The influences of row grade, ridge height and field slope on the seepage hydraulics of row sideslopes in contour ridge systems

- The seepage discharge process fit well to an exponential model.
- The maximum growth rates of the seepage discharges occurred within 14 min.
- The majority of the seepage discharges stabilized within 85 min.
- The ridge height and the quadratic term of the row grade significantly affected seepage.

Seepage plays an important role in soil erosion in contour ridge systems. Seepage generated from subsurface flow causes hillslope instability by reducing the soil shear strength and mobilizing soil particles and can produce cave-like features called seepage undercuts that can lead to contour failure. As the main threat to contour plowing, seepage results in large amounts of soil erosion due to row grade. Models used to predict seepage over a time series will provide a basis for modeling soil erosion resulting from seepage in contour ridging systems. Understanding seepage and its effects will advance our knowledge regarding seepage erosion mechanisms in contour ridge systems. In this study, 23 treatments were arranged using an orthogonal rotatable central composite design to model a seepage time series, build a simple seepage prediction model and investigate the effects of row grade, field slope and ridge height on seepage discharge.Most of the seepage discharge time series followed an S-shaped curve. The seepage discharge processes were fit by an exponential model with a determination coefficients (R2) greater than 0.995. Furthermore, the physical meaning of the exponential model was consistent with the experimental results. The seepage discharge continuously increased before the inflection point and then decreased. Finally, the seepage discharge approached a steady value. The maximum seepage discharge growth rate was achieved within 14 min, and the seepage discharge became steady within 106 min. Second-order polynomial regression models were used to determine the total and predicted steady seepage discharge using independent variables of row grade, field slope and ridge height, which produced R2 values of 0.66 (p < 0.05) and 0.68 (p < 0.05), respectively. Ridge height and row grade significantly affected (p < 0.05) the total and predicted steady seepage discharge. Field slope (and its related factors) was ignored because it had no significant effects on seepage discharge. The effects of row grade resulted in a concave curve with an increasing factor value, and ridge height exerted a positive linear effect on seepage discharge.