Simulation of Ground-Water Flow in the MojaveRiver Basin, California      
***By*Christina   L. Stamos, Peter Martin, Tracy Nishikawa,  and Brett F. Cox 
 U.S. GEOLOGICAL SURVEY 

 Water Resources Investigation Report 01-4002 **Version 2**

Sacramento, California 2001 
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A two-dimensional view of the model      simulation--simulation period 1931-99.(3MB)
ABSTRACT
 The proximity of the Mojave River ground-water basin to the highly urbanized Los Angeles region has led to rapid growth in population and, consequently, to an increase in the demand for water. The Mojave River, the primary source of surface water for the region, normally is dry-except for a small stretch of perennial flow and periods of flow after intense storms. Thus, the region relies almost entirely on ground water to meet its agricultural and municipal needs. Ground-water withdrawal since the late 1800's has resulted in discharge, primarily from pumping wells, that exceeds natural recharge. To better understand the relation between the regional and the floodplain aquifer systems and to develop a management tool that could be used to estimate the effects that future stresses may have on the ground-water system, a numerical ground-water flow model of the Mojave River ground-water basin was developed, in part, on the basis of a previously developed analog model. 

   The ground-water flow model has two horizontal layers; the top layer (layer 1) corresponds to the floodplain aquifer and the bottom layer (layer 2) corresponds to the regional aquifer. There are 161 rows and 200 columns with a horizontal grid spacing of 2,000 by 2,000 feet. Two stress periods (wet and dry) per year are used where the duration of each stress period is a function of the occurrence, quantity of discharge, and length of stormflow from the headwaters each year. A steady-state model provided initial conditions for the transient-state simulation. The model was calibrated to transient-state conditions (1931-94) using a trial-and-error approach. 

   The transient-state simulation results are in good agreement with measured data. Under transient-state conditions, the simulated floodplain aquifer and regional aquifer hydrographs matched the general trends observed for the measured water levels. The simulated streamflow hydrographs matched wet stress period average flow rates and times of no flow at the Barstow and Afton Canyon gages. 

   Steady-state particle-tracking was used to estimate travel times for mountain-front and streamflow recharge. The simulated travel times for mountain-front recharge to reach the area west of Victorville were about 5,000 to 6,000 years; this result is in reasonable agreement with published results. Steady-state particle-tracking results for streamflow recharge indicate that in most subareas along the river, the particles quickly leave and reenter the river. 

   The complaint that resulted in the adjudication of the Mojave River ground-water basin alleged that the cumulative water production upstream of the city of Barstow had overdrafted the ground-water basin. In order to ascertain the effect of pumping on ground-water and surface-water relations along the Mojave River, two pumping simulations were compared with the 1931-90 transient-state simulation (base case). The first simulation assumed 1931-90 pumping in the upper region (Este, Oeste, Alto, and Transition zone model subareas) but with no pumping in the remainder of the basin, and the second assumed 1931-90 pumping in the lower region (Centro, Harper Lake, Baja, Coyote Lake, and Afton Canyon model subareas) but with no pumping in remainder of the basin. 

   In the upper region, assuming pumping only in the upper region, there was no change in storage, recharge from the Mojave River, ground-water discharge to the Mojave River, or evapotranspiration when compared with the base case. In the lower region, assuming pumping only in the upper region, there was storage accretion, decreased recharge from the Mojave River, increased ground-water discharge to the Mojave River, and increased evapotranspiration when compared with the base case. 

   In the upper region, assuming pumping only in the lower region, there was storage accretion, decreased recharge from the Mojave River, increased ground-water discharge to the Mojave River, and increased evapotranspiration when compared with the base case. In the lower region, assuming pumping only in the lower region, there was less storage depletion, increased recharge from the Mojave River, increased ground-water discharge to the Mojave River, and increased evapotranspiration when compared with the base case. Overall, pumping in the lower region does not negatively affect the upper region; however, pumping in the upper region negatively affects the lower region by decreasing recharge from the Mojave River. 

   Streamflow, pumpage, and water-level data from   calendar years 1995-99 were used to validate the calibrated ground-water flow model, that is, to test that the ground-water flow model will duplicate measured data for a noncalibration period without modification of the model parameters. In general, the simulated results are in good agreement with the measured data, and the simulated hydrographs for wells in the floodplain and regional aquifers follow the measured water-level trends. Simulated streamflow data for the 1995-99 wet and dry stress periods at the Lower Narrows, Barstow, and Afton Canyon were compared with the measured data for average streamflow for the same periods; in general, the model reflects 1995-99 streamflow conditions. The simulation results also indicate that the streambed conductance values calibrated to the 1931-94 conditions reasonably simulate the 1995-99 conditions and therefore can be used for predictive purposes. 

   To visualize the magnitude, spatial distribution, and timing of water-level changes in the basin through time, simulated hydraulic heads for 1932-99 were compared with simulated hydraulic heads for 1931. Greater than average annual inflows to the Mojave River from the headwaters during the late 1930's and throughout much of the 1940's resulted in simulated hydraulic heads that were higher than the 1931 hydraulic heads along the Mojave River in most model subareas. Parts of the Baja and Harper Lake model subareas had declines in the simulated hydraulic head because of the increase in agricultural pumpage. By 1960, the simulated hydraulic heads were lower than the simulated hydraulic heads for 1931 in all model subareas of the floodplain and the regional aquifers because of pumpage. After 1960, the size and the magnitude of the areas of the regional aquifer for which simulated hydraulic heads were lower than those for 1931 continued to increase until the end of the simulation (1999). Along the Mojave River, hydraulic heads fluctuated in the floodplain aquifer in response to recharge during years with large inflows with little apparent effect on the simulated hydraulic heads in the regional aquifer. 

   Three water-management alternatives were evaluated to determine their effect on ground-water resources using the calibrated ground-water flow model. The water-management alternatives consider the artificial recharge of imported water allocated to the Mojave Water Agency (MWA): the first assumes that zero percent of the MWA allocation is available (alternative 1), the second assumes that 50 percent of the MWA allocation is available (alternative 2), and the third assumes that 100 percent of the MWA allocation is available (alternative 3). Each of the three water-management alternatives were evaluated for a 20-year drought. Streamflow conditions were simulated using the 20-year drought of 1945-64 with associated calibrated stream parameters. 

   Management alternative 1 results in a reduction in ground-water recharge from the Mojave River compared with average recharge for 1995-99; this reduction is reflected in simulated hydraulic-head declines between 1999 and 2019 of as much as 45 feet. Management alternatives 2 and 3 result in no change in recharge from the Mojave River for management alternative 2 and a small increase for management alternative 3 when compared with recharge for management alternative 1. The artificial recharge of imported water causes increases in simulated hydraulic head for both management alternatives at each of the artificial-recharge sites. Some of the increases are related to water that recharges into areas of low transmissivity which implies that the recharge operations may benefit from being distributed over a larger area. 
CONTENTS
Abstract 

Introduction

Purpose and Scope 

Description of Study Area 

Acknowledgments 

Surface-Water Hydrology 

The Mojave River 

Ungaged Tributary Streams 

Ground-Water Hydrology 

Geologic Setting 

Stratigraphic Units 

Definition of Aquifers 

Effects of Faulting on Ground-Water Flow 

Ground-Water Recharge and Discharge 

Recharge 

The Mojave River 

Mountain-Front Recharge 

Artificial Recharge 

Irrigation-Return Flow 

Fish Hatchery Discharge 

Treated Sewage Effluent 

Septic Systems 

Imported Water 

Discharge 

Pumpage

Evapotranspiration 

Transpiration by Phreatophytes and Hydrophytes 

Bare-Soil Evaporation from Dry Lakes 

Free-Surface Evaporation 

Underflow at Afton Canyon 

Ground-Water Flow Model 

Model Grid 

Model Boundary Conditions 

Aquifer Properties 

Transmissivity 

Storage Coefficient 

Vertical Leakance 

Stream-Aquifer Interactions

Simulation of Recharge 

Mountain-Front Recharge

Artificial Recharge 

Irrigation-Return Flow

Fish Hatchery Discharge and Imported Water

Treated Sewage Effluent

Septic Systems 

Simulation of Discharge 

Pumpage

Recreational Lakes in the Baja Model Subarea

Transpiration by Phreatophytes and Bare-Soil Evaporation

Dry Lakes 

Model Calibration

Simulation of Steady-State Conditions 

Simulation of Transient-State Conditions 

Simulation Results

Steady-State Ground-Water Flow Directions and Travel Times

Evaluation of Effects of Regional-Scale Pumping 

Upper Region Pumping Only

Lower Region Pumping Only

Summary of Effects of Regional-Scale Pumping 

Model Validation

Simulated Changes in Hydraulic Head, 1931-99

Model Limitations 

Evaluation of Selected Water-Management Alternatives 

Management Alternative 1: Zero Percent of Artificial Recharge   Allocation

Management Alternative 2: 50 Percent of Artificial Recharge   Allocation

Management Alternative 3: 100 Percent of Artificial Recharge   Allocation

Discussion of Management Alternatives 2 and 3 

Summary 

Selected References 

Appendix 1. Measured and model-simulated hydraulic heads at   selected wells in the Mojave River ground-water basin, southern California,   1931-99 

Appendix 2. Measured and model-simulated hydraulic heads at   multiple-well completion sites, Mojave River ground-water basin, southern California,   1992-99 
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