Quantifying hydrogeological drivers influencing daily fluctuations in shallow groundwater levels within an altiplanic pristine catchment in Chile.

Daily fluctuations in shallow groundwater levels provide valuable insights into hydro-ecological dynamics and aquifer hydraulic properties. These fluctuations usually depend on hydrological/hydrogeological processes, such as precipitation, evaporation, snow/ice melting/thawing, as well as soil characteristics that influence aquifer response times. The Salar del Huasco basin (20.2°S, 68.8°W; 4,164 m a.s.l.; 1,470 km²) is an endorheic system located in the arid Chilean Altiplano, hosting wetlands and a saline lagoon that sustain part of the region essential biodiversity such as chilean, andean, parina and chica’s flamingos, and it serves as a refuge for migratory birds (e.g., peregrine falcon, golden plover and baird's sandpiper). The area experiences extreme thermal oscillations (4–14°C daily averages; winter lows of -20°C), high potential evaporation (1,200 mm/year), and variable summer precipitation (11–400 mm/year). To explore shallow groundwater dynamics, we monitored for ~1 year two sites near the basin’s salt flat: the north and the south sites. Meteorological, soil, and groundwater levels data were collected at 30-min intervals. At the northern site, daily groundwater level fluctuations ranged from 6 to 45 mm, with a sharp and abrupt 300 mm rise in austral spring. In contrast, the southern site showed daily groundwater level fluctuations between 7 and 58 mm, with multiple rises during winter, ranging from 100 to 300 mm. Distinct patterns emerged at these sites: in the northern site, the maximum diurnal fluctuations correlated with solar radiation, while the southern site showed a more stable behavior, with no clear daily peaks. We applied a water balance to determine how the amplitude of possible input and output fluxes in the system altered the daily level fluctuations, and whether, despite the proximity of both sites (~9 km), soil texture, vegetation cover, and local meteorological-hydrogeological conditions explain the differences in groundwater level behavior.