Unexpected events frequency and magnitude in hydrology like occurrence probability, severity and duration of hydrological drought in most parts of world have great deal of importance so that evaluation of draughts occurrence probability and duratio serve as an stochastic analysis o dry spells. In the present study, two above-mentioned characteristics for drought were evaluated on hydrometric stations selected in Caspian Sea basin to predict and estimate drought severity and duration. To drought severity and its duration regional frequency analysis, linear moment’s method was used. Out of total 300 hydrometric stations located at Caspian sea basin, 180 stations were selected due to having suitable statistical period for regional frequency analysis and at the same time, maximum dry days and number of drought events were calculated as hydrological severity and duration parameters in every station respectively. After homogeneity and stochastic tests of data, homogeneity statistic H developed by Hosking and Wallis was calculated to test study area homogeneity, suggested that study area is completely heterogeneous. To achieve homogenous area, analysis of hierarchy process (AHP) was applied to left; MARGIN: 0cm 0cm 0pt; unicode-bidi: embed; DIRECTION: rtl" dir=rtl align=right Keywords : hydrological drought, drought severity and duration, regional frequency analysis, Dry Spells, linear moments, multivariate regression, Caspian Sea basin.
