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Factors Affecting Runoff Nutrient Losses

Rainfall can be a source of runoff and erosion. The potential for rainfall to create runoff depends on soil conditions (e.g., frozen versus thawed), soil type, rainfall intensity and volume, slope, ground cover, the soil’s water holding capacity and the soil’s structural integrity (e.g., compacted or not). Some of the most erosive events in Alberta have occurred due to large rainfall events that have overwhelmed the capacity of existing drainage paths, waterways and soil to absorb and hold water and maintain structure.

The Revised Universal Soil Loss Equation (RUSLE)

The Revised Universal Soil Loss Equation (RUSLE) lists the main factors that contribute to soil erosion. An understanding of these factors and how they affect soil erosion helps with the design of sound strategies to control runoff. The Universal Soil Loss Equation (USLE) was developed in the United States in the mid 1960’s, and was revised (RUSLE) for Canadian conditions in the 1990’s to estimate soil losses due to surface runoff: A = R × K × L × S × C × P Where,

A = soil erosion loss in tonnes/hectares R = rainfall factor K = soil erodibility factor L = length of slope factor S = slope factor

C = cropping system/ground cover factor P = management practices factor

Soil Properties

Soil properties (e.g., texture, structure and soil organic matter) affect the size and amount of soil pores, and determine how easily water infiltrates and is held in the soil. Larger soil pores and fissures present in coarse textured soils generally allow faster infiltration while water infiltrates more slowly in fine textured soils.

Soil Structure

Well-structured soil with stable aggregates and an extensive network of pores allows water to infiltrate much easier than a poorly structured, compacted soil. Organic matter in a soil also influences infiltration in a couple of ways:

Organic matter (especially coarse organic matter) •

is extremely porous so it allows water to infiltrate relatively easily. Organic matter enhances soil aggregate stability, •

which helps the soil to resist particle detachment by erosive forces and also promotes infiltration.

Soil Water Holding Capacity

The soil’s ability to handle water once it has infiltrated (i.e., water holding capacity) is determined by texture and organic matter content. Medium textured soils generally have the highest plant-available water content (Figures 8.1.2, 8.1.3). Organic matter content is positively related to water-holding capacity of the soil since organic materials act like a sponge and can absorb several times their dry weight in water. Increased water holding capacity reduces the potential for erosion that can occur as a result of poor soil structure or texture (e.g., eroded soils, low organic matter soils).

more info

To learn more about RUSLE and application to Canadian conditions, consult the following online document from Agriculture and Agri-Food Canada (AAFC):

Wall, G.E., Coote, D.R., Pringle, E.A. and Shelton, I.J. (Eds). 1997. Revised Universal Soil Loss Equation for Application in Canada: A Handbook for Estimating Soil Loss from Water Erosion in Canada. http://res2.agr. ca/CRECO/pubs/pdf/rusle_e.pdf