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**SIMULATION OF GROUND-WATER FLOW IN THE MIDDLE RIO GRANDE BETWEEN COCHITI   AND SAN ACACIA, NEW MEXICO WRIR 02-4200 [Report   pdf file 4.4 Mb][Model code and   data 5.7Mb]**

*By*Douglas P. McAda and Peggy Barroll

# ABSTRACT

This report describes a three-dimensional, finite difference,   ground-water-flow model of the Santa Fe Group aquifer system within the Middle   Rio Grande Basin between Cochiti and San Acacia, New Mexico. The aquifer system   is composed of the Santa Fe Group of middle Tertiary to Quaternary age and post-Santa   Fe Group valley and basin-fill deposits of Quaternary age.

 Population increases in the basin since the 1940's have   caused dramatic increases in ground-water withdrawals from the aquifer system,   resulting in large ground-water-level declines. Because the Rio Grande is hydraulically   connected to the aquifer system, these ground-water withdrawals have also decreased   flow in the Rio Grande. Concern about water resources in the basin led to the   development of a research plan for the basin focused on the hydrologic interaction   of ground water and surface water (McAda, D.P., 1996, Plan of study to quantify   the hydrologic relation between the Rio Grande and the Santa Fe Group aquifer   system near Albuquerque, central New Mexico: U.S. Geological Survey Water-Resources   Investigations Report 96-4006, 58 p.). A multiyear research effort followed,   funded and conducted by the U.S. Geological Survey and other agencies (Bartolino,   J.R., and Cole, J.C., 2002, Ground-water resources of the Middle Rio Grande   Basin, New Mexico: U.S. Geological Survey Circular 1222, 132 p.). The modeling   work described in this report incorporates the results of much of this work   and is the culmination of this multiyear study. 

The purpose of the model is (1) to integrate the components   of the ground-water-flow system, including the hydrologic interaction between   the surface-water systems in the basin, to better understand the geohydrology   of the basin and (2) to provide a tool to help water managers plan for and administer   the use of basin water resources. The aquifer system is represented by nine   model layers extending from the water table to the pre-Santa Fe Group basement   rocks, as much as 9,000 feet below the NGVD 29. The horizontal grid contains   156 rows and 80 columns, each spaced 3,281 feet (1 kilometer) apart. The model   simulates predevelopment steady-state conditions and historical transient conditions   from 1900 to March 2000 in 1 steady-state and 52 historical stress periods.   Average annual conditions are simulated prior to 1990, and seasonal (winter   and irrigation season) conditions are simulated from 1990 to March 2000. The   model simulates mountain-front, tributary, and subsurface recharge; canal, irrigation,   and septic-field seepage; and ground-water withdrawal as specified-flow boundaries.   The model simulates the Rio Grande, riverside drains, Jemez River, Jemez Canyon   Reservoir, Cochiti Lake, riparian evapotranspiration, and interior drains as   head-dependent flow boundaries. 

Hydrologic properties representing the Santa Fe Group aquifer   system in the ground-water-flow model are horizontal hydraulic conductivity,   vertical hydraulic conductivity, specific storage, and specific yield. Variable   horizontal anisotropy is applied to the model so that hydraulic conductivity   in the north-south direction (along model columns) is greater than hydraulic   conductivity in the east-west direction (along model rows) over much of the   model. This pattern of horizontal anisotropy was simulated to reflect the generally   north-south orientation of faulting over much of the modeled area. With variable   horizontal anisotropy, horizontal hydraulic conductivities in the model range   from 0.05 to 60 feet per day. Vertical hydraulic conductivity is specified in   the model as a horizontal to vertical anisotropy ratio (calculated to be 150:1   in the model) multiplied by the horizontal hydraulic conductivity along rows.   Specific storage was estimated to be 2 x 10-6 per foot in the model. Specific   yield was estimated to be 0.2 (dimensionless). 

A ground-water-flow model is a tool that can integrate the   complex interactions of hydrologic boundary conditions, aquifer materials, aquifer   stresses, and aquifer-system responses. This groundwater-flow model provides   a reasonable representation of the geohydrologic processes of the basin and   simulates many historically measured trends in flow and water levels. By simulating   these complex interactions, the ground-water-flowmodel described in this report   can provide a tool to help water managers plan for and administer the use of   basin water resources. Nevertheless, no ground-water model is unique, and numerous   sources of uncertainty remain. When using results from this model for any specific   problem, those uncertainties should be taken into consideration. 

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