Trend Analysis of Monthly Reference Crop Evapotranspiration of Zabol

In the recent decades the increasing of the air temperature due to changes of greenhouse gases of the atmosphere are noticed by many researchers and scientists. One of the major effects of these changes is increasing the amount of crop water requirement. The aim of this research isstudy of trend in monthly ET0 time series at Zabol station using Penman Montheis (PM) method. To do this data of this station are gathered and completed from 1963-2005. Then the amount of monthly ET0 for Zabol station was computed by PM method, separately for each months and year. Then trend analysis conducted using Mann-Kendal method for each of the obtained ET0 time series. Results showed thatthe average of ET0 varied from 2.08 for December to 14.8 mm/day for July. Positive trends are detected for all months, significant at 0.05. The weakest trend occurred in June, which is significant at 5% level, while the sever trend occurred in February, significant at 1% level. Results indicated that the main reason for positive trends of Zabol was attributed to the wind speed in proposed time period.