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Scientific Investigations Report 2010–5082

Prepared in cooperation with the Lost Creek Ground Water Management District
and the Colorado Water Conservation Board

Hydrogeology and Steady-State Numerical Simulation of Groundwater Flow in the Lost Creek Designated Ground Water Basin, Weld, Adams, and Arapahoe Counties, Colorado

By L.R. Arnold

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The Lost Creek Designated Ground Water Basin (Lost Creek basin) is an important alluvial aquifer for irrigation, public supply, and domestic water uses in northeastern Colorado. Beginning in 2005, the U.S. Geological Survey, in cooperation with the Lost Creek Ground Water Management District and the Colorado Water Conservation Board, collected hydrologic data and constructed a steady-state numerical groundwater flow model of the Lost Creek basin. The model builds upon the work of previous investigators to provide an updated tool for simulating the potential effects of various hydrologic stresses on groundwater flow and evaluating possible aquifer-management strategies.

As part of model development, the thickness and extent of regolith sediments in the basin were mapped, and data were collected concerning aquifer recharge beneath native grassland, nonirrigated agricultural fields, irrigated agricultural fields, and ephemeral stream channels. The thickness and extent of regolith in the Lost Creek basin indicate the presence of a 2- to 7-mile-wide buried paleovalley that extends along the Lost Creek basin from south to north, where it joins the alluvial valley of the South Platte River valley. Regolith that fills the paleovalley is as much as about 190 ft thick. Average annual recharge from infiltration of precipitation on native grassland and nonirrigated agricultural fields was estimated by using the chloride mass-balance method to range from 0.1 to 0.6 inch, which represents about 1–4 percent of long-term average precipitation. Average annual recharge from infiltration of ephemeral streamflow was estimated by using apparent downward velocities of chloride peaks to range from 5.7 to 8.2 inches. Average annual recharge beneath irrigated agricultural fields was estimated by using passive-wick lysimeters and a water-balance approach to range from 0 to 11.3 inches, depending on irrigation method, soil type, crop type, and the net quantity of irrigation water applied. Estimated average annual recharge beneath irrigated agricultural fields represents about 0–43 percent of net irrigation.

The U.S. Geological Survey modular groundwater modeling program, MODFLOW–2000, was used to develop a steady-state groundwater flow model of the Lost Creek basin. Groundwater in the basin is simulated generally to flow from the basin margins toward the center of the basin and northward along the paleovalley. The largest source of inflow to the model occurs from recharge beneath flood- and sprinkler-irrigated agricultural fields (14,510 acre-feet per year [acre-ft/yr]), which represents 39.7 percent of total simulated inflow. Other substantial sources of inflow to the model are recharge from precipitation and stream-channel infiltration in nonirrigated areas (13,810 acre-ft/yr) seepage from Olds Reservoir (4,280 acre-ft/yr), and subsurface inflow from ditches and irrigated fields outside the model domain (2,490 acre-ft/yr), which contribute 37.7, 11.7, and 6.8 percent, respectively, of total inflow. The largest outflow from the model occurs from irrigation well withdrawals (26,760 acre-ft/yr), which represent 73.2 percent of total outflow. Groundwater discharge (6,640 acre-ft/yr) at the downgradient end of the Lost Creek basin represents 18.2 percent of total outflow, and evapotranspiration (3,140 acre-ft/yr) represents about 8.6 percent of total outflow.

First posted September 8, 2010

For additional information contact:

Center Director, USGS Colorado Water Science Center
Box 25046, Mail Stop 415
Denver, CO 80225

http://co.water.usgs.gov

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Suggested citation:

Arnold, L.R., 2010, Hydrogeology and steady-state numerical simulation of groundwater flow in the Lost Creek Designated Ground Water Basin, Weld, Adams, and Arapahoe Counties, Colorado: U.S. Geological Survey Scientific Investigations Report 2010–5082, 79 p.



Contents

Abstract

Introduction

Purpose and Scope

Study-Area Description

Physiography and Climate

Soils

Land Use and Irrigation

Hydrogeology

Aquifer Extent and Thickness

Water Levels and Groundwater Flow

Hydrologic Properties

Recharge

Precipitation Infiltration

Stream-Channel Infiltration

Deep Percolation of Water Applied to Irrigated Agricultural Field

Passive-Wick Lysimeters

Water-Balance Method

Ditch and Reservoir Seepage

Subsurface Inflow

Discharge

Well Withdrawals

Evapotranspiration

Subsurface Outflow

Steady-State Numerical Simulation of Groundwater Flow

Previous Groundwater Flow Model

Design of Previous Model

Inflows and Outflows Simulated by Previous Model

Updated Groundwater Flow Model

Design of Updated Model

Boundary Conditions and Hydrologic Stresses

Model Calibration

Observations

Prior Information

Parameter Estimation

Calibration Assessment

Sensitivity Analysis

Model Nonlinearity

Simulation Results

Model Limitations and Data Needs

Summary

Acknowledgments

References Cited

Appendix 1.

Appendix 2.

Appendix 3.

Appendix 4.

Appendix 5.


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