Hydrologic Conditions and Assessment of Water Resources in the Turkey Creek Watershed, Jefferson County, Colorado, 1998–2001
By Clifford R. Bossong, Jonathan Saul Caine, David I. Stannard, Jennifer L. Flynn, Michael R. Stevens, and Janet S. Heiny-Dash
Available from the U.S. Geological Survey, Branch of Information Services, Box 25286, Denver Federal Center, Denver, CO 80225, USGS Water-Resources Investigations Report 03-4034, 140 p., 45 figs.
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Abstract
The 47.2-square-mile Turkey Creek watershed, in Jefferson County southwest of Denver, Colorado, is relatively steep with about 4,000 feet of relief and is in an area of fractured crystalline rocks of Precambrian age. Water needs for about 4,900 households in the watershed are served by domestic wells and individual sewage-disposal systems. Hydrologic conditions are described on the basis of contemporary hydrologic and geologic data collected in the watershed from early spring 1998 through September 2001. The water resources are assessed using discrete fracture-network modeling to estimate porosity and a physically based, distributed-parameter watershed runoff model to develop estimates of water-balance terms.
A variety of climatologic and hydrologic data were collected. Direct measurements of evapotranspiration indicate that a large amount (3 calendar-year mean of 82.9 percent) of precipitation is returned to the atmosphere. Surface-water records from January 1, 1999, through September 30, 2001, indicate that about 9 percent of precipitation leaves the watershed as streamflow in a seasonal pattern, with highest streamflows generally occurring in spring related to snowmelt and precipitation. Although conditions vary considerably within the watershed, overall watershed streamflow, based on several records collected during the 1940's, 1950's, 1980', and 1990's near the downstream part of watershed, can be as high as about 200 cubic feet per second on a daily basis during spring. Streamflow typically recedes to about 1 cubic foot per second or less during rainless periods and is rarely zero. Ground-water level data indicate a seasonal pattern similar to that of surface water in which water levels are highest, rising tens of feet in some locations, in the spring and then receding during rainless periods at relatively constant rates until recharged. Synoptic measurements of water levels in 131 mostly domestic wells in fall of 2001 indicate a water-table surface that conforms to topography. Analyses of reported well-construction records indicate a median reported well yield of 4 gallons per minute and a spatial distribution for reported well yield that has relatively uniform conditions of small-scale variability. Results from quarterly samples collected in water year 1999 at about 112 wells and 22 streams indicate relatively concentrated calcium-bicarbonate to calcium-chloride type water that has a higher concentration of chloride than would be expected on the basis of chloride content in precipitation and evapotranspiration rates. Comparison of the 1999 data to similar data collected in the 1970's indicates that concentrations for many constituents appear to have increased. Reconnaissance sampling in the fall of 2000 indicates that most ground water in the watershed was recharged recently, although some ground water was recharged more than 50 years ago. Additional reconnaissance sampling in the spring and fall of 2001 identified some compounds indicative of human wastewater in ground water and surface water.
Outcrop fracture measurements were used to estimate potential porosities in three rock groups (metamorphic, intrusive, and fault zone) that have distinct fracture characteristics. The characterization, assuming a uniform aperture size of 100 microns, indicates very low potential fracture porosities, on the order of hundredths of a percent for metamorphic and intrusive rocks and up to about 2 percent for fault-zone rocks. A fourth rock group, Pikes Peak Granite, was defined on the basis of weathering characteristics. Short-term continuous and synoptic measurements of streamflow were used to describe base-flow characteristics in areas of the watershed underlain by each of the four rock groups and are the basis for characterization of base flow in a physically based, distributed-parameter watershed model.
The watershed model, the Precipitation-Runoff Modeling System (PRMS), was used to characterize hydrologic conditions on the basis of precipitation and air temperature in 112 hydrologic response units for which physical characteristics were derived from mostly digital data. The watershed model also was used to characterize hydrologic conditions in subsurface portions of the watershed that are associated with streamflow. The model was conditioned, using a relatively small set of parameters, to match measurements of watershed and intrawatershed streamflow and point measurements of evapotranspiration, air temperature, and soil moisture. Results from the watershed model provide simulated estimates for water-balance terms in a contemporary simulation (January 1, 1999, through September 30, 2001) using precipitation and adjusted temperature data from within the watershed, and in a long-term simulation (October 1, 1948, through September 30, 1999) using precipitation and temperature data from near the watershed. The results of both simulations indicate that, on a watershed scale, base-flow reservoirs consistently contain about enough water to cover the watershed with 0.1 to 0.2 inch of water. The long-term simulations indicate that during a year with about 14 inches of precipitation, the watershed base-flow reservoir may have about a -0.06 inch change in contents during periods with relatively small amounts of recharge. The results from watershed simulations also indicate that contents of base-flow reservoirs vary within the watershed; base-flow reservoirs contain little or no recoverable water for significant portions of many years in about 90 percent of the watershed. In areas where base-flow reservoirs contain no water, the only source of water for wells is water that has percolated to relatively deep parts of the system that are not associated with local streamflow; water withdrawn under these conditions will need to be replaced before base flow can resume. Estimates of the amount of water withdrawn by wells in 2001 in the Turkey Creek watershed are equal to a watershed depth of about 0.43 to 0.65 inch (about 0.0012 to 0.0018 inch per day).
Contents
Glossary
Abstract
Introduction
Purpose and Scope
Location and Setting
Previous Investigations
Acknowledgments
Geologic Framework
Data Collection and Methods
Historical Data
Contemporary Data
Fracture Characteristics
Outcrop Selection
Methods of Fracture Measurement
Climatologic Data
Surface-Water Data
Ground-Water Data
Water-Quality Data
Runoff Modeling Methods
General Characteristics of the Precipitation-Runoff Modeling System
Watershed Characterization Using the Precipitation-Runoff Modeling System
Hydrologic Conditions
Evapotranspiration
Forest Site Results
Meadow Site and Comparison to Forest Site
Surface-Water Conditions
Watershed Conditions
Intrawatershed Conditions
Ground-Water Conditions
Time-Series Water Levels
Synoptic Water Levels
Analysis of State Engineer Well-Construction Data
Univariate Statistics
Variogram Analysis
Water-Quality Characteristics in the Turkey Creek Watershed
General Water-Quality Characteristics
Comparison to Historical Water-Quality Data
Tritium
Chloride to Bromide Ratios
Wastewater Compounds
Chloride Sources
Seasonality of Specific Conductance
Assessment of Water Resources
Estimates of Potential Fracture-Network Porosity
Runoff Modeling
Hydrologic Response Unit Delineation and Parameterization
Interflow and Base-Flow Reservoir Delineation and Parameterization
Distribution of Precipitation and Temperature Data
Model Calibration
Watershed Calibration Procedures
Intrawatershed Calibration Procedures
Final Calibration
Contemporary Simulations
Long-Term Simulations
Simulation Results and Their Implications to Water Supply
Summary
Selected References
Appendix
Brittle Structures of the Turkey Creek Watershed
Fracture Data Analysis, Model Construction, and Matching Models to Data
Fracture Set Designation
Fracture Length Simulation
Fracture Spacing
Fracture Intensity and Calibration of Fracture Network Models to Field Data
Spatial Analysis of Well-Construction Data and Ground-Water Levels
Evapotranspiration Measurement Methods
Quality Assurance and Quality Control
Milliequivalent Balances
Quality-Control Samples
Duplicate Samples
