Assessing Controls on Diffuse Groundwater Recharge Using Unsaturated Flow Modeling

Bridget Scanlon, principal investigator

Understanding climate, vegetation, and soil controls on recharge is essential for estimating potential impacts of climate variability and land use/land cover change on recharge. Recharge controls were evaluated by simulating drainage in 5-m-thick profiles using a 1-D unsaturated flow code (UNSAT-H), climate data, and vegetation and soil coverages from online sources. Vegetation parameters were obtained from the literature. Soil hydraulic properties were estimated from STATSGO/SSURGO soils data using pedotransfer functions. Long-term (1961–1990) simulations were conducted for 13 regions (county near meteorological station) that represent arid to humid climates and different vegetation (shrubs, grasses, forests, crops) and soil types, using data for Texas, U.S.A. Areally averaged recharge rates are most appropriate for water resources; therefore, Geographic Information Systems (GIS) was used to determine the spatial weighting of recharge rates for the combination of vegetation and soils found in each of the 13 regions. Simulated 30-yr average annual recharge in bare sand is high (51 mm/yr, arid to 709 mm/yr, humid) and represents 23 to 60% of mean annual precipitation (MAP) in arid to humid regions, respectively. Adding vegetation to bare sand had a similar effect on recharge as soil textural variability; vegetated sand reduced recharge by factors ranging from 2 to 30 (humidľarid), and soil textural variability reduced recharge by factors ranging from 2 to 11.

Vegetation and soil textural variability both resulted in high local variability in recharge within each region; however, spatially weighted, long-term average recharge rates were much less variable and were positively correlated with MAP (r2 =0.85, vegetated sand; r2 = 0.62, variably textured soils). The most realistic simulations included vegetation and variably textured soils, which resulted in a range of recharge rates from 0.2 to 118 mm/yr (0.1–10% of MAP). Mean annual precipitation explains 80% of the variation in recharge and can be used to map recharge.

Predicted recharge using the relationship between precipitation and simulated recharge for vegetated, texturally variable soils.

March 2006