Assessing rainfall erosion risk in southern Saskatchewan from daily rainfall records

A mean annual rainfall erosion index (R1daily) calculated from daily rainfall records satisfactorily compared with the mean annual erosion index (R) calculated using the more accepted method with hourly rainfall records. An R1daily contour map of Southern Saskatchewan was constructed from a greater number of weather stations keeping daily rainfall records compared to the R contour map constructed from very few stations that keep hourly rainfall records. It was concluded that the R1daily contour map provided a more reliable assessment of rainfall erosion potential than the R contour map because the fonner distinguished local areas with high values and interpolation of rainfall risk involved much shorter distances between point measurements. Since significant erosion requires a combination of rainfall detachment as well as runoff, a runoff model that operated from daily climatic records was used to determine which days with rainfall also produced runoff. The runoff model was run for 62 stations in southern Saskatchewan and only days that produced runoff were included in the annual erosion index total. This effective erosion index (Reff) varied from 3.4 to 83.9% of R1daily and was particularly sensitive to soil texture. Heavy-textured soils were, on average, more than four times as susceptible to water erosion than light-textured soils. It should be noted that the analysis does not include the effects of slope length and steepness, the crop canopy nor soil erodibility. INTRODUCTION Water erosion is closely related to rainfall since rainsplash is an important mechanism of soil detachment and rainfall patterns have a strong effect on runoff generation (Morgan 1986). In the Universal Soil Loss Equation (USLE), the erosivity of the rainfall is expressed as an index, R, based on the kinetic energy of the storm (Wischmeier and Smith 1958; Zanchi and Torri 1980). Calculation ofR requires a continuous record of rainfall intensity over a period of several decades. Unfortunately, records of this sort are usually not widely available. In Saskatchewan, hourly rainfall values are the most detailed records kept on a routine basis, but are available only for about a dozen major weather stations in the agricultural portion of the province. Wigham and Stolte (1986) used the procedure ofWischmeier and Smith to calculate the R value for a number of stations on the Canadian prairies. They commented on the limited length of the records for most of the stations. Daily rainfall records are available for much longer periods and for many more stations than just at the major weather stations. An estimate of the rainfall erosion index from daily rainfall records would be useful for providing input to erosion models and for assessing erosion risk in areas where detailed hourly records are not available for a sufficiently long period of time. Soil detachment and transport occurs by rainsplash and by runoff (Edwards and Burney 1987). The relative importance of splash and runoff is still subject to debate (Thompson et al. 1986). Foster et al. (1981) concluded that erosivity indices based on volume and rate of runoff may be better than the R value since the R value over-estimates the soil loss that occurs with negligible runoff, and conversely, under-estimates the erosion when runoff is great relative to rainfall. The objectives of this study were: (1) to compare rainfall erosion indices calculated from hourly data using different simplifying assumptions to previously published estimates by the Wischmeier and Smith approach, (2) to compare the erosion index calculated from hourly data with an index calculated from daily rainfall records, and (3) to combine the estimated daily erosivity with a daily runoff model to determine an "effective" erosion index for days in which there was runoff produced.