Change in Global Freshwater Storage.

Freshwater in both natural stores (i.e., glaciers, permafrost, groundwater, lakes, wetlands, and soils) and man-made stores (i.e., small and large reservoirs, and paddy fields) is critical for coping with spatial and temporal variations in water supply and demand. Its importance has grown as human populations have risen and socioeconomic development advanced. As a major component of many national adaptation strategies, the importance of water storage will continue to increase as the impacts of climate change become more pronounced. However, in large part because of the complex distribution over large spatial scales, and difficulties in monitoring, the aggregate volumes of water in terrestrial stores and the rates at which they are changing have, until recently, remained largely unknown. Recent advances in Earth observation and computer modeling enable greater disaggregation and better quantification of different storage elements. Based on literature and extrapolation, this paper provides a summary of estimates of different components and trends in terrestrial water storage. Such estimates are a useful starting point for more strategic thinking about how to manage and protect the Earth's freshwater sources in the future. Results indicate that, at the global level, direct human impacts on water storage remain very small compared to the total volumes stored, but are now approximately the same order of magnitude as climate-induced changes. Cumulative reduction in terrestrial water storage from 1971 to 2020 is estimated to be of the order of 27,079 Bm3 (more than annual total global freshwater use by humans). Excluding ice caps, the greatest depletion is in mountain glaciers (8,666 Bm3), groundwater (7,041 Bm3) and wetlands (3,920 Bm3). Although these changes are insignificant at the global scale, human impacts on 'operational' freshwater storage (i.e., the proportion of water storage that is sustainably utilizable by people) are much more significant. A conservative estimate is that there has been a decrease of more than 3% in operational freshwater storage since 1971. Declining water storage is a major contributor to local/regional water crises that ultimately threaten billions of people and many ecosystems worldwide. In many places, both natural and man-made water storage are declining simultaneously and exacerbating water stress. Decades of degradation of lakes, wetlands and soil, and sedimentation of reservoirs, as well as overabstraction of groundwater complicate future water resources management, reduce adaptive capacity, and undermine the resilience of 'thirsty' societies that are increasingly under threat from the impacts of climate change and increasing variability in freshwater availability. Actions to stop and reverse the loss of water from both natural and man-made water storage would help enhance water security, and buffer the negative impacts of multiple pressures, not only climate change, on ecosystems and society. [ABSTRACT FROM AUTHOR]