Universal Soil Loss Equation

1
Universal Soil Loss Equation

• Environmental Applications of GIS
• 3-1-2006

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.

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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.

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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. TN is 5 tons/acre/year
• Generally, soils with deep, uniform, stone free
  topsoil materials and/or not previous eroded have
  been assumed to have a higher rate

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Rainfall Erosivity 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,
• 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, j is the specific storm increment, n is the
  number of storm increments in a year.

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K Factor - Soil Erodibility

• Use soil layer information from Statsgo. Clip
  only the studied area. (Data is in
  G\4650\Data\3-1\, a shapefile in geographic
  coordinate system
• The layer comes with datum so you dont have to
  define it. However you may need to project it to
  UTM83, if not projected.
• Extra Soil data based on FW (Falling Water HUC10)
• Add the layer.dbf and comp.inf.
• Find the weighted average kffactor in each soil
  type and use this number for your analysis.

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STATESGO

• There are 14 soil polygons with only 9 different
  MUID.
• Relate layer.dbf to comp.inf through Museq and
  link layer.dbf to Attribute of Tennessee
  through Muid

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STATESGO soil data
Components Properties 1 2 . . 60
Components 1 2 . . 21
TN001
Layer Properties 1 2 3 . 28
Layers 1 2 . 6
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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, Pasture0.02, Wetland Scrubland0.1,
  urban/developed0, Open water0, 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, 12Strip Mines, 13
  Undefined
• 20 Mixed Bottomland Hwood, 23 Wetland Scrubland
• 28HW 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.

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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 G\4650\data\3-1\ and you should
  have the data now. Use Lookup table to
  reclassify your grid file from LULC layer.

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Computation of LS in GIS

• LS factor at a point r(x,y) on a hillslope is
• LS(r) (m1)A(r)/aom sinb(r)/bon
• 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 lt 100 m and
  slope angle lt 14 degree.

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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.

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Practice

• ArcGIS
• Calculate grid equation step-by-step, such as
  faccu0.45249 -gt Calculation1, (Question
  Why 0.45249?)
• Pow(Calculation1,0.6) -gtCalculation2
• Slope_degree0.01745 -gt Calculation3 (why
  0.01745?)
• Make the final grid a permanent grid file.

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Homework 5Due 3/16 (Thursday)

• Task 1 (10 points)
• Complete Chapter 14 in Changs book
• Task 2 (20 points)
• Calculate soil loss potential in your assigned
  HUC10 using default parameters discussed
  previously in the class.
• Data Land use and Land Cover data in
  G\4650\data\3-1\LULC. 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 Carthage rainfall data (carthage.txt, you
  need to convert it to excel to compute Intensity)
• LULC, and STATESGO files are required to process
  the equation.
• Compute soil loss in your watershed. Compute
  total soil loss and shade your results using
  different shading (such as high in red, medium in
  yellow and low in green)
• Describe your process and results on the same
  Words doc as in Task 1