Questioning the Endorheic Paradigm: Water Balance dynamics in the Salar del Huasco basin, Chile

Abstract. Arid endorheic basins exhibit limited water availability shaped by strong precipitation and evaporation variability. Understanding these processes is crucial for sustainable water resource management in such fragile environments. This study examines how rainfall and evaporation drive the spatial and temporal dynamics of groundwater recharge and water balance in an arid endorheic basin, using the Salar del Huasco in the Chilean Altiplano as a case study. For this, we implemented a modified semi-distributed rainfall-runoff model integrated with a 40-year record (1980–2019) of satellite-derived precipitation and evaporation estimates. Results show that, on average over the catchment, about 12 % of total rainfall (17 mm year^-1) recharges the aquifers, with a ∼35-day lag between rainfall and peak groundwater recharge. Spatial analysis reveals that most water infiltrates and recharges the groundwater system at high elevations (∼65 % of total recharge), while low-lying wetlands, shallow lagoons, and riparian zones lose up to 950 mm year^-1 via evaporation. Our findings highlight that when summer rainfall ceases, groundwater becomes the main water source supporting high evaporation rates, leading to a minimum in recharge by the end of autumn that persists until the end of the year. These results suggest competition between groundwater recharge and evaporation for available water during the dry season. Moreover, while the basin receives around 145 mm year^-1 of annual precipitation, evaporation reaches 230 mm year^-1. These values insinuate a substantial water loss or an unaccounted groundwater inflow, challenging the endorheic assumption of the basin's hydrogeological boundaries. Future research should revisit this assumption and incorporate fully coupled groundwater-surface water simulations to explicitly include interactions with lateral groundwater flows and groundwater levels, as well as with snow dynamics and vegetation processes currently omitted. Nonetheless, these results provide a valuable framework and a first-approximation for quantifying water balance components in an arid basin, offering insights for water resource management in a context of water scarcity and climate change.