Seepage And The Rill Morphological Features On Row Sideslopes In Contour Ridging System

Seepage plays an important role in the soil erosion of contour ridge systems. Understandingthe seepage process and the effects of various factors on seepage will advance our knowledgeregarding seepage erosion mechanisms in contour ridge systems. Concentrated flow can inducerill and even ephemeral gully erosion in contour ridging systems. The rill morphological featurescould provide basic information to prevent rill developing to gully, and help to predict soil lossmodulus induced by rill erosion. In this study,23treatments were arranged based on orthogonalrotatable central composite design to investigate the characteristics of seepage processes and theeffects of row grade, field slope and ridge height on seepage discharge. Meanwhile the rillmorphology was also researched under seepage condition in the contour ridging system.The seepage process curve can be divided into two types:(I) The majority of the seepagedischarge time series exhibited an “S”-shaped curve;(II) Only the treatment No.8expressed as asteady increasing trend. The seepage discharge processes fitted an exponential model well withdetermination coefficients (R2) greater than0.995. The maximum seepage discharge growth ratewas achieved within14min, whereas the seepage discharge stabilized within106min. Thesecond-order polynomial regression models were used to model the total and predicted stableseepage discharge with factors of row grade, field slope and ridge height as independentvariables, which produced values of0.66(p <0.05) and0.68(p <0.05), respectively. Theridge height and row grade quadratic term had significant effects (p <0.05) on the total andpredicted stable seepage discharge, whereas the field slope and its related factors had nosignificant effects that could be ignored in the regression models. The effect of the row gradepresented a concave curve with an increasing factor value. However, the ridge height exerted alinear positive effect.To better understand the morphological features of rills and deposition characteristics incontour ridging systems and their influencing factors, a linear equation was built with themaximum rill width as dependent variable and the maximum rill depth as independent variable(R2=0.69). An obvious linear relationship existed between the two variables. The erosion volumecould be predicted better by an simplified power equation with the maximum rill depth andlength considered than only the rill length involved, with the prediction accuracy R2=0.96andR2=0.61, respectively. Ridge height could promote the seepage generation and extend the maximum rill width and depth. Seepage presented a two-way effect on the maximum rill widthand the total soil erosion: when rill width and the total soil erosion less than14.3cm and0.7kg,respectively, the seepage presented a positive effect, however it exerted a negative effect whenthe values greater than them. Seepage had a positive effect on the maximum rill depth. Thesediment deposition could be divided into three types:(I) no deposition;(II) deposition at rilldownstream; and (III) deposition both at rill downstream and ridge furrow. Seepage has apositive contribution to the deposition amount. The row grade could make the depositionlocation up to ridge. Deposition at rill downstream and ridge furrow has obvious relationship.