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Universal Soil Loss Equation

Universal Soil Loss Equation. Environmental Applications of GIS Week 5, Fall, 2004. Soil Erosion in GIS. Universal Soil Loss Equation (USLE), an empirical equation designed for the computation of average soil loss in agricultural fields. A = RKLSCP

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Universal Soil Loss Equation

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  1. Universal Soil Loss Equation Environmental Applications of GIS Week 5, Fall, 2004

  2. Soil Erosion in GIS • Universal Soil Loss Equation (USLE), an empirical equation designed for the computation of average soil loss in agricultural fields. • A = RKLSCP • A: potential long term average annual soil loss in tons per acre per year. (Ton/acre-yr) • R = rainfall and runoff factor, the greater the intensity and duration of the rain storm, the higher the erosion potential • K = soil erodibility factor (from Statesgo soil layer) • LS = slope length-gradient factor. Represents a ratio of soil loss under given conditions to that at a site with the “standard” slope steepness of 9% and slope length of 72.6 ft. The steeper and longer the slope, the higher the risk for erosion.

  3. USLE - factors • C: crop/vegetation management factor. It is used to determine the relative effectiveness of soil and crop management systems in terms of preventing soil loss. • Corn : 0.40, Fruit Tree: 0.1, Hay and Pasture: 0.02 • P: Support practice factor. It reflects the effects of practices that will reduce the amount and rate of water runoff and thus reduce the amount of erosion. Most common used supporting cropland practices are cross slope cultivation, contour farming and strip cropping. • Up & Down Slope: 1.0, Cross Slope: 0.75, Contour Farming: 0.50, Strip Cropping: 0.37, Strip cropping and contour: 0.25.

  4. Soil Loss Tolerance Rates • A tolerable soil loss is the maximum annual amount of soil, which can be removed before the long term soil productivity is adversely affected. • Tolerance level varies, depending on the type and depth of soil. Generally, soils with deep, uniform, stone free topsoil materials and/or not previous eroded have been assumed to have a higher tolerance limit than soils which are shallow or previously eroded. • Tennessee’s soil is 5 tons/acre/year

  5. Rainfall Erosility Factor, R • Erosivity of rainfall events and is defined as the product of two rainstorm characteristics: kinetic energy and the maximum 30 minute intensity. (Kirkby and Morgan, 1980) • E = 1.213 + 0.89 logI • Where E = the kinetic energy, kg-m/m2 mm, muti • I = rainfall intensity, mm/hr • I30 = the maximum 30-minute rainfall intensity for the storm, mm/hr • Tj = the time period of the specific storm increment, hr. • And, jis the specific storm increment, n is the number of storm increments in a year.

  6. Weather Data • Download meteorological data from NCDC • http://hurricane.ncdc.noaa.gov/CDO/cdo • This link has precipitation data for 15-minute and 1hour. • Select Country and state and then Putnam (Monterey station, COOPID:406170) • Save file as Text file and use Excel to open this file. • Process the file to find out how many precipitation in Monterey. • (actual rainfall (inches) = value/100)

  7. K Factor - Soil Erodibility • Use soil layer information from Statesgo. Clip only the studied area. (Data is in Z:\Classes\4650\Data\9-29\, a shapefile in geographic coordinate system and Putnam.shp also in lat/long) • Clip Theme: Geoprocessing > Clip one theme based on anther. • Bring in the layer.dbf and comp.inf. • Link layer.dbf to comp.inf through “Museq” and link layer.dbf to “Attribute of Tennessee” through “Muid” • Bring in Putnam.shp and find the overlap portion of Putnam and Tennessee. Find the weighted average kffactor in each soil type and use this number for your analysis.

  8. C factor • C: crop/vegetation management factor. It is used to determine the relative effectiveness of soil and crop management systems in terms of preventing soil loss. • Tree: 0.1, Pasture:0.02, Wetland Scrubland:0.1, urban/developed:0, Open water:0, Cropland: 0.5. • Add “luc_sq_put” to your view and reclassify the grip based on the above definition. • Codes for LULC grid data: • 1: Open Water, 4: Pasture/Grassland, 5: Cropland • 10: Urban/Developed, 12:Strip Mines, 13: Undefined • 20: Mixed Bottomland Hwood, 23: Wetland Scrubland • 28:HW Forest, 29: Oak Forest, 30: Dry-Desic Oak, • 33: Southern Yellow Pine, 34: Xeric conifer • 35: White Pine, 38: E. Red-Cedar 42: Juniper/Oaks • Reclassify grid based on the values defined above.

  9. LULC data in grid format • If your working area is in Putnam county, you need to crop the grid theme into the shape of Putnam county. • You will need to crop LULC data to fit into your county area. • The data is in s:\classes\4650\data\tn-gap\ Copy this folder to your working folder. • Use Lookup table to reclassify your grid file from LULC layer.

  10. Computation of LS in GIS • LS factor at a point r(x,y) on a hillslope is • LS(r) = (m+1)[A(r)/ao]m [sinb(r)/bo]n • A(r) = upslope contributing area per unit contour width (use flow_accumulation) • ao = 22.1 m, bo = 0.09 (9%)= 5.16 degree • m = 0.6 and n = 1.3 for slope lengths < 100 m and slope angle < 14 degree.

  11. Slope from Grid • Click on the Spatial Analyst dropdown arrow, point to Surface Analysis, and click Slope. • Click the Input surface dropdown arrow and click the surface you want to calculate slope for. • Choose the Output measurement units. • Optionally, type a value for the Z factor. • Optionally, change the default Output cell size. (keep it as 10) • Specify a name for the output or leave the default to create a temporary dataset in your working directory. • Click OK.

  12. Practice • ArcGIS: • Calculate grid equation step-by-step, such as [faccu]*0.45249 -> [Calculation1], (Question – Why 0.45249?) • Pow([Calculation1],0.6) ->[Calculation2] • [Slope_degree]*0.01745 -> [Calculation3] (why 0.01745?) • Make the final grid a permanent grid file.

  13. ArcView: Procedures to compute soil loss Dimension of cell size • Flow Accumulation (if available) • In Map Calculator • build an expression • (([cookefaccu]*10/22.1).pow(0.6))*(((([slope_degree]*0.01745).sin)/0.09).pow(1.3))*1.6 • Press Evaluate, name new theme as “lsfac” • Build an expression within Map Calculator • R*[K]*[C]*[P]*[lsfac] • Give the new theme, soilloss • Color the new theme in differential color to indicate the soil loss risk. • *Notes: 2 / 360o = 0.01745

  14. Your homework – detailed to be given tomorrow • Calculate soil loss potential in your assigned county using default parameters discussed previously in the class. • Data: Land use and Land Cover data in “z:\classes\4650\data\9-29\gext1”. Remember copy two folders to your own folder (Gext1 and info). LandUse Land Cover file is a grid file. You should use a rectangle slightly larger than your county for cropping purpose. This file is in UTM27 zone 16. You need to reproject to UTM83. • Use NCDC link to download 1-hr precipitation data in your county and compute Erosivity for that county (use 1/1/02 - 12/31/02 range) • LULC, and STATESGO files are required to process the equation. • Compute soil loss in your county. Compute total soil loss and shade your results using different shading (such as high in red, medium in yellow and low in green)

  15. Next Week • Air Pollution • Aloha • TRI data • Census 2000 data • Risk Analysis • Wind speed, direction

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