Evaluation of Hydrological and Meteorological Drought Relationship and Reservoir Impacts (Case Study: Zayandeh Rood River Basin)

Regarding the occurrence of hydrological drought after the meteorological drought, determining the relationship between these two droughts is necessary. In this study, the Zayandehrood Basin in the central part of Iran was selected as the study area. Selected stations in this basin; Damane Fereydan station, Ghaleh Shahrokh station (upstream of dam) and Zamankhan bridge station (downstream of dam) were considered according to their location and trend of precipitation data. Initially, on the basis of monthly rainfall and flow data from 1360 to 1389, the standard precipitation index (SPI) and the standard flow index (SSI) (Meteorological droughts and hydrological drought, respectively), each with a three-month period were calculated. Then using the Run theory, the meteorological and hydrological drought characteristics, including the duration and magnitude of drought were identified. In the following, using R software and programming in it, the best model was sought to investigate the relationship between meteorological and hydrological drought. According to the number of samples, the model was also validated. The relationship in which a hydrological drought reacts to the meteorological drought is created using a nonlinear function model in Qaleh Shahrokh and Zaman Khan Bridge stations, which show the condition without the reservoir and the condition affected by the reservoir, respectively. The results showed that there is a nonlinear relationship between hydrological drought and meteorological drought and the threshold at which a hydrological drought begins to react to meteorological drought is achieved with respect to the nonlinear function model. The exponential function model with the appropriate validation indexes as well as high coefficients in both stations was selected as the best model. The duration and magnitude of hydrological drought at Qaleh Shahrokh station is 1.7 and 1.9, respectively. Also, the duration and magnitude of this drought at the Zaman Khan Bridge (under the influence of the dam reservoir) is 0.55 and 1.45. Originally, the time of occurrence of hydrological drought in the downstream of the dam will be occurred faster. These findings indicated that the operational activities of Zayandeh Rud reservoir significantly reduced the duration and amount of hydrological drought as compared to non-reservoir conditions.