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Technical Thematic Report No. 16. - Soil erosion on cropland – introduction and trends for Canada

Analysis

Soil erosion was calculated using landform data and the associated topographic data in the National Soil Database (Agriculture and Agri-food Canada, 2008). Each Soil Landscapes of Canada (SLC) polygon is characterized by one or more representative landforms, and each landform is characterized by hillslope segments (upper, mid, and lower slopes and depression), and each hillslope segment is characterized by a slope gradient and slope length. Soil erosion risk by wind, water, and tillage erosion was assessed individually as soil loss on the most severely eroding segment of a landform - the upper slope segment for wind and tillage erosion, the mid slope segment for water erosion. This was done to identify the most erosion affected portion of the field that would control the changes in management needed for the whole field. The final soil erosion indicator is assessed as an average soil loss over the total area of the upper and mid slope segments. The methods are discussed in Eilers et al. (2010).

Wind erosion was only estimated and included in the soil erosion risk for the Prairies and Boreal Plains ecozones+ (and the small area of CroplandFootnote6 in the Boreal Shield Ecozone+ that adjoins Cropland in the Boreal Plains Ecozone+). The Cropland in these ecozones+ has the greatest wind erosion risk due to its relatively dry climate and vast expanses of cultivated land with little protection from the wind. The contribution to erosion risk from wind erosion was judged to be insignificant for Cropland for other parts of Canada, although wind erosion may be significant in those parts in occasional years on the most sandy and peaty soils that are well exposed to wind.

Water erosion estimates are based on that from rill and inter-rill erosion from rainfall and do not include gully erosion that occurs in waterways. Although very localized, the erosion from gully erosion can be huge and a larger source of sediment than that from erosion on the Cropland contributing runoff. The erosion from snowmelt was not included and assumed to be less than that from rainfall over long term for most areas of Canada.

The change in risk of soil erosion over time was calculated by considering the effects of changes in land use and land management practices across Canada, such as fluctuations in Cropland areas, shifts in cropping systems used (crop rotations, including forages and summer fallow), and tillage systems used (conventional, conservation tillage, and no-till). This information was obtained from the Census of Agriculture for 1981, 1986, 1991, 1996, 2001, and 2006 (Statistics Canada, 1983; Statistics Canada, 1988; Statistics Canada, 1993; Statistics Canada, 1998; Statistics Canada, 2003; Statistics Canada, 2008) and also linked to each SLC mapping area. Cropping and tillage practices in the census database are grouped in classes (for example, grain corn after soybean under conventional tillage, grain corn after soybean under no-till, etc.). The proportion of Cropland falling into each of the risk classes outlined above was calculated for Canada and for each ecozone+. Changes over time in the percent value for each class in each area provides an indication of whether the overall risk of erosion is increasing or decreasing.

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Limitations

The erosion risk represents that due to long-term weather. Therefore, an area with very low erosion risk is expected to have very low rates when averaged over time. However, during extreme rainstorm or windstorm events, much higher erosion rates can be experienced.

The erosion risk from landslides is not estimated. Erosion from non-agricultural activities was not considered in this analysis. Although the area is relatively small, erosion from wind and water can be very severe on landscapes undergoing infrastructure construction and cause important off-site damage.

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Footnotes

Footnote 6

Cropland, as discussed throughout this report, also includes areas defined as Improved Pasture and Summerfallow in the Census of Agriculture.

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