ASSESSING REFERENCE EVAPOTRANSPIRATION USING PENMAN-MONTEITH AND PAN METHODS IN THE WEST REGION OF AFGHANISTAN

Optimal estimation of reference evapotranspiration (EEEE0) is extremely important to calculate irrigation scheduling in Afghanistan. In this study, a measured evapotranspiration from an A-class pan (EEEE) was selected as an index to discuss the error of EEEE0 which was calculated using the Penman-Monteith (FAO56PM) method in the west region of Afghanistan, which is exposed to extreme climate condition. Results obtained showed that the period from June to September was confirmed as extreme, with out-of-normal-range climatic data, for example, high temperature, low humidity, and relatively strong wind speed. While for the rest of the year, they were almost within the normal range of the climatic variables. By comparing the daily ave rage EEEE0 and EEEE, the differences between EEEE0 and EEEE were significantly large in the period from June to September, while this differentiation was very small outside of this period. By comparing the relationship of error with climatic variables, it was found that the relationship of the error with wind speed was strongest compared to the other three variables. The higher the wind, the larger the difference, and vice versa. Therefore, experimentally it was confirmed that this kind of error becomes larger when EEEE0 is greater than 10 mm d−1.