GROUNDWATER AND VEGETATION CONTROLS ON EPISODIC SOIL EROSION IN A DESERT ECOSYSTEM

Riparian zones and other localized regions with shallow groundwater support valuable biological communities that stabilize soil and maintain homogeneous resource distributions on desert landscapes. Such areas are often subjected to water extractions and diversions, especially during droughts, leading to vegetation loss, soil erosion, and dust emissions. Sustainable management of these systems requires datasets and methods that can link environmental disturbance with geomorphic response over appropriate timescales, but these are rarely available. Here we use dust collectors and fallout radionuclides ⁷Be, ²¹⁰Pb, and ¹³⁷Cs to quantify post-1965 soil stability in a groundwater-dependent desert ecosystem in central Owens Valley, CA. This region has a well-documented history of water withdrawals during the 1987-1992 drought, and we show that soil transport occurred in areas with depleted and stable groundwater resources during this time. However, episodic centimeter-scale soil loss occurred when vegetation cover collapsed by >25% for multiple years in areas where groundwater remained shallow enough to produce evaporite salt minerals. We show that erosion is most severe when the saturated zone falls below the 2 m effective rooting depth of native alkali meadow vegetation, but remains shallow enough (< 6 m) so that capillary action dilates the surface maintaining a loose layer of erodible sediment. By coupling erosion rate calculations made over different timescales, and drawing on groundwater, vegetation cover, and climate datasets, we show that erosion rates in Owens Valley were highest during 1987-1992, when groundwater extractions were the most significant, vegetation cover dropped at the steepest rate, soils were driest, and very high wind velocities were recorded. Interestingly, soils have since re-stabilized in many areas despite the encroachment of shrubs and an overall reduction in vegetation cover over the past 20-30 years.