U.S. Geological Survey, Water Resources of Pennsylvania

U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2005-5038

Comparison of Methods for Estimating Ground-Water Recharge and Base Flow at a Small Watershed Underlain by Fractured Bedrock in the Eastern United States

By Dennis W. Risser, William J. Gburek, and Gordon J. Folmar

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ABSTRACT

This study by the U.S. Geological Survey (USGS), in cooperation with the Agricultural Research Service (ARS), U.S. Department of Agriculture, compared multiple methods for estimating ground-water recharge and base flow (as a proxy for recharge) at sites in east-central Pennsylvania underlain by fractured bedrock and representative of a humid-continental climate. This study was one of several within the USGS Ground-Water Resources Program designed to provide an improved understanding of methods for estimating recharge in the eastern United States.

Recharge was estimated on a monthly and annual basis using four methods—(1) unsaturated-zone drainage collected in gravity lysimeters, (2) daily water balance, (3) water-table fluctuations in wells, and (4) equations of Rorabaugh. Base flow was estimated by streamflow-hydrograph separation using the computer programs PART and HYSEP. Estimates of recharge and base flow were compared for an 8-year period (1994-2001) coinciding with operation of the gravity lysimeters at an experimental recharge site (Masser Recharge Site) and a longer 34-year period (1968-2001), for which climate and streamflow data were available on a 2.8-square-mile watershed (WE-38 watershed).

Estimates of mean-annual recharge at the Masser Recharge Site and WE-38 watershed for 1994-2001 ranged from 9.9 to 14.0 inches (24 to 33 percent of precipitation). Recharge, in inches, from the various methods was: unsaturated-zone drainage, 12.2; daily water balance, 12.3; Rorabaugh equations with PULSE, 10.2, or RORA, 14.0; and water-table fluctuations, 9.9. Mean-annual base flow from streamflow-hydrograph separation ranged from 9.0 to 11.6 inches (21-28 percent of precipitation). Base flow, in inches, from the various methods was: PART, 10.7; HYSEP Local Minimum, 9.0; HYSEP Sliding Interval, 11.5; and HYSEP Fixed Interval, 11.6.

Estimating recharge from multiple methods is useful, but the inherent differences of the methods must be considered when comparing results. For example, although unsaturated-zone drainage from the gravity lysimeters provided the most direct measure of potential recharge, it does not incorporate spatial variability that is contained in watershed-wide estimates of net recharge from the Rorabaugh equations or base flow from streamflow-hydrograph separation. This study showed that water-level fluctuations, in particular, should be used with caution to estimate recharge in low-storage fractured-rock aquifers because of the variability of water-level response among wells and sensitivity of recharge to small errors in estimating specific yield. To bracket the largest range of plausible recharge, results from this study indicate that recharge derived from RORA should be compared with base flow from the Local-Minimum version of HYSEP.

Table of Contents

Abstract
Introduction
     Purpose and Scope

     Description of Study Area
Methods Investigated
     Recharge
          Unsaturated-Zone Drainage
          Water-Balance Equation
          Water-Table Fluctuations in Wells
          Rorabaugh Equations
     Base Flow as a Proxy for Recharge
          Streamflow-Hydrograph Separation—PART and HYSEP Programs
Recharge and Base-Flow Estimates
     Masser Recharge Site
          Unsaturated-Zone Drainage
          Water-Balance Equation
     WE-38 Watershed
          Water-Balance Equation
          Water-Table Fluctuations in Wells
               Variability in Water-Table Fluctuations
               Determining a Representative Specific-Yield Value
          Rorabaugh Equations with RORA and PULSE
               Determining the Recession Index (K)
               The Evapotranspiration Issue
          Base Flow from Streamflow Hydrograph Separation
     East Mahantango Creek Watershed
          Rorabaugh Equations with RORA
          Base Flow from Streamflow-Hydrograph Separation
Comparison of Results
     Period of Available Lysimeter Record (1994-2001)
     Period of Available Streamflow and Climate Records (1968-2001)
     Effect of Watershed Scale
          Recharge from RORA
          Base Flow from Streamflow-Hydrograph Separation
          Monthly Estimates
Summary and Conclusions
Acknowledgments
References Cited

Figures

1. Map showing location of the study area, WE-38, Masser Recharge Site, and East Mahantango Creek watershed, Pennsylvania
2. Graph showing cumulative percolate collected from seven gravity lysimeters at the Masser Recharge Site, 1994-2001
3. Graph showing annual mean percolate and standard deviation for seven gravity lysimeters at the Masser Recharge Site, 1994-2001
4. Graph showing variability of mean-monthly percolate for seven gravity lysimeters at the Masser Recharge Site, 1994-2001
5. Maps showing (A) soil group, (B) land cover, and (C) slope categories used to define landscape units within the WE-38 watershed and resulting estimates of (D) ground-water recharge, 1968-2001, from the HELP3 water-balance model
6. Graph showing determination of water-level rise in an observation well
7. Graph showing comparison of water-level fluctuations for 1999 relative to levels on January 1, in five observation wells in the WE-38 watershed
8. Graph showing mean-annual sum of all water-level rises in observation wells in the WE-38 watershed, 1994-2001
9. Graph showing simulated water-level rise for wells in headwaters, mid-slope, and near-stream locations
10. Graph showing change in water-level fluctuations in well 45-D in the WE-38 watershed probably caused by hydraulic testing in 1992
11. Graph showing recharge estimated from the WTF method at upland wells in the WE-38 watershed, 1994-2001
12. Graph showing days recharge was simulated by RORA with measurements of precipitation, unsaturated-zone drainage, ground-water altitude, and streamflow at the WE-38 watershed and Masser Recharge Site, 1998
13. Graph showing master recession curve for 20 recession segments from streamflow records from the WE-38 streamflow-gaging station, 1968-2001
14. Graph showing mean-monthly recharge estimates from RORA and PULSE at the WE-38 watershed, 1994-2001 18
15. Boxplots showing estimates of annual recharge and base flow, 1994-2001
16. Graph showing annual base flow and recharge estimates for WE-38 watershed and lysimeter percolate at the Masser Recharge Site, 1994-2001
17. Graph showing mean-monthly base flow and recharge estimates for the WE-38 watershed and lysimeter percolate at the Masser Recharge Site, 1994-2001
18. Boxplots showing results of comparing estimates of annual recharge and base flow at the WE-38 watershed, 1968-2001
19. Streamflow hydrographs for watersheds of 2.8 square miles (WE-38) and 162 square miles (East Mahantango Creek near Dalmatia),1998
20. Graph showing mean-annual recharge and base flow estimated at three watershed scales from the streamflow records at WE-38 (2.8 mi2), Klingerstown (45 mi2), and Dalmatia (162 mi2)
21. Graph showing effect of change in interval from 3 to 5 days on base-flow separation by the HYSEP Fixed-Interval method from streamflow records at WE-38 (2.8 mi2) and East Mahantango Creek at Klingerstown (45 mi2), April–May 1996
22. Graph showing mean-monthly recharge from RORA and base flow from PART for watershed scales of (A) 2.8 and (B) 162 square miles

Tables

1. Summary of methods investigated in this study for estimating recharge and base flow
2. Estimates of mean-monthly and mean-annual recharge and base flow, in inches, at Masser Recharge Site and WE-38 watershed for 1968-2001 and 1994-2001
3. HELP3 model input parameters used to simulate recharge at Masser Recharge Site, 1994-2001
4. Properties of landscape units used in the HELP3 water-balance model of the WE-38 watershed and simulated recharge for 1968-2001
5. Estimates of mean-monthly and mean-annual recharge and base flow, in inches, for streamflow-gaging stations on East Mahantango Creek, 1968-2001
6. Mean difference, in inches, between estimates of annual recharge or base flow by all methods, 1994-2001

This report is available online in Portable Document Format (PDF). If you do not have the Adobe Acrobat PDF Reader, it is available for free download from Adobe Systems Incorporated.

View the full report in PDF 2,257 KB

For more information about USGS activities in Pennsylvania contact:
Director
USGS Pennsylvania Water Science Center
215 Limekiln Road
New Cumberland, Pennsylvania 17070
Telephone: (717) 730-6960
Fax: (717) 730-6997
or access the USGS Water Resources of Pennsylvania home page at:
http://pa.water.usgs.gov/.


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