13. Soil Erosion SUMMARY Soil erosion, the movement of soil from one area to another, occurs through three main processes. It occurs naturally on cropland through the action of wind and water, which can be accelerated by some farming activities (e.g. summerfallow, row cropping). It is also caused directly by the farming practice of tillage, which causes the progressive downslope movement of soil, resulting in soil loss from hilltops and soil accumulation at the base of hills. Soil erosion is a major threat to the sustainability of agriculture in Canada. It removes topsoil, reduces soil organic matter and contributes to the breakdown of soil structure. These effects in turn adversely affect soil fertility, the movement of water into and from the soil surface and ultimately crop yields and profitability. Yields from severely eroded soils may be substantially lower than those from stable soil in the same field. Erosion can also have significant “off-site” adverse impacts on the environment through the physical transport and deposition of soil particles and through the nutri- ents, pesticides, pathogens and toxins that are released by erosive processes or carried by eroded sediments. This chapter deals with three distinct indicators that are used to assess the risk of soil erosion by the action of water, wind and tillage. A) Water Erosion AUTHORS: SUMMARY L.J.P van Vliet, G.A. Padbury, Soil erosion by water has long been recognized as a serious threat to agricultural sustainability in Canada, H.W. Rees and M.A. Matin albeit to a lesser extent on the Prairies. Results from the Risk of Water Erosion Indicator point to an overall decrease in water erosion risk in most provinces of Canada, between 1981 and 2001, contributing to a national decrease of 8%. This positive trend resulted mostly from changes in cropping measures and the INDICATOR NAME: increased use of conservation tillage and no-till. In 2001, 86% of cropland in Canada exhibited sustainable Risk of Water levels of erosion (very low risk class). The remaining 14% of Canadian cropland, which is still subject to Erosion unsustainable water erosion, typically consists of areas used for summerfallow and row cropping on sloping land. STATUS: National coverage, 1981 to 2001 THE ISSUE increase the turbidity (cloudiness) of the water and add to sediment build-up. This sedimenta- Rainfall and surface run-off are the driving tion may reduce the water’s suitability as habitat forces for water-induced soil erosion. Spring for fish and other aquatic organisms, alter the meltwaters and heavy summer storms have flow of the water and eventually clog channels, the greatest potential for causing water erosion; making clean-up necessary. In addition, crop however, water erosion can occur any time, nutrients, pesticides and bacteria are often contributing to large soil losses from farm fields attached to the eroding soil particles and so over time and soil degradation. Eroded soil is are also carried into water bodies, adding to the carried in run-off to agricultural drains, ditches water quality impacts. Curtailing water erosion and other waterways, where it can affect water can help to protect both soil and water quality. quality, given that suspended soil particles 90 Environmental Sustainability of Canadian Agriculture: Agri-Environmental Indicator Report Series—Report #2 THE INDICATOR calculated for Canada and for each province. Changes over time in the percent value for each The Risk of Water Erosion Indicator is used to class in each area provides an indication of identify areas at risk of significant water erosion whether the overall risk of erosion is increasing and to assess how this risk is changing over time or decreasing. under prevailing management practices. This risk is expressed in five classes: very low (less than 6 tonnes per hectare per year), low (6 to LIMITATIONS 11 t ha-1 yr-1), moderate (11 to 22 t ha-1 yr-1), The indicator is subject to the following high (22 to 33 t ha-1 yr-1) and very high (greater limitations: than 33 t ha-1 yr-1). Areas in the very low risk class are considered, under current conditions, • calculations did not account for some able to sustain long-term crop production and erosion control practices such as grassed maintain agri-environmental health. The other waterways, strip cropping, terracing, contour four classes represent the risk of unsustainable cultivation and winter cover crops. conditions that call for soil con- servation practices to support • Census data linked to map- crop production over the long ping areas were assumed to term as well as to reduce In 2001, 86% of be equally distributed over impacts on water quality. The Canadian cropland was the entire mapping area, performance objective for this in the very low water because crop types and indicator is to increase the tillage practices could proportion of cropland in erosion risk class. not be directly linked the very low risk class. to specific soil types or landscape features within each SLC mapping area. CALCULATION METHOD The rate of water erosion was estimated using • Census information is not detailed enough the Revised Universal Soil Loss Equation for to reflect the geographic distribution of Application in Canada (Wall et al. 2002). The management practices in landscapes where Soil Landscapes of Canada (SLC) maps (version farmland is fragmented. 3.0) and its attribute files provided information • slope lengths were determined for on the location of soils in the landscape and each landform, and did not account corresponding slope gradient (steepness) and for interception of run-off by roadways, length, along with soil properties for each field boundaries, ditches, ponds and mapping area. Data for soil properties, extracted drainage ways. from the SLC soil layer files, were used to calculate the inherent erodibility of each soil • the indicator is based on long-term average (K value). Values for the rainfall-run-off factor annual rainfall data that may not reflect (R) were tabulated from existing data sources single high intensity rainfall events that for each SLC mapping area. can cause significant soil erosion. The change in rate of water erosion over time was calculated by considering the effects of RESULTS changes in land use and tillage practices across The risk of water erosion in Canada and in each Canada, such as fluctuations in cropland areas, province is shown for 1981, 1986, 1991, 1996 shifts in the types of crops grown and the and 2001 in Table 13-1. Figure 13-1 shows the use of conservation tillage and no-till. This distribution of the various risk classes in information was obtained from the Census Western and Eastern Canada in 2001. of Agriculture for 1981, 1986, 1991, 1996 and 2001, and also linked to each SLC mapping Canada: In 2001, 86% of Canadian cropland area. The proportion of cropland falling into was in the very low (tolerable) water erosion each of the risk classes outlined above was risk class, up by 8% from the 1981 level. Hence, C. Soil Quality 91 Table 13-1: Share of cropland in various water erosion risk classes, 1981 to 2001 Share of Cropland in Different Water Erosion Risk Classes (in %) Province Very Low Low Moderate High Very High 81 86 91 96 01 81 86 91 96 01 81 86 91 96 01 81 86 91 96 01 81 86 91 96 01 BC 63 65 71 72 75 19 17 17 15 15 11 10 6 6 5 1 1 1 5 4 6 7 5 1 <1 AB 80 81 83 87 90 11 11 9 6 4 4 3 3 3 4 2 2 3 3 1 3 3 2 1 1 SK 8588899092 75544 43333 22 222 22211 MB 83 86 87 91 95 13 10 10 7 4 3 2 2 1 <1 <1 <1 <1 <1 <1 2 1 1 1 1 ON 44 45 45 56 56 22 23 23 15 15 15 16 14 16 16 11 9 10 6 7 8 8 7 7 6 QC 70 73 73 72 71 14 12 12 13 15 7 6 6 7 6 4 6 6 5 4 5 3 3 3 4 NB 54 54 55 54 55 18 20 19 20 21 24 23 24 23 21 4 3 2 3 3 <1 <1 <1 <1 <1 NS 61 64 65 64 65 21 22 24 23 23 13 10 8 9 10 4 4 2 2 1 1 1 1 2 1 PEI 52 51 52 50 51 38 38 37 39 39 3 4 4 4 3 5 5 5 5 5 2 2 2 2 2 Canada 78 80 82 84 86 11 9976 55444 33 332 33222 * In Newfoundland and Labrador, the proportion of agricultural polygons was not sufficient to provide reliable results. Therefore, results for NL were not reported. an overall decrease of 8% was recorded in the proportions (56%) of cropland in the very water erosion risk for Canada during the 20-year low risk class and the largest share (6%–8%) period under review, from a 1% decrease in of cropland in very high risk class during the cropland area in each of the moderate, high 20-year period. and very high risk classes and a 5% decrease in cropland area in the low risk class. Quebec: Only slight changes were observed in the various risk classes over the 20-year period. British Columbia: This province (like Alberta, The share of cropland in the very low risk class Manitoba and Ontario) posted the largest increased slightly between 1981 and 2001, from decrease in water erosion risk, with 12% of 70% to 71%.