Dynamic compound droughts in the Contiguous United States

Severe meteorological and hydrological drought synergy contributes to adverse and large-scale social, economic,and environmental impacts beyond the individual occurrences. The risk, memory, and causality of this combination can be expressed by a compound dynamic perspective under a changing climate. In this study, we show that the concurrent risk of hydrological and metrological droughts has increased by up to 10% to 20% for moderate and severe events, and up to 8% to 12% for extreme events in recent decades, across watersheds in the western and southeastern Contiguous United States (CONUS).
A bivariate Generalized Autoregressive Conditional Heteroskedasticity (GARCH) model also indicates that the dynamic compound droughts have strong short-term memory based on co-volatility, especially in the western CONUS. The results also suggest a short-memory causative dynamic mechanism, through which meteorologicaldroughts may exponentially increase the occurrence of long-lasting and severe compound droughts, especially in the western CONUS. Given the broad impacts of extreme compound droughts, our findings have critical relevance for the ongoing proactive and long-term adaptive plans to mitigate adverse consequences arising from droughts, especially in the western territories of the CONUS.