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Universal Soil Loss Equation

Environmental Applications of GIS

3-1-2006

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Soil Erosion in GIS• Universal Soil Loss Equation (USLE), an empirical equation

designed for the computation of average soil loss in agricultural fields.

• A = R*K*L*S*C*P

• 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 don’t 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

Components12..21

ComponentsProperties12..60

TN001

Layers12.6

LayerProperties123.28

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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, 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.

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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) = (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.

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Slope from Grid1. Click on the Spatial Analyst dropdown arrow, point to

Surface Analysis, and click Slope. 2. Click the Input surface dropdown arrow and click the

surface you want to calculate slope for. 3. Choose the Output measurement units. 4. Optionally, type a value for the Z factor. 5. Optionally, change the default Output cell size. (keep it

as 10)6. Specify a name for the output or leave the default to

create a temporary dataset in your working directory. 7. Click OK.

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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.

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Homework 5Due: 3/16 (Thursday)

• Task 1 (10 points)– Complete : Chapter 14 in Chang’s 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