The Trout Lake Watershed is in the Northern Highlands Lake District in 

north-central Wisconsin. The study area includes four subbasins with five 

lakes and two bog lakes. The objectives of the Trout Lake WEBB project 

are to (1) describe processes controlling water and solute fluxes in the 

Trout Lake watershed, (2) examine interactions among those processes 

and (3) improve the capability to predict changes in water and solute 

fluxes for a range of spatial and temporal scales (Elder and others, 1992).

The Trout Lake WEBB site is colocated with the National Science Foundation’s North Temperate 

Lakes Long-Term Ecological Research 

(NTL–LTER) project (Magnuson and 

others, 1984). The Wisconsin WEBB 

Trout Lake, Wisconsin

U.S. Department of the Interior 

U.S. Geological Survey

Fact Sheet-161-99

June 2000

A Water, Energy, and Biogeochemical Budgets Program Site

the glacial outwash promotes effective 

exchange of water between the lakes and 

the ground-water system. One manifestation of the strong connectivity of the 

lakes and the ground-water system is the 

predominance of lake-water plumes that 

move through the surficial sediments.

Modeling of Ground-Water Systems

Because the water budget of the Trout 

Lake watershed is dominated by the 

ground-water system, it is critically 

important to develop comprehensive 

tools to characterize the movement of 

water and the solutes it carries through 

the watershed. Numerical models are 

commonly used to provide a better 

understanding of ground-water systems. 

A numerical ground-water model is a 

mathematical description of the physical 

ground-water system that can be used 

to solve for water levels and flow rates 

(fluxes) anywhere within the watershed 

boundaries. Because of the high density 

Location of Trout Lake in Wisconsin, showing 3 basic hillslope sites, stream gages 

and flowpath study transects.

The landscape in the Trout Lake WEBB 

study area is characterized by a moderately 

dense distribution of lakes.

research, which is concerned primarily 

with watershed processes, was designed 

to complement the NTL–LTER research 

that focuses primarily on in-lake processes.

Nearly 80 percent of the land area and 

60 percent of the lake frontage in the 

study area lie within two state forests. 

Many of the lakes in this area have 

watersheds that are completely forested 

with a mixture of coniferous and deciduous species. The surficial geology of the 

area is dominated by 30-50 meters of 

unconsolidated glacial outwash overlying Precambrian igneous bedrock. The 

glacial outwash is predominantly sand 

and nearly carbonate free, a characteristic that distinguishes the watershed from 

otherwise similar glacial lake districts 

in the upper Midwest and Canada. The 

soils are generally thin with high organic 

content in the uppermost horizon. The 

topography of the area is generally flat, 

with a total relief of less than 50 meters 

across the Trout Lake watershed.

Coarse surficial deposits and low relief 

combine to yield a predominance of 

lakes in the Northern Highland area that 

have no surface-water inlets or outlets, 

and have hydrologic budgets that are 

dominated by direct precipitation, evapotranspiration and ground-water flow. In 

addition, the highly conductive nature of 

N

North Cr.

Trout Lake Trout

River

Trout

Bog

Sparkling Lake

Mann Cr.

Allequash

Cr.

Crystal

Lake

Crystal

Bog

Allequash

Lake

Stevenson Cr.

Trout Lake

0 1 km

0 1 mi

NTL-LTER study lakes

Other lakes

Lower

Middle

Isthmus

Big Musky

 Big

 Muskellunge

Lake

Upper

Basic hillslope site

Streamflow-gaging station

Flowpath study transects

Northern Highlands

WISCONSIN

The Water, Energy, and Biogeochemical Budgets (WEBB) Program was started 

in 1991 at five small watersheds in the United States to examine water, energy, 

and biogeochemical fluxes and to determine the effects of atmospheric 

deposition, climatic variables, and human influences on watershed processes. 

The five sites are at Loch Vale, Colorado; Luquillo Experimental Forest, Puerto 

Rico; Panola Mountain, Georgia; Sleepers River, Vermont; and Trout Lake, 

Wisconsin. These sites are supported, in part, by other programs in the 

USGS, other Federal and State Agencies, and Universities.

THE WEBB PROGRAM
Simulated and measured base-flow stream 

fluxes for the five streams tributary to Trout 

Lake. The estimated error for the measured 

baseflow is indicated by error bars (±10 

percent); the Mann Creek streamflow is 

estimated using the Allequash record, and 

the measurement error is likely greater 

than 10 percent. By improving the accuracy 

of the watershed boundaries, the accuracy 

of the stream flux and its distribution 

across the watershed is much improved.

of lakes in this area that are hydraulically connected to the ground-water system, these models also provide important information about lake hydrology.

One of the keys to understanding how 

water and energy flows through a system 

is knowledge of what happens at the 

boundaries of the watershed. Where 

the boundaries are located and how 

they interact with the watershed is 

crucial. Many times the ground-watershed boundaries are assumed to match 

closely the surface-watershed boundaries as defined by the topography of the 

landscape. In many cases, however, the 

boundaries of the ground-watershed and 

the surface-watershed do not align perfectly. This discrepancy can be important, especially in lake systems that have 

large ground-water interactions, such as 

the Trout Lake watershed. To address 

this factor, one focus of research efforts 

at the Trout Lake WEBB site has been 

the development of a numerical model of 

the ground-water system with an emphasis on better describing the boundaries of 

the watershed (Hunt and others, 1998). 

Previous NTL–LTER research resulted 

in a complex model of the Trout Lake 

watershed (Cheng, 1994) that is based 

on a traditional finite-difference modeling approach (complex FD model). 

The final model, which was calibrated 

against field measured water-levels in 31 

observation wells and stream base-flow 

measurements in the 5 tributary streams 

to Trout Lake, resulted in a complex 

set of model parameters to describe 

the hydraulic characteristics of the system. A different modeling approach 

(analytic-element 

model) was used 

to refine the watershed boundaries. 

This approach was 

used primarily 

because it easily 

allowed the model area to be greatly 

expanded, and did not require dividing 

the model area into specified grid cells 

or specifying conditions at the model 

boundaries. The resulting analytic-element model used a less complex set of 

hydraulic characteristics than the traditional finite-difference model.

The analytic-element model was used to 

calculate boundary fluxes for the original finite-difference-model grid used by 

Cheng (1994). The improved finite-difference model that resulted was less 

complex (Hunt and others, 1998). The 

differences in the observed and modeled 

water levels were similar for all three 

of the calibrations. The simulated stream 

fluxes, for both the analytic-element 

model and the improved finite-difference 

model, however, are much improved 

over the traditional model.

Water Sources and Flow Paths

To understand the chemistry of lakes, 

one must first understand where the 

water comes from, how it travels, and 

what happens chemically along its flowpath. In the Trout Lake watershed, 

there are two major sources of water 

to the ground-water system: precipitation recharge through the upland areas 

(terrestrial recharge) and recharge from 

upgradient surface-water features (lake 

recharge). Thus, the water budget and 

solute composition of lakes and streams 

at various points downgradient in the 

watershed will depend upon the relative 

contributions from these two sources. 

The goal of this research is to combine 

the knowledge of chemical reactions 

along each type 

of ground-water 

flowpath, with the 

knowledge of 

flowpaths obtained 

from the groundwater-modeling 

work to develop a watershed-scale 

model of solute flux.

The stable isotopes of water (18O/16O and 

2

H/1H) are commonly used as hydrologic 

tracers. Because evaporation selectively 

removes the lighter isotopes (16O and 1H), 

surface-water bodies become “enriched” 

in the heavier isotopes (18O and 2H) and 

are generally distinguishable from precipitation (rain and snow). In the Trout 

Lake watershed, ground-water derived 

through terrestrial recharge has a relatively “light” isotopic composition, and 

is very different than the “heavy” isotopic composition of the lakes. Because the 

hydrologic budgets are dominated by precipitation, evaporation and ground water, 

the Trout Lake WEBB site is an ideal 

location for using stable isotopes to determine the source location and timing of 

subsurface flowpaths in this watershed.

The area modeled by traditional finitedifference methods is within the larger 

domain of the analytic element model. 

Blue lines represent surface-water features 

(streams and lakes) that are modeled 

as sources/sinks in the analytic element 

model. Modeling an area larger than the 

watershed allows the watershed boundaries to be calculated rather than estimated.

The use of an analytic-element 

model, combined with flow and 

hydraulic head measurements, permitted construction of an improved 

and simplified flow model.

0.0

20,000

70,000

140,000

Allequash Stevenson North Mann Trout River

Discharge 

(cubic meters per day)

Measured baseflow

Analytic-element model

Improved FD model

Complex FD model

Area modeled

by Cheng (1994)

Area modeled by the

analytic-element model
Lines of equal percentages of Crystal Lake 

water in observation wells (grey dots) that 

are located beneath the isthmus between 

Crystal Lake and Big Muskellunge Lake. 

The lake-water plume flowpath is based on 

the large difference between the water isotope compositions of O and ¬H in lake- and 

terrestrial-recharge water.

Two flowpath studies are being conducted at the Trout Lake WEBB site—a 

transect between Crystal Lake and Big 

Muskellunge Lake 

(Isthmus) and a 

substantially longer 

transect between 

Big Muskellunge 

Lake and Allequash 

Creek (Big Musky). 

For the Isthmus 

study, water isotopes were used to 

delineate the dominant flowpath originating from the littoral zone in Crystal Lake, based on 

the proportion of Crystal Lake water in 

the observation wells (Bullen and others, 

1996). A similar distinction between the 

two water sources can be seen along the 

Big Musky transect; in both cases the 

large difference in water isotopes greatly 

enhances delineation of flowpaths, and 

aids in the calibration of the groundwater-flow model of the system. Note 

that the flowpath from the Crystal Lake 

littoral zone does not lead to Big Muskellunge Lake; it continues downgradient under the lake. This result is 

somewhat counter intuitive because Big 

Muskellunge is at a lower elevation than 

Crystal Lake. In this case the position of 

the lakes in the overall flow system has a 

striking influence on the flowpaths.

In addition to the water isotopes along 

the Isthmus, a focus of our research 

is to discern where the water gets its 

solutes—that is, the weathering of minerals. Strontium isotopes (87Sr/86Sr) were 

used together with solute chemistry to 

identify the chemical reactions that control ground-water chemistry along this 

flowpath. The striking conclusion from 

the study is that the mineral weathering 

(as shown by the wide range of 87Sr/86Sr) 

depended on whether the water came 

from terrestrial or lake recharge. For terrestrial recharge, the weathering is dominated by K-feldspar and/or biotite, 

whereas the lake 

recharge water is 

dominated by dissolution of plagioclase. Because the 

87Sr/86Sr signature 

is maintained 

along the different 

flowpaths, the 

combined use of strontium and water 

isotopes provides a powerful tool for 

discerning the source of waters and solutes in the Trout Lake watershed (Bullen 

and others, 1996). The Sr isotopes 

also demonstrated 

that the source of 

solutes can change 

along a flowpath 

even though mineral composition of 

the sediments does 

not change. Geochemical reaction-path models 

must include this 

phenomenon to 

describe adequately the chemical evolution of ground water along a flowpath.

Biogeochemical Processes at the 

Sediment-Water Interface

Several studies in the Trout Lake area 

have demonstrated that small-scale 

processes operating near the sedimentwater interface have a controlling influence on the chemistry—and subsequently the biology—of lakes and 

streams. Although biogeochemical processes occur throughout watersheds, the 

sediment-water interface is a zone of

intense bioreactivity and is particularly 

important for regulating the transport of 

bioreactive chemical species, including 

nutrients (carbon, nitrogen, phosphorus 

and sulfur) and other trace elements. 

Because one of the goals of WEBB 

research is to further our understanding 

of biogeochemical fluxes within watersheds, it is crucial to understand the processes that occur at the sediment-water 

interface.

An emphasis of research at the Trout 

Lake WEBB site focuses on the 

biogeochemical processes that operate 

in the hyporheic zone. The hyporheic 

zone in this study is defined as the 

area under the streambed where distinct 

chemical changes in upwelling ground 

water begin to occur. In a recent 

study, the hyporheic zone was 

sampled at the 

upper, middle and 

lower sites of 

Allequash Creek 

at a fine vertical 

scale, with increments as little as 

5 centimeters 

(Schindler and 

Krabbenhoft, 

1998). The most striking feature of the 

profiles immediately under the stream 

is the dramatic increase in all chemical 

constituent concentrations as the water 

moves upward through the hyporheic 

zone, yet the corresponding chemical 

constituent concentrations in the stream 

water are much lower than those in the 

hyporheic water. The apparent loss of 

bioreactive constituents in the stream, 

such as CO2 and CH4, as the hyporheic 

pore water discharges to the stream, 

is attributed to biologic uptake, oxidation and evasion to the atmosphere. If 

the dissolved organic carbon being proThe use of water isotopes has helped 

to distinguish water sources for 

flowpaths that range in length from 

several meters to about 1 kilometer 

(watershed scale). Establishing a 

reliable flowpath increases the confidence of results in related studies, 

such as determining the geochemical evolution within the watershed.

In general, more chemical change 

of bioreactive species occurs over 

the last 10 to 20 centimeters of the 

hyporheic zone than occurs over 

a flow path of 100’s of meters 

through the watershed. The nutrient 

status of lakes and streams is, 

therefore, largely regulated by the 

biogeochemical reactions at the 

sediment-water interface.

10%

30%

50%

70%

90%

90%

Lake-water

plume flowpath

Land surface Water table

General direction of

ground-water flow 3 m

10 m

Big 

Muskellunge 

Lake

Crystal

Lake
For more information about the Trout Lake Research 

Watershed WEBB study visit:

http://infotrek.er.usgs.gov/doc/webb/

For more information or for reprints, please contact:

WEBB Site Coordinator

U.S. Geological Survey

8505 Research Way

Middleton, WI 53562

REFERENCES

Bullen, T.D., Krabbenhoft, D.P., and Kendall, C., 

1996, Kinetic and mineralogic controls on the evolution of groundwater chemistry and 87Sr/86Sr in a 

sandy silicate aquifer, northern Wisconsin, USA: Geochimica et Cosmochimica Acta, v. 60, no. 10, p. 

1807-1821.

Cheng, X., 1994, Numerical analysis of groundwater 

and lake systems with application to the Trout River 

Basin, Vilas County, Wisconsin: Madison, Wisconsin, 

University of Wisconsin, PhD thesis, 177 p.

Elder, J.F., Krabbenhoft, D.P., and Walker, J.F., 1992, 

Water, energy, and biogeochemical budgets (WEBB) 

program: Data availability and research at the northern 

temperate lakes site, Wisconsin: U.S. Geological Survey Open-File Report 92-48.

Hunt, R.J., Anderson, M.P., and Kelson, V.A., 1998, 

Improving a complex finite-difference ground water 

flow model through the use of an analytic element 

screening model: Ground Water, v. 36, no. 6, p. 

1011-1017.

Magnuson, J.J., Bowser, C.J., and Kratz, T.K., 1984, 

Long-term ecological research (LTER) on north temperate lakes of the United States: Verhandlungen 

Internationale Vereinigung Limnologie, v. 22, p. 

533-535.

Schindler, J.E., and Krabbenhoft, D.P., 1998, The 

hyporheic zone as a source of dissolved organic carbon and carbon gases to a temperate forested stream: 

Biogeochemistry, v. 43, p. 157-174. 

—John F. Walker and Thomas D. Bullen

duced in the hyporheic zone and lost 

in the stream is being used by bacteria 

and algae, the biological productivity of 

these stream systems is larger than 

commonly thought. Additional research 

is ongoing at the three sites and in 

additional streams in the Trout Lake 

watershed to further understand the 

mechanisms that cause these important 

processes.

Streambed depth profiles 

for carbon-related chemical constituents in the 

hyporheic zone beneath 

the middle site along 

Allequash Creek. A depth 

of zero centimeters marks 

the sediment-water interface. Large increases in 

chemical constituent concentrations occur over a 

small depth just before 

the ground water discharges to the stream. For 

some bioreactive solutes 

the increases in concentration near the sedimentwater interface can be 

much larger those in the 

deeper and longer groundwater flowpath.

COLLABORATORS

Scientists with the WEBB research program at Trout 

Lake have collaborated with faculty members at the 

University of Wisconsin-Madison, Dartmouth College and Northern Michigan University. Several students at the University of Wisconsin-Madison have 

contributed to the WEBB project through their thesis 

research at the site. The WEBB program works 

closely with the National Science Foundation LongTerm Ecological Research (LTER) program in the Trout 

Lake watershed through the sharing of data and ideas 

and through the publication of research papers together.

200

150

100

50

0

0 0.4 0.8 1.2

0 1 2 3 0 20 40 60

Depth below sediment-water interface

(centimeters)

Dissolved

organic carbon 

(micromoles per liter)

Dissolved

inorganic carbon

(micromoles per liter)

0 200 400

Dissolved CH4

Dissolved CO2

Stream Stream

Groundwater 

flow

This portable sampling 

device allows centimeterscale-depth sampling of 

the pore waters beneath 

the sediment-water interface of lakes and streams.

 SUMMARY

• Combining an analytic-element model with traditional 

hydraulic measurements allows construction of an 

improved, simplified groundwater-flow model.

• A reasonable understanding 

of water sources and flowpaths can be accomplished 

with the combined use of 

water isotopes, strontium isotopes and traditional hydraulic 

measurements. With this approach, reliable prediction of 

flowpaths that range from several meters to nearly a kilometer can be made; these results 

are helpful for related studies, 

such as determining the geochemical evolution within a 

watershed.

• The sediment-water interface 

in streambeds—where ground 

water interacts with stream 

water—plays a major role in the 

carbon cycling in a watershed.