Improvement in the subterranean knowledge of the Limarí river basin to support decision making in the context of drought.

Water scarcity has 35% of Chile's population under a severe drought for over a decade. Local communities formed by users of surface or groundwater resources have been pushing to reach agreements prioritizing human consumption and ecosystem services over productive uses (e.g., Industry, Agriculture, and Mining). The new Chilean Water Code, in operation from April 2022, indicates that once a flow shortage evolves into a “Severe Drought”, the General Water Directorate of the country can request a “Redistribution Agreement” in coordination with the local water communities. In the last decades, in the Limarí Basin (semi-arid northern Chile), groundwater exploitation has increased significantly to maintain irrigation and drinking water supplies.  Therefore, a good knowledge about groundwater resources and their vulnerability is essential to develop sustainable water management strategies at a collective level. Also, because at the basin there is a coastal wetland (Salala) of high environmental interest. In this study, we aimed to characterize and model a mountainous watershed in the semi-arid Chilean Andes. The area of interest is distinguished by a high topographic gradient and narrow valleys filled with sedimentary deposits of various origins and surrounded by plutonic and volcanic-sedimentary rocks. To characterize the hydrostratigraphy of this complex sedimentary system and to estimate the volume of groundwater stored, we implemented a multidisciplinary approach integrating geophysical data from transient electromagnetic sounding (TEM), hydrogeological, geological, geomorphological and groundwater quality information. The results indicate the presence of two aquifer layers in most of the investigated areas: a superficial unconfined aquifer and a deeper confined (or semi confined) aquifer. We found that the width and depth of the sedimentary deposits increase with decreasing topography, while the proportion of fine material increases, in coherence with the sedimentation processes. Finally, we quantified the groundwater contribution of the different areas of the catchment and identified the main aquifer potential area in the pediplanes of the coastal mountain range (storing approximately 49% of the water available for extraction).The main contributions to the total uncertainties on the groundwater storage (ranging between 30 and 80% of the estimated volumes) are due to the propagation of the uncertainty on the thickness and porosity/specific yield of the modeled hydrostratigraphic layers. Due to the large spacing between TEM soundings and the limited number of stratigraphic bore logs in part of the studied area, the obtained characterization should be integrated with additional data for precise borehole sittings. Nevertheless, the implementation of TEM allowed us to cover an extensive area and to reach a large depth of exploration, so that it was possible to extract general information about the hydrostratigraphy of the different areas of the catchment.