Bivariate Frequency Analysis of Hydrological Drought Using a Nonstationary Standardized Streamflow Index in the Yangtze River

In order to reassess the hydrological drought in the Yangtze River under changing environment, a nonstationary standardized streamflow index was proposed in this study to fit the streamflow series at Yichang station; time and a modified reservoir index were introduced as covariates to assess the effect of reservoir regulation. The copula method was applied for bivariate modeling of drought duration and severity, in which joint and conditional return periods were considered for drought risk assessment. The results indicated that the monthly streamflow series at Yichang station have undergone great changes and the stationary assumption is no longer valid. The drought severity was more severe with its marginal distribution changed from the generalized extreme value to gamma when considering nonstationary properties. The Joe copula was selected for bivariate frequency analysis, and the correlation coefficient between drought characteristics increased in nonstationary models. Using the conditional return period, the model with reservoir index as covariate reported a worse drought condition compared with the stationary model, implying that the reservoirs may deteriorate the downstream hydrological drought at the Yichang station. By contrast, the time covariate may underestimate the drought risk. The nonstationary index is capable for drought modeling in the Yangtze River, and can be a useful tool in further research.