Ground-Water Resources Program

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, U.S. Geological Survey, and

William J. Gburek, and Gordon J. Folmar, U.S. Department of Agriculture

In cooperation with the U.S. Department of Agriculture, Agricultural Research Service

Scientific Investigations Report 2005-5038

U.S. Department of the Interior 

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2005

For sale by U.S. Geological Survey, Information Services 

Box 25286, Denver Federal Center 

Denver, CO 80225

For more information about the USGS and its products: 

Telephone: 1-888-ASK-USGS 

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report.

Suggested citation:

Risser, D.W., Gburek, W.J., and Folmar, G.J., 2005, Comparison of methods for estimating ground-water recharge and 

base flow at a small watershed underlain by fractured bedrock in the eastern United States: U.S. Geological Survey 

Scientific Investigations Report 2005-5038, 31 p.
 iii

Contents

Abstract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Purpose and Scope. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Description of Study Area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Methods Investigated . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Recharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Unsaturated-Zone Drainage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Water-Balance Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Water-Table Fluctuations in Wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Rorabaugh Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Base Flow as a Proxy for Recharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Streamflow-Hydrograph Separation—PART and HYSEP Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Recharge and Base-Flow Estimates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Masser Recharge Site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Unsaturated-Zone Drainage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Water-Balance Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

WE-38 Watershed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Water-Balance Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Water-Table Fluctuations in Wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Variability in Water-Table Fluctuations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Determining a Representative Specific-Yield Value. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Rorabaugh Equations with RORA and PULSE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Determining the Recession Index (K). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

The Evapotranspiration Issue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Base Flow from Streamflow Hydrograph Separation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

East Mahantango Creek Watershed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Rorabaugh Equations with RORA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Base Flow from Streamflow-Hydrograph Separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Comparison of Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Period of Available Lysimeter Record (1994-2001). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Period of Available Streamflow and Climate Records (1968-2001) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Effect of Watershed Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Recharge from RORA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Base Flow from Streamflow-Hydrograph Separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Monthly Estimates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Summary and Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

References Cited. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
iv 

Figures

1. Map showing location of the study area, WE-38, Masser Recharge Site, and East 

Mahantango Creek watershed, Pennsylvania . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2-4. Graphs showing:

2. Cumulative percolate collected from seven gravity lysimeters at the Masser Recharge 

Site, 1994-2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3. Annual mean percolate and standard deviation for seven gravity lysimeters at the 

Masser Recharge Site, 1994-2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4. Variability of mean-monthly percolate for seven gravity lysimeters at the Masser 

Recharge Site, 1994-2001. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

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 . . . . . . . . . . . . . . . . . . . . 11

6-14. Graphs showing:

6. Determination of water-level rise in an observation well. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

7. Comparison of water-level fluctuations for 1999 relative to levels on January 1, in five 

observation wells in the WE-38 watershed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

8. Mean-annual sum of all water-level rises in observation wells in the WE-38 watershed, 

1994-2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

9. Simulated water-level rise for wells in headwaters, mid-slope, and near-stream 

locations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

10. Change in water-level fluctuations in well 45-D in the WE-38 watershed probably 

caused by hydraulic testing in 1992. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

11. Recharge estimated from the WTF method at upland wells in the WE-38 watershed, 

1994-2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

12. 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

13. Master recession curve for 20 recession segments from streamflow records from 

the WE-38 streamflow-gaging station, 1968-2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

14. 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. . . . . . . . . . . . . . . . . . . . . . . . 20

16. Graph showing annual base flow and recharge estimates for WE-38 watershed and 

lysimeter percolate at the Masser Recharge Site, 1994-2001. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

18. Boxplots showing results of comparing estimates of annual recharge and base flow at the

WE-38 watershed, 1968-2001. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 

19. Streamflow hydrographs for watersheds of 2.8 square miles (WE-38) and 162 square

miles (East Mahantango Creek near Dalmatia), 1998. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

20-22. Graphs showing:

20. 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). . . . . . . . 27

21. 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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

22. Mean-monthly recharge from RORA and base flow from PART for watershed 

scales of (A) 2.8 and (B) 162 square miles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
 v

Tables

1. Summary of methods investigated in this study for estimating recharge and base flow . . . . . . . . . . . . .4

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. . . . . . . . . . . . . . . . . . . . . . .8

3. HELP3 model input parameters used to simulate recharge at Masser Recharge Site, 

1994-2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4. Properties of landscape units used in the HELP3 water-balance model of the WE-38

watershed and simulated recharge for 1968-2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5. Estimates of mean-monthly and mean-annual recharge and base flow, in inches,

for streamflow-gaging stations on East Mahantango Creek, 1968-2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

6. Mean difference, in inches, between estimates of annual recharge or base flow by all 

methods, 1994-2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
vi 

Conversion Factors and Datum

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C = (°F - 32) / 1.8

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.

*Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times 

foot of aquifer thickness [(ft3/d)/ft2

]ft. In this report, the mathematically reduced form, foot 

squared per day (ft2/d), is used for convenience.

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter

square foot (ft2) 0.09290 square meter (m2)

square inch (in2) 6.452 square centimeter (cm2)

square mile (mi2) 2.590 square kilometer (km2)

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

Pressure

bar 100 kilopascal (kPa) 

Hydraulic conductivity

foot per day (ft/d) 0.3048 meter per day (m/d)

Transmissivity*

foot squared per day (ft2/d) 0.09290 meter squared per day (m2/d)
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

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 GroundWater 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 streamflowhydrograph 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 unsaturatedzone 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. 

Introduction

Ground-water recharge is a fundamental component in the 

water balance of any watershed. However, because it is nearly 

impossible to measure directly, numerous methods, ranging 

widely in complexity and cost, have been used to estimate 

recharge (Lerner and others, 1990; Scanlon and others, 2002). 

Practicing hydrologists typically make the best estimates of 

recharge possible by the use of methods that are relatively 

straightforward in their application and require only commonly 

available hydrologic data. In the humid, eastern United States, 

where most streams are gaining and the water table is relatively 

shallow, recharge typically is estimated by an analysis of 

streamflow records, ground-water levels, or the water balance 

for a watershed. In some cases, base flow has been used as an 

approximation of recharge, with the acknowledgement that it is 

probably less than the amount recharging the ground-water system (Daniel, 1996; Holtschlag, 1997; Szilagyi and others, 

2003).

A common recommendation is that recharge should be 

estimated by the use of multiple methods and the results compared (Nimmo and others, 2003; Healy and Cooke, 2002). This 

is a prudent approach, though good-quality data usually are not 

available to make estimates from multiple methods. In east-central Pennsylvania, however, there are two hydrologic research 

sites where long-term monitoring of climate, ground water, surface water, and the unsaturated zone allows comparison of multiple methods for estimating ground-water recharge with avail-
2 Comparison of Methods for Estimating Ground-Water Recharge and Base Flow at a Small Watershed in the Eastern United States

able data. The sites are operated by the U.S. Department of 

Agriculture, Agricultural Research Service (ARS), as part of 

their Pasture Systems and Watershed Management Research 

Unit Research Watershed. Not only do these ARS sites afford 

long-term, continuous hydrologic records representative of the 

humid-continental climate of the northeastern United States, 

they include measurements of unsaturated-zone drainage from 

gravity-drainage lysimeters (a dataset rarely available) and 

streamflow data from gages in nested watersheds.

This study was conducted in cooperation with the ARS as 

part of the U.S. Geological Survey (USGS) Ground-Water 

Resources Program (Grannemann, 2001). It was one of several 

studies designed to provide an improved understanding of 

methods for estimating recharge in the humid, eastern United 

States.

Purpose and Scope

This report compares four methods for estimating groundwater recharge and four automated techniques for estimating 

base flow by hydrograph separation and discusses their application and limitations. The methods were evaluated using available data from 1968 to 2001 at the ARS Masser Recharge site, 

ARS WE-38 experimental watershed, and at two streamflowgaging stations within the East Mahantango Creek watershed in 

east-central Pennsylvania. 

Estimates of recharge and base flow were developed and 

compared for an 8-year period (1994-2001) and a 34-year 

period (1968-2001). The 8-year period was used because it corresponds to the period of record available for the gravity lysimeters at the Masser Recharge Site. The longer 34-year period of 

record was used to take advantage of the additional data from 

climatic stations, streamflow-gaging stations, and observation 

wells at the WE-38 experimental watershed. Streamflow data 

from gaging stations on East Mahantango Creek were used to 

evaluate the effects of watershed size on estimates of recharge 

and base flow.

Description of Study Area

The study area includes two hydrologic research sites 

operated by the ARS—watershed WE-38 and the Masser 

Recharge Site (fig. 1). WE-38 is a 2.8-mi2 sub-watershed of 

East Mahantango Creek that drains a rural, agricultural watershed of 162 mi2 in the unglaciated part of the Valley and Ridge 

Physiographic Province. The Masser Recharge Site is a 2-acre 

plot in an upland setting about 1 mi west of the WE-38 watershed. Ground water is present in folded and fractured shales,

siltstones, and sandstones of the Trimmers Rock Sandstone and 

Catskill Formation of Devonian and Mississippian age that are 

overlain by mostly silty loam soils. Depth to ground water 

ranges from about 80 ft below land surface beneath uplands to 

only several feet below land surface near streams. The climate 

of the study area is classified as humid continental. Average 

monthly temperature ranges from 25°F in January to 72°F in 

July. Annual precipitation averages about 42 in. and is distributed fairly evenly throughout the year. On average, annual 

potential evapotranspiration is about 26 in. (Waltman and others, 1997), so annual precipitation exceeds potential evapotranspiration by about 16 in.

The sites have been used in numerous investigations to 

characterize watershed hydrology and effects of agricultural 

activities on water quality. A good summary of site conditions 

and the ground-water system of the WE-38 watershed is contained in Urban (1977) and Gburek and others (1998). At 

WE-38, the ARS has collected meteorological and streamflow 

data since 1968 and ground-water data since 1973. WE-38 is 

nested within two larger gaged watersheds, providing the 

opportunity to study the effects of watershed scale on estimates 

of recharge and base flow. The 2.8-mi2 WE-38 watershed is 

nested within the 45-mi2 watershed of East Mahantango Creek 

upstream of the streamflow-gaging station at Klingerstown and 

the 162-mi2

 watershed upstream of the USGS streamflow-gaging station 01555500 near Dalmatia (fig. 1). The streamflowgaging station at Klingerstown has been operated continuously 

by ARS since 1968 and intermittently by USGS as station 

01555400 from 1993-95 and 1997-2000.

The Masser Recharge Site is described in detail in Gburek 

and Folmar (1999) and Stout and others (1998). At the Masser 

Recharge Site, unsaturated-zone drainage has been collected by 

the use of 28 gravity-drainage lysimeters (16 monitored continuously) since 1994. Data from seven of the 24-in. diameter 

lysimeters were used for this study. The lysimeters collect and 

monitor percolate at 3.3 ft below a grass-covered field plot.

Methods Investigated

Methods for estimating recharge and base flow in this 

study are summarized in table 1. Recharge was estimated on a 

monthly and annual basis by using four methods: (1) unsaturated-zone drainage, (2) a daily water balance, (3) water-table 

fluctuations (WTF) in wells, and (4) the equations of Rorabaugh (Daniel, 1976; Rorabaugh, 1964). Base flow was estimated from streamflow-hydrograph separation by the use of 

two computer programs—PART (Rutledge, 1993) and HYSEP 

(Sloto and Crouse, 1996). Unsaturated-zone drainage, collected 

by gravity lysimeters at the Masser Recharge Site, provides a 

direct measurement of downward water flux. The other 

recharge and base-flow methods were chosen for analysis 

because they are easy to apply and are widely used by practicing 

hydrologists in the humid eastern United States. 

The methods used in this study have inherent differences 

(summarized in table 1) that need to be considered when comparing their results. Methods in this study are used to estimate 

either recharge or base flow. The recharge methods attempt to 

quantify the water added to the water table (recharge), whereas 

base-flow methods separate part of the streamflow hydrograph 

attributed to ground-water discharge. The methods have other
Methods Investigated 3

Figure 1. Location of the study area, WE-38, Masser Recharge Site, and East Mahantango Creek watershed, Pennsylvania.
4 Comparison of Methods for Estimating Ground-Water Recharge and Base Flow at a Small Watershed in the Eastern United States

inherent differences—some provide estimates at a point location and others provide a spatially averaged value; some are 

indirect estimates and some are nearly direct measurements. In 

addition, estimates of recharge and base flow are derived by the 

use of differing data sources (streamflow, ground-water levels, 

or meteorological data); thus, any errors in those datasets are 

likely to propagate to the estimates of recharge or base flow.

Recharge

Recharge is defined for this study as any water that moves 

from land surface to the water table (Heath, 1983, p. 4). 

Although the four methods of estimating recharge in this study 

(table 1) are widely used, none directly measure the amount of 

water reaching the water table; thus, each has inherent advantages and disadvantages in its application.

Unsaturated-Zone Drainage

Recharge was estimated in this study from the unsaturatedzone drainage measured in gravity lysimeters. Gravity lysimeters are a method of estimating recharge by directly measuring 

the vertical flow of water through a large section of the unsaturated zone at a depth below most root systems (Lerner and others, 1990). Unsaturated-zone drainage from gravity lysimeters 

represents water that has not yet reached the water table, which 

has been termed “potential” recharge by Scanlon and others 

(2002). Percolate collected from the lysimeters ideally represents water that passed beneath the root zone and is assumed to 

closely represent a direct estimate of the volume of recharge 

reaching the water table, although not necessarily the timing of 

its arrival. The advantage of gravity lysimeters is that they are 

one of the few methods that provide an estimate of recharge by 

direct measurement of vertical water flux. Disadvantages of the 

lysimeters, in addition to expense and difficulty of installation, 

are that they provide only a point-estimate of recharge for a specific location and their installation disturbs the soil, which may 

affect the collection of percolate for several years (Lerner and 

others, 1990). 

Water-Balance Equation

Estimates of recharge from a daily water balance were 

computed in this study using the computer program Hydrologic 

Evaluation of Landfill Performance (HELP3) (Schroeder and 

others, 1994a). These estimates are probably best categorized as 

potential recharge because, as applied in this study, the program 

only routes water to the base of the root zone. The HELP3 

model was used to estimate recharge for the Masser Recharge 

Table 1. Summary of methods investigated in this study for estimating recharge and base flow.

Method Quantity 

estimated

Type of 

estimate

Extent of 

estimate

Computer program or

instrumentation used

Recharge estimated

for period

1994-2001 1968-2001

Unsaturated-zone drainage Recharge

(potential)

Direct Point Measurement of drainage from gravity 

lysimeters at Masser Recharge Site. 

Yes No

Daily water balance Recharge

(potential)

Indirect Point HELP3 Model at Masser Recharge Site 

using climate, land cover, and soils 

data.

Yes No

Areal HELP3 Model with GIS at WE-38 

watershed using climate, land cover, 

and soils data.

Yes Yes

Water-table fluctuation Recharge Indirect Point/Areal1 Observation wells at WE-38 watershed. Yes No

Rorabaugh equations Recharge 

(net)

Indirect Areal RORA—Computer program estimates 

recharge by recession-curve-displacement method from streamflow records.

Yes Yes

PULSE—Computer program estimates 

recharge by trial-and-error matching of 

simulated ground-water discharge to 

streamflow records. 

Yes No

Hydrograph separation for base 

flow

Base flow Indirect Areal HYSEP Program—Local-Minimum 

version

Yes Yes

HYSEP Program—Fixed-Interval 

version

Yes Yes

HYSEP Program—Sliding-Interval 

version

Yes Yes

PART Program Yes Yes

1

This method usually provides a “point” estimate of recharge, but in this study an “areal” estimate was developed from the weighted average of point 

values from 10 observation wells.
Methods Investigated 5

Site and the entire WE-38 watershed from the residual term in 

the general daily water balance:

R = P - (ET +RO +∆S ) (1)

where

R is recharge, in inches;

P is precipitation, in inches;

ET is evapotranspiration, in inches;

RO is direct runoff, in inches; 

and

∆S is change in storage, in inches.

HELP3 was developed by the U.S. Army Waterways 

Experiment Station to compute the water balance of landfills 

(Schroeder and others, 1994a). It estimates vertical recharge at 

a point in the watershed, but areal estimates can be obtained by 

summing recharge rates computed for subdivisions of the 

watershed with similar physical properties as described by 

Jyrkama and others (2002). HELP3 is a “quasi-two-dimensional” model that routes precipitation falling on the land to 

components of evapotranspiration, runoff, storage, and vertical 

infiltration (recharge) for a layered soil column on a daily basis. 

The lateral movement of water as overland and subsurface runoff is accounted for by an output from the model, but twodimensional flow is not explicitly modeled. The model algorithms are described in detail by Schroeder and others (1994b), 

and limitations are discussed by Berger (2000).

The water-balance method is attractive because it can be 

applied almost anywhere precipitation data are available. 

A major drawback of the method is that recharge is estimated as 

the residual term in an equation where the other budget terms 

usually are estimated with considerable error, which can result 

in large errors in the recharge estimate (Nimmo and others, 

2003).

The water-balance equation was applied at the Masser 

Recharge Site and the WE-38 watershed. At the Masser 

Recharge Site, it was used to estimate recharge at a single point 

location on the landscape; whereas, spatially variable estimates 

were derived for the WE-38 watershed. 

Water-Table Fluctuations in Wells

Water-table fluctuations (WTF) were used to estimate 

recharge from the water-level rise in a well multiplied by the 

specific yield of the aquifer (Rasmussen and Andreasen, 1959). 

This method actually measures the effect of recharge at the 

water table, so it should provide estimates that correspond most 

closely to our definition of recharge; however, the appropriate 

value of specific yield must be known to translate the measured 

water-level fluctuations into estimates of recharge. 

WTF in wells have been used by hydrologists for many 

years to estimate recharge (Meinzer and Stearns, 1929; Rasmussen and Andreasen, 1959; Gerhart, 1986). The WTF 

method assumes that a water-level rise is caused by recharge 

arriving at the water table and that the specific yield is constant. 

The method provides a point value of recharge computed from 

the water-level rise in a well multiplied by the specific yield of 

the aquifer as:

R = ∆h S × y (2)

where

R is recharge, in inches;

∆h is change in water-table altitude, in inches;

and

Sy is specific yield.

Although simple in concept, the WTF method has drawbacks in its application (Healy and Cooke, 2002). The method 

requires an estimate of specific yield and assumes this value is 

constant with time. Sophocleous (1985) challenged the validity 

of this assumption on a theoretical basis and Sloto (1990, p. 25) 

showed that specific yield decreased with water-table depth in 

an aquifer in southeastern Pennsylvania. The method should 

work best for wells that show a relatively rapid water-level rise 

in relation to the rate that water moves away from the water 

table. Other complications include water-level rises not associated with recharge—such as those caused by changes in atmospheric pressure, earth tides, and entrapped air.

Rorabaugh Equations

Equations described in Rorabaugh (1964) and Daniel 

(1976) were used to estimate recharge by analysis of streamflow records using two approaches—the computer programs 

RORA (Rutledge, 1993; 1998) and PULSE (Rutledge, 1997; 

2002). RORA provides estimates of ground-water recharge 

from the displacement of the streamflow-recession curve using 

an equation developed by Rorabaugh (1964). PULSE uses 

equations developed by Rorabaugh (1964) and Daniel (1976) to 

compute the ground-water discharge to a stream following an 

instantaneous pulse of recharge to the water table. Although 

ground-water recharge is not computed by the PULSE program, 

it can be obtained from the PULSE file of user-specified 

recharge that is created by adjusting recharge by trial and error 

until the PULSE program simulates a ground-water discharge 

hydrograph that is a good match to recession periods of gaged 

streamflow. Because the discharge recorded at a streamflowgaging station does not always include all recharge from the 

watershed, these estimates might appropriately be termed “net” 

recharge (Rutledge, 2000, p. 23). 

RORA and PULSE have the advantage of being able to 

estimate recharge from the Rorabaugh equations with the use of 

daily values of streamflow from any streamflow-gaging station. 

However, the PULSE program was not designed to analyze 

long periods of record, so it is generally impractical to estimate 

more than a few years of record with this method. The methods 

assume that streamflow recessions represent ground-water discharge from areal precipitation to the aquifer. Snowmelt runoff, 
6 Comparison of Methods for Estimating Ground-Water Recharge and Base Flow at a Small Watershed in the Eastern United States

streamflow regulation, and storage and release of water from 

wetlands or bank storage could be other sources that affect the 

shape of the recession curve. The methods estimate values of 

recharge for individual events on a daily basis, but Rutledge 

(2000, p. 31) recommends reporting results for RORA at no 

smaller than a seasonal (3-month) time scale.

Although RORA and PULSE use streamflow data to estimate ground-water recharge, they are not “hydrograph-separation” techniques. They are based on a one-dimensional analytical model of ground-water discharge to a fully penetrating 

stream in an idealized, homogenous aquifer with uniform 

recharge (Mau and Winter, 1996). Because of the simplifying 

assumptions inherent in the equations, Halford and Mayer 

(2000) suggest that RORA may not provide reasonable estimates of recharge for some watersheds.

Application of both RORA and PULSE requires an estimate of the slope of the streamflow-recession curve (recession 

constant K) representing periods when all streamflow is from 

ground-water discharge. The recession index is computed by 

constructing a master-recession curve from the streamflow 

record by use of the program RECESS (Rutledge, 1993).

Base Flow as a Proxy for Recharge

Base flow is that part of streamflow usually attributed to 

ground-water discharge (U.S. Geological Survey, 1989). 

Although base flow is not recharge, it is sometimes used as an 

approximation of recharge when underflow, evapotranspiration 

from riparian vegetation, and other losses of ground water from 

the watershed are thought to be minimal. When used as a proxy 

for recharge, base flow has sometimes been referred to as 

“effective recharge” (Daniel, 1996), “base recharge” (Szilagyi 

and others, 2003), or “observable recharge” (Holtschlag, 1997) 

to acknowledge that it probably represents some amount less 

than that which recharged the aquifer. 

The major assumptions in using base flow for estimating 

recharge are that base flow equals ground-water discharge, and 

that ground-water discharge is approximately equal to recharge. 

Implicit is the assumption that ground-water losses from the 

gaged watershed caused by underflow, ground-water evapotranspiration, and exports of ground water are minimal. If these 

conditions are met, base flow may provide a reasonable estimate of recharge for long time periods (1 year or more). Ultimately, though, different methods for separating base flow will 

provide different results and the user is left to determine which 

estimate (if any) is most representative of recharge.

Streamflow-Hydrograph Separation—PART and 

HYSEP Programs

Methods for separating streamflow hydrographs into components of base flow and direct runoff have been available for 

many years (Hall, 1968), and more recently, computer programs have automated the separation procedures (Pettyjohn and 

Henning, 1979; Nathan and McMahon, 1990; Rutledge, 1993; 

Arnold and others, 1995; Wahl and Wahl, 1988). Two computer 

programs for hydrograph separation—PART (Rutledge, 1993), 

and HYSEP (Sloto and Crouse, 1996)—were selected for investigation because they are automated computer programs that are 

widely used and are readily available from the USGS Internet 

software page (http://water.usgs.gov/software).

PART and HYSEP separate or “scalp” base flow from a 

streamflow hydrograph using somewhat arbitrary (though different) criteria. PART separates base flow by equating streamflow to base flow on those days after a storm meeting a requirement of antecedent-recession length greater than N and rate of 

recession less than 0.1 log cycle per day and uses linear interpolation to connect across periods that do not meet those tests. N 

is the approximate duration of surface runoff from Linsley and 

others (1982): 

N= (A)0.2, (3)

where 

N is the time after which surface runoff ceases, 

in days; 

and 

A is the watershed area, in square miles.

HYSEP uses three different versions developed by Pettyjohn 

and Henning (1979) to separate base flow—Local Minimum, 

Fixed Interval, and Sliding Interval. Each version searches the 

hydrograph for the minimum streamflow during an interval 2N* 

days. The width of the interval 2N* used for hydrograph separation in HYSEP is the nearest odd integer (between 3 and 11) 

to twice the value of N. The “*” notation is used by the authors 

of HYSEP to signify that the interval used is not exactly equal 

to twice the value of N.

Sloto and Crouse (1996) describe the three different 

HYSEP algorithms. The Local-Minimum version centers the 

interval 2N* on the day of interest. If it is the minimum streamflow within the interval, it is assigned as a local minimum and 

is connected by straight lines to adjacent local minimums. Base 

flow for days between local minimums is estimated by linear 

interpolation. The Fixed-Interval version assigns the lowest discharge to all days in the interval 2N*, starting with the first day 

of streamflow record; then the analysis is moved forward 2N* 

days, and the process is repeated. The Sliding-Interval version 

centers the interval 2N* on the day of interest. Base flow for 

that day is assigned the minimum streamflow within the interval; then the interval is moved forward 1 day, and the process is 

repeated.
Recharge and Base-Flow Estimates 7

Recharge and Base-Flow Estimates

The methods for estimating recharge and base flow (as a 

proxy for recharge) were applied at the Masser Recharge Site, 

WE-38 watershed, and East Mahantango Creek Watershed. Use 

of the methods at these sites provided insights into the benefits 

as well as possible shortcomings and limitations of each 

method.

Masser Recharge Site

Two methods were used to estimate recharge at the Masser 

Recharge Site—(1) unsaturated-zone drainage from lysimeters 

and (2) a water-balance equation (HELP3). Both methods provide a point estimate of infiltration below the root zone that is 

categorized as potential recharge for this study (table 1).

Unsaturated-Zone Drainage

Unsaturated-zone drainage was used to estimate recharge 

from direct measurements of percolate collected in zero-tension 

gravity-drainage lysimeters at the Masser Recharge Site during 

1994-2001 (table 2). The seven lysimeters at the Masser 

Recharge Site used to estimate recharge were selected because 

they had a relatively uninterrupted, continuous record of percolate. For periods of missing record at individual lysimeters, the 

monthly percolate volume was estimated from the operational 

lysimeter that correlated most closely. During three periods— 

June through August 2000, October 2000, and September 

through November 2001—none of the lysimeters were available because they were being used for other experiments or 

were not functioning. For those months, a qualitative amount of 

percolate was estimated from precipitation and ground-water 

fluctuations. The estimated percolate was 1.04 in. for JuneAugust 2000, 0.1 in. for October 2000, and 0.00 in. for September-November 2001. Because the missing record was during 

dry periods, annual estimates of recharge were not affected 

greatly.

The gravity lysimeters provide an estimate of recharge at a 

depth of 3.3 ft beneath the 3.1-ft2 surface area enclosed by each 

lysimeter. Variability of percolate collected among the seven 

lysimeters within the small (approximately 100 ft2) plot from 

1994 to 2001 is illustrated in figure 2. Although the general seasonal trends of recharge are represented similarly in all lysimeters, the volume of percolate collected by individual lysimeters 

varied. The mean-annual percolate from the seven lysimeters 

for the period 1994-2001 was 12.2 in. (table 2), although it varied by individual lysimeter from 10.8 to 13.1 in., indicating 

either the inherent spatial variability of the soils or differences 

caused by the lysimeters installation.

The variability of annual percolate among the seven lysimeters was largest during the first 3 years of operation (1994-96). 

The variability, expressed as standard deviation, ranged from 

2.4 to 3.9 in/yr from 1994 to 1996, but was only about 1 in/yr 

from 1997 to 2001 (fig. 3). The greater variability during the 

first few years of operation may be the result of the disruption 

of natural conditions caused by lysimeter installation in 1992. 

The variability does not seem directly related to the amount of 

percolate collected.

On a monthly basis, the variability in percolate among the 

seven lysimeters from 1994 to 2001 is shown in figure 4. The 

volume of percolate collected by the lysimeters varied most 

during the winter months January through March (standard 

deviation 0.26-0.45 in/month) and least during July and August 

(standard deviation 0.05-0.07 in/month). However, when the 

standard deviation is viewed relative to the magnitude of 

monthly percolate using the coefficient of variation, the lysimeter response is shown to be most variable during the summer 

months, June through August.

Water-Balance Equation

The HELP3 model was used to estimate recharge for conditions at the Masser Recharge Site for the period 1994-2001 

for which concurrent data were available from the gravitydrainage lysimeters. Mean-annual recharge for the period was 

12.3 in. (table 2). Input data used by the model in this study 

were daily precipitation, daily temperature, average seasonal 

wind speed and relative humidity, soil properties, and land 

cover. Solar radiation was synthesized by HELP3 from the 

WGEN weather-generation model of the U.S. Department of 

Agriculture (Richardson and Wright, 1984). The data used by 

the model for simulation of the Masser Recharge Site are summarized in table 3. The total depth of the soil profile corresponds to the 3.3-ft depth of the gravity lysimeters, and the soil 

properties were obtained directly or computed from properties 

measured at the Masser Recharge Site (Stout and others, 1998) 

or listed in the Northumberland County soil survey (Eckenrode, 

1985) (table 2).

WE-38 Watershed

Four methods were used to estimate recharge or base flow 

at the WE-38 watershed—(1) daily water-balance equation, (2) 

water-table fluctuations in wells, (3) Rorabaugh equations 

(RORA and PULSE), and (4) streamflow-hydrograph separation of base flow. The methods each provide an areal estimate 

of recharge or base flow for the 2.8-mi2 watershed (table 2).
Table 2. Estimates of mean-monthly and mean-annual recharge and base flow, in inches, at Masser Recharge Site and WE-38 watershed for 1968-200

Method Computer program or instrumentation Location Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. No

1968-2001 Recharge Daily water balance HELP3 Model WE-38 0.96 0.61 1.79 2.37 1.38 0.81 0.63 0.43 0.29 0.40 0.6

Rorabaugh equations RORA WE-38 1.69 1.91 2.72 1.84 1.66 .92 .33 .32 .60 .75 1.4Base flow Hydrograph separation with HYSEP Local Minimum WE-38 1.03 1.15 1.76 1.54 1.07 .60 .39 .24 .25 .43 .6

Sliding Interval WE-38 1.29 1.50 2.19 1.86 1.38 .83 .44 .30 .37 .60 .9Fixed Interval WE-38 1.30 1.45 2.17 1.90 1.39 .81 .44 .30 .38 .61 1.0

Hydrograph separation with PART PART program WE-38 1.19 1.41 2.10 1.82 1.37 .74 .43 .28 .29 .55 .8

1994-2001 Recharge Unsaturated-zone drainage Mean from 7 gravity- drainage lysimeters Masser 1.68 1.25 2.83 1.55 .69 .43 .06 .12 .63 .60 .9

Daily water balance HELP3 Model Masser .48 .72 3.03 2.07 .91 .56 .44 .52 .59 .63 1.0Rorabaugh equations PULSE program WE-38 1.24 1.39 2.00 1.47 .83 .56 .31 .20 .27 .35 .6

RORA program WE-38 2.10 1.91 3.10 1.48 .96 .87 .22 .22 .51 .47 1.0

Water-table fluctuations Weighted average from 10 wells WE-38 1.59 1.22 1.48 1.13 .62 .73 .24 .24 .47 .48 .7Base flow Hydrograph separation with HYSEP Local Minimum WE-38 1.07 1.11 2.18 1.24 .77 .45 .28 .16 .19 .27 .5

Sliding Interval WE-38 1.46 1.46 2.49 1.64 .98 .61 .32 .19 .25 .40 .7Fixed Interval WE-38 1.57 1.44 2.41 1.63 1.03 .59 .30 .19 .26 .40 .8

Hydrograph separation with PART PART program WE-38 1.18 1.42 2.39 1.65 .95 .55 .31 .19 .21 .36 .6

1Precipitation as measured at meteorological station RB-37.
Recharge and Base-Flow Estimates 9

Figure 2. Cumulative percolate collected from seven gravity lysimeters at the Masser Recharge Site, 1994-2001.

Figure 3. Annual mean percolate and standard 

deviation for seven gravity lysimeters at the 

Masser Recharge Site, 1994-2001.

Figure 4. Variability of mean-monthly percolate for seven 

gravity lysimeters at the Masser Recharge Site, 1994-2001.
10 Comparison of Methods for Estimating Ground-Water Recharge and Base Flow at a Small Watershed in the Eastern United States

Water-Balance Equation

The HELP3 model was used to estimate potential recharge 

for the entire WE-38 watershed from 1968 to 2001 in an 

approach similar to that described by Jyrkama and others 

(2002). A geographic information system (GIS) was used to 

divide the watershed into 26 landscape units on the basis of similar land cover, hydrologic soil group, and slope (fig. 5A-C). 

HELP3 provided estimates of recharge for each of the landscape units, which were weighted by their percentage of the 

WE-38 watershed, then summed to provide an estimate of average recharge for the watershed. Mean annual recharge for the 

period was 11.7 in. (table 2).

The properties used in the HELP3 model for each landscape unit are shown in table 4. Land cover was categorized as 

woods, crop, grass, or “developed” (farmlots and roadways) 

from the 1990 land-cover dataset of WE-38 compiled by ARS. 

Soils were categorized by hydrologic soil group B, C, or D 

(group C/D was lumped with D); and slopes were categorized 

as 0-8, 8-25, and 25-80 percent from the Northumberland 

County soil survey (Eckenrode, 1985). Of the possible 

36 landscape units, only 26 were present within the WE-38 

watershed. Runoff curve numbers were estimated from Natural 

Resources Conservation Service technical report TR-55 (U.S. 

Department of Agriculture, 1986, table 2). Soil properties were 

estimated from HELP3 default soil types and the county soil 

survey. Leaf area index was estimated using guidance in the 

HELP3 documentation (Schroeder and others, 1994b) and values from a worldwide survey of leaf-area index (Scurlock and 

others, 2001). Maximum depth of evapotranspiration was estimated for various crop types and hydrologic soil group from 

values given in Charles and others (1993, table 2). Daily precipitation and temperature data were used from the RB-37 meteorological station (fig. 1), and solar radiation was synthesized by 

HELP3.

There are several limitations in the application of HELP3 

at the watershed scale. Mean-annual recharge simulated by the 

HELP3 model for the WE-38 watershed during 1968-2001 

ranged from 10.7 to 13.6 in. across the 26 landscape units 

(fig. 5D and table 4), and averaged 11.7 in. for the watershed as 

a whole. These estimates are similar to estimates determined by 

other methods; however, because HELP3 does not route water 

from landscape units to a stream, it is difficult to compare model 

results directly to observations of streamflow on an event basis. 

For the period 1968-2001, the HELP3 model simulated 

3.0 in/yr of direct runoff, making the sum of simulated recharge 

plus runoff equal to 14.7 in/yr. Streamflow at the WE-38 gage 

was 20.1 in. during the same period, which suggests that evapotranspiration may be overestimated by HELP3 because the sum 

of annual recharge and direct runoff (14.7 in.) should approximately equal measured streamflow (20.1 in.) for this 34-year 

period.

Another questionable result is that the two landscape units 

having the greatest simulated ground-water recharge were those 

that represented developed areas within the WE-38 watershed. 

The implication of this result is that increased development will 

lead to increased ground-water recharge because evapotranspiration from vegetation is lessened. The large simulated 

recharge rates for some developed areas were caused by highly 

permeable soils in those areas and parameterization of the 

landscapes in HELP3 with a low leaf-area index (2), shallow 

limit of evapotranspiration (12 in.), and a runoff curve number 

(80) that might have been too small. Because it is difficult to 

know if these parameters are assigned properly and because the 

model is sensitive to these parameters (Jyrkama and others, 

2002), accuracy of the spatial distribution of recharge computed 

from HELP3 is difficult to evaluate.

Table 3. HELP3 model input parameters used to simulate recharge at Masser Recharge Site, 

1994-2001.

[vol, volume]

Model Input—General Data

Daily Precipitation and Temperature = Masser Recharge Site meteorological station

Daily Solar Radiation = Synthesized by HELP3 using temperature and precipitation data

Soil Profile Depth = 3.3 feet 

Number of Soil Layers = 4

Slope = 0%

Natural Resources Conservation Service Runoff Curve Number (CU) = 61

Leaf-Area Index = 3

Maximum Rooting Depth = 3.3 feet

Wilting Point = 0.085 vol/vol for all soil layers. Corresponds to moisture storage at suction of 15 bars

Soil Hydraulic Conductivity = 3.6 feet per day

Quarterly Relative Humidity = 69, 70, 78, 75 percent

Model Input—Data for Specific Soil-Profile Layers

Soil profile layer Layer thickness 

(feet)

Porosity

(vol/vol)

Field capacity

(vol/vol)

1 0.7 0.502 0.191

2 .5 .426 .134

3 1.3 .385 .139

4 .8 .351 .128
Figure 5. (A) Soil group, (B) land cover, and (C) slope categories used to define landscape units within the WE-38 watershed and resulting est

recharge, 1968-2001, from the HELP3 water-balance model.
Table 4. Properties of landscape units used in the HELP3 water-balance model of the WE-38 watershed and simulated recharge for 1968-2001. [vol, volume] Land- scape unit Categories used to define landscape units HELP3 model input parameters 

Land cover category Hydro- logic soil group Slope category, percent Number of soil profile layers Bottom of soil profile layer, inches below land surface Slope value used in HELP3, percent1 Natural Resources Conservation Service runoff curve number2 Leaf- area index3 Maximum depth of evapotrans- piration, in inches4 Saturated hydraulic conduc- tivity, feet per day Total porosity (vol/vol) 

ca

(v

1 Crop B 0 to 8 4 8/45/60/72 4 70 4 39.6 6.6 0.453 02 Crop B 8 to 25 4 8/45/60/72 17 70 4 39.6 6.6 .453 3 Grass B 0 to 8 4 8/45/60/72 4 59 3 50.4 6.6 .453 4 Grass B 8 to 25 4 8/45/60/72 17 59 3 50.4 6.6 .453 5 Woods B 0 to 8 4 8/45/60/72 4 55 5 72.0 6.6 .453 6 Woods B 8 to 25 4 8/45/60/72 17 55 5 72.0 6.6 .453 7 Woods B 25 to 80 4 8/45/60/72 55 55 5 72.0 6.6 .453 8 Developed B 0 to 8 4 8/45/60/72 4 80 2 12.0 6.6 .453 9 Developed B 8 to 25 4 8/45/60/72 17 80 2 12.0 6.6 .453 

10 Crop C 0 to 8 4 10/24/32/72 4 77 4 32.4 3.6 .501 11 Crop C 8 to 25 4 10/24/32/72 17 77 4 32.4 3.6 .501 12 Grass C 0 to 8 4 10/24/32/72 4 72 3 39.6 3.6 .501 13 Grass C 8 to 25 4 10/24/32/72 17 72 3 39.6 3.6 .501 14 Woods C 0 to 8 4 10/24/32/72 4 70 5 63.6 3.6 .501 15 Woods C 8 to 25 4 10/24/32/72 17 70 5 63.6 3.6 .501 16 Woods C 25 to 80 4 10/24/32/72 55 70 5 63.6 3.6 .501 17 Developed C 0 to 8 4 10/24/32/72 4 85 2 12.0 3.6 .501 18 Developed C 8 to 25 4 10/24/32/72 17 85 2 12.0 3.6 .501 19 Crop D 0 to 8 3 7/15/72 4 80 4 20.4 6.6 .471 20 Crop D 25 to 80 3 7/15/72 55 80 4 20.4 6.6 .471 21 Grass D 0 to 8 3 7/15/72 4 79 3 26.4 6.6 .471 22 Grass D 25 to 80 3 7/15/72 55 79 3 26.4 6.6 .471 23 Woods D 0 to 8 3 7/15/72 4 77 5 46.8 6.6 .471 24 Woods D 25 to 80 3 7/15/72 55 77 5 46.8 6.6 .471 25 Developed D 0 to 8 3 7/15/72 4 90 2 12.0 6.6 .471 26 Developed D 25 to 80 3 7/15/72 55 90 2 12.0 6.6 .471 

1 Slope is averaged value from county soil survey (Eckenrode, 1985). 2 Runoff curve number from U.S. Department of Agriculture (1986, table 2). 3 Leaf-area index estimated from HELP3 documentation (Schroeder and others, 1994b). 4 Maximum depth of evapotranspiration estimated from Charles and others (1993, table 2). 5 Wilting point is defined as the lowest moisture storage by soil at a suction of 15 atmospheres.
Recharge and Base-Flow Estimates 13

Water-Table Fluctuations in Wells

A nearly continuous record of ground-water levels is available since 1973 from 13 shallow wells within the WE-38 watershed (fig. 1). The WTF method was used to estimate meanmonthly and annual recharge for the watershed by analyzing 

hydrographs from 10 wells in upland settings (near-stream 

wells 01-I, 61-D, and 85-I were omitted) for 1994-2001. Mean 

annual recharge for the period was 9.9 in. (table 2). The watertable rise was computed graphically as the difference between 

the peak water level during a recharge event and the predicted 

level to which water levels would have declined if the recharge 

event had not occurred as illustrated in figure 6. For wells having incomplete water-level record, the monthly water-level rise 

was estimated from the well in which water levels correlated 

most closely. The average specific yield of the watershed was 

estimated from the watershed-wide water-table decline measured during periods of streamflow recession.

Variability in Water-Table Fluctuations

The response of water levels in observation wells varies 

within the WE-38 watershed as illustrated for several of the 

observation wells in figure 7. For comparison purposes, waterlevel data for each observation well shown in figure 7 were 

adjusted to zero on January 1, 1999, so the hydrographs show 

the water-level fluctuations relative to that date. In general, 

wells in upland settings have the largest water-level fluctuations.

The mean-annual sum of all water-table rises determined 

by the procedure shown in figure 6 during 1994-2001 ranged 

from as little as 8.2 ft at well 61-D to 368 ft at well 91-D (fig. 8). 

The mean-annual sum of all water-table rises for a well was 

determined by adding the water-table rise for each individual 

recharge event during 1994-2001, then dividing by the 8 years 

of record. For example, for well 43-D, the sum of all water-table 

rises during 1994-2001 was 1,201 ft, so the annual water-table 

rise averaged 150 ft (1,201 ft / 8 yr) as shown in figure 8. Therefore, if recharge is estimated by multiplying the water-level rise 

times the specific yield at each well, rates across the watershed 

would vary greatly (by a factor of about 45). Although recharge 

can vary spatially, the variability in water-level rise exhibited 

by these wells is mostly the result of location of the well within 

the watershed with respect to streams and the degree to which 

the well is connected to the aquifer through fractures intercepted by the well.

To illustrate the effect of location relative to a stream 

boundary on the water-table rise caused by a recharge event, a 

cross-sectional MODFLOW (McDonald and Harbaugh, 1998) 

model was constructed having a length of 1,000 ft, transmissivity of 1,000 ft2/d, specific yield of 0.01, and recharge rate of 

1 ft/yr. Although the model is general in nature, its properties 

were chosen to be representative of the WE-38 watershed (Gburek and others, 1998; table 4). Recharge of 0.1 ft was added to 

the model (in addition to the 1 ft/yr steady rate) for a period of 

1 day and the resulting water-table rise was plotted for

Figure 6. Determination of water-level rise in an 

observation well.

Figure 7. Comparison of water-level fluctuations for 1999 

relative to levels on January 1, in five observation wells in 

the WE-38 watershed.

Figure 8. Mean-annual sum of all water-level rises in 

observation wells in the WE-38 watershed, 1994-2001.
14 Comparison of Methods for Estimating Ground-Water Recharge and Base Flow at a Small Watershed in the Eastern United States

headwaters, mid-slope, and near -stream well locations (fig. 9). 

A water-table rise of 10 ft is predicted by the WTF method for 

a recharge event of 0.1ft in an aquifer with specific yield of 0.01 

(eqn. 2). As expected, a water-level rise of about 10 ft was simulated for the upland well location, but water levels at the midslope and valley locations rose less—only 8 and 1.8 ft, respectively. This result is caused by the movement of water away 

from the water table during the 1-day period of recharge, which 

is most rapid near the stream boundary. Such conditions are 

most pronounced for aquifers with high hydraulic diffusivity 

(transmissivity/storage coefficient) and high stream density 

(short distance from streams to divides), which are characteristics of many fractured-rock aquifers in the Valley and Ridge 

Physiographic Province. Thus, if all other factors are equal, 

wells in upland settings will be the best candidates for use in 

estimating ground-water recharge by the WTF method.

Measurement of the water-table rise in a fractured-bedrock 

aquifer is further complicated by the well/aquifer hydraulic 

connection. The degree to which an observation well is connected to conditions at the water table depends on the hydraulic 

connections provided by fractures that intercept the well. The 

hydrographs from wells in the WE-38 watershed exemplify 

some of this complexity. For example, wells 36-D and 59-D are 

only 158 ft apart, yet figure 7 shows that the water-level fluctuations measured in 59-D are significantly less than in 36-D. 

This difference is not because of well location relative to 

streams but likely is the result of differing hydraulic properties 

of fractures connecting each well to the bedrock aquifer. 

The importance of the well/aquifer hydraulic connection is 

further illustrated by changes in the water-level hydrograph of 

observation well 45-D following hydraulic testing. In 1992, the 

ARS conducted hydraulic testing of most of their observation 

wells by isolating depth intervals with packers and injecting 

water. Subsequent to the testing, the general water-level altitude 

and magnitude of fluctuations changed in many of the wells. 

The hydrograph of well 45-D is an example of the most extreme

change probably caused by the testing (fig. 10). Most likely, the 

hydraulic testing acted as a well-development mechanism, 

causing the well-aquifer connection to improve. If the entire 

period of record for this well were used to estimate recharge by 

the WTF method, it is not clear how to deal with the change in 

magnitude of water-level fluctuations beginning in 1992.

Determining a Representative Specific-Yield Value

Specific yield was computed by dividing the average 

water-table decline in the WE-38 watershed by the streamflow 

during recession periods when ground-water discharge was the 

only source of streamflow. The average water-table decline was 

estimated from a weighted average of water-level declines measured in 13 observation wells in the WE-38 watershed; streamflow volume was measured at the WE-38 streamflow-gaging 

station at the outlet of the watershed. Specific yield was computed as:

Sy = S / ∆h (4)

where:

Sy is specific yield;

S is streamflow volume during a recession 

period consisting of only ground-water discharge, 

in inches over the watershed area; and

∆h is the average decline in water-table altitude 

during the recession period, in inches.

This approach, described by Olmsted and Hely (1962, 

p. A-16), has the potential to underestimate specific yield 

because complete drainage of the geologic material is probably 

not attained during most recession periods. However, this estimate of specific yield may be appropriate for the purpose of 

estimating recharge with the WTF method because it is just as 

unlikely that the geologic materials are completely drained 

immediately prior to periods of water-table rise.

Figure 9. Simulated water-level rise for wells in headwaters, 

mid-slope, and near-stream locations.

Figure 10. Change in water-level fluctuations in well 45-D in the 

WE-38 watershed probably caused by hydraulic testing in 1992.
Recharge and Base-Flow Estimates 15

Specific yield was computed by the use of equation 4 for 

11 periods of streamflow recession from 1993 to 2001 during 

the months of October through May when evapotranspiration 

from ground water was expected to be minimal. The average 

specific yield for the watershed using this method was 0.013, 

which is nearly the same as the value of about 0.01 determined 

by Gburek and Folmar (1999) from water-table rises and lysimeter percolate at the Masser Recharge Site. Recharge determined from equation 2, using a specific yield of 0.013 and the 

water-level rises for upland (mid-slope and headwaters) wells 

shown in figure 8 indicate recharge ranged from 3.4 to 57 in. 

using this approach.

An alternate approach for use of the WTF method also was 

tested. Instead of using a uniform value of specific yield of 

0.013 for the entire WE-38 watershed, the apparent specific 

yield for each well was used in equation 2. The apparent specific yield was determined by applying equation 4 for the waterlevel decline at each well instead of for the watershed average. 

Three wells (61-D, O1-I, and 85-I) were not used because of 

their proximity to streams. Apparent specific yields for the 

remaining 10 individual upland wells (mid-slope or headwater 

settings) ranged from 0.0035 to 0.035. The apparent specific 

yields for each of the 10 upland wells was multiplied by the 

water-level rise on a monthly basis to compute monthly and 

annual recharge for the WE-38 watershed during 1994-2001 

(table 2). Estimates of mean-annual recharge computed from 

the individual upland wells ranged from 7.6 to 15.4 in. (fig. 11). 

Rorabaugh Equations with RORA and PULSE 

Monthly and annual recharge were estimated with the 

RORA and PULSE programs using streamflow data from the 

WE-38 streamflow-gaging station. Mean-annual recharge for 

1994-2001 was 14.0 in. from the RORA program and 10.2 in. 

from the PULSE program (table 2). Monthly and annual 

recharge also were estimated by the RORA program for the 

longer period 1968-2001 by the use of streamflow data from the 

WE-38 streamflow-gaging station. Mean-annual recharge for 

the period was 15.8 in. The PULSE program was not designed 

to analyze long periods of record, so it was impractical to estimate 34 years of record with that method.

The basic premise of the Rorabaugh equations is that 

recharge events occur concurrently with peaks in streamflow 

(Rutledge, 1998, p. 3). To verify that this assumption was reasonable, the coincidence of precipitation, unsaturated-zone 

drainage, ground-water rise, and streamflow peaks in 1998 at 

the WE-38 watershed and the Masser Recharge Site were plotted (fig. 12). All the major recharge events, as documented by 

lysimeter percolate, are represented by a corresponding 

increase in ground-water level or streamflow. The general minimal response to precipitation at the lysimeters, wells, and 

streamflow-gaging station during August-December consistently indicates a lack of ground-water recharge during those 

months. Such correspondence indicates an ideal situation for 

application of the RORA program, which is confirmed by 

examining the days on which RORA simulated recharge of 

greater than 0.1 in. (shown as triangles on the plot of streamflow 

in figure 12). The timing of the simulated recharge by RORA 

corresponds well to all the major recharge events as documented by the lysimeters, wells, and streamflow-gaging station.

Determining the Recession Index (K)

The recession index (K) was determined from streamflow 

records at the WE-38 gaging station from 1968 to 2001 by use 

of the RECESS program (Rutledge, 1993). Twenty recession 

segments were selected during the months of September 

through May to exclude periods of significant evapotranspiration from ground water. The recession index for the 20 individual segments ranged from 15.9 to 53.2 days. The median value 

of 26.9 days was used for application of the Rorabaugh equations. The master-recession curve computed from RECESS is 

shown in figure 13, which compares closely to the master recession curve constructed for a wider range of discharge by Gburek 

and others (1998, fig. 5). The curve shows some non-linearity, 

which deviates from the assumptions of the Rorabaugh equation.

The sensitivity of the computed value of recharge to the 

recession index was tested by applying the RORA program 

using the minimum, median, and maximum recession indices 

from the RECESS program. Mean-annual recharge computed 

for 1968-2001 was 15.8 in. when the median recession index of 

26.9 days was used. Estimates of mean-annual recharge varied 

from 14.2 to 16.1 in. for recession indices of 53.2 and 15.9 days, 

respectively, which indicates that the results are not very sensitive to the value of K, given the extreme values used for this 

test.

If the aquifer properties within the watershed can be determined, the recession index can be computed directly and compared to the value from analysis of the master-recession curve. 

Ideally, the result computed from aquifer properties should 

compare closely to that from the master-recession curve. The 

equation for the recession index derived from Rorabaugh and 

Simons (1966, p. 12) is:

Figure 11. Recharge estimated from the WTF 

method at upland wells in the WE-38 

watershed, 1994-2001.
16 Comparison of Methods for Estimating Ground-Water Recharge and Base Flow at a Small Watershed in the Eastern United States

Figure 12. 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.
Recharge and Base-Flow Estimates 17

K = 0.93 (a2) S/T (5)

where:

K is the recession index, in days;

a is average distance from stream to the hydrologic 

divide, in feet;

S is the average storage coefficient; and

T is average transmissivity, in feet squared per day. 

For the WE-38 watershed, the average distance “a” from 

streams to ground-water divides was estimated to be about 

1,000 ft as computed from 1/(2 * drainage density). The average storage coefficient (specific yield) of about 0.01 was determined by Gburek and Folmar (1999) and this study. Transmissivity of 470 ft2/day was used, which is the sum of transmissivity of all layers except the overburden from the calibrated 

ground-water flow model of Gburek and others (1998). Using 

these values, the computed recession index is about 20 days. 

Given the great uncertainty in values of aquifer properties, this 

result compares reasonably well to the median recession index 

of 26.9 days from the master-recession curve.

To further evaluate the recession index, master-recession 

curves were constructed for the 13 observation wells using 

water-level data from 1993 to 2001. Data prior to 1993 were not 

used because the well response might have been affected by 

aquifer-isolation (packer) tests conducted in 1992. Rorabaugh 

(1960) showed that the recession slopes of ground-water hydrographs should have the same recession index as the streamflow 

master-recession curve if water levels are referenced to altitude 

above stream level. Unfortunately, it is not usually apparent 

how to determine the appropriate stream altitude to use as a base 

reference. In the WE-38 watershed, water levels were referenced to the nearest stream intercepted along a hypothetical 

flowpath between the well and stream. Examination of the master-recession curves for the wells shows that the slopes are 

much less than indicated by the recession index of 26.9 days 

from the streamflow data. Recession indices from the wells 

ranged from about 50 to greater than 1,000 days. Although there 

is considerable ambiguity about the proper stream altitude that 

should be used as a reference for each well, reasonable stream 

altitudes could not be found that allowed the ground-water master-recession curves to have recession indices as small as 

26.9 days. The very large recession indices suggest that the 

complexity of the layered, fractured-bedrock hydrogeologic 

framework of the WE-38 watershed described in Gburek and 

others (1998) and Burton and others (2002) is significantly different than the simple strip aquifer assumed for the Rorabaugh 

equations.

The Evapotranspiration Issue

Rutledge (1993, p. 40) noted that RORA gives estimates of 

recharge that are greater than the estimates of base flow from 

the hydrograph separation by PART. Rutledge suggested the 

difference might be the result of ground-water evapotranspiration, which would lower the quantity of base flow estimated 

from PART but might not affect recharge estimates from 

RORA. Subsequently, Rutledge (2000, p. 23) has indicated that 

Figure 13. Master recession curve for 20 recession segments from streamflow records from the WE-38 

streamflow-gaging station, 1968-2001. 
18 Comparison of Methods for Estimating Ground-Water Recharge and Base Flow at a Small Watershed in the Eastern United States

estimates of recharge from RORA also are probably affected by 

evapotranspiration from ground water, so the reason for the 

higher estimates from RORA are not clear.

A comparison was made of mean-monthly recharge estimates from the WE-38 watershed for 1994-2001 by the use of 

RORA and PULSE (fig. 14). Recharge is estimated from 

PULSE by taking the user-supplied estimates of ground-water 

recharge that produced a simulated ground-water discharge 

hydrograph from PULSE fitting the recession segments of the 

streamflow hydrograph at the WE-38 streamflow-gaging station. Because the procedure involves fitting simulated discharge to the streamflow hydrograph, which is affected to some 

extent by ground-water evapotranspiration in summer months, 

monthly estimates of recharge from PULSE were expected to 

be less than estimates from RORA during summer months (if 

estimates from RORA are only minimally affected by groundwater evapotranspiration). However, figure 14 shows that 

RORA produces greater estimates of mean-monthly recharge 

for all months, with the greatest differences during winter 

months when ground-water evapotranspiration is small. Thus, it 

is unlikely that the higher estimates from RORA (compared to 

base flow determined from PART) can be attributed to evapotranspiration from ground water in the WE-38 watershed.

Base Flow from Streamflow Hydrograph Separation

Base flow was estimated from daily values of streamflow 

recorded at the outlet of the 2.8-mi2 WE-38 watershed. Values 

of monthly and annual base flow were estimated by streamflowhydrograph separation using the PART and HYSEP programs 

(table 2). Missing streamflow record at the WE-38 streamflowgaging station was estimated from the complete record near 

Dalmatia based on the drainage area upstream of each gage. 

Application of PART and HYSEP was straightforward, requiring no user input other than the drainage area. 

Mean-annual base flow at the WE-38 streamflow-gaging 

station during 1994-2001 was 10.7 in. from the PART program. 

Estimates from the HYSEP program were 9.0 in. (Local-Minimum version), 11.5 in. (Sliding-Interval version), and 11.6 in. 

(Fixed-Interval version). Mean-annual base flow for the longer 

period 1968-2001 was 13-15 percent greater than for 1994-

2001.

Because base flow does not account for losses of recharge 

caused by evapotranspiration of ground water, it might be reasonable to add an estimate of evapotranspiration from riparian 

vegetation to base flow as an approximation of ground-water 

recharge. Assuming that riparian vegetation extracted ground 

water from 50 to 100 ft on each side of streams within the 

WE-38 watershed at the rate of potential evapotranspiration, the 

loss would be on the order of 1.3 to 2.5 in/yr. Adding this to the 

base-flow estimates for the WE-38 watershed during 1994-

2001 gives a range for recharge from 10.3 (HYSEP Local-Minimum version) to 14.1 in/yr (HYSEP Fixed-Interval version). 

For the period 1968-2001, the range would be 11.5 to 15.6 in.

East Mahantango Creek Watershed

Recharge and base flow were estimated from daily values 

of streamflow during 1968-2001 at streamflow-gaging stations 

on East Mahantango Creek at Klingerstown and near Dalmatia 

to compare results from watersheds of different size. Recharge 

was estimated from the Rorabaugh equations using the RORA 

program, and base flow was estimated by streamflowhydrograph separation using the PART and HYSEP programs. 

Missing streamflow record at the Klingerstown streamflowgaging station was estimated from the complete record near 

Dalmatia based on the drainage area upstream of each gage. The 

methods provide estimates of net recharge or base flow averaged over the watershed area upstream of each streamflow-gaging station—45 mi2 for the Klingerstown station and 162 mi2

for the Dalmatia station (table 5). 

Rorabaugh Equations with RORA 

Values of monthly and annual recharge during 1968-2001 

estimated from the RORA program are summarized in table 5. 

Median recession indices (K) of 31.4 and 46.6 days were determined by use of the RECESS program for streamflow-gaging 

stations at Klingerstown and near Dalmatia, respectively. Estimates of mean-annual recharge determined by RORA were 

15.8 in. from the streamflow record at Klingerstown and 

15.6 in. from the streamflow record near Dalmatia.

Figure 14. Mean-monthly recharge estimates from RORA and 

PULSE at the WE-38 watershed, 1994-2001.
Table 5. Estimates of mean-monthly and mean-annual recharge and base flow, in inches, for streamflow-gaging stations on East Mahantango Creek, 

Method Computer program Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. 

East Mahantango Creek at Klingerstown

Recharge, in inches Rorabaugh EquationsBase flow, in inches 

RORA 2.10 1.97 2.55 1.65 1.45 0.64 0.28 0.19 0.60 0.87 1

Hydrograph Separation with HYSEPHydrograph Separation with PART

Local MinimumSliding IntervalFixed IntervalPART 

1.601.721.69

1.75

1.411.611.59

1.68

1.631.821.83

1.97

1.381.561.54

1.71

.91

1.051.03

1.17

.45.58.58

.60

.28.31.31

.35

.15.19.19

.19

.20.25.25

.25

.38.52.54

.57

East Mahantango Creek near Dalmatia

Recharge, in inches Rorabaugh EquationsBase flow, in inches 

RORA 1.63 2.00 2.91 1.59 1.41 .69 .35 .28 .70 .81 1

Hydrograph Separation with HYSEPHydrograph Separation with PART

Local MinimumSliding IntervalFixed IntervalPART 

1.101.211.18

1.25

1.121.341.34

1.40

1.762.002.03

2.10

1.661.821.81

1.94

1.161.281.27

1.36

.66.76.78

.79

.45.47.48

.51

.32.33.33

.34

.29.34.34

.33

.43.55.55

.59
20 Comparison of Methods for Estimating Ground-Water Recharge and Base Flow at a Small Watershed in the Eastern United States

Base Flow from Streamflow-Hydrograph Separation

Values of monthly and annual base flow were estimated by 

streamflow-hydrograph separation using the PART and 

HYSEP programs (table 5). Mean-annual base flow for East 

Mahantango Creek at Klingerstown during 1968-2001 was 

12.7 in. from the PART program. Estimates from the HYSEP 

program were 10.3 in. (Local Minimum version), 11.9 in. (Sliding Interval version), and 11.8 in. (Fixed Interval version). 

Mean-annual base flow for East Mahantango Creek near Dalmatia during 1968-2001 was 12.9 in. from the PART program. 

Estimates from the HYSEP program were 10.8 in. (Local Minimum version), 12.3 in. (Sliding Interval version), and 12.2 in. 

(Fixed Interval version). 

Comparison of Results 

Estimates of recharge and base flow were compared for an 

8-year period (1994-2001) and for a 34-year period (1968-

2001) (table 2). The short, 8-year period was used because it 

corresponds to the period of record available for the gravity 

lysimeters at the Masser Recharge Site. The longer 34-year 

period was used to take advantage of the additional data from 

climatic stations and streamflow-gaging stations at the WE-38 

experimental watershed.

Climatic conditions during the 8-year period were either 

about the same or drier than the 34-year period, depending on 

the criteria used for comparison. Precipitation at meteorological 

station RB-37 was only slightly greater during 1994-2001 

(average of 42.1 in.) than during 1968 to 2001 (average of 

41.8 in.); however, streamflow during 1994-2001 was about 

11 percent lower at the WE-38 streamflow-gaging station and 

7 percent lower at the streamflow-gaging station on East Mahantango Creek near Dalmatia than during 1968-2001. It follows 

that recharge and base-flow estimates determined from streamflow data might be lower during 1994-2001 than 1968-2001, 

but methods based on precipitation (daily water-balance) might 

be similar or slightly greater for 1994-2001. Comparison of 

results in table 2 shows that this was the case. Estimates of 

mean-annual recharge based on streamflow data for the WE-38 

watershed (Rorabaugh equations and base-flow methods) were 

11 to 13 percent less during 1994-2001 than 1968-2001, 

whereas recharge from the daily water balance increased by 

about 5 percent during 1968-2001.

Period of Available Lysimeter Record (1994-2001)

Indirect estimates of recharge and base flow were compared to the direct measurement of potential recharge from the 

lysimeters from 1994 to 2001. Estimates of mean-annual 

recharge for the 8-year period at the Masser Recharge Site and 

the WE-38 watershed ranged from 9.9 to 14.0 in. (24-33 percent 

of precipitation), and estimates of mean-annual base flow 

ranged from 9.0 to 11.6 in. (21-28 percent of precipitation) 

(fig. 15 and table 2). Mean-annual recharge was greatest from 

Figure 15. Estimates of 

annual recharge and 

base flow, 1994-2001.
Comparison of Results 21

the RORA program and least from the WTF method (fig. 15).

The variability of annual recharge also was greatest for the 

RORA program and least for the WTF method.

The mean differences in annual recharge or base flow 

between any two methods during 1994-2001 are shown in 

table 6. Methods that compare closely have small absolute values of mean difference. The mean difference compares the differences of the annual estimates of recharge or base flow as:

_ n

D = 1 ⁄ n ∑ ( ) r1 – r2 i

i = 1

(6)

where: _

 D is the mean difference, in inches;

n is number of years of record;

r1 is the annual recharge estimate from method 1 

for year i, in inches; and

r2 is the annual recharge estimate from method 2 

for year i, in inches.

On the basis of mean difference, the water-balance method 

and unsaturated-zone drainage from lysimeters compared most 

closely (mean difference of 0.07 in.). This result was 

unexpected because of the inherent error in water-balance 

models and because simulations were made without any 

attempt to calibrate the model to the lysimeter measurements

Table 6. Mean difference, in inches, between estimates of annual recharge or base flow by all methods, 1994-2001.

[Gray shading indicates that mean recharge or base flow for the two groups is significantly different on the basis of a paired-t-test at the 95-percent confidence

level. Negative values indicate that the value from the method along the top column is greater than for the method from the corresponding row.]

METHOD

Mean difference, in inches, between estimates of annual recharge or base flow

BASE-FLOW METHODS RECHARGE METHODS

HYSEP 

Sliding 

Interval

HYSEP 

Fixed 

Interval

HYSEP 

Local 

Minimum

PART

Rorabaugh 

equations 

(PULSE)

Rorabaugh 

equations 

(RORA)

Unsaturatedzone drainage 

(mean of

7 lysimeters)

Water-table 

fluctuations 

(weighted 

average 

from 10 

upland 

wells)

Water 

balance 

(HELP3 

model)

HYSEP Sliding 0

Interval

HYSEP Fixed .12 0

BASE-FLOW METHODS

Interval

HYSEP Local -2.5 -2.6 0

Minimum

PART -.78 -.91 1.7 0

Rorabaugh -.92 -1.1 1.5 -.14 0

equations 

(PULSE)

Rorabaugh 2.6 2.5 5.0 3.4 3.5 0

equations 

(RORA)

Unsaturated- .76 .63 3.2 1.5 1.7 -1.8 0

zone drainage 

RECHARGE METHODS

(mean of 7 lysimeters)

Water-table -1.8 -1.9 .66 -1.0 -.87 -4.4 -2.6 0

fluctuations 

(weighted 

average from

10 upland wells)

Water balance .83 .71 3.3 1.6 1.8 -1.7 .07 2.6 0

(HELP3 model)
22 Comparison of Methods for Estimating Ground-Water Recharge and Base Flow at a Small Watershed in the Eastern United States

by adjusting HELP3 model-input parameters. The Fixed-Interval and Sliding-Interval versions of HYSEP showed the closest 

overall similarity of annual base flow with a mean difference of 

0.12 in. The poorest overall correspondence on an annual basis 

was between estimates by the RORA program and the LocalMinimum version of HYSEP, with a mean difference of 5.0 in.

Differences between methods also were tested statistically 

by comparing annual estimates of recharge or base flow from 

1994-2001 using a paired-t-test (Helsel and Hirsch, 1992, 

p. 147). Prior to conducting the paired-t-test, a KolmogorovSmirnov Test indicated the assumption that the paired differences were normally distributed could not be rejected for any 

two methods. The statistical tests showed that mean recharge or 

base flow was significantly different at the 95-percent confidence level for 16 of the 36 possible pairs of methods (table 6).

All base-flow methods differed significantly from each other 

except for two of the HYSEP versions (Fixed Interval and Sliding Interval). The Local-Minimum version of the HYSEP 

method was the most different of the base-flow methods. It was 

significantly different than six other methods, with mean differences that were generally greater than for other base-flow methods (table 6). Of the recharge methods, results from RORA differed significantly from the most (six) other methods, and 

unsaturated-zone drainage from the gravity lysimeters differed 

from the fewest (zero) other methods. 

Annual variations in base flow and recharge from 1994 to 

2001 are shown for the different methods in figure 16. The 

mean-annual unsaturated-zone drainage from seven lysimeters 

at the Masser Recharge Site is shown in gray to allow it to be 

readily compared to other methods. Estimates of annual base 

flow ranged from 4.3 to 19.9 in.; annual recharge estimates 

ranged from 5.2 to 24.6 in. All methods gave small estimates for 

dry years with below normal precipitation (1995, 1997, 1999, 

and 2001) and larger estimates for wet years.

Seasonal variations in mean-monthly base flow and 

recharge from 1994-2001 are shown for different methods in 

figure 17. Estimates of mean-monthly base flow ranged from 

0.16 to 2.49 in., and estimates of mean-monthly recharge 

ranged from 0.06 in. (from lysimeters) to 3.10 in. All methods 

show the same general seasonal pattern in recharge or base 

flow—lowest in the summer and early autumn and greatest during winter and early spring. Because the seasonal distribution of 

precipitation is fairly even, seasonal variations in recharge are 

caused mainly by variations in the consumptive use of water 

through evapotranspiration.

Seasonal patterns in base flow are shown and estimates are 

compared among themselves and to the mean-monthly unsaturated-zone drainage from seven lysimeters in figure 17A. Meanmonthly base flow estimated from the Local-Minimum version 

of HYSEP was less than base flow from the other methods for 

all months. Base flow from the HYSEP Fixed Interval version, 

HYSEP Sliding Interval version, and PART compared closely 

during all months except January. Base-flow estimates were 

generally slightly less than values of unsaturated-zone drainage 

from September through March and greater during April 

through August. This probably reflects the slight lag in timing 

between recharge and ground-water discharge. Infiltration 

recorded at the lysimeters from September through January 

takes some time to reach the water table. Some of this water 

goes to satisfy deficiencies of soil moisture; thus, base flow is 

less than infiltration during these months. Base flow from April 

through August exceeds infiltration measured at the lysimeters 

because the base flow is contributed partly from ground-water 

recharge in previous months.

Seasonal trends for four different recharge methods are 

compared among themselves and to the mean-monthly unsaturated-zone drainage from seven lysimeters in figure 17B. Meanmonthly recharge followed the same seasonal trends as base 

flow; however, there was considerably greater variability 

among estimates of recharge. The daily water balance provided 

estimates of annual recharge corresponding closely to the unsaturated-zone drainage from lysimeters (fig. 16B), but on a 

monthly basis, correspondence to lysimeter measurements was 

not as good. Monthly recharge estimates differed from 

lysimeter results most greatly during January and February, 

probably because of difficulties in simulating the effects of 

frozen ground and snowpack. HELP3 (water-balance method) 

simulated 12.3 in. of annual recharge and 3 in. of direct annual 

runoff for 1994-2001. Nearly all the direct runoff was 

simulated during winter months January–March. However, 

direct runoff has not been observed at the site, so the waterbalance model may be overestimating runoff (and 

underestimating recharge) during winter months.

Recharge estimates from RORA followed the seasonal 

pattern of lysimeter percolate closely, but estimated more 

recharge than other methods during January and February. 

Recharge from water-table fluctuations showed the least seasonal variability and tended to estimate less recharge than the 

lysimeters. Variability among the recharge methods was greatest during months of greatest recharge (December through 

April).
Comparison of Results 23

Figure 16. Annual (A) base flow and (B) recharge estimates for WE-38 watershed and lysimeter percolate 

at the Masser Recharge Site, 1994-2001. 
24 Comparison of Methods for Estimating Ground-Water Recharge and Base Flow at a Small Watershed in the Eastern United States

Figure 17. Mean-monthly (A) base flow and (B) recharge estimates for the WE-38 watershed and lysimeter 

percolate at the Masser Recharge Site, 1994-2001.
Comparison of Results 25

Period of Available Streamflow and Climate Records 

(1968-2001)

Recharge and base-flow estimates during 1968-2001 for 

the WE-38 watershed are compared in table 2 and figure 18. 

Estimates of mean-annual recharge or base flow ranged from 

about 10.2 in. by the Local-Minimum version of the HYSEP 

method to 15.8 in. by RORA (fig. 18 and table 2). Differences 

in mean-annual recharge or base flow were shown to be statistically significant between any two methods, except the HYSEP 

Fixed-Interval/Sliding-Interval pair and the PART/Water-Balance pair, according to results from a paired-t-test at the 95-percent confidence level. Although fewer methods were compared 

for the 1968-2001 period than for the 1994-2001 period, the 

results were consistent with the 1994-2001 period, except for 

the water-balance method, which was shown to differ significantly from RORA and the Sliding-Interval and Fixed-Interval 

versions of HYSEP during 1968-2001, but not during 1994-

2001. This probably is because the water-balance point estimates of recharge for the Masser Recharge Site were used for 

the 1994-2001 comparisons and the areal-weighted estimates 

for the WE-38 watershed were used for 1968-2001. The similar 

results for the longer period of available data (1968-2001) indicate that valid conclusions probably can be drawn for methods 

that only were available for the shorter time period (1994-

2001).

Effect of Watershed Scale

Estimates of recharge and base flow in this report have 

been compared at two small sites. Estimates from the Masser 

Recharge Site are for a single upland location, and estimates 

from the small WE-38 watershed represent an average for a 

2.8-mi2 drainage area. Because recharge can vary spatially, estimates of recharge derived from watersheds of different size 

may not agree. Similarly, the conditions required for base flow 

to be a good surrogate for recharge (for example, negligible 

underflow and evapotranspiration from ground water) can be 

affected by the size of the watershed upstream of the streamflow-gaging station where base flow is determined. Thus, 

results from RORA and methods of hydrograph-separation for 

base flow were compared for nested watersheds at three scales 

from the streamflow record at WE-38 (2.8 mi2), East Mahantango Creek at Klingerstown (45 mi2), and East Mahantango 

Creek near Dalmatia (162 mi2).

Recharge from RORA

The premise of the Rorabaugh equations, that recharge 

events occur concurrently with peaks in streamflow, might not 

be correct for watersheds larger than the 2.8-mi2 WE-38 watershed. To test the effect of watershed scale on estimates of 

recharge from the recession-curve displacement approach of 

RORA, the program was applied to streamflow record collected 

from three watershed scales during 1968 to 2001. Comparison 

of hydrographs from streamflow-gaging stations for 1998 at 

WE-38 and East Mahantango Creek near Dalmatia (smallest 

and largest watersheds) indicates a close correspondence in timing and magnitude of streamflow peaks (fig. 19), which support 

the application of RORA across this range in watershed scales. 

Results from RORA indicate that this change in watershed 

scale did not appear to significantly affect the estimate of mean 

annual recharge (fig. 20). Recharge rates computed from the 

three streamflow records ranged from 15.6 to 15.8 in/yr (75 to 

79 percent of streamflow), which is within the 5-percent error 

inherent in the streamflow record.

Base Flow from Streamflow-Hydrograph Separation

The effect of watershed scale on estimates of base flow by 

use of the hydrograph-separation programs HYSEP and PART 

was evaluated by comparing results from streamflow-gaging 

stations at WE-38 (2.8 mi2), Klingerstown (45 mi2

) and near 

Dalmatia (162 mi2) during 1968-2001. The increase in watershed size caused a slight increase in the estimate of meanannual base flow computed by the use of PART and the LocalMinimum version of the HYSEP method as watershed size 

increased (fig. 20). Base flow computed from PART increased 

from 12.3 in/yr at the WE-38 gage to 12.9 in/yr at the Dalmatia 

streamflow-gaging station (61 to 63 percent of streamflow); 

recharge computed from the HYSEP Local-Minimum method 

increased from 10.2 in/yr at the WE-38 streamflow-gaging station to 10.8 in/yr at the Dalmatia streamflow-gaging station (50 

to 53 percent of streamflow). These increases could be caused 

by an increase in the contribution of ground-water discharge to 

streamflow, but the increase is within the 5-percent error inherent in the streamflow record. If not caused by measurement 

error, the increase could be the result of ground water that 

passed beneath the small 2.8-mi2 basin or the result of less 

evapotranspiration from ground water in the larger basin where 

drainage density is smaller. 

An increase in watershed size caused a noticeable decrease 

in the estimate of mean-annual base flow computed by use of 

the Fixed-Interval and Sliding-Interval versions of the HYSEP 

method at the 45-mi2 watershed scale (blue symbols) (fig. 20). 

The base-flow rate decreased from about 13.1 in/yr at the 

WE-38 streamflow-gaging station to 11.8 in/yr at the Klingerstown streamflow-gaging station (65 to 57 percent of streamflow). This decrease was not caused by watershed hydrology 

but was the result of the interval (2N*) used in the hydrographseparation technique changing from 3 days to 5 days. Both 

methods can be viewed as accomplishing the base-flow separation by moving a bar one-interval wide upward until it intersects 

the trace of the streamflow hydrograph. The longer the interval 

length, the less amount of flow is separated as the base-flow 

component, because a wider bar is unable to be moved upward 

beneath storm peaks on the hydrograph as far as a narrower bar. 

The interval changes from 3 days to 5 days at a watershed area 

of 32 mi2, to 7 days at 240 mi2, and to 9 days at about 1,000 mi2. 

The effect of changing the interval from 3 to 5 days can be seen 
26 Comparison of Methods for Estimating Ground-Water Recharge and Base Flow at a Small Watershed in the Eastern United States

Figure 18. Results of comparing estimates of annual recharge and base flow at the WE-38 

watershed, 1968-2001.

Figure 19. Streamflow hydrographs for watersheds of 2.8 square miles (WE-38) and 162 square miles (East 

Mahantango Creek near Dalmatia), 1998.
Comparison of Results 27

on the hydrographs of base-flow separations by the use of the 

Fixed-Interval version of the HYSEP method at the WE-38 and 

Klingerstown streamflow-gaging stations (fig. 21). During the 

period April-May 1996, the Fixed-Interval version assigned 

75 percent of the streamflow at the WE-38 streamflow-gaging 

station as base flow but only 60 percent of the streamflow at the 

Klingerstown streamflow-gaging station.

Of the base-flow methods tested, results from the FixedInterval and Sliding-Interval versions of HYSEP were statistically similar and both appear to be sensitive to watershed scale. 

Care should be used when comparing watersheds that cross the 

thresholds (32, 240, and 1,000 mi2) where the interval width 

used in the Fixed-Interval and Sliding-Interval versions of 

HYSEP changes. For this reason, PART or the Local-Minimum 

version of HYSEP may be the preferred methods for base-flow 

estimation of watersheds of differing size.

Monthly Estimates

Seasonal estimates of recharge from hydrograph-separation of base flow usually are not recommended because of the 

lag time between recharge and ground-water discharge as base 

flow. However, for the small 2.8-mi2 WE-38 watershed the 

same general seasonal trends were shown by the monthly estimates of both recharge and base flow from 1994 to 2001, indicating that monthly estimates from the base-flow methods may 

be as good as those derived from recharge methods (fig. 17). 

This result probably is due to the small size and quick hydrologic response (recession index of only 26.9 days) of the watershed. Therefore, the discrepancy between recharge computed 

by RORA and base flow from the PART method was compared 

using streamflow record from the WE-38 gaging station 

(2.8-mi2 watershed) and the East Mahantango Creek gaging station near Dalmatia (162 mi2) (fig. 22). As expected, the comparison indicated a greater discrepancy between the methods as 

watershed size increased, probably because of the increased lag 

time between ground-water recharge and discharge for the 

larger watershed with slower hydrologic response (recession 

index of 46.6 days).

Figure 20. 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).

Figure 21. 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.
28 Comparison of Methods for Estimating Ground-Water Recharge and Base Flow at a Small Watershed in the Eastern United States

Summary and Conclusions

Ground-water recharge is a fundamental component in the 

water balance of any watershed. However, because it is nearly 

impossible to measure directly, numerous methods have been 

used to estimate recharge, and in some cases, base flow has 

been used as an approximation of recharge. This report 

describes the results of a study by the U.S. Geological Survey 

(USGS), in cooperation with the U.S. Department of Agriculture, Agricultural Research Service (ARS), to compare commonly used methods for estimating ground-water recharge and 

base flow. The study used ARS data available from 1968 to 

2001 at their Masser Recharge Site and WE-38 experimental 

watershed and from streamflow-gaging stations on East Mahantango Creek, all of which are underlain by fractured bedrock 

and representative of a humid-continental climate in the northeastern United States. The ARS sites were chosen for study 

because they provide (1) long-term continuous hydrologic 

records, (2) measurements of unsaturated-zone drainage from 

gravity-drainage lysimeters (datasets that rarely are available) 

and (3) discharge data from the streamflow-gaging stations 

nested at three scales ranging from 2.8 to 162 mi2. 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 for this study was estimated on a monthly and 

annual basis from (1) unsaturated-zone drainage collected in 

gravity lysimeters, (2) daily water-balance equation, (3) watertable fluctuations in wells, and (4) the 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 a short 8-year period 

(1994-2001) that coincided with operation of the gravity lysimeters and a longer 34-year period (1968-2001) for which climate 

and streamflow data were available.

Figure 22. Mean-monthly recharge from RORA and base flow from PART for watershed 

scales of (A) 2.8 and (B) 162 square miles.
Summary and Conclusions 29

A common recommendation in the literature is that 

recharge should be estimated from multiple methods and the 

results compared, but in reality, comparing the results may be 

difficult because of differences inherent in the methods. In this 

study, the commonly used methods provided estimates not of 

recharge, but of some surrogate of recharge (potential recharge, 

net recharge, or base flow) representing differing segments of 

the watershed (point estimate or area estimate). For example, 

the unsaturated-zone drainage collected in gravity lysimeters 

provided an estimate of the potential recharge at a specific point 

location that does not compare directly to the net recharge for a 

watershed estimated from the Rorabaugh equations or base 

flow from hydrograph separation. Thus, recharge should be 

compared by multiple methods, but the inherent differences of 

each method must be given consideration when evaluating 

results.

Estimates of mean-annual recharge for 1994-2001 in the 

WE-38 watershed and at the Masser Recharge Site ranged from 

9.9 to 14.0 in. (24-33 percent of precipitation), and mean-annual 

base flow ranged from 9.0 to 11.6 in. (21-28 percent of precipitation). The magnitude and variability of mean-annual 

recharge estimates was notably smallest with the water-table 

fluctuation method and greatest from the recession-curve displacement method by use of the RORA program. All methods 

showed the same general patterns for wet and dry years during 

the 8-year period, but the mean-annual recharge or base flow 

was shown to be statistically different between methods for 16 

of 36 possible comparisons between methods. The Local-Minimum version of HYSEP and RORA were the most different of 

the methods—each was statistically different from six other 

methods.

The same general seasonal trends were shown by the 

monthly estimates of recharge and base flow for comparisons 

during 1994-2001; however, there was considerably greater 

variability among estimates of recharge. The variability among 

the recharge methods was greatest during months of greatest 

recharge (December through April). Base-flow estimates 

tended to generally be less than values of unsaturated-zone 

drainage from September through March and greater during 

April through August. This probably reflects the lag in timing 

between recharge and ground-water discharge. In general, 

monthly estimates from the base-flow methods were similar to 

those derived from the recharge methods for the WE-38 watershed. This result probably is due to the small size and fast 

hydrologic response of the watershed. The discrepancy between 

recharge and base-flow estimates became greater for the larger 

162-mi2 watershed of East Mahantango Creek, probably 

because of increased lag time between recharge and groundwater discharge.

Comparison of the results from the different methods of 

estimating recharge indicated that the mean-annual recharge for 

the 34-year period 1968 to 2001 at the WE-38 watershed ranged 

from 11.7 to 15.8 in., and mean-annual base flow ranged from 

10.2 to 13.1 in. Recharge and base-flow methods based on 

streamflow data at the WE-38 streamflow-gaging station gave 

results that were about 11-13 percent smaller for the 8-year 

period because streamflow was less by a similar amount. 

Recharge computed from the water-budget method increased 

slightly for the 8-year period because that method is based on 

precipitation record (not streamflow), which was slightly 

greater in 1994 to 2001 than in 1968 to 2001. This illustrates the 

sensitivity of results to the underlying hydrologic datasets used 

by the methods.

Some observations and general conclusions from the comparison of methods used in this study are listed below.

• Comparison of methods is recommended.—To 

bracket the largest range of plausible recharge, comparison of recharge from RORA with base flow from the 

Local-Minimum version of the HYSEP method is recommended. These methods consistently provided the 

greatest and smallest estimates respectively of longterm annual recharge and base flow at this study site. 

Another useful approach, in concept, is to compare 

results from two methods—one that estimates potential 

recharge entering the ground-water system and one that 

estimates base flow leaving the system. Thus, comparison of potential recharge from the water-balance equation with base flow from one of the hydrographseparation methods (PART or Local Minimum version 

of HYSEP) is recommended.

• Watershed size affected base-flow estimates for 

some methods.—The increase in watershed size 

caused a noticeable decrease in the estimate of meanannual base flow computed by the use of the FixedInterval and Sliding-Interval versions of the HYSEP 

hydrograph-separation method between the 2.8-mi2

WE-38 watershed and the 45-mi2 watershed upstream 

of the Klingerstown streamflow-gaging station. The 

base flow decreased from 13.1 in/yr at the WE-38 station to 11.8 in/yr at the Klingerstown station (65 to 

57 percent of streamflow). This decrease was not 

caused by watershed hydrology but mostly was the 

result of the change in “interval” used in the hydrograph-separation algorithm from 3 days to 5 days. It 

appears the HYSEP Fixed-Interval and Sliding-Interval 

results are artificially lessened when watershed size 

increases at thresholds of about 32, 240, and 1,000 mi2. 

Thus, if watersheds of various sizes are being compared, it may be advantageous to use PART or the 

Local-Minimum version of HYSEP because they did 

not seem to be artificially affected by watershed scale. 

• Long-term base-flow estimates are comparable to 

recharge estimates.—For determining mean-annual 

recharge, base-flow estimates are comparable to 

recharge estimates from most methods. Excluding estimates from RORA, recharge for the 8-year period 1994 

to 2001 at the Masser Recharge Site and the WE-38 

watershed ranged from 9.9 to 12.3 in., compared to 

estimates of 9.0 to 11.6 in. from base-flow methods. 

Mean-annual recharge estimated by the use of RORA 
30 Comparison of Methods for Estimating Ground-Water Recharge and Base Flow at a Small Watershed in the Eastern United States

was 14.0 in. It nearly always provided the greatest estimate of annual recharge among the methods compared 

in this study. 

• Water-level fluctuations in wells should be used 

with caution in low-storage fractured-rock aquifers.—Because of the variability of water-level 

response in observation wells in fractured rock and the 

sensitivity of recharge to small errors in estimating specific yield in low-storage aquifers, estimates of 

recharge from multiple observation wells should be 

used if possible. If all other factors are equal, wells in 

upland settings will be the best candidates for use in 

estimating ground-water recharge by the WTF method.

Acknowledgments

This project would not have been possible without access 

to hydrologic data from the Masser Recharge Site and WE-38 

watershed, which was provided by the U.S. Department of 

Agriculture, Agricultural Research Service (ARS). Kevin 

Breen, Geoff Delin, Edward Koerkle, and Ronald Sloto of the 

USGS provided technical assistance and review of the report. 

The USGS Ground-Water Resources Program is gratefully 

acknowledged for support of this study. 

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