| | On the farm landscape, slope is a stretch of ground forming an incline, or slant, that can be either in an upwards or downwards direction, and it is measurable at some angle. There are two components to slope that are important when considering how to measure it in the field. These components are slope length, or the stretch of ground, and the incline angle, or how steep the slope is. But why should producers care what a fields slope is?
"Producers need to consider slope because "stuff", in whatever form that may be, tends to flow downhill," says Jason Cathcart, Alberta Environmentally Sustainable Agriculture, (AESA) soil quality program coordinator, Edmonton. "This is particularly true in the agricultural world as slope plays a significant role in dictating whether erosion will occur or not. As slope length increases, erosion per unit area increases. Runoff accumulates as water runs downhill, that is, water is collected from ever increasingly large areas. The force and shearing strength of this running water also increases, leading to high erosive energy."
Field slope exerts significant effects on field variability, which is often expressed as a difference in crop response. For example, higher crop yields can occur in field depressions as a result of the accumulation and pooling of eroded topsoil and organic matter from the upper slope position. Furthermore, water tends to accumulate in the lower slope position, leading to better moisture conditions and crop growth. In comparison, the loss of valuable topsoil from the upper slope often exposes the less fertile sub-surface soil horizons, and this inherently reduces crop yield in those areas.
"In addition to crop response, field slope is important to field operations," says Cathcart. "Under the Agricultural Operation Practices Act (AOPA), slope is a consideration when determining setback distance during manure storage/application. Slope is also addressed in the Alberta Environmental Farm Plan (AEFP), where it is used in the assessment of water erosion and surface water contamination risk."
Slope length is considered to be the key component of slope as it represents the ground distance over which water can flow, leading to soil erosion. Slope length is not necessarily the distance between the highest and lowest point in the field. Natural water flow patterns must be considered in order to determine slope length. Once the natural drainage pattern is known, slope can be measured from the point where water flow begins. Slope is calculated as the distance from this point to the point where either slope decreases to the degree that soil deposition occurs (where the land flattens out); to where water becomes a concentrated flow, such as in a stream or ditch; or, to where flow enters some well-defined channel such as a grassed waterway.
A three-part series of articles on slope will be featured in Agri-News. In Part II of this series, four different methods for calculating field slope will be discussed, including the trigonometric approach, use of a clinometer or slope gauge, GPS, and the familiarity/visualization approach.
For more information on AOPA, visit Alberta Agriculture's website at www.agric.gov.ab.ca, and search under AOPA.
For more information on AEFP, see the AEFP website at www.albertaefp.com.
Contact: Jason Cathcart (780) 427-3432 |
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