Knowledge of low streamflow statistics is necessary for effective water management in regions prone to extreme hydrologic events such as Iran. This study employs a data set of 23 river flow time series from Sefidrood Drainage Basin, Iran, to examine regional hydrological drought based on the low flow index 7Q10. Hierarchical agglomerative cluster analysis was used to divide the 23 gauging stations into two homogeneous drought regions based on the similarity of the binary drought series. 7Q10 was determined using log-Pearson type-III and 2-parameter log-normal distributions selected as the best regional probability distribution functions in homogeneous drought region 1 and 2, respectively. The 7Q10 was related to principal components of catchment characteristics in each homogeneous drought region separately using backward stepwise regression. The resulting regression equations exhibit a coefficient of determination of 69 and 89%, respectively. The regression parameters are linked to a size factor related to catchment area, an elevation factor which is independent of catchment area, and geological formation variables, which can therefore be interpreted as important controls of low flow generation processes in the study area. The equations developed here are expected to provide robust estimates of 7Q10 values for watersheds in areas of similar geomorphology, geology and climate.

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