Unravelling the origins of precipitation over the world’s water towers

Mountain regions supply around 22% of the world's population with freshwater — from precipitation over these water towers to melt water from snow packs and glaciers. However, the frozen reservoirs of water that usually act to buffer precipitation deficits are diminishing as a result of climate change. As a consequence, precipitation will become the main source of freshwater supplied by these water towers. Yet, already today, precipitation deficits over many water towers frequently cause severe droughts that further induce supply deficits in downstream regions. 

Here, we unravel the origins of precipitation over the most important water towers worldwide and illustrate their dependency on upwind land regions. Using a moisture tracking framework constrained by satellite observations, we disentangle the local and remote surface drivers of drought over these water towers and highlight the role of forested and irrigated regions during these events. Our results indicate that many water towers can self-sustain their precipitation during drought events through an increased self-supply of moisture for precipitation: over the water tower of the Ganges-Brahmaputra, for example, around 80% of the precipitation during drought events is supplied by the water tower itself and its dependent downstream region. Our findings highlight the vulnerability of the world's most important water towers to drought from an atmospheric perspective and outline the potential of localized forest and land management practices to secure freshwater to billions of people in the future.