Water storage dynamics across different types of vegetated patches in rocky highlands of central Argentina

Abstract

Rivers originating in the granitic highlands of seasonally dry central Argentina provide water to 2 million people. These highlands comprise a complex landscape where a matrix of outcropping rock hosts vegetated patches that vary in response to long-term grazing and fire. We characterized water storage dynamics across 20 sites representing 6 vegetation-soil conditions with similar mid-upslope positions in the landscape. We measured temporal and spatial variability of water inputs (rain and fog) and water stored at the unsaturated and saturated zones. We found that rainfall was highly seasonal, but fog occurred year-round, likely representing an extra water source in areas with complex vegetation structure. Moreover, fog seems to regulate evapotranspiration/topsoil water dynamics. Water was stored in the saturated zone only during the rainy season. Piezometric response to rainfall was rapid but transient (dropping an average 15 cm/day), possibly buffering peak stormflows, reducing sediment yield, and delivering subsurface water downslope for potential storing throughout the year. Spatially, a reduction in soil depth (from 100 up to 9 cm) and vegetation structure (from woodlands to stonelands), with a degradation of topsoil conditions for infiltration were accompanied by a decline in water storage at the unsaturated (from 32% up to 14%) and saturated (from 46.3 up to 0 cm) zones. Taken together, our results support the infiltration trade-off hypothesis, which states that vegetation structure benefits fog interception, soil properties that enhance water infiltration, subsurface flow paths, and storage. Long-term disturbances have likely triggered a degradation of the hydrological function of seasonal highlands in central Argentina.