SEASONAL AND EVENT-DEPENDENT SURFACE WATER/GROUNDWATER INTERACTIONS ALONG A CRITICAL REACH OF THE RIO GRANDE

Riparian restoration efforts and interstate compact demands have motivated intense hydrologic characterization along a 50-mile stretch of the Rio Grande between San Acacia and Fort Craig, New Mexico. In collaboration with the New Mexico Interstate Stream Commission, the U.S. Army Corps of Engineers, and S.S. Papadopulos and Associates, New Mexico Tech has established a groundwater-surface water monitoring network within this critical reach of the Rio Grande Valley.

Pressure transducers have been installed at 70 surface and groundwater monitoring points to provide a high-resolution picture of seasonal, event-dependent, and diurnal water losses/gains from the river to the shallow aquifer. The magnitude and direction of vertical gradients are dependent on river stage, and vary seasonally. Monitoring wells adjacent to the river show an almost immediate response to monsoonal-induced changes in river stage and the return to base flow conditions. During a September 2003 flood event, deeper wells showed a stronger pressure response than water table wells, temporarily reversing the downward vertical gradient. Diurnal groundwater fluctuations are pronounced from early spring until late fall, and cease when stretches of the Rio Grande dry. Daily groundwater table fluctuations in July and August ranged from 5 to 95 mm/day, apparently controlled by evapotranspirative demand and site geology.