Rill morphology and deposition characteristics on row sideslopes under seepage conditions

Concentrated flow can induce rill and ephemeral gully erosion in contour ridging systems. To better understand the morphological features of rills, deposition characteristics in contour ridging systems and their influencing factors, rainfall simulation experiments were conducted considering four factors: field slope, row grade, ridge height and seepage discharge. The results showed that a simplified power equation that considered the maximum rill depth and length predicts the erosion volume better than an equation that only accounts for rill length, with prediction accuracies of R² = 0.96 and R² = 0.61, respectively. The ridge height could promote seepage and extend the maximum rill width and depth. The relationships between seepage and rill width, and seepage and soil loss could be fitted with quadratic equations. Seepage exerted a positive effect when the rill width and total soil loss were <14.3 cm and 0.7 kg, respectively, and presented a negative effect when the rill width and total soil loss were greater. Seepage had a positive effect on the maximum rill depth. Sediment deposition was divided into two types: (i) deposition downstream of the rill and (ii) no deposition on the row sideslope. Greater row grades could result in deposition up to the ridge. The simple rill volume prediction equation could help predicting the soil loss induced by rill erosion in contour ridging systems. The effect of field slope, row grade, ridge height and seepage discharge on soil loss could provide guidance for choosing appropriate ridge morphology on slopeland to reduce rill erosion.