In this paper, the possibility of studying the occurrence probabilities of hydrologic drought events has been explored. The obtained results were based on the case study of the annual inflow to Fatha discharge station on Tigris river, Iraq, over the period (1931-1988). This recorded time series is basically used to evaluate the observed duration and severity of the drought. The theory of runs is adopted to estimate, both, the probabilities of duration and severity of the drought. Three stochastic models have been used for modeling the hydrologic process having short-term time dependence to generate additional data for statistical inferring of the drought duration. Models such as(AR(1),AR(2),and ARMA(1,1))have been formulated by means of modern techniques called (SAMA-2007). The number of the generated series was ranging between 100 to 10000 series. This help in the estimation of the probabilities of drought severity for which no rigorous mathematical solution is exist yet. Discrepancy in the performances of the proposed models have been noticed, especially in their capabilities of preserving the statistical properties of the observed time series. It is concluded that while a certain model has a satisfactory performance in preserving certain parameters, it failed in the estimation of other parameters.
(Keywords: Drought, Stochastic Modeli