Evaluation of a Method for Comparing Phosphorus Loads from Barnyards

and Croplands in Otter Creek Watershed, Wisconsin

Figure 1. Otter Creek Watershed study areas.

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet FS-168-98

December 1998

Introduction

Control of phosphorus from rural nonpoint sources is a

major focus of current efforts to improve and protect water

resources in Wisconsin and is recommended in almost

every priority watershed plan prepared for the State’s

Nonpoint Source (NPS) Program. Barnyards and croplands usually are identified as the primary rural sources of

phosphorus. Numerous questions have arisen about which

of these two sources to control and about the method

currently being used by the NPS program to compare

phosphorus loads from barnyards and croplands. To evaluate the method, the U.S. Geological Survey (USGS), in

cooperation with the Wisconsin Department of Natural

Resources, used phosphorus-load and sediment-load data

from streams and phosphorus concentrations in soils from

the Otter Creek Watershed (located in the

Sheboygan River Basin; fig. 1) in conjunction with two computer-based models. This fact sheet describes:

• how stream data were used to simulate phosphorus loads in barnyard

and cropland runoff,

• how measured concentrations of

soil phosphorus were used to

modify estimates of the cropland

phosphorus loads,

• how the results of the model simulations and the soil phosphorus

analysis affect the relative amount

of phosphorus estimated to come

from barnyards and croplands, and

• how the relative proportion of phosphorus estimated to come

from barnyards increases as the number of barnyards in a

watershed increases.

Method for Comparing Phosphorus Loads From

Barnyards and Croplands

Two computer models are used by NPS to compare phosphorus

loads from barnyards and croplands. The BARNY model (Wisconsin Department of Natural Resources, 1994a) is used to estimate

loads of phosphorus and chemical oxygen demand in stormwater

runoff from individual barnyards. The WINHUSLE model (Wisconsin Department of Natural Resources, 1994b) is used to estimate

average annual loads of sediment (eroded soil) delivered to the

nearest channel from individual farm fields. WINHUSLE does not,

however, directly estimate the phosphorus loads from individual

fields; sediment loads from individual farm fields estimated from

WINHUSLE must be multiplied by a phosphorus enrichment factor

(pounds of phosphorus per ton of sediment). The enrichment factor

used by the NPS program is usually based on annual phosphorus and

sediment loads measured at outlets of watersheds from several

locations around Wisconsin. Median values for this watershedoutlet phosphorus enrichment factor have been about 4 and 8 lb/ton

(pounds per ton) for streams in the southwestern and southeastern

parts of Wisconsin, respectively (Corsi and others, 1998). Because

monitoring data are not available for most watersheds, enrichment

factors for unmonitored watersheds are based on data from monitored watersheds.

0 .5 1 MILE

0 .5 1 KILOMETER

EXPLANATION

0 50 100 MILES

0 50 100 KILOMETERS

Otter Creek

Watershed 

Sheboygan River

Basin 

WISCONSIN

44°

46°

45°

43°

92°

91°

90° 89°

88°

87°

Monitoring station

Rain gage

Upstream and downstream monitoring gage

Barnyard

Watershed boundary

Noncontributing areas

Watershed 

Area 1

Area 2 boundary

Area 3
Figure 2. Monitored and modeled phosphorus and sediment loads from

croplands and barnyards for selected storms in Otter Creek.

Testing the BARNY and WINHUSLE Models

An important part of evaluating the method used for comparing

phosphorus loads from barnyards and croplands is to test the

accuracy of the individual models used in the method. Although the

accuracy of a model is improved if the model is first tested with sitespecific data, it is not practical to collect phosphorus-load and

stream-sediment data in every watershed. Water-quality data collected in the Otter Creek Watershed (fig. 1) were used to evaluate the

models.

BARNY

Data on phosphorus loads from a barnyard in the Otter Creek

Watershed were used to test the BARNY model. Otter Creek was

monitored before pollution-control structures were installed at the

barnyard (Stuntebeck, 1995). Water samples were collected and

flow was measured just upstream and downstream from the barnyard for 11 runoff events (periods of runoff associated with storms)

in 1995. Phosphorus load in runoff from the barnyard was determined by subtracting the upstream load from the downstream load.

BARNY underestimated the phosphorus loads for most of the

individual runoff events, and the total simulated load for the 11

storms was about one-half of the monitored load (fig. 2).

Phosphorus loads estimated with BARNY were

about one-half the monitored value.

WINHUSLE

Sediment loads for runoff events measured for 5 years (1991 to

1995) at the mouth of Otter Creek were used to evaluate the

WINHUSLE model (Owens and others, 1996). Water samples were

collected during most of the significant runoff events during this

period along with routine bimonthly base-flow monitoring. Monitoring was done before most upland pollution practices were implemented. However, WINHUSLE was only applied to the farm fields

in one of Otter Creek’s subwatersheds, because two of the three

subwatersheds in Otter Creek are non-contributing. These noncontributing subwatersheds drain into lakes that are considered

effective sinks for sediment loads (fig.1). All the Otter Creek rainfall

values that were greater than 0.1 inch were used to run the model.

The data inputs into the WINHUSLE model were stored in and

derived from a detailed geographic information system land-use

data base. This data base was used to determine the acreage of the

fields and soil types, the overall watershed acreage, and the length

and elevation of the channels. Also, a large number of the farm fields

were inventoried to verify the values used in the model to calculate

soil loss. Croplands were approximately 60 percent of the total land

use/land cover, soils were predominantly Kewaunee silt-loams, and

average field slopes were 5 to 6 percent (Rappold and others, 1997).

Average annual sediment loads estimated by WINHUSLE were

compared with those measured at the mouth of Otter Creek between

1991 and 1995. WINHUSLE was found to overpredict sediment

loads by a factor of about 2.

Barnyard at Otter Creek before pollution-control structures were

installed.

Average annual phosphorus loads estimated by BARNY for

individual barnyards were particularly sensitive to the dimensions

of the yard, the size of the area draining to the barnyard, and the

distance of the barnyard to the stream. For example, the total

simulated load was six times lower than the monitored load before

the size of the yard was increased to include a cow path area between

the yard and the stream.

Sediment loads estimated with WINHUSLE were

about double the monitored value.

0

50

100

150

200

250

300

0

100

200

300

400

500

600

120.0

256.0

534.0

260.0

Modeled with BARNY

Monitored

Modeled with WINHUSLE

PHOSPHORUS SEDIMENT

1993–94 PHOSPHORUS LOADS (POUNDS PER YEAR)

1991–95 SUSPENDED SEDIMENT (TONS PER YEAR)
Farm field before implementation of pollution-control practices in Otter

Creek Watershed.

Table 1. Ratio of fields to barnyards and percentage of loads from barnyards for three study areas in

Otter Creek Watershed

This soil-concentration enrichment factor is about

one-fifth of the watershed-outlet enrichment factor

of 9.5 lb/ton for Otter Creek.

 Cropland Phosphorus Loads

As mentioned previously, the method for estimating cropland

phosphorus loads depends on using a phosphorus enrichment factor. A watershed-outlet phosphorus enrichment factor might not

represent the phosphorus concentration in the soils delivered to the

edge of a channel. Selective transport of different-size soil particles

could result in a phosphorus concentration in the vicinity of farmed

fields that is different from that at the mouth of a stream. In addition,

phosphorus loads at the mouth of Otter Creek could reflect the

effects of other phosphorus sources, such as barnyards.

Site-specific soils data from Otter Creek were used to determine

a soils-concentration enrichment factor. Soil samples were collected from five fields that could contribute runoff to the stream. The

samples were divided into sand, silt, and clay particle-size fractions.

Each fraction was then analyzed for phosphorus content following

the method of Dong and others (1979). The average soil concentrations for phosphorus for the sand-, silt-, and clay-size fractions were

142, 835, and 1,850 μg/g (micrograms of phosphorus per gram of

soil), respectively. The total concentration of phosphorus in the soil

was 737 μg/g, which is equivalent to 1.5 pounds of phosphorus per

ton of soil. The total soil phosphorus concentration was then

adjusted for the deposition of the sand-sized particles before they

reached the edge of the channels. Particle-size analysis of the soil in

the water samples collected from Otter Creek showed the sand

content of the soil was reduced from 42 to 8 percent by the time the

soil reached the channel. By decreasing the sand-size fraction to 8

percent, the particle size distributions were shifted to higher proportions of clay and silt in the soil. The soils phosphorus concentration

representing this new particle-size distribution was 1,074 μg/g, or

2.1 lb/ton.

Application of the Model Tests Results and Soil

Phosphorus Concentrations to Three Otter Creek

Study Areas

Results from testing the models and the determination of phosphorus enrichment factors in Otter Creek indicates that the application of these results would significantly change previous estimates

of how much phosphorus is coming from barnyards and croplands.

To adjust the modeled load estimates for Otter Creek, the phosphorus load from BARNY should be doubled and the sediment load

from WINHUSLE should be reduced by one-half. Assuming that

the soil-concentration phosphorus enrichment factor produces a

more accurate estimate of cropland phosphorus load than the watershed-outlet enrichment factor, the sediment loading value for Otter

Creek would have to be multiplied by 2.1 lb/ton instead of 9.5 lb/ton.

The magnitude of these adjustments will probably vary between

watersheds, depending on the soil phosphorus concentrations and

on the particle-size distributions of the farm-field sediment.

Corrections applied to the model outputs and the phosphorus

enrichment factor increased the relative importance of the barnyards

as phosphorus sources in three study areas (fig. 3). Study area 1 is the

Otter Creek watershed used to evaluate the model (fig. 1). The other

two study areas are nested within study area 1, and have a lower

farm-field to barnyard ratio (that is, a higher density of barnyards)

(table 1). Ratios of the number of farm fields to barnyards range from

32:1 to 10:1. This ratio includes only the fields and barnyards

classified as high priority for management (Category I and II

management sources eligible for funding or technical assistance;

Wierl and others, 1996). Percentages of each land-use/land-cover

category and soil types and slopes are similar for all the study areas.

The correction increased the relative importance of the barnyards as

phosphorus sources in the three study areas (fig. 3). In study area 1,

for example, the barnyard loads increased from 2 percent of the total

load to 24 percent of the total load when the barnyard and cropland

corrections were applied. In terms of relative proportion of the total

phosphorus load, barnyards go from being a negligible or small

source of phosphorus (before the application of the corrections) to

a significant source in all three study areas.

Study

area

Number of

Category I and II

fields (acres)

Number of

barnyards

Ratio of

fields to

barnyards

Percentage of phosphorus

from barnyard

before corrections

Percentage of phosphorus

from barnyard

after corrections

1 161 (1,395) 5 32:1 2 24

2 83 (794) 4 21:1 4 41

3 20 (149) 2 10:1 15 76
Information

For more information, please contact:

District Chief

U.S. Geological Survey

8505 Research Way

Middleton, WI 53562

(608) 828-9901

www: http://wi.water.usgs.gov/

Changes in Phosphorus Loads With Increases in the

Number of Barnyards in a Watershed

If phosphorus loads are computed with corrections to both the

model loading values and phosphorus enrichment factors, the percentage of the phosphorus coming from barnyards increases by a

factor of 3 from study area 1 to study area 3 (table 1). Controlling

phosphorus in barnyard runoff appears to be as important as reducing phosphorus in cropland runoff where the ratio of farm fields to

barnyards is about 20:1 or less. The ratios of fields to barnyards in

watersheds around the State commonly fall in the range of the ratios

of study areas 2 and 3. For example, the ratio for Eagle Creek in

Buffalo County is about 10:1, and for Bower Creek in Brown County

is about 15:1 (Wierl and others, 1996). Reducing barnyard phosphorus loads in watersheds with field-to-barnyard ratios of about 30:1

or higher, such as Otter Creek, does not appear to be as important as

reducing cropland phosphorus loads.

Implications of Access to Site-Specific Data

Site-specific water-quality monitoring and soil phosphorus concentration data could improve the accuracy of estimating barnyard

and cropland runoff. The computed relative loads of phosphorus

from barnyards increased significantly when site-specific data were

used for Otter Creek. This effect would be magnified in watersheds

with small ratios of fields to barnyards. The differences in determining phosphorus loads with and without site-specific data were large

enough to change the conclusion about which of the phosphorus

sources was more important. Methods for estimating barnyard and

cropland phosphorus loads could be improved by:

• verifying the accuracy of BARNY inputs, such as size of the

barnyard, size of the areas draining to the barnyard, and the

distance between the barnyard and the stream,

• calibrating WINHUSLE on existing monitored data or collecting new data in regional areas where gaps are apparent,

and

• creating State regional site-specific soil-concentration phosphorus enrichment factors by occasionally processing (using

the method of Dong and others, 1979) farm-field soil samples

for the NPS priority watershed program.

References Cited

Corsi, S.R., Graczyk, D.J., Owens, D.W., and Bannerman, R.T., 1998, Unitarea loads of suspended sediment, suspended solids and total phosphorus

from small watersheds in Wisconsin: U.S. Geological Survey Fact Sheet

FS-195-97, 4 p.

Dong, Allen, Chesters, Gordon, and Simsiman, G.V., 1979, Dispersibility

of soils and elemental composition of soils, sediments, and dust and dirt

from the Menomonee River Watershed: U.S. Environmental Protection

Agency Report EPA-905/4-79-029-F, 56 p.

Owens, D.W., Corsi, S.R., and Rappold, K.F., 1996, Evaluation of nonpoint

contamination, Wisconsin; selected topics for water year 1995: U.S.

Geological Survey Open-File Report 96-661A, 41 p.

Rappold, K.F., Wierl, J.A., and Amerson, F.U., 1997, Watershed characteristics and land management in the nonpoint-source evaluation monitoring watersheds in Wisconsin: U.S. Geological Survey Open-File Report

97-119, 39 p.

Stuntebeck, T.D., 1995, Evaluating barnyard best management practices in

Wisconsin using upstream-downstream monitoring: U.S. Geological

Survey Fact Sheet FS-221-95, 4 p.

Wierl, J.A., Rappold, K.F., and Amerson, F.U., 1996, Summary of the landuse inventory for the nonpoint-source evaluation monitoring watershed

in Wisconsin: U.S. Geological Survey Open-File Report 96-123, 23 p.

Wisconsin Department of Natural Resources, 1994a, BARNY 2.2—The

Wisconsin barnyard runoff model, inventory instructions and user’s

manual: Report WR-285-91, 35 p.

Wisconsin Department of Natural Resources, 1994b, WINHUSLE—Model

documentation and user’s manual, version 1.4.4: Report WR-294-91

[variously paged].

Figure 3. Changes in proportion of cropland and barnyard contributions to

watershed phosphorus loads using uncorrected and corrected phosphorus loads for three areas in the Otter Creek Watershed. AREA 1 AREA 3 AREA 2

Before corrections After corrections

2%

98%

4%

96%

15%

85%

24%

76%

41%

59%

24%

76%

Cropland

Barnyard

Authors: Judy A. Wierl, and Elise M.P. Giddings, U.S. Geological Survey,

Water Resources Division;

and Roger T. Bannerman, Wisconsin Department of Natural Resources

Layout and illustrations: Michelle Greenwood

Banner graphic: Karen Lonsdorf