GIS Analysis of Gully Head Erosion Rates on High Ridge Tree Farm in Winona County, Minnesota Lisa M. Worrell Department of Resource Analysis, Saint Mary s University of Minnesota, Winona, MN 55987. Keywords: Gully Head Erosion, Hydrology, Slope, Driftless Area, Private Land Ownership Abstract High Ridge Tree Farm is a 108 acre, privately owned and managed wooded property located outside of Lewiston, MN. Existing within what is known as the Driftless Area, High Ridge Tree Farm is susceptible to soil loss from gullies that form from run-off on surrounding agricultural land. The unique topography, steep sided ridges, springs and vast number of coldwater streams make the Midwest s Driftless Area an ecologically distinct and interesting zone. Because of the large size (24,000 square miles) of the Driftless Area and the varying abilities and funding for numerous management agencies, very few of the watersheds and streams are functioning properly within the region. Historic clearing of the land for agricultural purposes led to upland soils accumulating in the lower valleys which created shallower, warmer streams with steep, high banks. High Ridge Tree Farm, located in the heart of the Driftless Area, represents the importance of private land management in controlling erosion and restoring watershed health. Comprehensive analysis of the gully head erosion rates in the sub-watershed encompassing the High Ridge Tree Farm will locate areas of high concern for soil loss and identify conservation measures appropriate for curbing erosion. Introduction Gully erosion is a highly noticeable form of soil erosion and can affect soil productivity and impair roads and waterways. Soil eroded from gullies can cause siltation of streams, culverts, dams, and reservoirs. Suspended sediments, which may have attached nutrients and pesticides, can adversely affect water quality. Furthermore, these fine particles remain in suspension and can clog groundwater aquifers, pollute water courses, and disturb aquatic life. Gully erosion is caused when run-off concentrates and flows at a velocity particles. Water run-off increases with energy as it spills over a gully head and water splashback at the base erodes the subsoil, advancing the gully head up a slope. Gully head development may be initiated by several different factors including: cultivation or grazing on sites susceptible to gully erosion (as is the case throughout the Driftless Area), increased run-off due to landcover changes, poor vegetative cover or removal of preferred vegetative cover, and improper design, construction or maintenance of waterways in cropping areas (Welter, 2005). Over one hundred sufficient to detach and transport soil Worrell, Lisa. 2007. GIS Analysis of Gully Head Erosion Rates on High Ridge Tree Farm in Winona County, Minnesota. Volume 10, Papers in Resource Analysis. 6 pp. Saint Mary s University of Minnesota Central Services Press. Winona, MN. Retrieved (date) http://www.gis.smumn.edu
years of landcover alterations, including clearing land for agricultural purposes in the Driftless Area, have led to soil movement from the steep hillsides down into the valleys. This process has dramatically altered watershed and stream health in the region. Many large scale restoration projects completed by various conservation and government agencies have seen marked success in improving stream and watershed quality. This success can be enlarged by increasing the focus on privately held lands, particularly in sensitive watersheds, which are the most at risk for soil loss and stream siltation. High Ridge Tree Farm (HRTF) is located within the Garvin Brook watershed in Winona County, MN, which has been identified as a target watershed for restoration efforts by the Natural Resource Conservation Service (NRCS). This research project will focus on the subwatershed that encompasses HRTF and the smaller, micro-watersheds within the subwatershed. Analysis involved data collected in the field including GPS waypoints for each gully head and specific measurements recorded at each gully head location, along with GIS generated watershed delineation and digitized micro-watershed and gullyshed layers. A ranking system from 1-7 for each of three categories: microwatershed acreage, pooled gully head soil loss in tons per year for each microwatershed, and average slope for each micro-watershed was created using criteria supported by soil conservation experts. Rankings for each category were aggregated to produce a microwatershed rating to focus conservation practices on the most critical microwatersheds within the larger subwatershed. Methods Watershed Delineation The first step in delineating the watershed included downloading the streams layer along with the DRG (digital raster graphic), and DEM (digital elevation model) layers from the Minnesota Department of Natural Resources Data Deli. Using the Watershed tool within the Hydrology toolset in the Spatial Analyst toolbox, a grid layer was created for all of the subwatersheds in Winona County. From this raster, the sub-watershed encompassing HRTF could be delineated into a vector shapefile by onscreen digitizing as seen in Figure 1. Figure 1. Digitized sub-watershed from watershed raster (in shades of pink). From this shapefile, using the topographic map, micro-watersheds were digitized according to contour lines and elevation changes. Finally, gullysheds were further digitized to delineate the drainage area to each gully head. The X-Tools extension was used to calculate acreages for each level of 2
watershed, sub-watershed, microwatershed and gully-shed. Calculating Gully Head Erosion Rates Gully head locations around HRTF were collected using a Garmin GPS; measurements including top width, bottom width, and depth were recorded at each gully head. These measurements were integrated into the following equation, obtained from the NRCS to determine gully head erosion rates in tons per year (Cheetham, 1982): d = gully head depth bw = gully head bottom width tw = gully head top width 10 = constant; recession rate/year average for HRTF.045 = constant; soil weight (90 #/cubic foot for clay/silt found on HRTF over 2000 #/ton) d x bw + tw x 10 x.045 2 Each gully head erosion rate was calculated individually; then aggregated rates were calculated for each microwatershed and gully-shed. Calculating Average Slope The average slope for each microwatershed and gully-shed was calculated by using the Extract by Mask tool within the Extract toolset in the Spatial Analyst toolbox. Using a slope (degree) grid, individual slope grids were extracted for each micro-watershed and gully-shed. The statistics for each new grid were calculated through their attribute tables, and mean values were used as average slopes for analysis and ranking purposes. Ranking Each Micro-Watershed A rank between 1 and 7 was assigned to each micro-watershed for each of the three grid criteria: acreage, average slope, and gully head erosion rate, with 1 as the lowest micro-watershed when sorted in descending order and 7 as the highest. For example, the acreages of each micro-watershed ranged from 26.41 acres to 78.88 acres. The microwatershed with 26.41 acres was assigned a value of 1, while the micro-watershed with 78.88 acres was assigned a value of 7. Each individual category rank was then multiplied using the Raster Calculator in the Spatial Analyst extension to establish an overall microwatershed ranking. The final rankings were added to the attribute table for the micro-watersheds and symbolized in ArcMap for easy identification of critical areas of soil erosion. Structure/No Structure Comparison Of the seven micro-watersheds within the sub-watersheds of HRTF, two have soil erosion control structures, or dams. The logical assumption would be that micro-watersheds with dam structures in place would have significantly reduced gully head erosion rates. This comparison involved the overall gully head erosion rate in tons per year per acre both with and without dam structures, and also a comparison of two sets of micro-watersheds with similar acreages and average slopes, but with dam structures as the test variable. Results/Discussion Ranking the Micro-Watersheds 3
Results of ranking each micro-watershed based on three different criteria are illustrated in figures 2, 3, and 4. Acreages of the micro-watersheds ranged from 26.41 acres to 78.88 acres. Gully head soil loss ranged from 0 Tons/Acre/Year to 19.9908 Tons/Acre/Year. Average slope for each micro-watershed ranged from 5.83 degrees to 17.48 degrees. that of the micro-watersheds indicated, those two micro-watersheds on the east side of the sub-watershed are the highest concern for gully head erosion. Figure 4. Watersheds 1-7 ranked by gully head soil loss in tons/acre, symbolized as lowest = green (0 Tons/Acre/Year) and highest = red (19.9908 Tons/Acre/Year). Figure 2. Watersheds 1-7 ranked by acreage, symbolized as lowest = green (26.41 acres) and highest = red (78.88 acres). Figure 5. Aggregate ranking of microwatersheds. Red = Most Critical, Green = Least Critical. Structure/No Structure Comparison Figure 3. Watersheds 1-7 ranked by average slope in degrees, symbolized as lowest = green (5.83 degrees) and highest = red (17.48 degrees). Results of the aggregated ranking, as illustrated in figure 5, show This study compared overall gully head erosion rates in tons per acre per year. The result found that soil erosion rates from gully heads without any dam structures were 5.43 times higher than 4
soil erosion rates from gully heads with dam structures. Using two sets of two like micro-watersheds (similar to each other in acreage and average slope), but with soil loss as the variable, it was established that soil erosion was 7-10 times greater in micro-watersheds without dam structures than those with dam structures. This supports the theory that conservation measures involving dams significantly reduce the amount of soil erosion from gully heads. from gully heads on HRTF could be reduced by 58% to 5.7 tons per year (Figure 8). Discussion According to county soil conservation district guidelines, it is recommended that soil erosion be reduced by 60% on average (Cheetham, 1982). At a sum of approximately 45 tons of soil per year, loss through gully head erosion from the sub-watershed encompassing HRTF, it is advised that conservation practices be implemented to curb nearly 27 tons per year of soil loss. Concentrating on the individual gully-sheds, two sites were identified as most suited for dam structures (Figure 6). By implementing this conservation practice on just two gully head sites, soil loss from gully head erosion would be reduced by 25 tons per acre per year. These specific gully head locations however, are outside the HRTF boundary, and would involve cooperation with neighboring landowners (Figure 7). When focusing solely on gully heads within or on the boundary of HRTF, it is noted that of the four gully heads directly adjacent to the HRTF boundary, two have existing dam structures. These four gully heads have an annual soil loss of 9.84 tons per year, with 7.3 tons per year (74%) originating from the two gully heads without dams. By constructing a single dam, soil loss Figure 6. Proposed dam structure locations shown in green and existing dam structure locations shown in red. Figure 7. Gully-sheds directly adjacent to HRTF boundary shown in green. 5
Conclusions This project was designed to analyze gully head erosion rates in the subwatershed encompassing the High Ridge Tree Farm to both identify critical areas of soil loss due to gully head erosion and to compare management on HRTF to that of accepted management practices in the Driftless Area. Because of the unique topography in the Driftless Area, special measures need to be taken to abate soil erosion which causes sedimentation in the characteristically deep, cold water trout streams found in the region. and a positive conservation measure in the Driftless Area. Acknowledgements I would like to thank Bob Micheel for his insight and technical assistance on this project, as well as Johnny Micheel for his on-sight field assistance. I would also like to thank John Ebert of the Resource Analysis staff at Saint Mary s University of Minnesota for his guidance through this process. References Cheetham, R. N. Jr., 1982. Determining Sediment Volumes from Sheet, Rill, and Gully Erosion in Wisconsin. Natural Resource Conservation Service. p 21. http://www.wi. nrcs.usda.gov Welter, D. 2005. Trout Unlimited: The Driftless Area-A Landscape of Opportunities. p 3. http://www.tu.org/ Figure 8. Proposed dam structure for HRTF shown in yellow. According to the results of this study, High Ridge Tree Farm is in accordance with common management practices within the Driftless Area. The HRTF owners have installed two dams to curb soil erosion, and according to the calculations found in this study, that has reduced soil loss by approximately 4 tons per year. This is significant at the private landowner level 6