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

Unit-Area Loads of Suspended Sediment, Suspended Solids, and Total Phosphorus From Small Watersheds in Wisconsin

by Steven R. Corsi1, David J. Graczyk1, David W. Owens1, and Roger T. Bannerman2

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

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Introduction

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 compared 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:

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:

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 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 low-flow 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 unit-area 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.

Estimates of loads of total suspended sediment (or solids) and total 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.

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 14 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 11,900 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 unit-area 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 model involving several additional variables could be used. Most existing 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.

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., 1 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, 1990, Land use and land cover digital data from 1:250,000- and 1:100,000-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 1. Land use, drainage area, and unit-area loads summary statistics for selected monitored
watersheds in Wisconsin
[sq mi, square miles; Ag, agriculture]
-------------------------------------------------------------------------------------------------------------
		                          USGS        Drainage                Land-use percentage
Watershed and                          Downstream     Area     ----------------------------------------------
monitoring station                    order number    (sq mi)  Urban    Ag    Forest  Water  Wetland  Other
-------------------------------------------------------------------------------------------------------------
                          Northern Lakes and Forests Ecoregion
-------------------------------------------------------------------------------------------------------------
1  Bear River  near Manitowish Waters   05357335       81.3     0.9     0.1   60.0   22.3   16.4    0.3
2  Little Balsam Creek  near Patzau     04024315        5.2     0.0     6.0   81.2    0.6   12.1    0.0
3  Little Balsam Creek Tributary        04024318        0.5     0.0    49.9   50.1    0.0    0.0    0.0
    near Patzau
4  Pine Creek  Tributary near Moquah    04026348        0.6     0.0    15.9   84.1    0.0    0.0    0.0
5  Pine Creek near Moquah               04026349       21.5     0.0    27.7   72.3    0.0    0.0    0.0
-------------------------------------------------------------------------------------------------------------
                       North Central Hardwood Forests Ecoregion
-------------------------------------------------------------------------------------------------------------	
6  Duncan Creek Tributary near Tilden   05364850        4.2     0.0    91.9    8.1    0.0    0.0    0.0
-------------------------------------------------------------------------------------------------------------
                               Driftless Area Ecoregion
-------------------------------------------------------------------------------------------------------------
7  Apple River near Shullsburg          05418731        9.3     0.0    99.2    0.8    0.0    0.0    0.0
8  Black Earth Creek at Cty P           05406460       14.6     2.6    68.1   27.9    0.3    0.0    1.1
9  Brewery Creek at Cross Plains        05406470       10.5     0.9    81.9   16.7    0.5    0.0    0.0
10 Bruce Valley Creek near Pleasantville 05379288      10.1     0.0    66.4   33.6    0.0    0.0    0.0
11 Eagle Creek near Fountain City       05378185       14.3     0.0    37.1   62.9    0.0    0.0    0.0
12 Elk Creek  near Independence         05379305       99.7     0.1    66.6   33.3    0.0    0.0    0.0
13 Garfoot Creek at Cross Plains        05406491        5.4     0.0    56.0   43.8    0.0    0.0    0.2
14 Joos Valley Creek near Fountain City 05378183        5.9     0.0    36.7   63.3    0.0    0.0    0.0
15 Kickapoo River at Ontario            05407500      151       0.4    62.4   37.2    0.0    0.0    0.0
16 Kuenster Creek near North Andover    054134435       9.6     0.0   100.0    0.0    0.0    0.0    0.0
17 Madden Branch near Meekers Grove     05414920       15       0.0   100.0    0.0    0.0    0.0    0.0
18 Madden Branch Tributary near Belmont 05414915        2.8     0.0   100.0    0.0    0.0    0.0    0.0
19 Pats Creek near Belmont              05414894        5.4     0.0   100.0    0.0    0.0    0.0    0.0
20 Rattlesnake Creek near North Andover 05413449       42.4     0.0    99.2    0.8    0.0    0.0    0.0
21 Steiner Branch near Waldwick         05433510        5.9     0.0    71.0   29.0    0.0    0.0    0.0
22 Trout Creek Site A near Barneveld    05406573        8.4     0.6    56.3   43.1    0.0    0.0    0.0
23 Yellowstone River near Blanchardville 05433500      28.5     0.0    99.0    1.0    0.0    0.0    0.0
-------------------------------------------------------------------------------------------------------------
                    Southeastern Wisconsin Till Plains Ecoregion - Rural
-------------------------------------------------------------------------------------------------------------
24 Bower Creek near DePere              04085119       14.8     0.0    99.4    0.6    0.0    0.0    0.0
25 Delevan Lake Tributary at Delevan    05431018       10       1.0    94.7    2.2    0.4    1.7    0.0
26 Green Lake Inlet near Green Lake     04073468       53.5     5.6    85.1    1.9    0.6    6.3    0.5
27 Jackson Creek near Elkhorn           05431016       16.8    13.2    86.3    0.0    0.5    0.0    0.1
28 Little Menomonee River near Friestadt 04087050       8      10.4    84.5    3.6    0.0    0.0    1.6
29 Onion River near Sheboygan Falls     04085845       91.8     1.1    92.8    4.1    0.3    1.5    0.2
30 Otter Creek near Plymouth            040857005       9.5     5.7    85.9    6.1    1.4    0.6    0.2
31 Pheasant Branch at Middleton         05427948       18.3     8.4    90.3    0.1    0.0    0.0    1.1
32 South Branch Manitowoc River at Hayton 04085395    109       2.6    87.2    5.8    0.1    4.2    0.1
33 South Fork Pheasant Branch at  Hwy 14 05427945       5.7    15.1    84.3    0.0    0.0    0.0    0.5
34 Silver Creek near Ripon               040734644     36.2     7.9    85.2    0.8    0.2    5.8    0.2
35 Turtle Creek near Clinton             05431486     199       5.0    88.5    3.6    1.9    0.8    0.3
36 White Creek near Green Lake           04073462       3.1     0.3    92.7    7.1    0.0    0.0    0.0
37 Yahara River at Windsor               05427718      73.6     2.7    96.0    0.7    0.2    0.2    0.2
-------------------------------------------------------------------------------------------------------------
                    Southeastern Wisconsin Till Plains Ecoregion - Urban
-------------------------------------------------------------------------------------------------------------
38 Hawley Road Storm Sewer at Wauwatosa  04087130       1.8   100.0     0.0    0.0    0.0    0.0    0.0
39 Honey Creek at Wauwatosa              04087119      10.3    94.0     5.6    0.0    0.0    0.0    0.4
40 Jackson Creek Tributary near Elkhorn  054310157      4.3    39.0    59.3    0.0    1.2    0.0    0.5
41 Jefferson Park Drainage at Germantown 04087019       1.8    19.6    80.4    0.0    0.0    0.0    0.0
42 Kinnickinnic River at Milwaukee       04087159      20.2    95.7     4.3    0.0    0.0    0.0    0.0
43 Lincoln Creek at Milwaukee            040869415      9.6   100.0     0.0    0.0    0.0    0.0    0.0
44 Little Menomonee River at Milwaukee   04087070      19.7    26.5    68.9    3.3    0.0    0.0    1.3
45 Menomonee River at Wauwatosa          04087120     123      49.8    45.7    3.0    0.0    0.7    0.8
46 Monroe St. detention pond inlet   430309089260701    0.4   100.0     0.0    0.0    0.0    0.0    0.0
       at Madison
47 Nine Springs Creek tributary storm    05429268       0.2   100.0     0.0    0.0    0.0    0.0    0.0
       sewer at Madison
48 Noyes Creek at Milwaukee              04087060       1.9    79.5    19.0    0.0    0.0    0.0    1.4
49 Schoonmaker Creek at Wauwatosa        04087125       1.9   100.0     0.0    0.0    0.0    0.0    0.0
50 Spring Harbor at Madison              05427965       3.3    57.5    36.9    0.0    0.0    0.0    5.6
51 Underwood Creek at Wauwatosa          04087088      18.2    82.4    10.3    4.3    0.3    2.2    0.6
52 Willow Creek at Madison               05427970       3.2    95.6    4.3    0.1    0.0    0.0    0.0
-------------------------------------------------------------------------------------------------------------


Table 1. -- Continued
--------------------------------------------------------------------------------------------
       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
Site    Min     Max     Median       Record        Min      Max      Median       Record
--------------------------------------------------------------------------------------------
1        2       3        3             3          23        26        24           3
2       29     116       73             2           -         -       372           e
3       31     133       82             2           -         -       320           e
4        5      27       16             2           -         -       106           e
5      113     348      231             2           -         -       943           e
6       17     275      146             2         387      1600       992           2
7      137     282      209             2         461      1400       929           2
8       31      70       50             2         106       312       209           2
9        4     251       63             8          67      1020       346           8
10       -       -      215             1           -         -      1600           1
11     274     557      429             4         623      1250       813           4
12       -       -      200             1           -         -      1460           1
13      37     216       63             7         324      1310       528           7
14     237     493      304             4         537       109       752           4
15      78     191      135             4           -         -       803           e
16      36    1010      332             3         240      3960       957           3
17     239     684      462             2         749      2290      1520           2
18     229     740      485             2         543       207      1310           2
19     200     309      254             2         681      1750      1210           2
20     139     837      200             3         722      3670       821           3
21      85     369      227             2         231       708       469           2
22      45     266      175             4           -         -       709           e
23      54     220      137             2           -         -       683           e
24      49     751      131             4         618      1800       685           4
25       4      12        8             2          41        59        50           2
26       7      67       15             9           -         -         *           e
27      15     103       17             3         141       438       194           3
28      15      62       39             2         201       455       328           2
29      78      90       84             2           -         -         *           e
30       9      91       26             6          85       588       246           6
31      14     350       81            14         183      1440       650           3
32       5       6        5             2           -         -        84           e
33      61      97       63             3           -         -       340           e
34      11      48       19             9         176       666       283           9
35      45     177      111             2           -         -       722           e
36      24    1710      338             7          85      1400       458           7
37       8     116       22             6          86       526       154           6
38       -       -       17             1           -         -       127           e
39     163     169      166             2           -         -       698           e
40      27     184       52            13         133      1210       291          13
41       -       -      451             1           -         -      1150           e
42     264     329      297             2           -         -      1110           e
43       -       -      100             1           -         -       328           1
44      75     140      107             2           -         -       555           e
45      21      85       74             5           -         -       524           e
46      77     205      141             2         171       446       308           2
47     177     303      240             2         585      1000       794           2
48     165     230      197             2           -         -       662           e
49      54      63       59             2           -         -       291           e
50      61     163      130             5           -         -       526           e
51      45      57       51             2           -         -       332           e
52      80     293      143             6           -         -       558           e
--------------------------------------------------------------------------------------------


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                  Unit-area loads of
                                        total suspended solids or           total phosphorus
                                        sediment (tons per square mile)     (pounds per square mile)
----------------------------------------------------------------------------------------------------------
                                                                Number of                       Number of
    Ecoregions                           Min     Max  Median   Watersheds   Min    Max  Median  Watersheds
----------------------------------------------------------------------------------------------------------
Northern Lakes and Forests               2.00    348     73         5          -     -      -        -
Driftless Area                           4.29   1010    209        17       66.7   3960   875       14
Southeastern Wisconsin Till Plains-Rural 4.40   1710     32.4      14       40.7   1800   283        9
Southeastern Wisconsin Till Plains-Urban 17.0    451    130        15      133     1210   318        4
State Summary-Rural                       2.32  1710    111        36       23.1   3960   650       24
State Summary                             2.32  1710    120        52       23.1   3960   499       28
----------------------------------------------------------------------------------------------------------
For more information please contact:

District Chief
U.S. Geological Survey
8505 Research Way
Middleton, WI 53562-3586
(608)828-9901

Acknowledgements

Layout and illustrations: Gail Moede and Michelle Greenwood
Banner graphic: Karen Lonsdorf
Web layout: Dan Sullivan

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

USGS Water Resources of Wisconsin Home Page
Last modified: Fri May 15 14:40:14 1998