New Mexico Geological Society Annual Spring Meeting — Abstracts

A small drainage bas in on the Sevilleta National Wildlife Refuge (about 25 km north of Sococro, New Mexico) was intensively instrumented with ground-water, meteorological, and soil-physical monitoring equiment to estimate natural ground-water recharge. The soil monitoring instrumentation was installed in a variety of topographic, lithologic, and vegetative conditions in an attempt to collect data at representative points a cross the spatially variable site. In this study, soil-moisture data was analyzed with special attention to defining the dependence of recharge on the topographic location of the point of measurement. The locations studied were uriderlain principally by fine to medium aeolian sands. Recharge was estimated by two different techniques: neutron logging and Darcy's Law. In addition, tracer experiments were conducted on a sandy hillslope.

The results indicate that there is a strong lateral component to unsaturated flow on a hillslope, even in the absence of strongly impeding sub-layers. This lateral flow leads to markedly increased recharge beneath topographically concave locations. Darcian calculations estimate steady deep flux beneath concave locations to range bet ween 4% and 45% of an assumed mean annual precipitation of 20 cm, increasing with size of the basin above point of measurement. Long-duration wet periods were found to temporarily increase the recharge rates by several orders of magnitude. Darcy's Law calculations of deep flux using gradients determined by tensiornetric data tend to give better results than neutron-logging mass-balance approaches.