Soil Erosion Estimates for the Moody Creek Watershed, Idaho

Introduction: Moody Creek is a mountainous stream located in Southeastern Idaho near Rexburg, Idaho. Its source begins in the Big Hole mountain range and flows north northwest to the South Fork of the Teton River. According to the National Hydrography Dataset, the actual Moody Creek watershed has a stream network length of 301.7 km, and a drainage area of 297.7 km². Figure 1 is a map of the land use of the Moody Creek watershed. It can be see that much of the land associated with Moody Creek is irrigated and non-irrigated farm land. The farm land consists mostly of potatoes, barley, and wheat. The land with the land use of exempt is Bureau of Land Management and Targhee National Forest land.

Figure 1. Land Use for Moody Creek Watershed

In 1998, Idaho’s Department of Environmental quality placed Moody Creek on the 303(d) list. The 303(d) list, later the Surface Water Integrated Report, is a list of waterbodies that do not meet the water standard of the state or the US Environmental Protection Agency (EPA). For Moody Creek, nutrients were listed as the impairment, specifically nitrate and total phosphorus. Although not proven, it is assumed that the total phosphorus found in stream is from soil erosion. In September 2006, a Total Maximum Daily Load was approved by the US EPA for total phosphorus in Moody Creek. The TMDL calculates the maximum amount of a pollutant allowed to enter a waterbody, so that the waterbody will meet and continue to meet water quality standards for that particular pollutant. Therefore the TMDL acts as a pollutant budget and pollutant management plan (Idaho DEQ, 2008).

Currently the Natural Resource Conservation Service (NRCS) is working with local farmers to reduce the amount of erosion as part of the management plan. By reducing the amount of erosion, it is assumed that the concentration of phosphorus in the stream will also reduce. The purpose of this project was to assist the NRCS in identifying locations of high erosion rates using ArcGIS and the Revised Universal Soil Loss Equation (RUSLE).

The Revised Universal Soil Loss equation is an equation used to estimate the annual erosion due to rill and interrill erosion based on site specific conditions. The details of the equation along with input values will be shown later. As defined in the Scientific Documentation of RUSLE2, rill erosion is defined as the erosion that is caused by overland flow. Interrill erosion is the erosion caused by rain droplets (USDA, 2008). Figure 2 below shows rill erosion from the pivot sprinkler and similar erosion takes place during runoff event. Little gullies form and carry soil down these small channels.

Figure 2. Example of rill erosion in the Moody Creek Watershed

Procedure: Several steps were taken to complete this project. Data acquisition, calculating soil loss, interpreting results, and understanding restraints will be discussed below.

1. Data Acquisition: To determine the amount of erosion that takes place in the Moody Creek, certain data were obtained through various agencies and organizations. First the catchments and drainage lines were obtained from the National Hydrography Dataset (NHD). Although the catchments could have been derived from the DEM, the NHD provided an adequate description of the Teton River watershed, HUC number17040204. Using the capabilities of ArcGIS’s geometric network and utility network analyst, the catchments associated with Moody Creek could easily be selected and exported into their own shapefile. Next, a 10 meter digital elevation model (DEM) was obtained though the University of Idaho’s website. This covered an area much greater than the actually watershed so I masked it with the outline of the catchments. The DEM was then used to calculate the slope and slope lengths. Through the help of Madison County’s GIS Department, land use data was obtained for Moody Creek. Finally, soil data was obtained from the NRCS website. The analysis was highly dependent on this information. The NRCS also supplied a scanned copy of RUSLE factors. The RUSLE factors were determined in 1975 under the 1975 Farm Bill. They use these factors to calculate soil loss. The scanned pages were put into a spreadsheet for use in the calculation.

2. Calculation

The Revised Universal Soil Loss equation as stated in the Handbook for RUSLE 2 program is: A=R*K*LS*C*P,

where A is the average soil loss in mass per acre year, R is rainfall runoff factor in erosivity units per acre year, K is the soil erodibility in mass per erosivity unit, LS is the slope length steepness factor (unitless), C is the cover management factor (unitless), P is the support practice factor (unitless).

In this analysis the factors for the RUSLE Equation were provided for the various soil types. These factors were predetermined based on others research. Desmet and Grover developed an algorithm to calculate LS factor in GIS. Chen and Zhou have developed models to find K factors. Wischmeier and Smith have developed was to calculate the R factor for a given storm event (Zhang, 2006). For the values presented here, the R factor is determined from a 10 year 1inch per hour event.

Using the soil map and the table of RUSLE factors, the soil map symbol from the soil shapefile and the soil table were joined. A new column was created and the field calculator was used to evaluate the universal soil loss equation. The rainfall runoff factor, soil erodibility factor, and slope length/steepness factor varied spatially based on soil type. See the table in the Appendix for values used. For an initial analysis, cover management practice factors and support practices factor were assumed to be 0.1 for cover management practice and 1 for support practice throughout the watershed.

3. Results: Figures 3-6 show the various ranges of RUSLE factor used for the calculations. The higher values for each factor usually appear in the southwest corner of the watershed. This area is associated with a Turnerville soil class.

Figure 3. R factors for Moody Creek

Figure 4. K values for Moody Creek Watershed

Figure 5 Slope Length Steepness Factor

Figure 6. Soil Erosion Estimates for Moody Creek

As a worst case scenario, take the 16 tons per acres per year. Assuming that soil weighs 110 pounds per cubic foot (1762.2 kg per cubic meter), the worst case scenario means that the area loses 0.08 inches per year or 2.0 mm per year.

4. Validation: A suggested way to validate the results of this analysis is to find out how much soil the farmers have to reapply to their land each year. This varies from year to year and soil to soil. The loss has been measured to be a large as 22 tons per acre per year in the spring of the year (Cleve Bagley, personal communication, November 22, 2008).

5. Limitations:

Scaling effects: To effectively use RUSLE, the cover management factor and support practice factor should be evaluated on a field by field basis from year to year. Support practice factors involve various methods to prevent erosion from taking place Examples of support practice involve things such as sediment basins, contour farming, and tillage equipment use. These vary yearly and spatially from field to field, and farmer to farmer.

Figure 7 and 8 below shows examples of current implementations on the Moody Creek watershed. Figure 7 is a sediment basin used to reduce the LS factor. Figure 8 is a tillage practice called surface roughing.

Figure 7. Sediment Basin

Figure 8. Surface Roughing

Land type: RUSLE was developed with the intent of it to be used on agricultural land, where the soil is exposed to the effects of rainfall erosion (i.e. rill and interrill erosion). Currently, RUSLE is not accepted among rangeland managers and the forest service; however, the developers of the equations and program agree that the RSLE could be used on forest land. (Carrie Jensen-Smith, personal communication, December 5, 2008).

Conclusion: The purpose of this report was to identify quantitatively the areas in the Moody Creek watershed that have high erosion potential. Knowing this, the NRCS can actively visit with land owners to suggest land practices that will reduce the amount of soil loss. Much uncertainty lies in the factors presented in this paper. The factor obtained through the NRCS could very well be outdated. As has been mentioned in the limitation, much of the uncertainty of the results lies in inability to define C and P factor for an entire watershed. RUSLE should therefore only be considered when know cover management and support practice values can be obtained for parcels. This analysis resulted in values that closely matched actual measured values of erosion and could assist the NRCS in reducing phosphorus in Moody Creek.