Sensitivity Analysis of the Penman-Monteith reference Crop Evapotranspiration to Climatic Variables in Iran

Sensitivity analysis of reference crop evapotranspiration (ET 0 ) in Iran is an important necessity due to better management of water in such an arid and semi-arid country. It is important to analyze the sensitivity of ET 0 to weather parameters as climate seems to be changed to some degree everywhere. In this study, sensitivity of ET 0 to climatic variables at the eight selected stations of Iran was investigated. To estimate the ET 0 , the most recommended form of the FAO-56 Penman-Monteith method was used. Sensitivity of ET 0 was analyzed in terms of change in mean air temperature (T mean ), actual vapor pressure (e a ), wind speed (u 2 ) and net solar radiation (R n ) within a range of ±20 %, in 5 % steps, from their corresponding normal long-term values (1965–2005). Results showed that ET 0 was most sensitive to T mean at the six of the stations in annual time scale. Increase in T mean by +20 %, caused the most increase in ET 0 at Chabahar. In contrast ET 0 was least sensitive to e a in most selected stations. In this study, we investigated the interaction parameters in different scenarios on ET 0 at all stations in monthly and annual time scale. Results showed in annual time scale increasing T mean and WS by 20 % and decreasing e a by 20 % yielded ET 0 increasing 36.4 % at Chabahar. But, in monthly time scale 10 % increase in T mean and WS as well as 10 % decrease in R n yielded ET 0 to be increase to about 30.5 % in December at Bandar Anzali.