water resources in the region.