Long-Term Hydrologic Responses To Shrub Removal In A SW Texas Rangeland: Using Soil Chloride To Estimate Deep Drainage

The Carrizo-Wilcox aquifer is a valuable groundwater resource, situated in a semi-arid landscape of Southwest Texas, where over-use by dependent farming practices has lowered aquifer levels. In semi-arid regions, rates of groundwater recharge are predominantly low due to high potential evapotranspiration rates; however, least understood is the role that vegetation plays in soil-plant-water dynamics. Vegetation management potentially plays a major role in countering the loss to recharge because evapotranspiration (ET) varies with vegetation type and cover. The conversion from shrubland to grassland likely reduces rooting depths and total plant cover. Subsequently, deep drainage (percolation below the root zone) will likely increase and lead to groundwater recharge, at least temporarily. The primary aims of the study were to identify those biotic and abiotic factors facilitating deep drainage and to examine differences in recharge for the years following clearing of natural shrub vegetation. Soil chloride was examined to estimate long-term recharge rates, since its concentration in the soil is influenced by the movement of water. Short-term soil moisture trends were also monitored for any water movement deep in the soil profile in response to individual rain events. Rooting depths decreased following removal of vegetation; yet root biomass unexpectedly increased due to successful grass establishment during the first five years after treatment. Soil properties did not vary between treatments, indicating that the majority of chloride differences seen were a consequence of vegetation change. Peak and total soil chloride concentrations were expected to decrease and occur deeper in the soil profile 15-30 years following the clearing of woody vegetation. Total chloride decreased by up to 65% after 30 years and resulted in an estimated 14.9 mm/yr more recharge compared to adjacent untreated controls. Evidence in this study suggest that much of this chloride is leached during the first five years following treatment and that more leaching occurs in especially wet periods. During the wet 2007 growing season, soil moisture below the root zone increased by up to 17% after vegetation clearing. The results of this study indicate that hydrologic changes following brush removal were evident in this system and are likely to positively influence groundwater recharge in the long-term.