Glacier contributions to river discharge during the current chilean megadrought

The current Chilean megadrought has led to acute water shortages in central Chile since 2010.Glaciers have provided vital fresh water to the region's rivers, but the quantity, timing and sustainability ofthat provision remain unclear. Here we combine in-situ, remote sensing and climate reanalysis data to showthat from 2010 to 2018 during the megadrought, unsustainable imbalance ablation of glaciers (ablation notbalanced by new snowfall) strongly buffered the late-summer discharge of the Maipo River, a primary sourceof water to Santiago. If there had been no glaciers, water availability would have been reduced from Decemberthrough May, with a 31 ± 19% decrease during March. Our results indicate that while the annual contributionsof imbalance ablation to river discharge during the megadrought have been small compared to those fromprecipitation and sustainable balance ablation, they have nevertheless been a substantial input to a hydrologicalsystem that was already experiencing high water stress. The water-equivalent volume of imbalance ablationgenerated in the Maipo Basin between 2010 and 2018 was 740 × 10 6 m 3 (19 ± 12 mm yr −1), approximately 3.4times the capacity of the basin's El Yeso Reservoir. This is equivalent to 14% of Santiago's potable water use inthat time, while total glacier ablation was equivalent to 59%. We show that glacier retreat will exacerbate riverdischarge deficits and further jeopardize water availability in central Chile if precipitation deficits endure, andconjecture that these effects will be amplified by climatic warming.