Changes in the Probability of temporally compound wet and dry events in a warmer world
Authors: Ansari Rubina , Casanueva Ana , Grossi Giovanna 
Published: February 2023

The temporal compounding of two contrasting extremes of the hydrological spectrum (droughts and floods) reflects a volatile hydrological cycle and makes water resources management more challenging. To this end, the present study examines extreme wet-dry events and their temporal compounding over the Upper Jhelum Basin (UJB) in the future climate context based on simulations of climate models from three modeling initiatives (CMIP6, CORDEX - WAS-44 domain and CORDEX-CORE - WAS-22 domain) under low, medium and high emission scenarios for two-time segments i.e., near future (2040-2059) and far future (2080-2099). Wet and dry events are characterized by using a multivariate drought index (namely the Standardized Precipitation Evapotranspiration Index, SPEI), which is derived from daily precipitation and maximum and minimum temperatures. The temporally compound event (hereafter referred as compound event-CE), which is defined as a successive transition from one powerful state to another, includes dry to wet (D-to-W) events and wet to dry (W-to-D) events in the adjacent month. Therefore, the minimum duration of a compound event is 2 months. A D-to-W compound event is defined as a dry spell (SPEIi &#8804;&#8722;1) abruptly changing into a wet spell (SPEIi+1 &#8805;1) in the next month. Conversely, a W-to-D compound event is defined as a wet spell (SPEIi &#8805;1) abruptly changing into a dry spell (SPEIi+1 &#8804;&#8722;1) in the next month. The statistical interdependency of temporally compound wet and dry events (CEs) and their statistical significance are investigated using event coincidence analysis (ECA). The significance test is performed based on the assumption of a Poisson process with the null hypothesis that the successive occurrence of wet and dry event is randomly distributed. Results show that the probability of D-to-W CEs is much higher than the W-to-D CEs under both retrospective (i.e., past) or prospective (i.e., future) climate contexts. Specifically, the probability of D-to-W events is high in the southwest of the basin (up to 80 %, statistically significant at 5% level) both in the historical and projections. In contrast, the W-to-D CEs are found to be statistically non-significant for a 95% confidence level (up to 40 %) with no clear pattern of occurrence. There are some differences depending on the climate model ensembles used. CORDEX models (WAS44 and WAS22) show decreasing probability of D-to-W CEs in the southwest part of the basin by the end of the century whereas the CMIP6 ensemble shows a negligible increase from near to far future especially under the highest emission scenario. Overall, the CMIP6 ensemble presents higher probability of CEs under all scenarios and time segments. Climate projections of this kind of extreme events, spanning different scenarios and all sources of uncertainty are essential to fully characterize their impacts on water-related sectors and to plan possible adaptation strategies, such as developing more efficient reservoir operation rules or agricultural planning.