Field Study of Natural Ground Water Recharge in a Semi Arid Lowland

A desert area near Socorro, New Mexico was studied to determine the amount of recharge that could occur by direct infiltration of precipitation. Recharge variability due to topography, vegetation and surficial geology was also studied. Except on the dune slopes, the depth to the water table is shallow, on the order of several meters. The instrumentation network included a meteorological station, soil-moisture sensors, and observation wells. Data were collected from as early as November 1982, but the network was not complete until about September 1983.

Precipitation during both summer and winter generates soil-water movement below the root zone. At one location, recharge calculated from Darcy’s equation is at least 4 cm/yr, or roughly 20 percent of mean annual precipitation. By calculation of deep drainage at 15 locations, during a 19 month period, the amount of precipitation that may become recharge may vary by an order of magnitude due to the combined effects of vegetation, topography and soil type. Even when the potential for evaporation greatly exceeds the available precipitation, significant amounts of rainfall may infiltrate thick, unconsolidated soils to become recharge. This finding is in contrast to previous speculation by some hydrologists and geologists thatgroundwater recharge in dry climates is negligible.

Infiltration of precipitation is strongly affected by topography. Tracer experiments on sandy hillslopes clearly show evidence for lateral movement even in the absence of an underlying low-permeable horizon. Lateral flow in the unsaturated zone is also believed to cause substantial variability in moisture content within a homogeneous sand dune. Hydrologic models and water budgets of sloping terrain should recognize the significance of lateral flow in topographically convergent areas. Such areas would be expected to he areas of enhanced recharge potential.