Abstract

The Andes of southern Chile have isolated glaciers on volcanic edifices that, together with the precipitation, contribute to several important river systems in the region. The decrease in the extension of glaciers and the negative trend in precipitation will reduce the downstream water availability. Nonetheless, studies on the sensitivity of hydrological systems due to climate change focus mainly on arid and semi-arid areas of the Andes, compared to wet regions with isolated mountain glaciers. Despite the extension and hydrological importance of the Wet Andes and significant trends of decreasing flow that have affected the region for several years, studies have yet to assess the importance of its glaciers in runoff generation. Through isotopic monitoring, we estimate the contribution of glacial melt in the Allipen headwater basin of the Wet Andes (between 38 degrees 40'S and 39 degrees S). We performed isotopic measurements of 18 O and 2 H on a seasonal basis during three seasons in snow, glacial melt, lagoons, groundwater, and stream flow in nested basins. Stable isotope data were analyzed in a Bayesian framework using the MixSIAR model, where spatial and temporal relationships were incorporated. The contribution of groundwater (springs) is prominent in three of the four nested basins throughout the analyzed period. In one of the basins, lagoons correspond to the main contributing source. Additionally, the contribution of glacier thaw reached up to 30% in another of the nested basins. Meanwhile, precipitation was less important in all basins. When analyzing the complete basin (without the nested basin approach), the results show the dominance of groundwater contributions as the main source, varying between 56% and 62%, followed by ponds, which vary between 13.5% and 23%. In contrast, glacier thaw varies between 11% and 18%, even though it only covers approximately 1.5% of the total basin area. These results show the variability of hydrological processes at different spatial scales and the importance of small mountain glaciers in the basins of the Wet Andes. Furthermore, the analysis of the physical variables of the basins allows for identifying the importance of the volcanic landscape in the hydrological dynamics of the region. Our results suggest that due to climate change, it is necessary to evaluate and quantify changes in flow dynamics in humid regions, especially in basins that receive significant contributions from retreating glaciers. These results will allow for the identification of new monitoring points and techniques to discriminate and quantify the contribution of different sources and the identification of areas or hydrological processes relevant in terms of conservation or integrated management.