Frequency Analysis of Precipitation Anomaly Percentage and Stream Flow Drought Using Copula Functions

The purpose of this research is to joint frequency analyze of precipitation anomaly percentage as a meteorological drought index and flow rate at Chalekhmaz station located in the Zarinerood basin at south of Lake Urmia in the period of 1995-2016, which is based on the duration of the mentioned indicators. The results of the analysis of investigated copula functions in Zarinerood basin showed that, based on evaluation criteria, Frank's copula function describes well the dependence between two variables of the duration of anomaly percentage and the duration of hydrological drought. In Chalekhmaz station, the expectation of drought duration of 4 to 7 months for the hydrological variable and 9 to 12 months for the meteorological variable in the coming years is not far from reality. The results of the study of the return period of drought characteristics showed that in the case of the frequency of the stream flow drought index, the return period also increases with the increase in the severity of the drought. The joint frequency analysis of drought characteristics shows how meteorological and hydrological drought characteristics can be determined simultaneously in one station by using joint probabilities. This can provide users and researchers with very useful information related to the probable behavior of drought characteristics in order to optimally use of surface water. For the duration of a certain meteorological drought in a station, the duration of the hydrological drought in the hydrometric station can be determined based on the conditional probability of occurrence and also certain return periods.