International Symposium on
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Estimation of Monthly Discharge
Based on Thornthwait Water Balance Model in Semi-arid Region of Iran

M. Azarakhshi1 &  M. Mahdavi2

Watershed Management, Natural Resources Faculty, Tehran University, Iran

Natural Resources Faculty, Tehran University, Iran


Monthly discharge is one of the most important factors in designs and hydrological works. Some of basins do not have hydrometric equipment; in this case average monthly discharge could be estimated by 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 in PE type models the input series are precipitation and potential evapotranspiration. In this study PE type models have been used in 12 basins in semi-arid climate in region of Azarbaijan and North of Khorasan provinces in Iran. After collecting data of temperature, precipitation and average monthly discharge of these basins, the potential evapotranspiration is calculated from Thornthwait formula.

Remaining parts of water balance equation including: Actual evapotranspiration, soil moisture supply of basin for each month and later month are estimated from Thornthwait model. Then, by regarding the logical relationship between parameters, the regression relationships between various parameters of water balance are calculated. The models are analyzed at 5 % confidence level. The results of the t- test showed that difference between observed runoff and estimated runoff by models was not significant .Consequently by using the monthly water balance models, the series of mean monthly discharge could be generated.

Keywords: Monthly water balance, Small basin, PE type models, Azarbaijan, Khorasan