Development of a Numerical Groundwater Flow Model of Selected Basins within the Colorado Regional Groundwater Flow System, Southeastern Nevada (Version 1.0)

Tetra Tech, Inc.

A report that describes the development of a three-dimensional numerical groundwater flow model of parts of 13 hydrographic basins within the Colorado Regional Groundwater Flow System in southeastern Nevada. A new three-dimensional model has been completed of all or parts of 13 hydrographic areas within the Colorado Regional Groundwater Flow System in southeast Nevada. This model simulates the movement of groundwater in an area ranging from the Clover and Delaware Mountains on the north to the Las Vegas Valley Shear Zone and Lake Mead on the south, and from the Sheep Range on the west to the Virgin and Beaver Dam Mountains on the east. The impetus for the DOI bureaus to develop this model is to have a tool with which to make quantitative estimates of the future impacts on springs and streams throughout the model area as a result of ongoing pumping of existing groundwater rights and additional groundwater applications currently pending before the Nevada State Engineer. The DOI bureaus are interested in the potential adverse effects to springs and streams on federal lands for which they have responsibility. Development of this new model was based, in part, on studies funded by the DOI bureaus: (1) geophysical studies, geologic mapping, and new geologic cross-sections; (2) synoptic discharge-measurement runs on the Muddy and Virgin rivers; (3) an ET study over the whole model area; (4) boundary flux estimates; and (5) preliminary hydrogeochemical investigations. Other information, such as lithologic information from new drillholes and recent water-level and water-use data, has been provided by other (non-federal) parties and organizations with interest in the groundwater resources of the study area. MODFLOW-2000 was used for the groundwater model. The model has 18 layers, representing different depths below the water table, with a total thickness of 15,630 feet. The model grid is oriented north-south to east-west, has 314 rows and 209 columns, with the model cells ranging in length from 1,500 meters (4,921 feet) down to 250 meters (820 feet). The grid spacing is finest (250 meters) in the Muddy River Springs Area, Coyote Spring Valley, and California Wash to take advantage of the greater number of drillholes, and better information on pumping rates, groundwater discharge, and water-level changes in these areas. The model was calibrated based on many different types of information, including measurements of water levels and drawdown, discharge rates for springs, streamflow measurements, reported pumping rates that varied through time, seasonal estimates of ET based on field measurements and satellite mapping of plant communities, and estimates of model boundary fluxes based on regional information. A “pre-production” model was developed to match water levels and water-budget information. The model was also calibrated to the effects of time-varying pumping and seasonal ET during the period October 2008 through December 2011, primarily in the area of the Muddy River Springs, Coyote Spring Valley, and California Wash. The simulated drawdowns agree reasonably well with the observed drawdowns. In California Wash, the seasonal variation observed in the measurements is not present in the simulated water levels, but the longer term trends are present...