عنوان مقاله [English]
Drought is one of the catastrophic events which has always imposed a lot of irreparable damages on human societies. Drought is not only related to a specific climate condition, but also it can occur in any areas under any climate regime. Therefore, investigating drought and its effects is a critical issue.To minimize the negative effects of drought, prioritizing risk management instead of crisis management and also drought monitoring and forecasting are essential. Drought indices have specific importance in assessing drought conditions as they are extensively used in drought literature. Many drought indices have been developed for evaluating drought conditions all around the world. Although different drought indices have no preference to each other, some of them have a better performance under a specific situation. In some cases, it is more appropriate to assess drought conditions with several indices. Therefore, most of the designers, planners, and decision-makers in water resources management consider more than one index and do not rely on a single index before any decision making.Drought analysis based on a single variable may not be sufficient, because drought is a multi-dimensional phenomenon that is related to multiple variables (e.g., runoff, soil moisture, and precipitation).Considering the importance of indices in drought risk management, in this study, a Two-variate Standardized Index (TSI), based on rainfall-runoff data (1972-2010), regarding the concept of copula, has been developed for two river basins located in Guilan Province and Urmia Province, Iran. Moreover, the mentioned index is compared with two popular drought indices, i.e., the standardized precipitation index (SPI) and standardized streamflow index (SSFI), in order to assess drought conditions.Results showed that TSI determines more severe drought conditions than SPI and SSFI in a specific drought event. Therefore, the disadvantages of SPI and SSFI are not found in TSI, making it more appropriate for drought frequency analysis. This analysis also indicated that, for specific conditions, Urmia basin experiences more severe drought than Rasht basin.