Summary of U.S. Geological Survey ground-water-flow models of basin-fill aquifers in the southwest alluvial basins region, Colorado, New Mexico, and Texas

Four ground-water-flow models of basin-fill aquifer systems in Colorado, New Mexico, and Texas were completed in conjunction with the Southwest Alluvial Basins study that is part of the Geological Survey's Regional Aquifer-System Analysis program. The modeled areas are the San Luis Valley in Colorado, the Albuquerque-Belen Basin and the Animas Valley in New Mexico, and the Mesilla Basin in New Mexico, Texas, and Mexico. These flow models and 10 additional models of basin-fill aquifer systems in areas of Colorado, New Mexico, and Texas are described in this report. The models are summarized to identify the common simulated hydrogeologic characteristics and to isolate preferred approaches to simulating ground-water flow in the basin-fill aquifer systems. On the basis of attributes that are common to most of these models, a set of guidelines was developed that enables the rapid construction of groundwater-flow models of specific basin-fill aquifer systems. The feasibility of this modeling approach was tested and the guidelines were refined by developing test case generalized models of the Albuquerque-Belen and La Jencia-Socorro Basins. The generalized models met the objective of being adequate representations of both the function and the response of the basinfill aquifer systems; this demonstrates the reliability of the guidelines. The guidelines for construction of a generalized ground-water-flow model of a specific basin within the Southwest Alluvial Basins region are as follows: (1) Perform a literature search to determine basin geometry, geologic structure, and lithology. (2) Use a three-dimensional model to simulate the aquifer to a depth of approximately 4,000 feet or to the total depth of the basin if less than 4,000 feet. Use at least five model layers, the top layer being 200 feet or less in thickness. (3) Simulate the basin-fill aquifer system as having a horizontal hydraulic conductivity of 20 to 45 feet per day in the open-drainage basins and 2 to 10 feet per day in the closed-drainage basins, except where field data indicate otherwise. Simulate fine-grained playa or lake deposits as having a hydraulic conductivity of 0.25 to 10 feet per day and floodplain alluvial deposits as having a hydraulic conductivity of 50 to 70 feet per day. (4) Do not vary horizontal hydraulic conductivity as a function of depth unless specific lithologies are being simulated. Compaction of the aquifer and increases in temperature with depth need not be simulated as affecting the apparent hydraulic conductivity, except where these specific problems are being addressed. The two factors have opposite, and potentially offsetting, effects. (5) Use a horizontal to vertical hydraulic-conductivity ratio of from 200:1 to 1,000:1 except where geologic features or steeply dipping beds exist. such as faults, clay sequences, (6) Simulate aquifer specific storage to bo in the range of 2 x 10 to 5 x 10 per foot and specific yield in the range of 0.10 to 0.20. (7) Include rivers and drains, if presentj in the simulations as headdependent-flux boundaries, preferably With flow routing to allow the location of the boundary to change with time. (8) Include estimated mountain-front and evapotranspiration, and net irrigation fltux. (9) Include historical ground-water withdrawals. tributary recharge, is The general models may be rapidly assembled yet retain an accuracy that much greater than might be expected fi'om the small expended effort. Appropriate uses of these general models are collection program customized to the needs of to aid in the design of a dataa specific study area, to make an initial evaluation of a specific problem, or to test hypotheses regarding the hydrologic responses in a basin. A generalized flow model needs to be viewed as a preliminary effort that will be superseded by a refined and calibrated model.