Drought Characteristics Encompassing Climate Indices in the Yangtze River Basin Using Nonstationary and Copula-Based Methods

Abstract

Abstract In this study, a nonstationary standardized precipitation index (NSPI) is calculated by fitting precipitation data to the nonstationary gamma model with climate indices as covariates and compared with stationary standardized precipitation index (SSPI) in fitting observed precipitation and identifying meteorological droughts during 1964–2016 in the Yangtze River basin (YRB). Results show that changing trends of NSPI are roughly consistent with those of SSPI, but the NSPI performs better than SSPI for fitting precipitation. Using the NSPI, spatiotemporal variations and joint return period of drought characteristics are investigated by using modified Mann–Kendall and copula function. It is found that the west YRB experiences mostly severe droughts but shows a mitigating tendency, while in the north-central region, moderate droughts are dominant but have an aggravating tendency. Drought peak shares a spatial distribution pattern similar to intensity, with higher averages in the west and south-central basin and lower averages in the north-central part, but they exhibit a higher occurrence frequency of droughts. The joint return periods of drought characteristics reveal that slight and moderate droughts with duration D of no more than 3 months are more likely to occur in the YRB with return periods of 2–25 years. The severe- and extreme-intensity droughts with more than 3-month duration occur rarely in the YRB, but extreme peak droughts with slight or moderate intensity could hit most of the basin in 100 years. In addition, for most drought scenarios, droughts exhibit longer return periods in the north-central and southeast parts relative to other regions of the YRB. Significance Statement Computation of a widely used standardized precipitation index depends heavily on the assumption of stationarity in precipitation, but this assumption is questionable under a changing climate. This study is the first time that a nonstationary standardized precipitation index is calculated to comprehensively assess drought in the Yangtze River basin (YRB). Results show that the basin is prone to no more than 3-month-duration slight and moderate droughts with return periods of 2–25 years, while extreme-intensity droughts longer than 3 months occur rarely. In addition, for most drought scenarios, the droughts exhibit longer return periods in the north-central and southeast parts relative to other regions of the YRB. These results are important for drought monitoring and early warning in the YRB.