Overview of the Groundwater Hydrology of the Rio Grande Basin

Figure 3. Conceptual hydrogeologic model showingundrained basins, partly drained basins, drained basins,and regional sinks (modified from Eakin et al. 1976;Hibbs et al. 1998). Phreatic playas are restricted toundrained and partly drained basins; and vadoseconditions exist in “dry playa” areas.Figure 3 illustrates the Bryan-Tolman conceptualmodel in a more general hydrogeologicsense for the entire Basin and Range province,and it incorporates subsequent work in the GreatBasin section (e.g., Mifflin 1968, 1988; Eakin etal. 1976), and in the Trans-Pecos Texas andChihuahua bolson region (Hibbs et al. 1998). Thetopographic terms closed and open are here usedonly in reference to the surface flow into, through,and from intermontane basins, whereas the termsundrained, partly drained, and drained designateclasses of groundwater flow involving intrabasinand/or interbasin movement. Phreatic and vadose,respectively, indicate saturatedand unsaturated subsurfaceconditions. Phreatic playas(with springs and seeps) arerestricted to floors of closedbasins (bolsons) that are undrainedor partly drained, andvadose playas occur in bothclosed and open, drained basins.In the Rio Grande rift studyregion, as well as in most otherdesert basins of western NorthAmerica, the intermediate basinclass referred to as partlydrained is probably the majorgroundwater-flow regime. Fewintermontane basins (bolsonsand semibolsons) are truly undrained in termsof groundwater discharge, whether or not theyare closed or open in terms of surface flow.Under predevelopment conditions, groundwaterdischarge in the region occurred mainlythrough subsurface leakage from one basinsystem into another, discharge into the gainingreaches of perennial or intermittent streams,discharge from springs, or by evapotranspirationfrom phreatic playas and cienegas(valley-floor wetlands). Most recharge tobasin-fill aquifers occurs by two mechanisms,(1) “mountain front,” where some precipitationfalling on bedrock highlands contributesto the groundwater reservoir along basinmargins (Figure 4); and (2) “tributary,” wherethe reservoir is replenished and along losingreaches of larger intrabasin streams (Hearneand Dewey 1988; Anderholm 1994; Kernodle1992; Wasiolek 1995). The upland networksof major stream valleys in the Sangre deCristo, San Juan, and Jemez Mountains ofsouthern Colorado and northern New Mexicoare the primary source areas for recharge ofbasin-fill aquifers in the RGR region. Secondarycontributors to these groundwater reservoirsare the few high and massive mountainranges that form isolated highlands borderingindividual basin units. Recharge estimates inthis paper are based on the assumption that (1)less than 5% of average annual precipitationcontributes to recharge, and (2) this contributionis distributed very unevenly over higherwatersheds and in major stream valleys.Figure 4. Two-dimensional conceptual model of a groundwater rechargesystem in a Basin and Range by hydrogeologic setting (from Wasiolek 1995,modified from Feth 1964, and Mifflin 1968).The Rio GrandeCompact:It’s the Law!Overviewof theHydrogeologyandGeohydrologyof theNorthern RioGrande Basin -Colorado,New Mexico,andTexasWRRIConferenceProceedings19994