Droughts are intrinsically regional. The critical conditions in the dry periods occur when there is a severe shortage of water resources for a long period on a wide area. Therefore, an investigation of the regional and not of a local drought offers better understanding of this phenomenon. This study aims at a regional analysis of the hydrological drought in East Azarbaijan Province for a period of 45 years (1972– 2017) using the data collected at 25 hydrometric stations with proper spatial distribution across the province. Homogeneous hydrological areas was identified based on hierarchical analysis method in the SPSS software using the following independent variables: Area, Perimeter, Length of the main basin, Main channel gradient, Drainage density, Basin shape factor, Total hydrographic network, Weighted average slope of the basin, Maximum, Minimum and Median height of the basin, and its length. Frequency analysis and probability distribution of the dependent variables (duration and intensity of hydrological drought spells) was performed on the basis of the two year return period for each hydrometric station. The factor analysis was performed using the PCA method in the SPSS software. The suitability of the data for the factor analysis was evaluated using the KMO coefficient. The value of this coefficient was estimated as 0.546, which indicates that the data were suitable for factor analysis. The result of factor analysis showed that the contribution of perimeter, height and shape of the basin were 40.13, 26.42 and 13.12% (Sum 79.68 % ) respectively in explaining the variance of the variables (i.e. the most important factors in zoning). Results of regional analysis using multivariable regression (stepwise method) indicated that perimeter, avrage height and the basin shape coefficient have a decisive role in estimating the drought severity and its duration with two-year return period in East Azarbijan. In the scarcity of hydrometric stations in most of the watersheds of the East Azarbaijan province, the regional equation presented in this study can be used to estimate hydrological drought characteristics of watersheds without observed surface runoff data.
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