This USGS program addresses the need for improved methods of quantifying inflow to ground water systems, and the issue of regional synthesis of information on the interaction of ground water and surface water. Our project's contributions center around the relation of the geologic nature of unsaturated-zone materials to recharge rate, making use of point recharge measurements by the Darcian-SSC approach. A major goal is to permit the use of properties that are easier to measure than unsaturated K in order to explore the spatial variation of recharge rates over regions of diverse terrain. Ultimately this should permit basin-scale recharge determinations from information that can be obtained relatively easily. This study will be done as a set of investigations at multiple locations in the Southwest over the five years of the program (1999-2003). Research at the Mojave Basin, Middle Rio Grande Basin (see below), southeastern Utah (Navajo Sandstone), and the Amargosa Desert Research Site (ADRS) will complement additional studies pursued in connection with the SWGWRP.
Aquifer recharge is of great interest in this arid basin (as in the Mojave Basin), mainly because local population pressures are putting increasing demand on ground water supplies. For the problem described by Nimmo (1997a), estimating recharge rates in a hydrologically significant basin by a variety of independent methods, we are collaborating with the New Mexico District and with other NRP projects [Stonestrom and Constantz] in the multidisciplinary MRGB Program.
The main objective for our work here is the measurement and correlation of recharge rates with variations in topography and geology. We are applying the Darcian-SSC method, and developing new point-scale unsaturated flow modeling and hydraulic-property interpretation techniques. We are also developing modeling techniques that estimate total recharge for a particular subbasin. We have developed a method to assess basin-scale recharge rates and variability, and have applied it first at the watershed of Abo Arroyo, a substantial basin in the Southeastern portion of the MRGB.
Results based on 16 Darcian estimates of local recharge rate suggest that recharge within Abo Arroyo is substantial (1 to 2,700 cm/yr), while recharge rates in the inter-arroyo region are very low (< 0.013 cm/yr) and drop off sharply outside the channel. Within Abo Arroyo, recharge is variable along the entire reach, in some cases to a surprising degree. These studies are an important step toward understanding the topographic and geologic factors that determine recharge rates at a given location (Lewis and others, 1999, Nimmo and others, 2000). On the basis of hydrogeomorphic characteristics and the set of Darcian recharge estimates, we divided the 640-square-km region encompassing Abo Arroyo into four subregions, each having relative uniformity of major recharge-related characteristics. Recharge estimates for these subregions, appropriately integrated, give a total recharge estimate for the arroyo and its basin. Results fall within the range of previous estimates by water-budget, chloride mass-balance, and tracer methods, and with the advantages of high spatial resolution and potentially great accuracy of the Darcian technique.
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