Ministry of Agriculture, Food and Rural Affairs

Measuring Field Slopes to Estimate Soil Erosion

1. Introduction
2. Some definitions
3. What tools can be used to measure slope?
4. Where in the field should I measure the slope?
5. Conclusion
6. References

Introduction

The topography of a field greatly determines how susceptible the soil is to erosion by water. In general, the steeper and longer the slopes are in a field, the greater the soil erosion potential. To estimate a field's long-term average soil erosion potential, soil conservation planners and agronomists often use the Universal Soil Loss Equation (USLE), or a revised version of this equation called the Revised Universal Soil Loss Equation (RUSLE2). The USLE is embedded in the Ministry of Agriculture, Food and Rural Affairs' nutrient management planning software (NMAN). NMAN uses USLE calculations to assess the potential for phosphorus loss from a field receiving manure. The calculations also give NMAN users a relative comparison of soil loss potential for the field under different crop and management practices.

Both the USLE and RUSLE2 use the field's slope as an input. However, it can be difficult to get an accurate slope value when there are many slope length and steepness measurements in a single field. The information below will help to solve this problem.

Some definitions

It is important to understand how erosion estimation equations define a field slope if you want to use them to make accurate estimates of soil erosion. For conservation planning and nutrient management planning purposes, the USLE defines a slope length as the distance from the origin of overland flow to the point where deposition begins. This is the traditional definition of field slope length and can be used as input to either the USLE equation or the RUSLE2 equation. RUSLE2 can also accommodate a more complex field slope description, called the overland flow path length. The overland flow path length is the distance runoff water travels as it moves across a field's soil surface from its point of origin to the point where it becomes concentrated flow. For both definitions, runoff is assumed to be perpendicular to hillslope contours.

Figure 1 illustrates the difference between the USLE slope definition and the overland flow path slope definition that can be represented in RUSLE2. The USLE definition of slope length stops where deposition begins. As a rule, deposition along a slope length in a field begins at the point where slope steepness is one-half of the average steepness of the field slope above this point.

Figure 1. An illustration of the difference between slope length and overland flow path length. (Source: United States Department of Agriculture - Agricultural Research Service (USDA-ARS), 2008.)

Normally, we measure the slope as the average straight line slope (see the dotted line on each sketch in Figure 2). In reality, however, Figure 2 shows that overland flow paths can have a variety of shapes.

Figure 2. Types of overland flow path profiles. (Source: USDA-ARS, 2008.)

RUSLE2 can operate using the USLE definition of slope length, but can also allow users to input various slope shapes that may be present along the entire overland flow path length. In this way, RUSLE2 is capable of predicting the rate of soil erosion or deposition occurring at any point along the overland flow path. With this capability, RUSLE2 is able to estimate the amount of sediment that gets delivered to the bottom of an overland flow path and enters the concentrated flow area, such as a gully or watercourse (see Figure 3).

Figure 3. A field illustration of the difference between an overland flow area and a concentrated flow area. (Source: USDA-ARS, 2008.)

What tools can be used to measure slope?

There are a variety of tools and approaches that can be used to measure field slopes. Perhaps the simplest approach for estimating slope is to obtain a copy of the soils report for the field. Most modern soil reports include an estimate of the range of field slopes characteristic of each soil polygon mapped. For example, Figure 4 shows a clip of mapping from the Middlesex County soils report. It shows some soil polygons in the extracted area having field slopes in the range of 0.5 to two per cent (shown as "b" on the map). One polygon has a greater proportion of slopes in the two to five per cent range (shown as c>d on the map).

Figure 4. Example of field slope estimation from soil mapping for a site. (Source: Hagerty and Kingston, 1992.)

Slope length is difficult to obtain from a soil map. Uppercase letters suggest the field has slopes more than 50 metres (m) (160 feet (ft.)) in length. Lowercase letters indicate the slope lengths are less than 50 m (160 ft.).

Soil conservation planning practitioners in the United States, who have measured slope length and gradient in the field for years, have developed a reliable way to approximate the maximum slope length from known slope gradients. Figure 5 shows this relationship of maximum slope length for a given slope gradient.

Figure 5. Approximation of slope length from slope gradient. (Source: United States Department of Agriculture-Natural Resources Conservation Service (USDA-NRCS).)

In the example shown in Figure 4, if a five per cent slope gradient is measured, the maximum slope length that is typically expected before the flow becomes concentrated is 53 m (175 ft.).

Another useful way to calculate field length and steepness is to use information from a detailed field contour or topographic map. Such contour maps can be obtained from a variety of data sources, including:

• tile drainage installation mapping
• Digital Elevation Model (DEM) data obtained from a variety of sources, including:
  • precision farming tools that collect Real Time Kinematic field data
  • published digital elevation model data or data gathered using Light Detection and Ranging (LiDAR) equipment.

Figure 6 shows an example of a detailed contour map, prepared from LiDAR data. The thin black lines are the contour lines for the field. The thick blue lines mark the concentrated flow pathways. The high points, or sub-watershed boundaries, shown by the contour line data identify the starting points for the overland flow path profiles. The black arrows on the map that run perpendicular to the contour lines represent selected overland flow pathways. These overland flow pathways could be used as slope input into RUSLE2. The dashed line arrows beside the solid line arrows represent the slope lengths that would typically be used to represent slopes in the USLE and NMAN. Note that these dashed lines end at points where the contour spacing is at least double compared to the contours above this end point. This change in contour spacing indicates a flattening of the slope, and therefore a point where soil deposition starts.

Figure 6. Extracting slope information from detailed contour mapping.

The practical approach for arriving at slope length and gradient, when no mapping is available, is to do some direct field measurements. Slope steepness can be measured using an Abney level (Figure 7) or a clinometer (Figure 8). Also, useful slope measurement "apps" exist for smartphone devices. To ensure accuracy, be sure to test these smartphone tools and compare their results to standard measuring procedures. A standard engineer's rod and level can also be used, but is more cumbersome. The length of slope can be paced off (most common) or measured with a measuring tape. If an engineer's rod and level is used to measure slope steepness, then stadia can be used to calculate slope length.

Figure 7: Abney Level. Figure 8: Clinometer.

Where in the field should I measure the slope?

Given the range of topography in most fields, it can be difficult to decide which slope to measure or from where in the field the slope should be measured. The USDA-NRCS recommends using the overland flow path profile that represents the topography of one-third to one-quarter of the most erodible part of the field. Figure 9 illustrates this concept. While many flow paths can be identified, the flow path identified as "A" in Figure 9 represents the flow paths that characterize much of the within-field watershed.

Figure 9. Overland flow path identification and selection.

This logic can be used for the field shown in Figure 6. Assuming the map represents a field, the slope in the lower portion of the map would be the profile selected for conservation planning purposes due to its steepness. It also covers approximately one-third of the field area.

Conclusion

The above information provides guidance on how tools like the USLE and RUSLE2 are used to define

• field slopes
• where a slope starts and ends
• ways to measure the slope of a field

Even with practice and experience, no two people will arrive at exactly the same measurements for slope length and gradient for a field because of the complexity of field topography. With experience and knowledge, however, the values each individual chooses should result in long-term average soil loss estimates that are similar in magnitude.

References

Hagerty, T.P. and M.S. Kingston. 1992. The Soils of Middlesex County, Report No. 56 of the Ontario Centre for Soil Resource Evaluation. Ontario Ministry of Agriculture and Food, Agriculture Canada, Guelph.

Nowell, P. and S. Sweeney. 2012. LiDAR Identification of Water and Sediment Control Basins in the Gully Creek Watershed. Ontario Ministry of Agriculture, Food and Rural Affairs. Unpublished Data, Guelph.

United States Department of Agriculture-Agricultural Research Service. 2008. User's Reference Guide - Revised Universal Soil Loss Equation Version 2 (Draft). Washington D.C.