Hydrological drought intensity classification with SMMD30 index (Case study of snowy-icy flow regime rivers in Mazandaran province)

Drought occurrence is more than wet years in the North of Iran, while no specific climatic trend has been observed in the region in terms of declining precipitation in the years leading up to recent decades. In terms of spatial pattern, the occurrence of droughts and wet years did not follow a specific trend and mostly did not have special synchronicity and coordination between different regions of the province. In this study, the intensity of hydrological droughts in the rivers with a snowy-icy flow regime in Mazandaran province was studied and classified (weak, severe, and very severe) by sequence theory method with the standard index of minimum flow of 30 days. The independence of the hydrological behavior of the rivers, especially in drought status, the differences in the topographic and physical attributes of watersheds, the independence of the minimum discharge threshold in drought conditions, and consequently the difference in the microclimates caused that this index was different even in different seasons of the year. The very severe intensity index of the Chalous river was -1.486 in the autumn and this index was -0.743 in Cheshmeh Kile river with a similar hydrologic regime and at the same time, in the regional analysis, it was possible to express the classification index of the very severe intensity of hydrological drought in the snowy-icy flow regime was -1.385 in the autumn.