assessment of the monthly water balance in an arid region using tm model and gis (case study: pishkouh watershed, iran)

monthly discharge is one of the most important factors considered in designs and hydrological works. some watersheds 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 watersheds. in this study, thornthwaite & mather (tm) model were used in the pishkouh watershed in arid climate of yazd, iran. the water balance was used for computing seasonal and geographical patterns of water availability to facilitate better management of available water resources. the water balance study using the tm model with the help of remote sensing and geografhic information systems (gis) is very helpful in finding out the periods of moisture deficit and moisture surplus for an entire basin. this study indicates that there is an annual deficit of 442.7 mm in the study basin and an annual surplus of 26.4 mm. the pishkouh watershed has a period of moisture surplus from june to august and the remaining months are a period of deficit. generally these mean estimated runoff values fall between the 90% confidence intervals for the measured runoff and quality of the model was satisfactory (qual=2.86). these results indicate that the tm model can be satisfactorily applied to estimate mean monthly stream flow and potential runoff map in the arid regions of central iran, too.