Laboratory Study On Soil Surface Roughness Effect On Soil Infiltration, Runoff And Sediment Production At The Loess Hillslope

Soil surface roughness is an index for describing the surface microtopography induced by tillage and soil management at the agricultural hillslope. Due to its effect on soil infiltration, surface runoff generation, flow concentration, detachment and transport capacity of flow, it have been considered as one of the focused topics in hillslope hydrology and soil erosion in the past and present.The objective of this study was to investigate the hydrological effect of soil surface roughness at the agricultural sloping filed. The study area locates at the Loess Plateau in China. The soil surface roughness was formed through tillage at study plot such as contour ploughing, deep hoeing, which are the common tillage practice in this area. Based on the knowledge from previous studies, I have conducted a series of experiments to measure the spatial characteristic of soil surface roughness and the effects of soil surface roughness on soil infiltration, surface runoff generation, flow concentration, soil particle distribution of eroded sediment and detachment and transport capacity of flow and to simulate runoff hydrograph using a travel time method. The experiments were conducted in laboratory with the simulated rainfall system. The goal of this study was to investigate hydrological effect, which is mentioned five subjects above, of soil surface roughness. To achieve this objective, some theory, technique and method including soil erosion, mathematics, soil science, hillslope hydrology, three-dimensional laser scanning and GIS were used to measure, process and analyze experimental data. The main results list below.1. Tillage practices increase the non-uniformity in soil surface roughness. Spatial variability and multi-fractal are two natural properties of soil surface roughness. Fractal dimension is 2.79, 2.72 and 2.87 for the soil surface roughness at shallow hoeing, deep hoeing and contour ploughing and, meanwhile, crossover length is 1.09, 1.03 and 1.01. Results show that the greater the soil surface roughness, the greater the fractal dimension and the spatial variability but lower the crossover length.2. The initial characteristics of soil surface roughness play a control role in changes of the size and direction(i.e., increase or decrease) of soil surface roughness in erosion process. For a rough surface, the size of soil surface roughness decrease with the increased accumulated rainfall. The change could be described by a power function(P<0.05). The greatest change in soil surface roughness triggered by 20 mm of accumulated rainfall from the beginning of rainfall, which is approximately 20% to 40% of the total variation. After that, the effect of rainfall erosion on soil surface roughness decrease. Furthermore, the change in roughness is also related to its position in an eroded slope. This is because spatial variability is a natural property of roughness, resulting in response differently to rainfall erosion process for roughness in different position.3. Soil with rougher soil surfaces experienced a larger effect of roughness on soil wetting front. However, the effect of roughness on soil wetting front decreases as the increased depth of infiltration. For a rough surface, in the vertical direction of the slope surface, the process of infiltration changed from a non-uniform pattern to a uniform pattern as time passed during a rainfall event. Overall, soil surface roughness can increase soil infiltration by more than 10% compared with that of smooth surface.4. Soil surface roughness affects surface runoff and sediment production. The initiation time of surface runoff from a rougher surface was delayed approximately 10 min compared with that of smooth surface. After beginning of runoff, the runoff rate of a rough surface was in general lower than that of the smooth surface at the raised duration of hydrograph. The difference in runoff rate between the rough surface and smooth surface decrease as the increased rainfall time. Sediment production shows a similar relationship between the rough surface and smooth surface as that of runoff production. In additional, the gradient of slope and rainfall intensity both decrease the effect of soil surface roughness on surface runoff and sediment production.5. The fraction of coarse particles in sediment from a rough surface is lower than that from the smooth surface. The fraction of coarse particles in sediment decrease and the fraction of fine particles in sediment from rough and smooth surfaces both increase with the increased rainfall time. And more importantly, the increase rate of the fraction of fine particles in sediment from the rough surface is greater than that of the smooth surface. In additional, rainfall intensity, runoff and sediment rate from upslope contributing area decrease the effect of soil surface roughness on sediment particle size distribution.6. Soil surface roughness contains two elementary forms, i.e., depresions and mounds, which affect water flow on the surface differently. While depresions serve as temporary water storage, mounds diverge water away from their local summits. A runoff model was proposed to simulate hydrograph on soil surface with different microtopographys. Three important input parameters including rainfall intensity(I), mean flow velocity(vm), ponding time in depression area(TPi) were applied to predicted runoff rate with the growth of the time based on travel time theory. A good agreement is obtained between the measured and predicted runoff hydrograph. This model provides a new idea for understanding soil surface roughness effect on the menchism of runoff production on surface.