Sensitivity of groundwater levels and chemistry to partial removal of vegetation in Prosopis woodlands of the Monte Desert, Argentina

Abstract

Land use changes affecting ecological and hydrological processes often shape the biogeochemistry of deserts. As highly productive hotspots, Prosopis flexuosa woodlands located in interdune lowlands of the Monte Desert are subject to intense transformations by humans and livestock, and offer a unique opportunity to understand how phreatophyte vegetation, groundwater, rainfall variability, and vertical solute fluxes connect with each other. In this ecosystem, we evaluated the consequences of heavy disturbance by livestock settlements (partial tree removal, intense grazing) on shallow (7–15 m deep) groundwater recharge/discharge dynamics and vertical solute (nutrients and salts) transport. We compared three relatively undisturbed woodlands with three paired livestock stations in the Telteca Reserve (Mendoza, Argentina). At these locations we monitored groundwater levels and chemical and isotopic compositions during six years (2011–2017). Seasonal groundwater level fluctuations reflected shifts in water demand (lower in the cold season) rather than rainfall (high in the warm season), as shown by fall-winter rises and spring-summer declines. These seasonal fluctuations were more intense in undisturbed woodlands than in livestock stations. At a shorter time scale, intense rainfall events were associated to groundwater level raises, suggesting a partial decoupling of phreatophyte vegetation from the free aquifer and, less likely, recharge episodes. Livestock stations also affected groundwater chemistry by locally increasing chloride and nitrate concentrations, and salinity, suggesting higher water percolation and nitrate leaching to the aquifer in livestock stations than in control woodlands. This study addressed ecohydrological processes, which are necessary to understand the coupling between phreatophyte vegetation and groundwater, and its relationships with human and livestock activities.