Characterization of Suspended Solids and Total Phosphorus 

Loadings from Small Watersheds in Wisconsin 

Scientific Investigations Report 2010–5039

U.S. Department of the Interior

U.S. Geological Survey
Cover: U.S. Geological Survey gaging station on Eagle Creek near Fountain City, 

Wisconsin (USGS streamflow-gaging station No. 05378185).

Clockwise from top left:

5/20/2004 Event Photograph by Mari Danz, U.S. Geological Survey

3/15/2006 Winter Photograph by Troy Rutter, U.S. Geological Survey

5/10/2006 Summer Photograph by Troy Rutter, U.S. Geological Survey
Characterization of Suspended Solids and 

Total Phosphorus Loadings from 

Small Watersheds in Wisconsin

By Mari E. Danz, Steven R. Corsi, and David J. Graczyk, U.S. Geological Survey; 

and Roger T. Bannerman, Wisconsin Department of Natural Resources

Scientific Investigations Report 2010–5039

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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Suggested citation:

Danz, M.E., Corsi, S.R., Graczyk, D.J., and Bannerman, R.T., 2010, Characterization of suspended solids and 

total phosphorus loadings from small watersheds in Wisconsin: U.S. Geological Survey Scientific Investigations 

Report 2010-5039, 16 p.
iii

Contents

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

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

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

Methods...........................................................................................................................................................2

Watershed and Ecoregion Characteristics ......................................................................................2

Loading and Streamflow Computation..............................................................................................6

Distribution of Loadings................................................................................................................................7

Annual Loading and Stormflow Contribution ...................................................................................7

Monthly Loading Distribution..............................................................................................................7

Daily Loading Distribution..................................................................................................................10

Variability in Loadings ........................................................................................................................10

Summary and Conclusions.........................................................................................................................13

Acknowledgments.......................................................................................................................................13

References Cited..........................................................................................................................................13

Figures

Figure 1. Map showing location of selected watersheds and streamflow-gaging 

stations in Wisconsin ..................................................................... 3

Figure 2. Graphs showing percent of the average annual solids and total phosphorus 

load and streamflow contributed by stormflow for selected watersheds in 

Wisconsin .................................................................................... 8

Figure 3. Graphs showing percentage of the average annual solids and total 

phosphorus loads and average annual streamflow contributed by base flow 

and stormflow for selected watersheds in Wisconsin, by month ..................... 9

Figure 4. Graphs showing cumulative distribution of ranked daily loading as a percent 

of annual solids and total phosphorus for selected watersheds in Wisconsin 

ecoregions.................................................................................... 11

Figure 5. Box plots showing distribution of the percent of total solids contributed 

by stormflow and base flow each month for selected watersheds in the 

Driftless Area ecoregion of Wisconsin ................................................... 12

Tables

Table 1. Land use, drainage area, and years of record for selected watersheds in 

Wisconsin..................................................................................... 4
iv

Conversion Factors and Datum

Conversion Factors

Multiply By To obtain

mile (mi) 1.609 kilometer (km)

square mile (mi2) 2.590 square kilometer (km2) 

Datum

Horizontal coordinate information is referenced to the North American datum of 1983 (NAD 83).
Abstract

Knowledge of the daily, monthly, and yearly distribution 

of contaminant loadings and streamflow can be critical for the 

successful implementation and evaluation of water-quality 

management practices. Loading data for solids (suspended 

sediment and total suspended solids) and total phosphorus 

and streamflow data for 23 watersheds were summarized for 

four ecoregions of Wisconsin: the Driftless Area Ecoregion, 

the Northern Lakes and Forests Ecoregion, the North Central 

Hardwoods Ecoregion, and the Southeastern Wisconsin Till 

Plains Ecoregion. The Northern Lakes and Forests and the 

North Central Hardwoods Ecoregions were combined into 

one region for analysis due to a lack of sufficient data in each 

region. Urban watersheds, all located in the Southeastern 

Wisconsin Till Plains, were analyzed separately from rural 

watersheds as the Rural Southeastern Wisconsin Till Plains 

region and the Urban Southeastern Wisconsin Till Plains 

region. Results provide information on the distribution of 

loadings and streamflow between base flow and stormflow, 

the timing of loadings and streamflow throughout the year, 

and information regarding the number of days in which the 

majority of the annual loading is transported.

The average contribution to annual solids loading from 

stormflow periods for the Driftless Area Ecoregion was 

84 percent, the Northern Lakes and Forests/North Central 

Hardwoods region was 71 percent, the Rural Southeastern 

Wisconsin Till Plains region was 70 percent, and the Urban 

Southeastern Wisconsin Till Plains region was 90 percent. 

The average contributions to annual total phosphorus loading 

from stormflow periods were 72, 49, 61, and 76 percent for 

each of the respective regions. The average contributions to 

annual streamflow from stormflow periods are 20, 23, 31, and 

50 percent for each of the respective regions.

In all regions, the most substantial loading contributions 

for solids were in the late winter (February through March), 

spring (April through May), and early summer (June through 

July), with fall (October through November) and early 

winter (December through January) contributing the smallest 

loadings. The Northern Lakes and Forests/North Central 

Hardwoods region had some substantial loading in September. 

There was a similar pattern for total phosphorus loading in all 

regions, with the pattern somewhat less pronounced in urban 

watersheds. As with the loading results, average monthly 

streamflow values were greatest in late winter, spring, and 

early summer, with the lowest values typically in fall and early 

winter. Loading contributions were greater from stormflow 

than from base flow in all instances, except total phosphorus 

in the Northern Lakes and Forests/North Central Hardwoods 

region, which had equal or greater base-flow contribution for 

several months. Base flow constituted a greater percentage of 

the total streamflow than stormflow in all rural watersheds for 

all regions.

Only a few storms each year dominated the annual 

loading totals for solids and total phosphorus. When daily 

loading values were ranked for the year, all regions reached 

50 percent of the annual solids loading in the 5 highest loading 

days and nearly 50 percent of the annual total phosphorus 

loading in the 14 highest loading days.

Introduction

Federal, State, and local watershed managers and 

scientists use loading estimates of solids and total phosphorus 

in streams for numerous management purposes. Loading data 

are the foundation upon which water-quality management 

practices are built. Loading data are used to establish goals 

for reducing contaminant loadings, to estimate the relative 

magnitude of nonpoint sources compared to point sources, 

and to estimate nutrient loadings to lakes (Lathrop and 

others, 1998; Lathrop and others, 2007). Information on total 

loading potential is important in designing and evaluating 

water-quality management practices (Corsi and others, 1997). 

Further information on the seasonal timing and distribution 

of loadings could provide valuable insight for watershed 

managers to accomplish these water-quality management 

actions.

Characterization of Suspended Solids and 

Total Phosphorus Loadings from 

Small Watersheds in Wisconsin

By Mari E. Danz1, Steven R. Corsi1, David J. Graczyk1, and Roger T. Bannerman2

1

 U.S. Geological Survey, Middleton, Wisconsin

2

 Wisconsin Department of Natural Resources, Madison, Wisconsin
2 Characterization of Suspended Solids and Total Phosphorus Loadings from Small Watersheds in Wisconsin

The effectiveness of management practices for improving 

water quality can be influenced by hydrologic conditions 

as well as the timing and relative magnitude of loading 

contributions throughout the year (Stuntebeck and Bannerman, 

1998; Corsi and others, 2005). Therefore, an understanding 

of the temporal distribution of loading contributions is 

critical in the design and evaluation of these practices. Some 

management practices may function successfully only during 

certain times of the year. For example, if the majority of the 

annual loading is transported during periods when the ground 

is frozen, the effectiveness of management techniques that rely 

on emergent vegetation may be reduced. These seasonal issues 

can affect the evaluation and monitoring of management 

practices as well. For example, if the majority of the loading is 

transported during winter months (December through March), 

the monitoring plan needs to be designed for sampling during 

snowmelt periods. 

Many management practices are designed specifically 

for controlling nonpoint pollution during storm-runoff periods 

(Graczyk and others, 2003). Quantification of the fraction of 

loading contributed during storm runoff, as opposed to that 

contributed during base-flow periods, can provide important 

information on potential loading reductions. 

To effectively design, implement, and evaluate the 

effects of management practices, determination of the number 

of days that contribute the majority of the annual loading 

is also important. If a substantial proportion of the annual 

loading is transported during a few days that have large runoff 

events, management practices would need to be designed 

to accommodate a large volume of water. In addition, the 

monitoring plan would need to include sampling activities 

that will ensure accurate concentration estimates for high-flow 

events as well as for base-flow periods in order to adequately 

estimate annual loadings. 

The U.S. Geological Survey (USGS) has monitored 

water quality in many watersheds throughout Wisconsin in 

cooperation with national, regional, State, and local agencies. 

Solids and total phosphorus are the two most commonly 

recognized and monitored nonpoint contaminants with 

elevated concentrations resulting from agricultural and 

urban runoff. Solids data collected from USGS monitored 

watersheds are represented as suspended-sediment 

concentration or total suspended solids. Analytical methods 

for determination of suspended-sediment concentration and 

total suspended solids are different. For water samples that 

contain large particles (sand size or greater), the reported value 

of total suspended solids may be slightly less than the value 

reported for total suspended sediment. For the purposes of 

this report, however, the two constituents are considered to be 

interchangeable and will be referred to as “solids” throughout 

the report.

Purpose and Scope

This report presents the results of a study conducted by 

the U.S. Geological Survey in cooperation with the Wisconsin 

Department of Natural Resources (WDNR). The objective of 

this report is to provide Wisconsin water-resource managers 

with information describing the daily, monthly, and yearly 

distributions of solids and total phosphorus loadings and 

streamflow. This report examines these distributions and 

relative magnitudes of loading occurrence. Annual stormflow 

and base-flow contributions, monthly stormflow and baseflow distributions, and cumulative daily loadings are analyzed 

and summarized. The information in this report is compiled 

from data from 23 watersheds throughout Wisconsin that 

are grouped into regions based on ecoregion and land-use 

characteristics. The period of record used at each watershed 

varies between 1 and 29 full water years1 for solids and 

between 1 and 23 full water years for total phosphorus. The 

data collection occurred between 1976 and 2006. The analysis 

took place in 2007 and 2008.

Methods

Daily loading and daily streamflow data were used as 

the base data for all calculations presented in this report. 

In order to capture the various ways loading distributions 

can be characterized, an array of basic statistical analyses 

were performed on the data. The data were separated into 

groups likely to have similar trends by summarizing them by 

ecoregion and land use.

Watershed and Ecoregion Characteristics

A total of 23 small watersheds throughout Wisconsin 

were chosen for loading characterization (fig. 1, table 1). 

Because land use, geologic setting, geographic location, slope, 

soil type, and climate are factors that can affect the magnitude 

and variability of loadings, watersheds were grouped and 

summarized based on the ecoregions of Wisconsin (Omernik 

and Gallant, 1988). The four main ecoregions of Wisconsin are 

the Driftless Area Ecoregion, the Northern Lakes and Forests 

Ecoregion, the North Central Hardwoods Ecoregion, and the 

Southeastern Wisconsin Till Plains Ecoregion (fig. 1). These 

ecoregions are defined based on land use, potential natural 

vegetation, land-surface form, soils, climate, and growing 

season.

1

 A water year is defined as the 12-month period from October 1, for any 

given year, through September 30, of the following year. The water year is 

designated by the calendar year in which it ends. Annual references in this 

report refer to the water year.
Methods 3

Figure 1. Location of selected watersheds and streamflow-gaging stations in Wisconsin.

EXPLANATION

Ecoregion

Driftless Area

North Central Hardwood Forests

Northern Lakes and Forests

Southeastern Wisconsin Till Plains



 







 

 

 















U.S. Geological Survey gaging station

Watershed area

Base from USEPA Level III Ecoregions of the Continental United States ecoregion 

map derived from Omernik (1987) and from refinements of Omernik's framework 

that have been made for other projects. 

Watershed boundaries were manually defined during selected watershed study 

onto 1:24,000 (7.5 minute) USGS topographic quadrangles (paper maps). The maps 

are part of the USGS-Wisconsin Water Science Center Drainage Area Map file. 

The boundaries were digitized and joined to create a statewide basin coverage.

88°30

90°

91°30'

46°30'

45°

43°30'







Pine Creek

Bear River

Bark River

Bower Creek

Eagle Creek

White Creek Otter Creek

Duncan Creek

Yahara River

Halfway Creek

Brewery Creek

Garfoot Creek

Jackson Creek

Kuenster Creek

Steiner Branch

Pheasant Branch Menomonee River

Joos Valley Creek

Rattlesnake Creek

Kinnickinnic River

Delavan Lake Inlet

Little Balsam Creek

Jackson Creek Tributary

0 20 40 60 MILES

0 20 40 60 KILOMETERS
4 Characterization of Suspended Solids and Total Phosphorus Loadings from Small Watersheds in Wisconsin

Table 1. Land use, drainage area, and years of record for selected watersheds in Wisconsin.

[ -- , no data available ]

U.S. Geological Survey 

downstream order number Watershed and monitoring station name Drainage area(square miles)

Driftless Area Ecoregion

05406470 Brewery Creek at Cross Plains 10.5

05378185 Eagle Creek at County Highway G near Fountain City 14.3

05406491 Garfoot Creek near Cross Plains 5.39

05382240 Halfway Creek at County Highway ZN near Onalaska 34.1

05378183 Joos Valley Creek near Fountain City 5.89

054134435 Kuenster Creek at Muskellunge Road near North Andover 9.59

05413449 Rattlesnake Creek near North Andover 42.4

05433510 Steiner Branch near Waldwick 5.9

Northern Lakes and Forests Ecoregion/North Central Hardwood Forests Ecoregion

05357335 Bear River near Manitowish Waters 81.3

05364850 Duncan Creek Tributary near Tilden 4.17

04024314 Little Balsam Creek at Patzau 5

04026349 Pine Creek near Moquah 21.5

Southeastern Wisconsin Till Plains Ecoregion (Rural Watersheds)

05426067 Bark River at Nagawicka Road at Delafield 35.9

04085119 Bower Creek at County Trunk Highway MM near De Pere 14.8

05431017 Delavan Lake Inlet at State Highway 50 at Lake Lawn 21.8

05431016 Jackson Creek at Mound Road near Elkhorn 16.8

040857005 Otter Creek at Willow Road near Plymouth 9.5

04073462 White Creek at Spring Grove Road near Green Lake 3.05

05427718 Yahara River at Windsor 73.6

Southeastern Wisconsin Till Plains Ecoregion (Urban Watersheds)

054310157 Jackson Creek Tributary near Elkhorn 4.34

04087159 Kinnickinnic River at S. 11th Street at Milwaukee 18.8

04087120 Menomonee River at Wauwatosa 123

05427948 Pheasant Branch at Middleton 18.3
Methods 5

Full years of record

Solids Total Phosphorus Land use as a percentage of watershed1

Number of 

years

Years monitored Number of 

years

Years monitored Urban Agriculture Forest Water Wetland Other

Driftless Area Ecoregion

12 85, 90-98, 00-01 10 85, 90-98 6 70 21 0 0 2

8 91-94, 03-06 8 91-94, 03-06 4 46 49 0 0 1

9 85, 90-93, 95-98 9 85, 90-93, 95-98 4 43 50 0 0 3

2 05-06 2 05-06 7 39 53 0 0 1

8 91-94, 03-06 8 91-94, 03-06 5 47 46 0 0 2

4 92-95 4 92-95 6 93 1 0 0 0

4 91-94 4 91-94 5 93 2 0 0 0

2 78-79 2 78-79 4 56 37 0 0 3

Northern Lakes and Forests Ecoregion/North Central Hardwood Forests Ecoregion

3 92-94 3 92-94 6 0 46 24 24 0

2 88-89 2 88-89 4 77 17 0 1 1

2 77-78 -- -- 2 0 74 0 23 0

2 77-78 -- -- 3 18 71 0 3 4

Southeastern Wisconsin Till Plains Ecoregion (Rural Watersheds)

1 04 3 04-06 19 54 16 1 6 3

4 91-94 4 91-94 5 87 5 0 1 1

8 84-85, 90-95 23 84-06 18 75 5 2 0 1

13 94-06 13 94-06 20 75 3 1 0 1

10 91-97, 00-02 10 91-97, 00-02 6 71 17 1 3 2

16 82-87, 97-06 16 82-87, 97-06 6 83 8 0 0 3

16 91-06 16 91-06 10 84 3 1 1 1

Southeastern Wisconsin Till Plains Ecoregion (Urban Watersheds)

23 84-06 23 84-06 47 48 2 1 0 2

3 83-84, 05 1 05 98 0 1 0 0 0

5 76-77, 83-84, 05 1 05 61 25 8 0 5 1

29 78-06 13 93, 95-06 26 67 5 0 1 1

1

 Land cover was derived for watersheds from the National Land Cover Data Set (NLCD) using a geographic information system (GIS) (Homer and others, 

2007; U.S. Geological Survey, 2008).
6 Characterization of Suspended Solids and Total Phosphorus Loadings from Small Watersheds in Wisconsin

Due to a lack of qualifying streamflow-gaging and 

water-quality monitoring stations available in the northern 

ecoregions, the data from the selected watersheds in the 

Northern Lakes and Forests Ecoregion and the North Central 

Hardwoods Ecoregion were combined. These two ecoregions 

have similarities in their soils, land use, vegetation and 

inclusion of numerous wetland areas. The major differences 

between the two are that the Northern Lakes and Forest 

Ecoregion has less agriculture, more wetlands, larger 

watersheds, and more coniferous forests. The North Central 

Hardwood Forests Ecoregion is a transitional area between 

the Northern Lakes and Forests Ecoregion and the highly 

agricultural ecoregions of the southern part of the state.

Wang and others (1997) have indicated a degradation of 

biota for Wisconsin streams in watersheds with greater than 

10- to 20-percent urban land use. To address this, land-use data 

were compiled for each watershed (table 1) and watersheds 

with greater than 20-percent urban land use were grouped 

and analyzed separately. All of the watersheds that met these 

criteria were located within the Southeastern Wisconsin Till 

Plains Ecoregion.

Throughout the report, the four regions that are analyzed 

are the Driftless Area Ecoregion, the Northern Lakes 

and Forests/North Central Hardwoods region, the Rural 

Southeastern Wisconsin Till Plains region, and the Urban 

Southeastern Wisconsin Till Plains region.

To generate valid comparisons, the following criteria 

were used to select the watersheds:

• Drainage area was less than 200 square miles;

• Two or more years of continuous loading data were 

available for solids or total phosphorus;

• Watersheds were chosen from each of the four regions; 

and

• Loads were previously published in USGS WaterResources Data Annual Reports.

Watershed boundaries were digitized from 1:24,000 

USGS topographic quadrangles and land cover was derived 

for watersheds from the National Land Cover Database 

(NLCD) using a geographic information system (GIS) (Homer 

and others, 2007; U.S. Geological Survey, 2008).

Loading and Streamflow Computation

Daily loading and daily streamflow values were used 

for all calculations presented in this report. Each watershed 

and its associated data are represented in USGS databases 

and published in “Water Resources Data, Wisconsin” for 

suspended sediment concentration or total suspended solids, 

and total phosphorus loadings (U.S. Geological Survey, 

1977–1982; Holmstrom and others, 1983–2000; Garn and 

others, 2001; Waschbusch and others, 2002–2006; and U.S. 

Geological Survey, 2007).

Loadings of solids and total phosphorus were computed 

using one of two methods depending on the sampling 

protocol for each individual sampling site. For most sites, 

multiple samples were collected during periods of storm 

runoff, and additional samples were collected during lowflow periods. For these sites, the integration method was 

used to determine the load for each day and daily loads were 

summed to determine the annual load (Porterfield, 1972). For 

the remainder of the sites, multiple samples were collected 

during each storm runoff period and composited into a single 

sample; analyses of these composite samples resulted in an 

“event-mean concentration.” Samples also were collected 

during low-flow periods. “Event loadings” were computed by 

multiplying the event mean concentrations and the stormflow 

volumes. The low-flow loadings were computed by use of 

the integration method. Total loadings were computed by 

summing the event and low-flow loadings. Streamflow at each 

site was computed using standard USGS methods (Rantz and 

others, 1982). Loading and streamflow values were included in 

this analysis only for water years with complete daily records.

Daily mean streamflow and loadings data were separated 

into daily mean base-flow and stormflow contributions by use 

of the USGS Hydrograph Separation Program (HYSEP). This 

is a computer program that is used to separate a streamflow 

hydrograph into base-flow and surface-runoff components. It 

includes three methods of hydrograph separation: the fixedinterval, sliding-interval, and local-minimum methods. These 

methods can be described conceptually as three different 

algorithms to systematically draw connecting lines between 

the low points of the streamflow hydrograph. The sequence 

of these connecting lines defines the base-flow hydrograph. 

The hydrograph-separation techniques were developed by 

Pettyjohn and Henning (1979). The fixed-interval method 

(Sloto and Crouse, 1996) was used in this report; it assigns 

the lowest discharge in a calculated time interval to all days in 

that interval starting with the first day of the period of record. 

The assigned values are then connected to define the base-flow 

hydrograph. 
Distribution of Loadings 7

Distribution of Loadings

Solids, total phosphorus, and streamflow data were 

analyzed on annual, monthly, and daily time scales. The 

contributions of these constituents from stormflow and base 

flow also were analyzed. The following distributions were 

determined: the distribution of annual loading and streamflow 

between base flow and stormflow, the monthly distribution of 

loads and streamflow, and the daily cumulative distribution of 

loads.

Annual Loading and Stormflow Contribution

For each of the selected watersheds, the average 

percentage of annual loading contributed by stormflow 

was calculated for solids and total phosphorus; the average 

percentage of total annual streamflow contributed by 

stormflow also was calculated (fig. 2). For most watersheds, 

stormflow contribution to total loading is substantially greater 

than base-flow contribution. For solids, stormflow contributes 

greater than 50 percent of the loading at 21 of 23 watersheds, 

greater than 70 percent at 18 watersheds, and greater than 

90 percent at 6 watersheds. For total phosphorus, stormflow 

contributes greater than 50 percent of the loading at 18 of 

21 watersheds, greater than 70 percent at 13 watersheds, 

and greater than 80 percent at 5 watersheds. In contrast, 

stormflow contribution to total annual streamflow generally 

is less than base flow. Stormflow contributes less than 50 

percent of the annual streamflow at 20 of 23 watersheds, less 

than 30 percent at 13 watersheds, and less than 20 percent at 

9 watersheds. Even though base flow is the major contributor 

to annual streamflow, the majority of annual constituent load is 

transported during stormflow periods. 

The average contribution to annual solids loading 

from stormflow periods for the Driftless Area Ecoregion is 

84 percent, the Northern Lakes and Forests/North Central 

Hardwoods region is 71 percent, the Rural Southeastern 

Wisconsin Till Plains region is 70 percent, and the Urban 

Southeastern Wisconsin Till Plains region is 90 percent. The 

average contributions to annual total phosphorus loading 

from stormflow periods are 72, 49, 61, and 76 percent for 

each of the respective regions. The average contributions to 

annual streamflow from stormflow periods are 20, 23, 31, and 

50 percent for each of the respective regions.

The error bars in figure 2 illustrate the wide variability 

in stormflow contributions from year to year at individual 

watersheds. For watersheds with at least 3 years of data, the 

average maximum-to-minimum range is 31 percent for solids, 

33 percent for total phosphorus, and 17 percent for streamflow. 

Variability among watersheds may be explained by several 

factors, such as land use, drainage area, slope, soil type, 

climate, and time period monitored. 

Monthly Loading Distribution

Average monthly percentages of annual loadings and 

annual streamflow were computed by averaging the percent 

monthly contributions for all available years at individual 

watersheds (fig. 3). For all rural watersheds, results show 

substantial solids and total phosphorus loading contributions 

in the late winter (February through March), spring (April 

through May), and early summer (June through July), with 

fall (October through November) and early winter (December 

through January) contributing the smallest loadings (fig. 3, 

rows A and B). This effect was less pronounced in the urban 

watersheds, where the total phosphorus loads were distributed 

more evenly throughout the year (fig. 3, row B). In the urban 

watersheds, less than 15 percent of the annual total phosphorus 

loading was contributed each month. Similar to loading 

results, average monthly streamflow for rural and urban 

regions is greatest in late winter, spring, and early summer, 

with the lowest values typically in fall and early winter (fig. 3, 

row C). This similarity between loads and streamflow is a 

result of the direct influence of streamflow on loadings.

These monthly data also were separated into stormflow 

and base-flow contributions and averaged for each region 

(fig. 3). Loading contributions were found to be greater 

from stormflow than from base flow in all instances, except 

total phosphorus in the Northern Lakes and Forests/North 

Central Hardwoods region, which has equal or greater 

base-flow contribution for several months. In contrast, for 

total streamflow each month, base flow contributes a greater 

percentage of the monthly streamflow than stormflow in all 

rural watersheds for all regions. For example, for the Driftless 

Area Ecoregion, solids loading for March accounts for 

22 percent of the total annual solids loading, with 19 percent 

from stormflow and 3 percent base flow. However, the 

reverse is true for total streamflow contribution, where March 

accounts for 13 percent of the total streamflow, with 9 percent 

from base flow and 4 percent from stormflow. This is a direct 

result of greater solids and total phosphorus concentrations 

during stormflow periods than during periods of base flow. In 

the urban watersheds, where there is more impervious surface 

area, stormflow and base flow each constitute 50 percent of the 

streamflow.
8 Characterization of Suspended Solids and Total Phosphorus Loadings from Small Watersheds in Wisconsin

Figure 2. Percent of the average annual solids and total phosphorus load and streamflow contributed by stormflow for 

selected watersheds in Wisconsin.

WATERSHED

Driftless Area

Northern Lakes and 

Forests/North Central 

Hardwoods

Southeastern 

Wisconsin 

Till Plains

(Rural watersheds)

Southeastern 

Wisconsin 

Till Plains

(Urban watersheds)

Brewery

Eagle

Garfoot

Halfway

Joos

Kuenster

Rattle

Steiner

Bear

Duncan

Little Balsam

Pine

Bark

Bower

Delavan

Jackson Creek

Otter

White

Yahara

Jackson Creek Trib.

Kinnickinnic

Menomonee

Pheasant

0

10

20

30

40

50

60

70

80

90

100

PERCENT OF ANNUAL LOAD

CONTRIBUTED BY STORMFLOW

EXPLANATION

Solids

Phosphorus

Streamflow

Maximum watershed value

Minimum watershed value

Data range
Distribution of Loadings 9 Figure 3. Percentage of the average annual solids and total phosphorus loads and average annual streamflow contributed by base flow and stormflow for selected watersheds in Wisconsin, by month.

BaseflowStormflow

BaseflowStormflow

Stormflow

MONTH MONTH MONTH MONTH

Baseflow

BaseflowStormflow

BaseflowStormflow

BaseflowStormflow

BaseflowStormflow

BaseflowStormflow

BaseflowStormflow

BaseflowStormflow

BaseflowStormflow

BaseflowStormflow

25

2015

10

5

0

25

2015

10

5

0

25

2015

10

5

0

25

2015

10

5

0

35

3025

2015

10

0

86420

16141210

6420

8.

16141210

0 08642

16141210

8642

16141210

5

35

3025

2015

10

5

0

35

3025

2015

10

5

0

35

3025

2015

10

5

0

Driftless Area Northern Lakes and Forests/ North 

Central Hardwoods

Southeastern Wisconsin Till Plains 

(Rural watersheds)

Southeastern Wisconsin Till Plains 

(Urban watersheds)

A

B

C

L AUNNAEGAREVAF OT NECREP

WOLF MAERTS

L AUNNAEGAREVAF OT NECREP

DAOL SUROHPSOHPL ATOT

L AUNNAEGAREVAF OT NECREP

DAOL SDI L OS

Jan. Feb. Mar. Apr. MayJune JulyAug. Sep. Oct. Nov.Dec. Jan. Feb. Mar. Apr. MayJune JulyAug. Sep. Oct. Nov.Dec. Jan. Feb. Mar. Apr. MayJune JulyAug. Sep. Oct. Nov.Dec. Jan. Feb. Mar. Apr. MayJune JulyAug. Sep. Oct. Nov.Dec.
10 Characterization of Suspended Solids and Total Phosphorus Loadings from Small Watersheds in Wisconsin

Daily Loading Distribution

A cumulative distribution of daily constituent loading 

was determined for each watershed and for each year. For each 

watershed, the daily loading values were ranked from highest 

to lowest each year with the greatest value given a ranking of 

1 and the smallest value given a ranking of 365 (366 for leap 

years). A running summation of these values was computed 

to represent the greatest 1, 2, 3, 4, 5, 6, 7, 14, 30, 60, 90, 120, 

180, 270, and 365 (annual) day cumulative loadings for each 

watershed for each year of record. Values were averaged 

over the period of record and final values were expressed as 

a percent of the annual loading in each region (fig. 4). The 

maximum and minimum values from all regions were used 

to construct cumulative distributions to represent the range of 

variability (fig. 4).

The magnitude of loading for the first few highest 

loading days of each curve indicates that a few storms each 

year dominate the annual loading totals for solids and total 

phosphorus. All regions reach 50 percent of the annual solids 

loading in the 5 highest loading days and nearly 50 percent 

of the annual total phosphorus loading in the 14 highest 

loading days. Differences among regions are also apparent 

in these data. For example, the distribution for the Driftless 

Area Ecoregion, characterized by steeper watersheds, 

increases more rapidly than other regions for solids and total 

phosphorus, with 80 percent of the annual solids loading and 

69 percent of the annual total phosphorus loading contributed 

in the 14 highest loading days on average. The solids 

distribution for the urban watersheds, characterized by more 

impervious areas, also increases rapidly, with 57 percent of the 

annual solids loads reached in the 5 highest loading days and 

77 percent reached in the 14 highest loading days.

This cumulative loading information emphasizes the 

importance of understanding the timing of loadings when 

considering watershed-management options. A small number 

of large storms transport a high percentage of the annual load. 

These few days with the highest loadings must be considered 

to provide the best chance at achieving load-reduction goals. 

For example, the management plan for Eagle Creek in the 

Driftless Area Ecoregion calls for a 50-percent reduction in 

upland sediment (Wisconsin Department of Natural Resources, 

1990). The 3 highest loading days represent 53 percent of the 

annual solids load in this region (fig. 4). The reduction goal 

may not be met if management practices are not effective 

during these 3 days. Similarly, water-quality sampling used to 

estimate loadings in these small watersheds would need to be 

planned carefully to achieve an acceptable level of confidence 

in final results. If substantial focus is not placed on sampling 

or estimating concentrations during these few highest days of 

loading, annual loadings may be incorrectly estimated.

It is important for watershed managers to understand 

the seasonality in loading as well as the timeframe in which 

the largest storms occur. For example, a site analysis of 

Otter Creek in the Rural Southeastern Wisconsin Till Plains 

region reveals that an average of 55 percent of the annual 

solids loading occurred in the 7 highest loading days of 

the year over the 10 years that data were collected. Further 

examination of these 70 days reveals that 44 of them occurred 

within 3 months: March (15 days), April (18 days), and June 

(11 days). This is consistent with the timing of the three 

largest monthly solids loading contributions where a total 

of 57 percent of the annual solids loading in the Otter Creek 

watershed occurred in the same months: March (18 percent), 

April (23 percent), and June (16 percent) (Corsi and others, 

2005). Management practices in this watershed would need to 

be effective during these 3 months to achieve load-reduction 

goals. Use of the cumulative distributions of daily loading 

(fig. 4) together with the monthly contributions (fig. 3) can 

provide watershed managers with a valuable tool to help 

choose the capacity and type of management practice needed 

to best achieve load-reduction goals. 

Variability in Loadings

Substantial variability exists among the watersheds 

within each region (fig. 2). There also is substantial interannual variability for individual watersheds (fig. 2). This 

variability also is observed in the monthly values used to 

compute the averages presented in figure 3 and the cumulative 

distributions presented in figure 4. Figure 5 has been included 

to illustrate the variability in monthly solids loadings among 

the eight watersheds within the Driftless Area Ecoregion. In 

this region, an average of 19 percent of the total solids loading 

is delivered as stormflow in March (fig. 3, first graph, row 

A). For individual watersheds, this value ranges from 7 to 45 

percent (fig. 5). It also should be noted that the data from each 

watershed was averaged over the period of record that was 

available, and some watersheds have numerous years of data, 

but some have only a few (table 1). Results for watersheds 

with more years of data may provide a more accurate 

representation of average contributions. Collectively, this 

intersite and intrasite variability indicates that caution should 

be exercised when attempting to utilize these data for specific 

site applications.
Distribution of Loadings 11

Figure 4. Cumulative distribution of ranked daily loading as a percent of annual solids and total phosphorus for selected 

watersheds in Wisconsin ecoregions.

0

10

20

30

40

50

60

70

80

90

100

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30

PERCENT OF ANNUAL SOLIDS LOAD

DAY

SOLIDS

TOTAL PHOSPHORUS

5 day 14 day

0 5 10 15 20 25 30

PERCENT OF ANNUAL PHOSPHORUS LOAD

DAY

All regions maximum

All regions minimum

1

 Driftless area

2 Northern lakes and forests/North central hardwoods

3 SE Wisconsin till plains (rural)

4 SE Wisconsin till plains (urban)

1-day 2-day 3-day 4-day 5-day 6-day 7-day 14-day 30-day 60-day 90-day 120-day 180-day 270-day 365-day

DL1 21% 35% 42% 47% 51% 55% 57% 69% 77% 83% 87% 90% 94% 98% 100%

NLF/NCH 2 15% 28% 33% 36% 38% 40% 41% 48% 57% 66% 72% 78% 86% 94% 100%

SWTP (R) 3 15% 23% 29% 34% 38% 41% 44% 56% 69% 80% 85% 89% 94% 98% 100%

SWTP (U) 4 12% 20% 26% 30% 34% 37% 40% 55% 70% 83% 88% 91% 95% 98% 100%

Region 1-day 2-day 3-day 4-day 5-day 6-day 7-day 14-day 30-day 60-day 90-day 120-day 180-day 270-day 365-day

DL1 29% 45% 53% 59% 64% 67% 70% 80% 87% 91% 93% 95% 97% 99% 100%

NLF/NCH 2 26% 38% 47% 54% 57% 60% 62% 71% 78% 85% 89% 92% 95% 99% 100%

SWTP (R) 3 25% 36% 44% 49% 52% 55% 57% 68% 78% 86% 90% 93% 96% 99% 100%

SWTP (U) 4 24% 38% 46% 52% 57% 60% 64% 77% 87% 93% 96% 97% 98% 100% 100%

5 day 14 day
12 Characterization of Suspended Solids and Total Phosphorus Loadings from Small Watersheds in Wisconsin

Figure 5. Distribution of the percent of total solids contributed by stormflow and base flow 

each month for selected watersheds in the Driftless Area ecoregion of Wisconsin.

Stormflow

PERCENT CONTRIBUTION TO TOTAL SOLIDS LOAD

x

x

x

x

o

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

NovDec

30 number of

values 

EXPLANATION

Schematic boxplot

x

x

o

o

75th percentile

median

25th percentile

lower adjacent

lower outside

lower detached

upper detached

upper outside

upper adjacent

Baseflow

MONTH

MONTH

PERCENT CONTRIBUTION TO TOTAL SOLIDS LOAD

x

x

x

o x

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

NovDec

0

8 8 8 8 8 8 8 8 8 8 8 8

8 8 8 8 8 8 8 8 8 8 8 8

10

1

0

20

30

40

50

2

3

4

5

6

7
References Cited 13

Summary and Conclusions

To ensure the adequate design and evaluation of waterquality management practices, watershed managers need to 

be knowledgeable about loading trends in their watershed. 

The purpose of this report is to provide Wisconsin waterresource managers with information about daily, monthly, and 

yearly distributions of solids and total phosphorus loadings 

and streamflow. Loading and streamflow data for 23 small 

watersheds in Wisconsin were used in the generation of this 

report, and the data have been summarized for four regions 

of the State. These regions were assigned on the basis of 

ecoregion and land use and were as follows: the Driftless Area 

Ecoregion, the Northern Lakes and Forests/North Central 

Hardwoods region, the Rural Southeastern Wisconsin Till 

Plains region, and the Urban Southeastern Wisconsin Till 

Plains region.

Results indicate that stormflow contributions to annual 

load were often greater than base-flow contributions for solids 

and total phosphorus. The average contribution of annual 

solids loading by stormflow for the Driftless Area Ecoregion 

was 84 percent, the Northern Lakes and Forests/North Central 

Hardwoods region was 71 percent, the Rural Southeastern 

Wisconsin Till Plains region was 70 percent, and the Urban 

Southeastern Wisconsin Till Plains region was 90 percent. 

The average contributions of annual total phosphorus loading 

by stormflow were 72, 49, 61, and 76 percent for each of 

the respective regions. Stormflow contributions to annual 

streamflow were often less than base-flow contributions. The 

average contributions to annual streamflow from stormflow 

periods were 20, 23, 31, and 50 percent, respectively, for each 

of the regions.

Monthly summary results indicated that in all regions, 

the most substantial loading contributions for solids were in 

the late winter, spring, and early summer months, with fall 

and early winter contributing the smallest loadings. Total 

phosphorus loading showed similar patterns; however, this 

effect was less pronounced in the urban watersheds than 

it was for the rural watersheds. Similar to loading results, 

average monthly streamflow was greatest in late winter, 

spring, and early summer, with the lowest values typically in 

fall and early winter. Loading contributions were greater from 

stormflow than from base flow in all instances, except for 

total phosphorus loading in the Northern Lakes and Forests/

North Central Hardwoods region, which had equal or greater 

base-flow contribution for several months. In contrast, base 

flow constituted a greater percent of the total streamflow than 

stormflow in all rural watersheds for all regions. 

The magnitude of loading for the first few highest 

loading days of the year indicated that a few storms each 

year dominated the annual loading totals for solids and 

total phosphorus. All four regions reached 50 percent of the 

annual solids loading in the 5 highest loading days and nearly 

50 percent of the annual total phosphorus loading in the 

14 highest loading days.

Acknowledgments

Support for this work was provided by the Wisconsin 

Department of Natural Resources. The authors thank all field 

and office personnel from the USGS and cooperating agencies 

who have contributed to the collection, analysis, and archival 

of the 31 years of data used in this report. The manuscript 

benefited from the suggestions made by the technical and 

editorial reviewers, Dale Robertson (USGS), Richard Lathrop 

(Wisconsin Department of Natural Resources), and Marilyn 

Billone (USGS). Michelle Greenwood (USGS) coordinated 

the publication of the report. Laura Nelson (USGS) assisted 

with the report illustrations. Dorothy Tepper (USGS) was the 

approving official.

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16 Characterization of Suspended Solids and Total Phosphorus Loadings from Small Watersheds in Wisconsin

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Danz and others— Characterization of Suspended Solids and Total Phosphorus Loadings from Small Watersheds in Wisconsin—Scientific Investigations Report 2010–5039