Effect of Brush Vegetation On Aquifer Recharge Using Chloride Mass Balance.

Increased groundwater recharge has been documented where native vegetation or forest/shrub land was converted to grassland or pasture, or where the land was cleared for agricultural purposes. The basic argument for increased recharge is that evapotranspiration, primarily interception and transpiration, is higher in shrublands than grasslands. Chloride mass balance (CMB) has been used to estimate ancient recharge, but recharge from recent land-use change has also been documented, specifically where vegetation has been altered and deep-rooted species replaced with shallow-rooted grasses. Chloride concentrations are inversely related to recharge rates: low chloride concentrations indicate high recharge rates as chloride is leached out of the system; high chloride concentrations indicate low recharge rates since chloride accumulates as a result of evapotranspiration. The CMB for a pedon at steady state can be written as: P(Cl_p) = R(Cl_s) where P is the mean annual precipitation (mm yr^-1); Cl_p is the mean Cl^- input from all sources (wet and dry fallout, mg L^-1); Cl_s is the average Cl^- concentration of pore water below the root zone (mg L^-1); and R is the annual recharge rate (mm yr^-1). Soil chloride concentration profiles from paired sites in the Rolling Plains near Abilene, TX will be presented and groundwater recharge will be calculated. Each site consisted of a pair of similar soils with contrasting vegetative cover: grassland and shrubland. Soils were sampled to a 3-m depth for bulk sample, clods for bulk density, and in situ water content. In addition to standard soil characterization analyses, chlorides were extracted by adding 30 ml of deionized water to 15-g of dried, ground soil and shaken for 4 h. Chlorides were determined by ion chromatography. The 24-year data record (1983-2007) of the National Atmospheric Deposition Program (NADP) indicated an annual mean chloride input of 0.215 mg L^-1 to the area.