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Slope Length (Overland Flow)Length of Overland Flow can be calculated in several ways, including.

A. Field measurements.B. Regional estimation method (Troeh et al. 1991).

C. Unit stream power method (Mitasova 1993, Mitasova et al. 1996, Moore and Wilson, 1992,

Moore and Burch 1986).

D. Upslope contributing area proxy method (Desmet and Govers 1995, 1996).

E. Slope length from high points (ridges/peaks) and the flow direction raster (Hickey 2000).

F. Cell-by-cell Non-cumulative Slope Length (NCSL) method: FlowAcc * SR , where FlowAcc =

Flow accumulation raster (orientation of maximum downhill slope), SR = Cell size of raster (i.e., 30

x 30m).

G. TIN analysis (Cowen 1993).

H. Average length of overland flow by the equation:

Average Length of Overland Flow = 1 / (2D d 1 (S c / S g)2 or 1 / 2D d GIS Method

** Below is my quick and dirty translation of instructions by Hickey (2000, p. 4-7) . I may have

missed something. Please let me know if so. **

- Acquire a DEM.

- Mosaic multiple DEMs if necessary.- Fill sinks. Name output raster Fill_DEM.

- Run Flow Direction on filled DEM. Name output raste r FlowDir

- Reclassify FlowDir using 2 classes to isolate cardinal flow direction pixels. Cell VALUES of 2, 8,

32, and 128 = NoData. VALUES of 1, 4, 16, and 64 = 1. Thus the new output values will be 1 and

NoData for the two classes. Name output raster Cardinal_out.

- Reclassify FlowDir using 2 classes to isolate diagonal flow direction pixels. Cell VALUES of 1, 4,

16, and 64 = NoData. Cell VALUES of 2, 8, 32, and 128 = 1. Name output raster Diagonal.

- Run Flow Accumulation from Flow Direction raster . Name output raster FlowAcc.

http://onlinegeographer.com/slope/hickey_slope_length.pdfhttp://onlinegeographer.com/slope/hickey_slope_length.pdfhttp://onlinegeographer.com/slope/hickey_slope_length.pdfhttp://gis4geomorphology.com/mosaic-dems/http://gis4geomorphology.com/mosaic-dems/http://gis4geomorphology.com/mosaic-dems/http://gis4geomorphology.com/mosaic-dems/http://onlinegeographer.com/slope/hickey_slope_length.pdf

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- Delineate pixels of zero flow accumulation (peaks, ridges) by reclassifying the FlowAcc raster. Set

Break Values at 1 and the maximum FlowAcc VALUE (New Values = 1, NoData). Name output

raster Peaks. Reclassification may identify some pixels that do not truly occupy peak and ridgeline

positions. Eliminate any pixels that fail this conceptual test or slope length calculation will be

underestimated (this may be difficult to automate).

- In Raster Calculator, build this equation: Peaks* (DEM pixel size in meters / 2) . Name outputraster Peak_out.

- In Raster Calculator, build this equation: Diagonal* 1.4142 . Name output raster Diagonal_out. - Create the slope length raster with Raster Calculator and this equation:

Peak_out+ (Diagonal_out* pixel size) + (Cardinal_out* pixel size) - Convert cumulative slope lengths to feet (1 meter = 3.28084 feet).

- Not sure about this stepThe USLE equation for LS values is:

LS = (/ 72.6) m (65.41 sin 2 + 4.56sin+ 0.065) Name output raster SL_Cumulative.

is the cumulative slope length in feet

is the downhill slope angle

m is a slope contingent variable:

0.5 if the slope angle is > 2.86 degrees

0.4 on slopes of 1.72 2.86 deg

0.3 on slopes of 0.57 1.72 deg

0.2 on slopes < 0.57 deg

Refs:

Cowen (1993) Proceedings 13th ESRI User Conference

Desmet and Govers (1995) Catena 25

Desmet and Govers (1996) Journal of Soil and Water Conservation 51

Hickey, R. (2000) Cartography 29

Mitasova (1993) GRASSclippings 7Mitasova et al. (1996) International Journal of GIS 10

Moore and Burch (1986) Soil Science Society of America Journal 50

Moore and Wilson (1992) Journal of Soil and Water Conservation 47

Renard et al. (1997) USDA Agricultural Handbook #703

Troeh et al. (1991) Soil and Water Conservation (2nd edition), Chapter 6

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Wischmeier and Smith (1978) Predicting rainfall erosion losses-A guide to conservation planning:

Agricultural Handbook #537