An Overview of Stream Depletion and How It Is Determined Using Numerical Groundwater Flow Models

1. An Overview of Stream Depletion and How It Is Determined Using Numerical Groundwater Flow Models Clint Carney – NPPD/COHYST Technical Committee North Platte NRD Board Meeting April 10th, 2003 Bridgeport, NE

2. What Is Stream Depletion ? Jenkins (1968) defines stream depletion as “direct depletion from the stream or reduction of return flow to the stream” In other words…… • “Pulling” water directly from the stream into the aquifer towards a pumping well or other sink • Removing water from the aquifer that would eventually reach the surface water feature

3. Example of Typical Stream- Aquifer System

4. Jenkins Method for Determination of Stream Depletion • Analytical technique used to determine stream depletion in many early analyses • Uses many assumptions that may not hold true in the real system • Does not account for complex geology or other hydrologic features

5. Groundwater Flow Models Applied to Stream Depletion • A more technical alternative to the Jenkins Equation for determining stream depletion • Groundwater models can account for – • Complex geology and hydrologic characteristics • Variable pumping regimes • Better representation of the overall system

6. What is a Groundwater Flow Model? • Numerical representation of the real-world system using computer processors to solve groundwater flow equations • Simplification of the real system that accounts for: • Geologic strata • Hydrologic conditions • Climatic conditions • Historic conditions • Prediction capabilities

7. Example of a Groundwater Model Simulation • Transient Simulation – 40 years • 3 layer model representing typical geology found in the Platte Valley • No pumping wells simulated

8. Example of a Groundwater Model Simulation (cont.) • Same model as previously shown • Now includes 14 pumping wells screened in Layer 3 (Ogallala Group) • Pumping rate set to 500 gallons per minute for each well

9. How Does a Groundwater Model Determine Stream Depletion? CALIBRATED FLOW MODEL SIMULATE PUMPING CALCULATE DEPLETION

10. How Does a Groundwater Model Determine Stream Depletion? • Simple process of determining water balances calculated by the flow model • Over any specified time, the stream depletion caused by a well(s) can be calculated for any pumping rate • Percent of depletion determined by volume of depletion/volume of water pumped from the aquifer. • Stream depletion line determined when specified time of pumping has caused and X percentage depletion to the stream

11. Example of Stream Depletion Values in Relation to Proximity of a River • “Mock” flow model • 40-year simulation • Pump rate at 500 gallons/minute for 120 day interval • No pumping during remainder of year

12. Effects of Variation in Pumping Rates • 1 pumping well • 4 miles from river • Simulation Statistics • 40 years • Test rates/duration • 500 gal/min for 120 days • 1,000 gal/min for 60 days

13. Effects of Variation in Pumping Rates After 40 Years…. Each pumping rate produced 10,690 acre- feet of water. Reduction in simulated stream flow – 3,021 acre-feet (28% of pumped volume)

14. What About More Than One Well? Another 40 Years…. Two wells pumping 500 gal/min (~1 cfs) daily for 120 day pumping period Wells 1 mile apart Total volume pumped – 21,379 acre-feet Reduction in simulated stream flow – 6,250 acre-feet 29% of pumped volume 30% Depletion New Well 28% Depletion Old Well

15. In Conclusion…. • COHYST will produce lines delineating 28% depletion over 40 years throughout the entire study area using calibrated multi-layer groundwater flow models • Stream depletion is an affect of volume of water removed from aquifer over time • If volume of removal is constant, stream depletion is independent of pumping rate or duration of pumping period • Distance of the stream depletion lines to the river will vary depending on local hydrologic properties of the aquifer

16. QUESTIONS??