Severity and Duration Variations of Monthly Stream Flow in Gorganroud Watershed, Golestan Province

The assessment of wet and dry hydrological periods is the primary and necessary basis of water resources management and planning. The aim of this study was to determine the hydrological drought spells, and its severity and duration variations at monthly time scale. For this purpose, 38-year recorded data (1975-2013) were used over 20 hydrometric stations in Golestan Province. The weighting factor of drought importance for each month was calculated based on the flow distribution in different months. The subtracted mean discharge of particular month from the long-term average was multiplied by the weighting factor to determine the effective discharge. The difference of mean monthly discharge and effective discharge was considered as monthly flow deficit. The onset and end of hydrological drought periods were defined and the cumulated flow deficits were calculated. According to the results, the NodeKhandoz and Sarmoo stations had no deficit amount in all months of the year which located at upland portions of the study area; while, the flow deficit durations were observed in July and August in almost all stations. The variations of flow deficit volume showed that the frequency of 4-5 month durations were higher than other duration spells. According to the clustering results, the similar stations (Taghiabad and Basirabad) had the highest flow deficit severity; while, the lowest flow deficit severity was observed in NodeKhandoz and Sarmoo stations.