Global analysis of the hydrologic sensitivity to climate variability

Climate change is driving an increase in the frequency and intensity of extreme weather events, with alterations to the functioning of the water cycle. Global-scale assessments of shifts in the hydrologic response to climatic perturbations (i.e. hydrologic sensitivity) are needed to identify regions where mitigation efforts will be necessary to avert detrimental changes on terrestrial water resources. In this work, we quantified the hydrologic sensitivity of the terrestrial planet by assessing year to year changes in hydrologic response to the interannual variability in climate forcing during the 2001-2016 period, while evaluating the role of major topoclimatic factors in modulating these responses. Using a metric derived from the inverse of the elasticity concept in Budyko's space, we produce a Hydrologic Sensitivity Index (HSi) evaluating the hydrologic behavior of a location under variable climatic conditions by examining the extent of the changes in Evaporative Index (AET/P) against the interannual variation in the Dryness Index (PET/P) for consecutive years. This approach produces HSi > 1 for hydrologic sensitive regions and HSi <= 1 for hydrologically resilient locations while also allowing the detection of changes in water yields as well as wetting or drying conditions. Globally, the results indicate that hydrologic sensitive areas are clustered at high and low latitudes; at high latitudes, boreal and arctic zones show heightened hydrologic sensitivity accompanied by increasing water yields, while at low latitudes, tropical rainforests show the largest hydrologic sensitivity with the majority of their sensitive area leaning towards decreasing water yields. We found that hydrologic sensitivity is amplified at high elevations and steep-sloped terrain, outlining the importance of topography in modulating the impacts of varying climatic forcing on hydrologic response. We direct the attention towards climate warming resulting in rapid snowmelt and increasing precipitation in Arctic tundra and boreal forests and reduced tree cover in tropical forests, as probable mechanisms driving the observed patterns. Our study highlights the regions with greatest hydrologic sensitivity to interannual climatic variability, motivating further regional and basin-scale investigations on their cascading effects on ecosystems and water resources and their attribution.