Glacier melt content of water use in the tropical Andes

Accelerated glaciers melt is expected to affect negatively the water resources of mountain regions and their adjacent lowlands, with tropical mountain regions being among the most vulnerable. In order to quantify those impacts, it is necessary to understand the changing dynamics of glacier melting, but also to map how glacier melt water contributes to current and future water use, which often occurs at considerable distance downstream of the glacier terminus. While the dynamics of tropical glacier melt are increasingly well understood and documented, major uncertainty remains on how tropical glacier meltwater contribution propagates through the hydrological system, and hence how it contributes to various types of human water use in downstream regions. Therefore, in this paper we present a detailed regional mapping of current water demand in regions downstream of the major tropical glaciers. We combine these maps with a regional water balance model to determine the dominant spatiotemporal patterns of glacier meltwater contribution to human water use at unprecedented scale and resolution. We find that the number of users relying continuously on water resources with a high (>25%) long-term average glacier melt contribution is low (391 000 domestic users, 398 km2 of irrigated land, and 11 MW of hydropower production). But this reliance increases sharply during drought conditions (up to 3.92 million domestic users, 2096 km2 of irrigated land, and 732 MW of hydropower production in the driest month of a drought year). A large share of domestic and agricultural users is located in rural regions where climate adaptation capacity tends to be low. Therefore, we suggest that adaptation strategies should focus on increasing the natural and artificial water storage and regulation capacity to bridge dry periods.