Erosion Assessment Goes Geospatial

While managing soil microbes might be the newest frontier to improving soil health, one thing has not changed: you cannot improve soil you no longer have! Minimizing erosion is the first step in a soil health management plan.

The old adage says: "you can only manage what you measure", but actually measuring soil erosion is very complicated, even for experts in this field. There's no need to measure Cesium isotopes along that slope in your field. What needs to be managed is the risk of erosion.

It used to be that in order to estimate the erosion risk of a field you had to download and learn to use specific software to perform the calculations required for the Revised Universal Soil Loss Equation (RUSLE2). The calculations require specific inputs like slope length and steepness that must be measured in the field or derived from topographic data collected on the farm. These are all barriers to actually doing an erosion risk assessment, and even then, the result is a single number. Is this good enough? We have moved to much more refined ways of determining fertilizer requirements, why can't we do the same for erosion risk? Now we can.

A water risk erosion tool has been added to the Agricultural Information Atlas (i.e. AgMaps). The tool runs RUSLE2 in the background to calculate the amount of soil lost to water erosion at a sub-field resolution. The results are an inherent water erosion potential map that calculates the risk based on soil and landscape factors, and an annual water erosion estimate map that takes the current crop and tillage practices into account (the 'C' factor from RUSLE 2). This article will explain how to run the tool.

Running the Tool

Step 1. Navigate to the tool

Once you have opened AgMaps, click on the 'Markup and Printing' tab. Then, click the 'Create Map' button and scroll down to 'Water Erosion Potential Map'.

Figure 1. Navigating to the Water Erosion Potential tool.

Figure 1. Navigating to the Water Erosion Potential tool.

Step 2. Select the field

Pan to your field on the map. You may want to switch to satellite imagery view (right of the Google street map button) as you get closer.

Once the field is on the screen, click the polygon button under 'Field Boundary / Area of Interest'. To outline the field, first click on one corner, then click again every time you end a line. To complete the outline, double click the last point - the last line between your last and first points will be created automatically to complete the polygon. You may now enter your farm and field name.

The tool will now automatically calculate the inherent water erosion potential for the entire field, colour coded by severity. This is the amount of erosion that would occur if the field were left bare for a year. You should recognize patterns that correspond to slope length, steepness, and soil type.

Figure 2. Inherent Water Erosion Potential.

Figure 2. Inherent Water Erosion Potential.

Step 3. Determine C factor

To calculate the erosion estimated to occur with current practices, the 'C' factor must be determined. This is a factor that determines how well the current crop and tillage practices mitigate the inherent erosion potential. Once you have selected your crop and tillage method, click beside 'Display Annual Water Erosion Estimate Map' to show how much erosion potential has been mitigated. You can change tillage methods and recalculate to see what effect a different system would have.

Soybeans with 'mulch-tillage' (i.e. minimum tillage) provide a C factor of 0.3 and reduce the erosion from the field from 12.2 ton/ac/yr for bare soil to 3.7 ton/ac/yr (Figure 3). If the soybeans were no-tilled, that could be further reduced to 1.5 ton/ac/yr (Figure 4).

Figure 3. Annual Water Erosion Estimate Map with soybeans and minimum tillage.

Figure 3. Annual Water Erosion Estimate Map with soybeans and minimum tillage.

Figure 4. Annual Water Erosion Estimate Map with soybeans and no tillage.

Figure 4. Annual Water Erosion Estimate Map with soybeans and no tillage.

Limitations

There are clearly more factors that determine erosion in a field than the current crop and tillage method. Organic matter levels and compaction status will influence how much water infiltrates or runs off. The tool currently does not include these factors.

This example field clearly has a complex topography and slopes that converge into a draw. These are conditions that increase the risk of gully erosion from concentrated flows, but the RUSLE2 program that lies behind this tool currently only calculates sheet erosion.

Updates

The current tool is published in the first of four planned phases. The second phase will make it possible to calculate the risk of gully erosion, while the third will integrate tillage erosion risk, and a final fourth phase will indicate the best placement of erosion control measures such as grassed waterways. These updates are planned for the next 2-3 years.


For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca