Simulation of response law for soil surface roughness and hydraulics parameters of runoff

Abstract: Soil surface roughness (SSR) is used to describe irregularities in the soil surface at a small scale. It is affected by tillage systems, soil properties, runoff, micro topography, and climate. The objective of this paper was to study the mutual influence of SSR and characteristic of runoff as well as its hydraulic parameter under four different tillage systems. Artificial shallow plowing (ASP), artificial deep plowing (ADP), contour plowing (CP), and no treatment tillage (CK) were simulated in the laboratory to form four different soil surface roughness. Soil sample was filled into a 2.0 m × 1.0 m × 0.5 m iron slope-adjustable box. The soil in the box were exposed to 60 mm/h and 120 mm/h simulated rainfall for 90 min at slope 36%. Soil surface roughness was measured before and after each rainfall event. Runoff was measured in every two min since runoff occurred. The shape of runoff and runoff pattern were observed during the rainfall events. Reynolds number, Froude number, Resistance coefficient and Flow shear stress were calculated. Results showed that SSR was in an order of CP > ADP > ASP > CK before and after the rainfall simulation. In the same condition of rainfall intensity and duration, soil surfaces with different SSR showed significant difference in runoff. The higher the initial SSR was, the easier the runoff was in stable state and the flow was a laminar flow. On the contrary, the smaller initial SSR was, the easier the runoff was in turbulent flow state. Tillage with bigger initial SSR showed smaller quantity of flow, runoff, and sediment yield. Under 60 mm/h rainfall, the initial SSR of CP was 6.51 mm, which had the smallest runoff volume of 75.79 L. The initial SSR of ADP and ASP were 4.90 mm and 4.17 mm, respectively. The runoff volume of ADP and ASP were respectively 85.93 L and 87.13 L. The initial SSR of CK was the smallest one (0.36 mm). Its runoff volume was 97.83 L. The initial SSR was negatively correlated to the runoff volume significantly as well as Reynolds number, Froude number, and flow shear stress, but it showed a positive correlation with the resistance coefficient of runoff. The 120 mm/h rainfall had the same variation trend as 60 mm/h intensity. The SSR change rate was also calculated from the SSR before and after the rainfall events. This parameter can reflect the effect of runoff on SSR change. The pattern of runoff and flow shear stress affected SSR during runoff process. On the one hand, runoff can cut the surface peak area and fill the depression area to decrease SSR at inter-rill area. On the other hand, runoff scoured rills, shear rills bed off, corroded soil particles of rills to increase SSR. Both the decrease and the increase coexisted in the process of the runoff. However, after rill erosion occurred, inter-rill area was domination while rill erosion was subordinate. Hence, runoff showed decreasing effect on SSR apparently after a complete rainfall. The Reynolds number and flow shear stress of runoff presented a negative correlation with the decreasing effect on SSR. The Reynolds number and flow shear stress of CK were the largest, it were 239 and 8.62 Pa for 60 mm/h rainfall, 1893 and 27.23 Pa for 120 mm/h rainfall. The SSR of CK decreased 27.61% for 60 mm/h rainfall and 70.48% for 120 mm/h rainfall, which were the lowest among the four tillage systems. Reynolds number and flow shear stress of CP were the smallest, it were 82 and 6.67 Pa for 60 mm/h rainfall, 738 and 20.05 Pa for 120 mm/h rainfall. But the SSR of CP decreased by 34.49% for 60 mm/h rainfall and by 84.25% for 120 mm/h rainfall, respectively. The research results provide reference for further understanding of the interaction between surface roughness and its hydrological characteristics.