U.S. Department of the Interior U.S. Geological Survey Fact Sheet FS-195-97

science 

USGS

for a changing world

Unit-Area Loads of Suspended Sediment, Suspended Solids, and Total 

Phosphorus From Small Watersheds in Wisconsin

By Steven R. Corsi 1 , David J. Graczyk1 , David W. Owens 1 , and Roger T. Bannerman2 

Introduction

45° 

Watershed planners in the Wisconsin Department of Natural Resources 

(WDNR) and in Wisconsin county governments use estimates of loads of 

total solids and total phosphorus in streams for numerous management 

purposes. A few examples of these are to establish load reduction 

goals, to estimate the relative magnitude of nonpoint sources com- 4i 

pared to point sources, and to estimate phosphorus loads to lakes. 

Solids and phosphorus are two of the most common nonpoint 

contaminants resulting from agricultural activity. Loads can be 

estimated either by monitoring the water quantity and water quality 

in a watershed or by modeling those same factors. Monitoring is the 

most accurate method for load estimation, but it is also time consuming and expensive. A simple method of estimating loads of chemical 

constituents or suspended solids in a watershed is to use unit-area loads that 

have been calculated from monitored data to estimate loads in watersheds 

where monitoring data are not available. A "unit-area load" is defined as the 

mass of a particular constituent transported by a stream, divided by the 

drainage area of the watershed.

The U.S. Geological Survey (USGS). in cooperation with the WDNR, is 

studying the factors that affect the loads of total solids and total phosphorus 

in Wisconsin watersheds. The objectives of that study are to:

 Tabulate unit-area loads and land-use characteristics for selected 

monitored watersheds in rural and urban areas of Wisconsin.

 Evaluate the effects of land-use characteristics, drainage area, and 

ecoregion on unit-area loads.

 Determine an appropriate grouping of unit-area loads for

applications in different watersheds in Wisconsin. 

This fact sheet summarizes unit-area loads of total suspended sediment 

or total suspended solids (a measure similar to total suspended sediment), 

and unit-area loads of total phosphorus from monitored watersheds in 

Wisconsin.

Watershed Characteristics

The USGS has monitored water quality in a number of watersheds in 

Wisconsin as part of studies conducted in cooperation with national, 

regional, state, and local agencies. All watersheds listed in this fact sheet are 

represented in USGS data bases for total suspended sediment, total suspended solids, and total phosphorus loads (fig. 1). The methods for analyzing total suspended sediment and total suspended solids are somewhat 

different. Thus, for water samples that contain large suspended particles 

(sand size or greater), the reported value of total suspended solids may be 

slightly less than the value reported for total suspended sediment analysis. 

For the purposes of this fact sheet, however, the two constituents are 

considered to be interchangeable.

The following criteria, which were met by 52 watersheds, were used to 

select the watersheds included in this summary:

 Data were collected from 1975 through 1996.

 Drainage areas are less than 200 square miles.

 One or more years of continuous data on sediment (or solids) and 

phosphorus loads are available.

 Point sources contribute less than 15 percent of the total monitored 

yearly load.

1 U.S. Geological Survey; 2 Wisconsin Department of Natural Resources

EXPLANATION 

Ecoregions

^jj Northern Lakes 

and Forests

North Central 

Hardwood Forests

Driftless Area

Southeastern Wisconsin 

Till Plains

Load monitoring and 

stream gaging stations

A Rural watershed 

A Urban watershed

38.39, 

42.43, o 

44,45, L 

48,49.

51

25 50 MILES 

0 25 50 KILOMETERS

Figure 1 . Location of watersheds and gaging stations.

Land use. drainage area, ecoregion, and other watershed characteristics 

such as slope, soil type, and climate affect the magnitude and variability of 

unit-area loads. Land-use data for each of the watersheds (table 1) were 

compiled by the WDNR Research Center on the basis of aerial-photograph 

interpretation from photographs taken throughout the 1970's and 1980's 

(U.S. Geological Survey, 1990). Data follow the format of the Land Use and 

Land Cover classification system (Anderson and others, 1976). Of the 52 

selected watersheds, 7 are rural with 50 percent or more land in forest, and 

30 are rural with 50 percent or more land in agriculture; the remaining 15 

watersheds are more than 20 percent urban. Watersheds are grouped in table 

1 by ecoregions (Omernick and Gallant, 1988) in areas of similar climate, 

landforms, soil, natural vegetation, hydrology, or other ecologically relevant variables. All but six watersheds monitored were in either the Driftless 

Area or the Southeastern Wisconsin Till Plains. All watersheds shown as 

having 20 percent or greater urban land use are in the Southeastern Wisconsin Till Plains in the Milwaukee and Madison areas; the data from these 

watersheds are summarized separately from those for rural watersheds. 

Previous studies indicate that the quality of biota and habitat of Wisconsin 

streams draining watersheds with greater than 10 to 20 percent urban land 

use is not as good as the quality of biota and habitat of more rural streams 

(Wang and others, 1997).

Calculated Unit-Area Loads

Loads of total suspended sediment (or solids) and phosphorus from the 

selected watersheds 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 low-flow periods. For these sites, the 

integrator method (Porterfield, 1972) was used to compute total annual 

loads. For the remainder of the sites, multiple samples were collected during 

each storm runoff period and composited into a single sample; analyses of
Table 1. Land use, drainage area, and unit-area loads summary statistics for selected monitored watersheds in Wisconsin

Watershed and 

monitoring station

USGS 

Downstream Drainage Area

Land-use percentage

order number (square miles) Urban Agriculture Forest Water Wetland Other

Unit-area loads of total suspended solids Unit-area loads of total phosphorus

or sediment (tons per square mile) (pounds per square mile)

Years of Years of

Minimum Maximum Median Record Minimum Maximum Median Record

1 Bear River near Manitowish Waters

2 Little Balsam Creek near Patzau

3 Little Balsam Creek Tributary near Patzau

4 Pine Creek Tributary near Moquah

5 Pine Creek near Moquah

6 Duncan Creek Tributary near Tilden

Apple River near Shullsburg 

Black Earth Creek at Cty P 

Brewery Creek at Cross Plains 

Bruce Valley Creek near Pleasantville 

Eagle Creek near Fountain City 

Elk Creek near Independence 

Garfoot Creek at Cross Plains 

Joos Valley Creek near Fountain City 

Kickapoo River at Ontario 

Kuenster Creek near North Andover 

Madden Branch near Meekers Grove 

Madden Branch Tributary near Belmont 

Pats Creek near Belmont 

Rattlesnake Creek near North Andover 

Steiner Branch near Waldwick 

Trout Creek Site A near Barneveld 

Yellowstone River near Blanchardville

24

25

26

27

28

29

30

31

32

33

34

35

36

37

Bower Creek near DePere

Delevan Lake Tributary at Delevan

Green Lake Inlet near Green Lake

Jackson Creek near Elkhorn

Little Menomonee River near Friestadt

Onion River near Sheboygan Falls

Otter Creek near Plymouth

Pheasant Branch at Middleton

South Branch Manitowoc River at Hayton

South Fork Pheasant Branch at Hwy 14

Silver Creek near Ripon

Turtle Creek near Clinton

White Creek near Green Lake

Yahara River at Windsor

05357335

04024315

04024318

04026348

04026349

05364850

05418731

05406460

05406470

05379288

05378185

05379305

05406491

05378183

05407500

054134435

05414920

05414915

05414894

05413449

05433510

05406573

05433500

04085119

05431018

04073468

05431016

04087050

04085845

040857005

05427948

04085395

05427945

040734644

05431486

04073462

05427718

81.3

5.2

0.5

0.6

21.5

4.2

9.3

14.6

10.5

10.1

14.3

99.7

5.4

5.9

151

9.6

15

2.8

5.4

42.4

5.9

8.4

28.5

14.8

10

53.5

16.8

8

91.8

9.5

18.3

109

5.7

36.2

199

3.1

73.6

Northern Lakes and Forests Ecoregion

0.9 

0.0 

0.0 

0.0 

0.0

0.1

6.0

49.9

15.9

27.7

60.0

81.2

50.1

84.1

72.3

22.3 

0.6 

0.0 

0.0 

0.0

16.4

12.1

0.0

0.0

0.0

0.3 

0.0 

0.0 

0.0 

0.0

North Central Hardwood Forests Ecoregion

0.0 91.9 8.1 0.0 0.0 0.0

Driftless Area Ecoregion

o.o

2.6 

0.9 

0.0 

0.0 

0.1 

0.0 

0.0 

0.4 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.6 

0.0

99.2

68.1

81.9

66.4

37.1

66.6

56.0

36.7

62.4

100.0

100.0

100.0

100.0

99.2

71.0

56.3

99.0

0.8

27.9

16.7

33.6

62.9

33.3

43.8

63.3

37.2

0.0

0.0

0.0

0.0

0.8

29.0

43.1

1.0

0.0 

0.3 

0.5 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0

0.0

1.1

0.0 

0.0 

0.0 

0.0 

0.2 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0 

0.0

Southeastern Wisconsin Till Plains Ecoregion-Rural

0.0

1.0

5.6

13.2

10.4

1.1

5.7

8.4

2.6

15.1

7.9

5.0

0.3

2.7

99.4

94.7

85.1

86.3

84.5

92.8

85.9

90.3

87.2

84.3

85.2

88.5

92.7

96.0

0.6 

2.2 

1.9 

0.0 

3.6 

4.1 

6.1 

0.1 

5.8 

0.0 

0.8 

3.6 

7.1 

0.7

0.0 

0.4 

0.6 

0.5 

0.0 

0.3 

1.4 

0.0 

0.1 

0.0 

0.2 

1.9 

0.0 

0.2

0.0 

1.7 

6.3 

0.0 

0.0 

1.5 

0.6 

0.0 

4.2 

0.0 

5.8 

0.8 

0.0 

0.2

0.0 

0.0 

0.5 

0.1 

1.6 

0.2 

0.2 

1.1 

0.1 

0.5 

0.2 

0.3 

0.0 

0.2

2

29

31

5

113

274

37

237

78

36

239

229

200

139

85

45

54

3

116

133

27

348

17 275

137 282

31 70

4 251

557

216

493

191

1010

684

740

309

837

369

266

220

3

73

82

16

231

146

209

50

63

215

429

200

63

304

135

332

462

485

254

200

227

175

137

49

4

7

15

15

78

9

14

5

61

11

45

24

8

751

12

67

103

62

90

91

350

6

97

48

177

1710

116

131

8

15

17

39

84

26

81

5

63

19

111

338

22

4

2

9

3

2

2

6

14

2

3

9

2

7

6

23

461

106

67

623

324

537

240

749

543

681

722

231

618

41

141

201

85

183

176

85

86

26

387 1600

1400

312

1020

1250

1310

109

3960

2290

207

1750

3670

708

1800

59

438

455

588

1440

666

1400

526

Southeastern Wisconsin Till Plains Ecoregion-Urban

24

372

320

106

943

992

929

209

346

1600

813

1460

528

752

803

957

1520

1310

1210

821

469

709

683

685

50

194

328

*

246

650

84

340

283

722

458

154

38

39

40

41

-*. 42

^x 43

}/44

/ 45

I 46

s l 47

48

49

50

51

52

Hawley Road Storm Sewer at Wauwatosa

Honey Creek at Wauwatosa

Jackson Creek Tributary near Elkhorn

Jefferson Park Drainage at Germantown

Kinnickinnic River at Milwaukee

Lincoln Creek at Milwaukee

Little Menomonee River at Milwaukee

Menomonee River at Wauwatosa

Monroe St. detention pond inlet at Madison

Nine Springs Creek tributary storm sewer

at Madison

Noyes Creek at Milwaukee

Schoonmaker Creek at Wauwatosa

Spring Harbor at Madison

Underwood Creek at Wauwatosa

Willow Creek at Madison

04087130

04087119

054310157

04087019

04087159

40869415

04087070

04087120

430309089260701

05429268

04087060

04087125

05427965

04087088

05427970

1.8

10.3

4.3

1.8

20.2

9.6

19.7

123

0.4

0.2

1.9

1.9

3.3

18.2

3.2

100.0

94.0

39.0

19.6

95.7

100.0

26.5

49.8

100.0

100.0

79.5

100.0

57.5

82.4

95.6

0.0

5.6

59.3

80.4

4.3

0.0

68.9

45.7

0.0

0.0

19.0

0.0

36.9

10.3

4.3

0.0

0.0

0.0

0.0

0.0

0.0

3.3

3.0

0.0

0.0

0.0

0.0

0.0

4.3

0.1

0.0

0.0

1.2

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.3

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.7

0.0

0.0

0.0

0.0

0.0

2.2

0.0

0.0

0.4

0.5

0.0

0.0

0.0

1.3

0.8

0.0

0.0

1.4

0.0

5.6

0.6

0.0

163

27

264

75

21

77

177

165

54

61

45

80

169

184

329

140

85

205

303

230

63

163

57

293

17

166

52

451

297

100

107

74

141

240

197

59

130

51

143

1

2

13

1

2

1

2

5

2

2

2

2

5

2

6

 127

 698

133 1210 291

 1150

 1110

 328

 555

 524

171 446 308

585 1000 794

 662

 291

 526

 332

 558

 i^HM

e

e

13

e

e

1

e

e

2

2

e

e

e

e

e

[e, estimated; , not available; *, not used because point discharge 

greater than 15 percent of total load]

these composite samples resulted in an "event mean concentration." Samples 

also were collected during low-flow periods. "Event loads" were computed by 

multiplying the event mean concentrations and the stormflow volumes. The lowflow loads were computed by use of the integrator method. Total annual loads 

were computed by summing the event and low-flow loads. Unit-area loads were 

then computed for all watersheds by dividing total annual load by the watershed 

drainage area.

All of the unit-area loads presented in this fact sheet represent the sum of loads 

from low-flow periods and storm-runoff periods. Because many best-management practices are designed specifically for controlling nonpoint pollution 

during storm-runoff periods, it would be useful to have an estimate of what 

fraction of the total load originates from storm runoff. Storm-runoff loads were 

separated from total loads for Otter Creek near Plymouth and Silver Creek near 

Ripon in the Southeastern Wisconsin Till Plains and for Joos Valley Creek near 

Fountain City and Rattlesnake Creek near North Andover in the Driftless Area 

to give an indication of the magnitude of low-flow load and storm-runoff load as 

a percentage of total load. Median annual storm-runoff loads as a percentage of 

annual total suspended-sediment or suspended-solids loads are. for Otter Creek, 

66%; Silver Creek. 59%; Joos Valley Creek, 93%; and Rattlesnake Creek, 95%. 

Median annual storm-runoff loads as a percentage of annual total phosphorus 

loads are, for Otter Creek, 56%; Silver Creek, 36%; Joos Valley Creek, 87%; and 

Rattlesnake Creek, 82%. Storm-runoff percentages for the two watersheds in the 

Driftless Area are notably higher than those for the two sites in the Southeastern 

Wisconsin Till Plains.

Minimum, maximum, and median unit-area loads for each watershed listed in 

this fact sheet are presented in table 1. Several rural watersheds were not 

monitored for total phosphorus. For these watersheds, a regression analysis of 

monitored data was used to relate median unit-area loads of total suspended 

solids or sediment to median unit-area loads of total phosphorus and was further 

used to estimate unit-area loads of total phosphorus for rural watersheds without 

total phosphorus data. Not enough data on total phosphorus were available to do 

a similar regression for urban watersheds.

Unit-area loads of total suspended sediment (or solids) and total phosphorus 

within watersheds can vary greatly from year to year depending mainly on 

climatic conditions. Unit-area loads for several watersheds change by more than 

an order of magnitude from the year with minimum loads to the year with 

maximum loads (table 1). The statistics for watersheds with a long period of 

record are more representative of the variability of data from that particular 

watershed than those for watersheds with shorter periods of record. Data for 

watersheds with only a few years of record give some information about general 

loads, but variability is not well characterized.

Unit-area loads of total suspended sediment (or solids) and total phosphorus 

between watersheds also can vary greatly. This variability probably is due to a 

combination of many different watershed characteristics and climatic factors. 

For the watersheds listed in this fact sheet, no relation was apparent between unitarea loads and percent agriculture, percent forest, or drainage area. There were, 

however, differences between the unit-area loads of rural watersheds in the 

Southeastern Wisconsin Till Plains ecoregion and the Driftless Area ecoregion 

(fig. 2). Unit-area loads in the Driftless Area are typically greater than unit-area 

loads from rural watersheds in the Southeastern Wisconsin Till Plains. Watersheds in the Driftless Area tend to be steeper; consequently, runoff and stream 

velocities and thus erosion potential are higher, resulting in larger sediment and 

phosphorus loads. Total suspended sediment or solids for the Southeastern 

Wisconsin Till Plains indicate that median loads are slightly higher for urban 

watersheds than for rural watersheds in the same ecoregion, but that overall 

variability in urban unit-area loads is less.

Estimating loads

Minimum, maximum, and median unit-area loads of total suspended sediment 

(or solids) and total phosphorus for each ecoregion are presented in table 2. The 

minimum and maximum values represent extremes of all annual unit-area loads 

shown in table 1, whereas the median values are computed from the values in the 

median columns in table 1. The median unit-area loads for monitored watersheds 

can be used to estimate loads for watersheds where monitoring data are not 

available. From the variability in median unit-area loads shown in table 2, it is 

apparent that this method produces only a gross estimate of loads that should be 

used with caution.
9 ^ 1,000

w LU

LLJ <

10

LU <S)

w£ 100

LLJ O

D <

LLJ LLJ 

OCt 

Z < 

LU u. 

D- 5

10

I I I

D

O

-I LLJ 

<=!

10.000

b < 1,000

=) CL 

CC LU

IO

CL CL

100

10

EXPLANATION

Data value less than or equal to 

1.5 times the interquartile range 

outside the quartile

75th percentile

Median

25th percentile

/ /

/ /

* *

Figure 2. Distribution of unit-area 

loads of total suspended sediment/ 

solids and total phosphorus in 

ecoregions in Wisconsin.

Estimates of loads of total suspended sediment (or solids) and total model involving several additional variables could be used. Most existing

phosphorus can be made by using the following method:

From figure 1, identify the ecoregion of the watershed of interest. 

Then, find the median unit-area load for that ecoregion in table 2 

and multiply it by the drainage area of the watershed of interest.

watershed models of this type, however, are time consuming and expensive 

to use. Another way to improve the estimates would be to increase 

monitoring of loads of total suspended sediment (or solids) and total 

phosphorus in watersheds with more diverse land uses and watershed 

characteristics.

As an example, consider Pheasant Branch at Middleton, whose drainage 

area is 18.3 square miles. Suspended sediment loads at Pheasant Branch 

were monitored for 1 4 years, and total phosphorus loads were monitored for 

3 years. During these periods, the median annual total suspended solids load 

was 1480 tons, and the median annual total phosphorus load was 1 1,900 

pounds. To estimate the loads using the method described above, one would 

find that Pheasant Branch is in the Southeastern Till Plains ecoregion (fig. 

1) and that the median unit-area loads for the Southeastern Till Plains are 

32.4 tons per square mile for total suspended sediment and solids and 283 

pounds per square mile for total phosphorus (table 2). Multiplying the unitarea loads by the drainage area results in total annual load estimates of 593 

tons for total suspended sediment or solids and 5,180 pounds for total 

phosphorus. This example demonstrates that loads determined by this 

method are gross approximations total suspended sediment and total 

phosphorus are underestimated by 60 percent and 57 percent, respectively.

In order to estimate loads more accurately, a more elaborate watershed

References Cited

Anderson, J.R., Hardy, E.E., Roach, J.T., and Witmer, R.E., 1976, A land use and land 

cover classification system for use with remote sensor data: U.S. Geological Survey 

Professional Paper 964. 28 p.

Omernick, J.M., and Gallant, A.L., 1988, Ecoregions of the Upper Midwest States: 

Corvallis, Oreg., U.S. Environmental Protection Agency. Environmental Research 

Laboratory, EPA/600/3-88/037, 56 p., I map.

Porterfield, George. 1972, Computation of fluvial-sediment discharge: U.S. Geological Survey Techniques of Water- Resources Investigations, book 3, chap. C3, 66 p.

U.S. Geological Survey, 1 990, Land use and land cover digital data from 1 :250,000- 

and 1 : 1 00,000-scale maps. Data user guide 4: Reston. Va., U.S. Geological Survey, 

25 p.

Wang, L., Lyons, J., Kanehl, P., and Gatti, R., 1997, Influences of watershed land use 

on habitat quality and biotic integrity in Wisconsin streams: Fisheries, v. 22, no. 6, 

June 1997, p. 6-12.

Table 2. Minimum, maximum, and median unit-area loads of total suspended sediment or solids and total phosphorus for ecoregions in Wisconsin 

[ , insufficient data; to convert tons per square mile to pounds per acre, multiply by 3.12; to convert tons per square mile to kilograms per hectare, multiply 

by 3.50; to convert pounds per square mile to pounds per acre, divide by 640; to convert pounds per square mile to kilograms per hectare, multiply by 0.00175]

Unit-area loads of 

total suspended solids or 

sediment (tons per square mile)

Unit-area loads of 

total phosphorus 

(pounds per square mile)

Ecoregions Mjnimum

Northern Lakes and Forests Ecoregion 

Driftless Area Ecoregion 

Southeastern Wisconsin Till Plains Ecoregion Rural 

Southeastern Wisconsin Till Plains Ecoregion Urban 

State Summary Rural 

State Summary

2.00 

4.29 

4.40 

17.0 

2.32 

2.32

Maximum

348 

1010 

1710 

451 

1710 

1710

Number of 

Median Watersheds

73 

209 

32.4 

130 

111 

120

5 

17 

14 

15 

36 

52

Minimum

66.7 

40.7 

133 

23.1 

23.1

Maximum

3960 

1800 

1210 

3960 

3960

Number of 

Median Watersheds

875 

283 

318 

650 

499

14 

9 

4 

24 

28

For more information, please contact:

District Chief

U.S. Geological Survey

8505 Research Way

Middleton, Wl 53562

(608) 828-9901

www: http://wwwdwimdn.er.usgs.gov

Layout and illustrations: Gail Moede and Michelle Greenwood 

Banner graphic: Karen Lonsdorf

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet FS-195-97