Brendan James Moran, David F Boutt, Lee Ann Munk, Joshua D. Fisher

Deciphering the dominant controls on interconnections between groundwater, surface water, and climate is critical to understanding water cycles in arid environments, yet persistent uncertainties in the fundamental hydrology of these systems remain. The growing demand for critical minerals such as lithium and associated water demands in these arid environments has amplified the urgency to address these uncertainties. We present an integrated hydrological analysis of the Dry Andes region utilizing a uniquely comprehensive set of tracer data (3H, 18O/2H) for this type of environment, paired directly with physical hydrological observations. We find two strongly decoupled hydrological systems that interact only under specific hydrogeological conditions where preferential conduits have developed. The primary conditions in these conduits form are when laterally extensive fine-grained evaporite and/or lacustrine units or perennial flowing streams exist in connection with groundwater discharge sites. These conduits which efficiently capture and transport modern or “contemporary” water (weeks to years old) within the system control the interplay between modern hydroclimate variations and groundwater aquifers. Modern waters account for a small portion of basin budgets but are critical to sustaining surface waters due to the existence of these conduits. As a result, surface waters near basin floors are disproportionally sensitive to short-term climate and anthropogenic perturbations. This framework describes a new understanding of the dominant controls on natural water cycles intrinsic to these arid high-elevation systems which improves our ability to manage critical water resources.