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
Thomthwait formula.
Remaining parts of water balance equation including: Actual evapotranspiration, soil moisture
supply of basin in each month and later months are estimated from Thomthwait model .Then, by
regarding the logical relation ship 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.