تعيين مدل بيلان آبي مناسب ماهانه در حوزه هاي آبخيز كوچك كشور ( مطالعه موردي : استان آذربايجان شرقي و شمال خراسان )

A Determination of An Appropriate Monthly Water Balance In Small Watersheds of Iran (Case Study: Eastern Azarbayejan And North of Khorasan)

Monthly discharge is one of the most important factors to be considered in designs and hydrological works. Some basins are not equipped with needed hydrometric equipment. In such a case average monthly discharge could be estimated from regional monthly water balance models of representative basins. These models are of two types: P and PE. In P type the only input is precipitation and whereas PE type the input series are precipitation as well as potential evapotranspiration. In this study, PE type models were used in 12 basins in the semi-arid climate of Azarbayejan as well as North of Khorasan province in Iran. Following collection of data in temperature, precipitation and average monthly discharge in these basins, the potential evapotranspiration was calculated using Thornthwait formula. Remaining parts of water balance equation including: Actual evapotranspiration, soil moisture supply of basin in each month and later months are estimated from Thornthwait model .Then, by regarding the logical relationship between parameters, the regression relationships between various parameters in water balance are established. The models are analysed at 5% confidence level. The results of the tstudent-test showed that diffrence between observed and estimated runoff, from models, was not significant and thus by using the monthly water balance models, the series for mean monthly discharge could be generated.