Focused groundwater discharge in closed basins provides opportunities to investigate mechanisms for closing hydrologic and solute budgets in arid regions. The Salar de Atacama (SdA), adjacent to the Central Andean Plateau in the hyperarid Atacama Desert, provides an extreme example of halite (>1800 km3) and lithium brine (~5000 ppm) accumulation spanning late Miocene to present. Minimum long‐term water discharge needed to sustain halite accumulation over this timescale at SdA is 9–20 times greater than modern recharge (and double wet‐climate paleorecharge) within the topographic watershed. Closing this imbalance requires sourcing water from recharge on the orogenic plateau in an area over 4 times larger than the topographic watershed. Prolonged water discharge at SdA requires long residence times, deep water tables in recharge zones coupled with persistent near‐surface water tables in discharge areas, and large contributing areas characterized by strong gradients in landscape and climate resulting from plateau uplift.
Rates of water discharge often exceed groundwater recharge in arid catchments. This apparent mass imbalance within a catchment may be reconciled through either regional-scale groundwater flow between topographic drainages and/or the draining of stored groundwater recharged during pluvial periods. We investigate discrepancies in the modern hydrologic budget of catchments along the west flank of the Andes in northern Chile (21–25° S), focused on the endorheic Salar de Atacama basin, and adjacent basins. Our new, uncertainty bounded, estimates of modern recharge rates do not come close to balancing observed modern groundwater discharge within topographic catchments. Two geologically realistic conceptualizations of hydrogeologic catchments discharging to Salar de Atacama were explored with a 2D groundwater model. Results from models support the interpretation that both regional flow and transient drainage of groundwater from storage are required to balance water budgets along the plateau margin. The models further examine whether this system is still responding to climatic forcing from pluvial periods and highlight general characteristics for similar plateau margin systems including: (1) water level changes at the plateau margin are highly sensitive to changes in recharge on the plateau, (2) extent and magnitude of the changes in water table are controlled by the distribution of hydraulic conductivity at the margin, (3) contributing area to the lower elevation catchment is itself dynamic, and not coincident with the topographic boundary, and (4) difficulty in reconciling the modern position of “the water table” on the Andean plateau with the regional groundwater flow conceptualization and modern discharge to low lying catchments.
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