Calculating LS Factor of RUSLE Using USPED in ArcMap: A Step-by-Step Guide

Welcome to the tutorial on calculating the LS factor of the Revised Universal Soil Loss Equation (RUSLE) using the Unit Stream Power Erosion and Deposition (USPED) method in ArcMap. This method is essential for understanding soil loss in a given area and involves several steps, including generating flow accumulation and slope maps.

Overview of the LS Factor

The LS factor is a crucial component of the RUSLE, which estimates soil erosion by water. It is influenced by the length (L) and steepness (S) of the slope. In this tutorial, we will use raster calculations to derive the LS factor from the flow accumulation and slope of the watershed or area of interest.

Step 1: Preparing Your Data

To begin, you need to launch ArcGIS and add your Digital Elevation Model (DEM) of the study area. This DEM will serve as the basis for generating the necessary maps.

Clipping the Area of Interest

1. Add the shapefile of your area of interest to the ArcMap.
2. Use the "Extract by Mask" tool under the Spatial Analysis toolbox to clip the DEM to your area of interest.
3. Input the raster (DEM) and the shapefile as the mask data, then save the output.

Step 2: Calculating Flow Accumulation and Slope

Flow Accumulation

1. Under the Spatial Analysis toolbox, navigate to Hydrology and select the "Fill" tool to fill any sinks in your DEM.
2. Next, use the "Flow Direction" tool to calculate the flow direction from the filled DEM.
3. Finally, calculate the flow accumulation using the flow direction raster generated in the previous step.

Slope Calculation

1. Still under the Spatial Analysis toolbox, select the "Slope" tool.
2. Input the clipped DEM and ensure the output measurement is set to degrees.
3. Save the slope map output.

Step 3: Calculating the LS Factor

Now that we have both the flow accumulation and slope maps, we can calculate the LS factor using the raster calculator.

Using the Raster Calculator

1. Open the Raster Calculator under the Map Algebra section of the Spatial Analysis toolbox.
2. Input the formula for calculating the LS factor:
   • LS = (Flow Accumulation * Cell Resolution / 22.1) ^ 0.4 * (sin(Slope) / 0.09) ^ 1.4
3. Ensure to replace the cell resolution with the appropriate value for your DEM (e.g., 30 meters).
4. Name the output raster as your LS factor map.

Step 4: Classifying the LS Factor Map

Once the LS factor map is generated, you can classify it for better visualization.

1. Open the properties of the LS factor layer and navigate to the Symbology tab.
2. Choose to classify the data into intervals (e.g., eight classes) based on a geometric interval.
3. Select a color ramp that effectively represents the data.

Conclusion

You have successfully calculated the LS factor of the RUSLE using the USPED method in ArcMap. This step-by-step guide has walked you through the process of generating flow accumulation and slope maps, performing raster calculations, and classifying the results for analysis. If you found this tutorial helpful, please give it a thumbs up and leave your comments below. Thank you for watching!