مدل آماري برآورد از سطح مخزن چاه نيمه سيستان به روش بيلان حجمي

Statistical model of free water surface evaporation using the volume balance method in Chahnimeh reservoir, Sistan-Iran

The water budget modeling is an important primary task in hydrologic studies of lakes and reservoirs. With the aid of the water budget equation, one can calculate the amount of free surface water evaporation. This method was used to determine evaporation from the Chahnimeh Reservoir in Sistan region, east of Iran. Nine years of daily inflow and outflow, water surface fluctuation, rainfall, and pan evaporation data were used in this study. A detailed investigation showed that daily water budget survey is not appropriate because of extreme fluctuation of water surface. Therefore, 5-day, I0-day, and monthly time steps were selected to form the water budget equations. The water budget equation was used to calculate pan coefficient in two cases: Annual and time dependent within the year. A regression model was formed to determine the coefficients. The model was confirmed by F test in 99 percent significant level in development phase and was verified by K2 test in 99 percent significant level in the evaluation phase, except for the monthly time step due to insufficient number of samples. Classification of data into two homogeneous groups, dry years and normal-wet years, showed that pan coefficient decreases during the 120 days wind blowing in Sistan from May to October. The reduction in pan coefficient is larger, by as much as 50 percent, in the dry years than normal-wet years.