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| Utah Water Science Center |
By Lynette E. Brooks and James L. Mason
Cedar Valley, located in the eastern part of Iron County in southwestern Utah, is experiencing rapid population growth. Cedar Valley traditionally has supported agriculture, but the growing population needs a larger share of the available water resources. Water withdrawn from the unconsolidated basin fill is the primary source for public supply and is a major source of water for irrigation. Water managers are concerned about increasing demands on the water supply and need hydrologic information to manage this limited water resource and minimize flow of water unsuitable for domestic use toward present and future public-supply sources.
Surface water in the study area is derived primarily from snowmelt at higher altitudes east of the study area or from occasional large thunderstorms during the summer. Coal Creek, a perennial stream with an average annual discharge of 24,200 acre-feet per year, is the largest stream in Cedar Valley. Typically, all of the water in Coal Creek is diverted for irrigation during the summer months. All surface water is consumed within the basin by irrigated crops, evapotranspiration, or recharge to the ground-water system.
Ground water in Cedar Valley generally moves from primary recharge areas along the eastern margin of the basin where Coal Creek enters, to areas of discharge or subsurface outflow. Recharge to the unconsolidated basin-fill aquifer is by seepage of unconsumed irrigation water, streams, direct precipitation on the unconsolidated basin fill, and subsurface inflow from consolidated rock and Parowan Valley and is estimated to be about 42,000 acre-feet per year. Stable-isotope data indicate that recharge is primarily from winter precipitation. The chloride mass-balance method indicates that recharge may be less than 42,000 acre-feet per year, but is considered a rough approximation because of limited chloride concentration data for precipitation and Coal Creek. Continued declining water levels indicate that recharge is not sufficient to meet demand. Water levels in many areas are at or close to historic lows.
In 2000, withdrawal from wells was estimated to be 36,000 acre-feet per year. About 4,000 acre-feet per year are estimated to discharge to evapotranspiration or as subsurface outflow. Prior to large-scale ground-water development, ground-water discharge by evapotranspiration and discharge to springs was much larger.
Ground water along the eastern margin of the valley between Cedar City and Enoch is unsuitable for domestic use because of high dissolved-solids and nitrate concentrations. The predominant ions of Ca and SO4 in this area indicate dissolution of gypsum in the Markagunt Plateau to the east. Data collected during this study were compared to historic data; there is no evidence to indicate deterioration in ground-water quality. The spatial distribution of ground water with high nitrate concentration does not appear to be migrating beyond its previously known extent.
No single source can be identified as the cause for elevated nitrate concentrations in ground water. Low nitrogen-15 values north of Cedar City indicate a natural geologic source. Higher nitrogen-15 values toward the center of the basin and associated hydrologic data indicate probable recharge from waste-water effluent. Excess dissolved nitrogen gas and low nitrate concentrations in shallow ground water indicate that denitrification is occurring in some areas.
A computer ground-water flow model was developed to simulate flow in the unconsolidated basin fill. The method of determining recharge from irrigation was changed during the calibration process to incorporate more areal and temporal variability. In general, the model accurately simulates water levels and water-level fluctuations and can be considered an adequate tool to help determine the valley-wide effects on water levels of additional ground-water withdrawals and changes in water use. The model was used to simulated water-level changes caused by projecting current withdrawal rates, increased withdrawal rates, and a 10-year drought. Water levels declined 20 to 275 feet in the southern and central parts of the valley and less than 20 feet north of Enoch.
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Systems Incorporated.
Abstract
Introduction
Purpose and Scope
Acknowledgments
Previous Investigations
Physical Characteristics of the Study Area
Surface Water
Hydrology of the Ground-Water System
Hydrogeology
Unconsolidated Basin Fill
Consolidated Rocks
Ground-Water Conditions In the Unconsolidated Basin Fill
Aquifer Properties
Movement
Recharge
Seepage from Streams
Seepage from Irrigation Water
Precipitation
Subsurface Inflow
Discharge
Wells
Evapotranspiration
Springs and Subsurface Outflow
Trends in Water-Level Fluctuations
Residual and Uncertainty
Water Quality and its Implications for Ground-Water Flow
Major-Ion Chemistry
Isotope Chemistry
Ground-Water Age Dating and Recharge Temperature
Nitrate-Source Determination
Chloride Mass Balance
Numerical Simulation of Ground-Water Flow in the Unconsolidated Basin Fill
Recharge from Irrigation
Recharge during 1938-49
Recharge during 1950-2000
Percent-Recharge Method
Consumptive-Use Method
Model Construction
Spatial Discretization
Temporal Discretization
Distribution of Aquifer Characteristics
Hydraulic Conductivity and Horizontal-to-Vertical Anisotropy
Specific Yield and Specific Storag
Horizontal-Flow Barriers
Boundary Conditions
No-flow Boundaries
Recharge Boundaries
Discharge Boundaries
Model Calibration
Parameter Adjustment and Sensitivity
Irrigation Zones
Hydraulic Conductivity
Evapotranspiration
Inflow from Consolidated Rock and Parowan Valley
Other Parameters
Specific Yield
Steady-State Calibration
Transient Calibration
Parameter Correlation, Sensitivity Analysis, and Need for Additional Data
Model Projections
Results
Summary
References Cited
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