Overview of the Groundwater Hydrology of the Rio Grande Basin

Basin (Kernodle et al. 1987) and the TularosaBasin contributes underflow to the Hueco Bolson(Bedinger et al. 1989; Hibbs et al. 1997).Even before 20 th -Century exploitation ofmajor groundwater resources, every intra-basinsource of water plus a portion of flow in the RioGrande went to evaporation from open water or totranspiration. Each basin along the “string ofpearls” with the possible exception of theAlamosa and Sunshine Valley subbasins of theSan Luis Basin, caused a diminished flow in theriver except during periodic local flood events.After groundwater development began, morewater was lost from the surface-water system (again to groundwater) and less was lost to evapotranspiration.No efforts have yet been made toaugment other sources of recharge to the basins’aquifers.Models – Past, Present, and FutureThe earliest model of a northern Rio GrandeBasin was the one by Reeder and others (1967) ofa portion of the Albuquerque Basin. It was basedon some still valid concepts and others that areobsolete; but they made it work with a handcrankedcalculator. Over the following years,many government-financed and private modelswere completed of this and other basins in the rift.Each progressive step took advantage of technologicalimprovements in computing power andcollective improvements in the overall understandingof Rio Grande rift-basin flow systems.The SWAB RASA (Southwest Alluvial BasinsRegional Aquifer-Systems Analysis program–Wilkins 1986; 1998) addressed the geohydrologyof 22 basin-fill aquifers in the Rio Grande rift andadjacent parts of the southeastern Basin andRange province in New Mexico, western Texas,and southern Colorado. As part of that study, fourmodels were commissioned to explore the practicaland economic feasibility of different approachesto modeling rift basins.A model of the Alamosa subbasin of the“Valley” tested a superposition approach (Hearneand Dewey 1988) as well as a two-dimensionalvertical cross-section model to determine thenecessary depth of simulation of the subsequentareal three-dimensional model. A model of the“Albuquerque-Belen Basins” (Kernodle and Scott1986; Kernodle et al. 1987) tested the feasibilityof using a deep (200 feet) constant hear boundarythroughout the 2- to 5-mile wide flood plain torepresent the Rio Grande. A third model wascontracted to the New Mexico Bureau of Minesand Mineral Resources (O’Brien and Stone 1983)to use flow-net analysis to guide transmissivityestimates for a two-dimensional model. The fourthSWAB model to be formally documented was ofthe Mesilla Basin (Frenzel and Kaehler 1992).That model aspired to include every hydrologicdetail of even the remotest importance.An early objective of the SWAB RASA wasto construct a groundwater-flow model of theentire rift system. As the study progressed, itbecame very clear that the “string of pearls” couldnot be simulated from a groundwater perspective.Hence, a different approach was taken: to evaluateall existing public-domain (e.g., USGS orgovernment contract) models in an attempt toanalyze the assets, flaws, common attributes, andvarious calibration approaches (Kernodle 1992).Altogether, 14 models were evaluated, withselected information on five of them included inTable 4. The critique resulted in a set of nineguidelines that were tested in new models for abasin with an already existing model (Albuquerque)and for joined basins (San Agustin-Socorro),which had not previously been modeled. A thirdmodel of the Palomas-Engle Basin was leftincomplete. Each model was allocated approximatelythree weeks for completion. The experimentalmodel of the Albuquerque Basin was,statistically, an improvement over its predecessoreven though the first took years to complete andthe other, only weeks. Still, both are seriouslyoutdated in their portrayal of the current understandingof the hydrogeologic framework of thebasin.The nine guidelines (Kernodle 1992) are listedbelow, but, be aware that technological improvementsand recent data acquisition have expandedthe envelope on some of them (cf. Tables 2-4):1. Perform a literature search to determine basingeometry, geologic structure, and lithology.2. Use a three-dimensional model to simulate theaquifer to a depth of approximately 4,000 feetor to the total depth of the basin fill if lessthan 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 ashaving a horizontal hydraulic conductivity of20 to 45 feet per day in the open-drainageThe Rio GrandeCompact:It’s the Law!Overviewof theHydrogeologyandGeohydrologyof theNorthern RioGrande Basin -Colorado,New Mexico,andTexasWRRIConferenceProceedings199917