Influence of ridge height, row grade, and field slope on soil erosion in contour ridging systems under seepage conditions

•Soil erosion in contour ridge systems under seepage conditions was investigated.•Row grade and field slope had a greater effect on sediment yield than on runoff.•Ridge height exerted an increasing positive effect on runoff.•Interactions of the row grade, field slope, and ridge height were not significant.

Seepage in contour ridge systems is a common phenomenon that can exacerbate soil erosion, however, the characteristics of soil erosion under seepage conditions in contour ridge systems are not clear. The objective of this study was to analyze the soil erosion process under seepage conditions and quantify the effects and interactions between the ridge height, row grade, and field slope on runoff and sediment yield. Twenty-three treatments for these three factors were arranged by an orthogonal rotatable central composite design. A new type of experimental plot for simultaneously changing the row grade and field slope and creating seepage conditions was used to imitate the microtopographic relief of contour ridge systems. In each run, seepage samples from the row sideslope were collected every 2 min for 60 min, and then artificial rainfall simulation was performed for 30 min during which runoff samples were collected every 1 min. The results showed that four soil erosion sub-processes were observed, including interrill erosion, headward erosion, contour failure, and rill erosion. Second-order polynomial regression models predicted the sediment yield (R2 = 0.74) better than the runoff (R2 = 0.56). Interactions between these factors did not significantly affect the runoff or sediment yield even at p < 0.1. The row grade and field slope exerted a greater effect on the sediment yield than on the runoff, whereas the ridge height influenced the runoff more with an increasing positive effect. The effect of these three factors on sediment yield revealed a convex curve with an increasing factor value. The field slope exhibited a greater increasing effect before the maximum sediment yield occurred and a greater decreasing effect after that than the other two factors did. The maximum runoff and sediment yield occurred at similar row grades (7.5° and 7.1°, respectively) and field slopes (10.9° and 10.8°, respectively). However, the minimum runoff occurred at a ridge height of 6.7 cm, and the maximum sediment yield at a ridge height of 12 cm. The findings have important implications for assessing and modeling soil erosion in contour ridge systems.