The hydrological change under extreme drought in the United States: Separating climate and landscape impacts

Abstract

Current climate change characterized by increasing temperature has led to an increase in the intensity and frequency of extreme droughts that have more prolonged and profound ecohydrological and social impacts. By paying attention to the hydrological change before and after extreme drought and the patterns of drought recovery of ecohydrological system, it is possible to better understand the consequences of extreme drought on ecohydrological system. Both climate change and landscape change have an influence on catchment hydrological condition. The drought-related hydrological change is, therefore, the combination of changes induced by these two drivers. To further explore extreme drought impacts and the root causes of hydrological change under extreme drought events, it is necessary to separate the impact of drought-related climate change from the impact of landscape change. This study aims to characterize the variations in hydroclimatic conditions before and after extreme drought by studying the hydroclimatic movements in Budyko space, explore post-drought ecohydrological system recovery, and further separate and investigate the effects of climate and landscape change on catchment hydrological conditions. Monthly SPEI at a 12-month timescale was used to characterize and define the extreme drought events. The Budyko framework was applied to study the hydroclimatic changes of 63 basins in the United States induced by extreme drought events from 1990 to 2013 by quantifying the hydroclimatic movements in Budyko space. The climate effect on precipitation partitioning was distinguished from the landscape effect that is mainly related to vegetation response to extreme drought events. The contributions of precipitation and potential evaporation were quantified to further understand the effect of climate change which is caused by alterations of these climatic variables. To understand the effect of drought-related vegetation change on catchment precipitation partitioning, NDVI was applied to examine the response of vegetation to drought in terms of alteration in vegetation greenness and patterns of vegetation recovery. There were significant hydroclimatic changes in the basins before and after extreme drought. In post-drought period, more precipitation tended to be partitioned into evaporation in most basins. Change in streamflow was larger than the change in evaporation. 63.5% of all the basins experienced wetter conditions and more precipitation after drought. All basins gradually recovered in post-drought period, but not fully restored to their pre-drought states. The hydrological change under extreme drought was not explained by climate change alone in these basins, suggesting the existence of landscape drivers. The climate and landscape effects on precipitation partitioning could either enhance or counteract each other. The landscape drivers contributed more to change in catchment precipitation partitioning. In terms of the climatic effect that associated with the change in aridity index, climate change affects catchment precipitation partitioning by changing the precipitation and potential evaporation, among which precipitation is a more crucial climatic driver. From a vegetation-related landscape perspective, vegetation greenness reverted to pre-drought level within three years in most basins. The rapid or slow recovery, regrowth and even degradation of vegetation in post-drought period cause landscape-driven changes in catchment precipitation partitioning through directly changing vegetation transpiration and streamflow.