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Utah Water Science Center

U.S. GEOLOGICAL SURVEY
Scientific Investigation Report 2005-5170—ONLINE ONLY

Hydrology and Simulation of Ground-Water Flow in Cedar Valley, Iron County, Utah

By Lynette E. Brooks and James L. Mason

ABSTRACT

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 Water withdrawn from the unconsolidated basin fill is the primary source for public supply and is a major source of water for irrigation.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.

This report is contained in the following files:

SIR2005_5170.pdf (3.2 mb)

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CONTENTS

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

Send questions or comments about this report to the author, Lynette Brooks at lebrooks@usgs.gov, 801.908.5014.

For more information about USGS activities in Utah, visit the USGS Utah Water Science Center home page.

U.S. Department of the Interior, U.S. Geological Survey
Persistent URL: http://pubs.water.usgs.gov/sir20055170
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Last modified: Thursday, January 10 2013, 04:56:17 PM
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