Groundwater Recharge in Wisconsin—Annual Estimates 

for 1970–99 Using Streamflow Data

By Warren A. Gebert, John F. Walker, and Randall J. Hunt

Introduction

The groundwater component of streamflow is important because it is indicative of the sustained flow of a 

stream during dry periods, is often of better quality, and 

has a smaller range of temperatures, than surface contributions to streamflow. All three of these characteristics 

are important to the health of aquatic life in a stream. If 

recharge to the aquifers is to be preserved or enhanced, 

it is important to understand the present partitioning of 

total streamflow into base flow and stormflow. Additionally, an estimate of groundwater recharge is important for 

understanding the flows within a groundwater system—

information important for water availability/sustainability 

or other assessments. 

The U.S. Geological Survey operates numerous 

continuous-record streamflow-gaging stations (Hirsch and 

Norris, 2001), which can be used to provide estimates of 

average annual base flow. In addition to these continuous 

record sites, Gebert and others (2007) showed that having 

a few streamflow measurements in a basin can appreciably reduce the error in a base-flow estimate for that basin. 

Therefore, in addition to the continuous-record gaging 

stations, a substantial number of low-flow partial-record 

sites (6 to 15 discharge measurements) and miscellaneousmeasurement sites (1 to 3 discharge measurements) that 

were operated during 1964–90 throughout the State 

were included in this work to provide additional insight 

into spatial distribution of annual base flow and, in turn, 

groundwater recharge.

How Were Estimates of Baseflow and 

Recharge Obtained?

The methods used in this study are briefly summarized here; detailed descriptions of the data and methods 

are given in Gebert and others (2011). The recorded daily 

discharge at gaging stations can be separated into base 

flow and stormflow using the Base Flow Index (BFI) 

automated hydrograph separation approach (Institute of 

Hydrology, 1980 a, b). A FORTRAN implementation of 

the BFI method (Wahl and Wahl, 1995) was applied to the 

daily streamflow at 123 gaging stations with continuous 

records for the 1970–99 period to determine the average 

annual base flow and total streamflow for each station. 

The resulting base-flow index, defined as the base flow 

divided by total streamflow, is presented in figure 1. 

Figure 1. Spatial distribution of base-flow index, 1970–99, at 

streamflow-gaging stations in Wisconsin.

0 25 50 MILES

0 5025 KILOMETERS

87°W

92°W

EXPLANATION

Base-flow index,

in cubic feet per second

greater than 0.90

0.81–0.90

0.71–0.80

0.61–0.70

0.51–0.60

0.41–0.50

0.31–0.40

0.21–0.30

0.11–0.20

less than 0.11

Subbasin boundary

Open water

Stream

County boundary

47°N

43°N

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2009–3092

August 2011
Estimated average annual base flow can be divided 

by the basin area to express the base flow, in inches, over 

the basin. Assuming that the groundwater contributing 

area is the same as the surface-water drainage area, the 

long-term average base flow value, in inches, is a reasonable estimate for the recharge to the groundwater system. 

The resulting annual recharge values based on the 123 

streamflow-gaging stations are shown in figure 2A. 

For a low-flow partial-record site, estimates of 

average annual base flow can be obtained by relating a 

“snapshot” of streamflow at a location without a streamflow gaging station to daily discharge from a correlated 

gaging station (or “index station”) recorded on the same 

day. In some cases the relation line between the two is not 

very good; thus, it provides an unreasonable estimate of 

annual base flow. For those cases, the statewide regression 

equation published by Gebert and others (2007) was used 

to estimate the average annual base flow.

For miscellaneous-measurement sites where only 1 to 

3 measurements exist, insufficient discharge measurements preclude establishing a relation line with a nearby 

index station. As a surrogate, we used nearby low-flow 

partial-record sites and local knowledge of the regional 

hydrology to associate index sites with the miscellaneousmeasurement sites. With an associated index site, a 

discharge measurement made during low-flow conditions 

can be used to estimate average annual base flow using 

the statewide regression equation (Gebert and others, 

2007). The resulting annual recharge values for the 

low-flow partial-record sites and the miscellaneous-measurement sites, collectively referred to as partial-record 

sites, are shown in figure 2B.

What Was Learned from This Study?

The results for continuous-record gaging stations 

(fig. 2A) and partial-record sites (fig. 2B) illustrate a wide 

range of estimated recharge rates for Wisconsin. Combining the areas covered by the two maps accounts for nearly 

72 percent of the surface area of the State. The weighted 

average annual base flow for the State was 6.8 inches per 

year. The weighted average total streamflow for the State 

was 10.9 inches per year; thus, 62 percent of the annual 

total streamflow is comprised of base flow.

Generally, regional recharge rates exceeding 

12 inches annually were considered unrealistic given the 

magnitude of precipitation and evapotranspiration across 

Wisconsin. These unrealistic rates were likely due to 

violations of the limiting assumptions (Gebert and others, 

2011). In some cases, the groundwater and surface-water 

watershed boundaries did not align very closely, thus 

resulting in high values. In other cases, an exceptionally high value may be caused by error introduced by the 

method used to extrapolate base flow to basins without 

gaging stations. Gebert and others (2011) provides a likely 

cause for each basin with annual recharge in excess of 

12 inches. 

Figure 2. Spatial distribution of average annual groundwater recharge, 1970–99, on the basis of data from A, streamflow-gaging stations, and 

B, partial-record sites, in Wisconsin.

87°W

92°W

47°N

43°N

0 25 50 MILES

0 5025 KILOMETERS

87°W

92°W

EXPLANATION

Recharge, in inches

Subbasin boundary

Open water

Stream

County boundary

47°N

43°N

≥ 12

11–11.9

10–10.9

9–9.9

8–8.9

7–7.9

6–6.9

5–5.9

4–4.9

3–3.9

2–2.9

1–1.9

< 1

A B
increasing trends in streamflow over the period of 

record (Gebert and others, 2007). The increasing trends 

in base flow are consistent with changes observed 

after 1970 in lakes, groundwater levels and streams 

around the State (Magnuson and others, 2003). Likewise, McCabe and Wolock (2002) found large-scale 

increases in annual minimum and median streamflows 

over large portions of the eastern U.S., indicating climate as a likely driver. Alternatively, Gebert and Krug 

(1996) suggest that changes in agricultural practices 

are the primary driver for these changes, thus basins 

that contain more agriculture by area are more likely 

to show increases in base flow over time. Although the 

exact relation among climate, land use, and recharge 

are still active areas of research, the results in table 1 

underscore the need to match the time period used to 

estimate base flow and calculate recharge to the timeframe of the question being asked.

Has Annual Recharge Increased Since the Early 1900s?

Recharge is not expected to be constant over time 

as both changes in climate and land use can affect how 

precipitation falling on the land surface is distributed 

between surface-water stormflow and recharge-derived 

base flow. The time period used to calculate base 

flow can affect the calculated recharge rate, because 

changes in streamflow and base flow directly affect 

estimates of recharge using the methods described 

herein. For example, Gebert and others (2007) examined temporal trends in base flow for the period of 

record for 22 streamflow-gaging stations in Wisconsin. 

A comparison of the average annual base flow for the 

period of record to the average annual base flow for 

the 1970–99 period is presented in table 1. Although 

base flow was calculated using the same technique, 

the values for the two periods differ by as much as 

39 percent. Interestingly, the stations with differences 

of 10 percent or more all had statistically significant 

Table 1. Comparison of the average annual baseflow for the period of record to the 1970–99 period.

[Bold values indicate statistically significant trends in baseflow for the period of record (Gebert and others, 2007)]

Station 

number Station name Period of record

Length of 

record

(years)

Average annual baseflow

(cubic feet per second)

Full record 1970–99 Percent difference

04025500 Bois Brule River – Brule 1943–99 57 149 152 2.0

04063700 Popple River – Fence 1964–99 36 69.7 68.5 -1.7

04069500 Peshtigo River – Peshtigo 1954–99 46 566 604 6.7

04073500 Fox River – Berlin 1900–99 100 941 1,130 20.1

04074950 Wolf River – Langlade 1967–99 33 349 351 .6

04086000 Sheboygan River – Sheboygan 1917–99 83 122 146 19.7

04087000 Milwaukee River – Milwaukee 1915–99 85 209 290 38.8

05362000 Jump River – Sheldon 1916–98 83 173 190 9.8

05368000 Hay River – Wheeler 1951–98 48 232 263 13.4

05379500 Trempealeau River – Dodge 1915–99 85 327 404 23.5

05381000 Black River – Neillsville 1906–99 94 162 201 24.1

05394500 Prairie River – Merrill 1915–99 85 115 114 -.9

05397500 Eau Claire River – Kelly 1915–99 85 127 134 5.5

05399500 Big Eau Pleine River – Stratford 1915–99 85 30 32 6.7

05405000 Baraboo River – Baraboo 1915–99 85 219 271 23.7

05406500 Black Earth Creek – Black Earth 1955–98 44 29.1 32.2 10.7

05408000 Kickapoo River – LaFarge 1939–99 61 123 144 17.1

05413500 Grant River – Burton 1935–99 65 111 137 23.4

05414000 Platte River – Rockville 1935–99 65 65.1 78.5 20.6

05426000 Crawfish River – Milford 1932–99 68 229 271 18.3

05432500 Pecatonica River – Darlington 1940–99 60 121 143 18.2

05436500 Sugar River – Brodhead 1915–99 85 234 297 26.9
Printed on recycled paper

The difference in drainage area for continuous-record 

streamflow-gaging stations (average 305 square miles) 

and partial-record sites (average 50 square miles) can 

be used to illustrate the effect of basin scale on spatial 

variability of recharge. Values for large basins will likely 

be less variable than values for small basins, because the 

value reported for a particular type of site represents an 

average over the entire drainage area. Thus, one would 

expect more variability for the partial-record sites (fig. 

2B) than the gaging stations, which generally represent 

large basins (fig. 2A). This is particularly evident when 

comparing a value for a gaging station to the values for 

partial-record sites that are contained within the gaging 

station drainage boundary. For example, there are several 

gaging stations in the northwest part of the State that have 

a number of partial-record sites nested within the drainage 

boundary for each gaging station. The variability of the 

values for the smaller nested basins is appreciably higher 

than for the larger gaging stations that contain the partialrecord sites.

From the results shown in figure 2, general patterns 

can be seen in the State. The northern forested portion of 

the State contains some of the highest values, due in part 

to highly permeable sandy soils, a thick transmissive aquifer, and somewhat lower evapotranspiration. Fairly high 

values also are found in the central portion of the State 

(east of the Wisconsin River). This portion of the State, 

referred to as the Central Sand Plains, contains fairly 

thick highly conductive sandy sediments and a transmissive aquifer. These basin properties, along with the area’s 

flat terrain, facilitate the infiltration of water into the 

groundwater system rather than becoming stormflow or 

evapotranspiration. There is a fairly uniform distribution 

of recharge in the southwest portion of the State, often 

referred to as the “Driftless Area,” with a predominant 

range from roughly 6 to 9 inches per year. This is consistent with the relatively uniform underlying geology in 

the Driftless Area. There are somewhat low values in the 

central portion west of the Wisconsin River, where the 

aquifers are thin and surficial soils have low permeability. 

These factors are expected to result in less aquifer storage 

and increased surface runoff. This portion of the State also 

has some of the lowest base flow index values (fig. 1), 

which also indicates that water not infiltrated and stored 

in the aquifer contributes to streamflow as surface runoff. 

Low values also are evident along the southern border 

of Lake Superior and the western edge of Lake Michigan, likely the result of clayey surficial soils that inhibit 

infiltration.

References Cited

Gebert, W.A., and Krug, W.R., 1996, Streamflow trends 

in Wisconsin’s driftless area: Journal of the American 

Water Resources Association, v. 32, no. 4, p. 733–744.

Gebert, W.A., Radloff, M.J., Considine, E.J., and Kennedy, J.L., 2007, Use of streamflow data to estimate 

base flow/groundwater recharge for Wisconsin: Journal 

of the American Water Resources Association, v. 43, 

no. 1, p. 220–236.

Gebert, W.A., Walker, J.F., and Kennedy, J.L., 2011, Estimating 1970–99 average annual groundwater recharge 

in Wisconsin using streamflow data: U.S. Geological 

Survey Open-File Report 2009–1210, 17 p. plus appendixes..

Hirsch, R.M., and Norris, J.M., 2001, National streamflow 

information program–Implementation plan and progress 

report: U.S. Geological Survey Fact Sheet FS–048–01, 

6 p.

Institute of Hydrology, 1980a, Low flow studies: Wallingford, Oxon, United Kingdom, Report No. 1, 41 p.

Institute of Hydrology, 1980b, Low flow studies: Wallingford, Oxon, United Kingdom, Report No. 3, p. 12–19.

Magnuson, J.J., Krohelski, J.T., Kunkel, K.E., and Robertson, D.M., 2003, Wisconsin’s waters and climate–

historical changes and possible futures, in Meine, C., 

ed., Wisconsin’s waters—a confluence of perspectives: 

Madison, Wis., Wisconsin Academy of Sciences Arts 

and Letters, p. 23–36. 

McCabe, G.J., and Wolock, D.W., 2002, A step increase in 

streamflow in the conterminous United States: Geophysical Research Letters, v. 29, no. 24, p. 2185–2188.

Wahl, K.L. and Wahl, T.L., 1995, Determining the flow 

of Comal Springs at Braunfels, Texas, in Texas Water 

’95: August 16–17, 1995, San Antonio, Tex., American 

Society of Civil Engineers, p. 77–86.

For more information, please contact:

Center Director

USGS Wisconsin Water Science Center

8505 Research Way

Middleton, WI 53562

(608) 828-9901

http://wi.water.usgs.gov