Plastic Covered Cropping Systems: Runoff Patterns and Soil Erosion Rates

Dryland farming practices in mountainous regions of South Korea are dominated by cultivation of vegetables with plastic film covered ridges (plastic mulch). Primarily, furrows are oriented perpendicular to the main slope of the field site. In terms of soil erosion, this cultivation strategy could be regarded as a contour tillage conservation practice because it diminishes surface runoff along the steepest slopes. Contour farming is expressed in the Universal Soil Loss Equation with the P-Factor. The effectiveness of contouring depends primarily on slope angle and slope length, which results in P-Factors between zero (maximum efficiency) and one (no effect). Alternatively, tillage in mountainous areas of South Korea is typically at an angle to the contour lines, and runoff flows along the furrows, although not parallel to the steepest flow path. Therefore the erosion reduction efficiency is reduced with angle from contour. The contouring effectiveness depends not only on the slope angle and flow distance, but also on the field site morphology. On fields with a straight or concave surface, runoff flows along the ridges from the field to its edge with sediment only derived from the corresponding furrow. On field sites with a convex surface, runoff is focused along the furrows to the steepest point where flow accumulation occurs. During intense monsoonal events, the accumulated runoff can induce ridge breakovers resulting in increased erosion rates. Additional erosion can occur due to the plastic cover, which inhibits infiltration and accelerates runoff generation in the inter-rows. This results in both, loss of ridge conservation effectiveness, and higher field site total erosion. In this case P-Factors larger than one are necessary to describe the erosive effect of this cropping system. The objective of this study was to verify and quantify the impact of this cultivation strategy on soil erosion rates from mountainous farmland areas in South Korea.