Erosion Of Brown Soil In Contour Ridging Systems And Its Influencing Factors

Field slope is one of the most important areas for sediment generation effected by tillage cultivation. Contour ridging, generally shaped as ridge and furrow, is an effective soil conservation practice for increasing crop yield used throughout the world. Among the existing soil erosion models, the soil conservation benefit for contour ridging has been considered to the greatest extent in the Revised Universal Soil Loss Equation, Version2(RUSLE2). In RUSLE2, the factors of rigde height and row grade is used for assess the benefit of contour ridging (Pc) as subfactors. The accumulation of rainwater in the ridge and furrow system may couse ridge collapse and seepage generation, which could increase soil erosion. Laking of sufficient observation data, the soil erosion process on row siderslopes, erosion induced by ridge failure and erosion characteristic under seepage conditions in contour ridging systems has not been carefully considered in RUSLE2model, and the effect of factors (e.g. microtopography, ridge geometry) and their interactions on soil erosion is not quantitatively interpreted and need further studies.Through field investigation of slope land microtopography and ridge geometry in the hilly and mountainous areas of central and southern Shandong province, a new type of experimental plot was designing to imitate microtopographic relief of ridge and furrow system. In such plot, the row grade and field slope can be changed simultaneously and seepage conditions can be created. In this study,32rainfall simulation experiments were performed in drainage conditions to analyze the effects and interaction of two ridge geometry indices (ridge width and ridge height), two microtopography indices (field slope and row grade), and rainfall intensity on soil erosion with two replications. To address the importance of seepage in soil erosion, a total of23treatments with3factors (e.g., ridge height, row grade and field slope) in5levels were arranged in an orthogonal rotatable central composite design. To predict the sediment yield and evaluate the significance of the effects and interactions of these factors, second-order polynomial regression models were built and the regression coefficients were tested. The main results and conditions were listed as bellows: (1) Before contour failure, soil erosion process on the row sideslope could be classified as interrill erosion period and rill erosion period.The runoff generated during the two periods accounted for about44.2%and55.8%of the total runoff, respectively. Sediment yield in the rill erosion period was the main source for the entire sediment with the contribution of87.2%. The duration for the interrill erosion period was longer than that of rill erosion period and occupied72.3%of the entire duration. In the interrill period, the runoff and sediment yield per min were positively affected by ridge width and rainfall intensity, with the contributions of33.1%and28.7%for runoff and14.8%and17.0%for sediment yield, respectively. Ridge height had significant and positive effect on runoff per min but not on sediment yield per min. runoff per min was mainly influenced by ridge geometry factors, while the sediment yield per min mainly by the microtopography relief. During the rill erosion period, runoff per min was significantly and negatively affected by ridge height with the reason that higher ridge height could retain more rain water for a longer time under a higher water head to lead more water infiltration. Through reducing the soil cohesiveness and slope stability, a greater row grade could significantly increase the sediment yield per min. The interactions between some factors played an important role in the soil erosion on row sideslopes, e.g., the negative interaction between field slope and rainfall intensity on runoff during both periods, the positive interaction between rainfall intensity and ridge width on sediment yield in the rill erosion period, the positive interaction between row grade and ridge width on the duration of the rill erosion periods.(2) When the contour failure occurred during the erosion process in ridge system, except for row grade, all of the factors in this study had significant effect on runoff and sediment yield at p<0.01. The runoff mainly affected by rainfall intensity with the highest contribution of68.1%, and then followed by the factor of ridge height, field slope, and ridge width. The interaction between field slope and rainfall intensity significantly and negatively affected runoff with a contribution of5.4%. under a lower rainfall intensity, the runoff showed a increasing trend with the field slope increasing, while under a higher rainfall intensity, the the runoff showed a decreasing trend. Some interactions also exerted significant effect on runoff, e.g., the negative interaction between ridge height and width and the positive interaction between field slope and ridge height. Ridge height, with a negative effect, had a greater influence on sediment yield than rainfall intensity with the contribution of21.4%and19.4, indicating that adjusting microtopography relief and ridge geometry may have a better controlling benefit on sediment yield than on runoff. Additionally, the effect of row grade and its interaction with ridge width on sediment yield were positive and significant. According to the contribution of the effect and interactions, the optimal combinations of factors for runoff and sediment controlling were determined. To control runoff, the optimal combinations were FS1, H2, and W2under lower rainfall intensity, and RG1, FS2, H2, and W2under higher rainfall intensity. The optimal combinations for sediment controlling were RG1, FS1, H1, and W2. Here, the subscripts1and2represented the lower and higher factor level, respectively.(3) Under seepage conditions, soil erosion on row sideslope could be classified as four period:interrill erosion, headward erosion, contour failure, and rill erosion. Compared with the water supplying conditions, seepage dischare became smaller during the simulated rainfall conditions probably caused by rainfall pressure on soil matix and splashed soil partical clogging soil porosity. Taking row grade, field slope and ridge height as input variable, the second-order polynomial regression models for runoff and sediment yield were built, with the determination coefficient R2-0.743and0.545, respectively. Compared to runoff, the effect of row grade and field slope was greater on sediment yield. Ridge height had a greater influence on runoff than sediment yield with an increasing positive effect. The impact of row grade, ridge height, and field slope on sediment yield showed as a convex curve with factor level increasing. From the convex curve for each factor, the maximum sediment yield could be calculated out and the monofactor level where the maximum sediment yield occurred could be determined accordingly. Compared to the other two factors, field slope presented a greater increasing impact on sediment yield before the maximum sediment yield occurred, and after that field slope exerted a greater decreasing effect. Even at p<0.1, the interactions between the field slope, ridge height, and row grade had no significantly effect on both runoff and sediment yield. The results indicated that avoiding the factor level where the maximum sediment yield occurred, could better use contour tillage to control soil erosion.(4) Sediment yield under seepage conditions was as higher as about15times than under drainage conditions. Seepage disarge could be estimated using the second-order polynomial regression models with row grade, field slope and ridge height as input parameters. The dermination coefficient of the seepage discharge estimation model is0.759with significance at p<0.01.The seepage discharge were mainly affected by row grade, quadratic terms of row grade and ridge height. The interaction between these factors had no significant effect on seepage discharge. Using the measured or predicted seepage discharge as an input variable, the coefficient of determination (R2) increased from0.743to0.915or0.893and the root-mean square error (RMSE) decreased from0.67to0.38or0.43, respectively. The impoved sediment yield regression model combined with measured seepage discharge showed a greater significance than that combined with predicted seepage discharge, and the p value was0.007and0.016, respectively. With measured seepage discharge combined, the regression model presented more significanct effect and interactions, e.g., the row grade and the interaction between row grade and ridge height, field slope, and seepage discharge. The quadratic terms of field slope and the interaction between field slope and row grade and seepage discharge were also detected as significant items. With predicted seepage discharge combined, the regression model only included two significant items, i.e., quadratic terms of seepage discharge and the interaction between row grade and seepage discharge. Therefore, through removing the non-significant items, the predicted seepage discharge combined regression model could be simplified to a concierge form that could be easily used, especially when the seepage discharge could not be measured.