Drought is a climatological hazard and is a part of the climate of a region. This climatological phenomenon has some properties that distinguish it from other natural disasters. Drought is a gradual phenomenon and when the amount of moisture and precipitation become less than normal value in a certain period, its damages are non- structural in this condition and it is said that drought has occurred. Researches show that drought damages are the highest among natural disasters. Therefore the extensive monitoring or creating an early warning system in the drought prone area is inevitable. One of the basic instruments of drought monitoring is the application of its index. In this paper Palmer Drought Severity Index (PDSI) has been assessed as a valid and applicable model. The PDSI uses temperature (T), precipitation (P) and available water holding capacity (AWC) of the soil. Using these data for each period, potential amount of evapotranspiration is calculated based on two layer model for soil moisture computations. Then, other parameters such as potential of recharge and potential of runoff are being calculated. Finally the model output results which are monthly deviation moisture (Z) and drought severity index (X3i) have been determined to use designed software for mashed synoptic station for the statistical period (1971-2003).

Keywords: Drought, drought index, Mashad, Palmer drought severity index, temperature, precipitation.