The Characteristic Of Surface Microrelief Changes And Erosion Response Of Red Soils During Rainfall

Soil surface microrelief is an important factor which affect the process of soil erosion, such as runoff, infiltration, soil sealing, flow concentration and sediment transport and so on. It's great important to study the relationship between the characteristics of soil surface microrelief and erosion for soil erosion control and environmental protection in China. Field experiments of three successive rainstorm simulations with the same initial condition and different rainfall intensity were conducted on three typical red soils derived from Quaternary red clay, Shale. Combining the use of mm-level high-precision laser scanning and computer digital image processing method, this paper studies the characteristics of soil surface microrelief changes and erosion response during the simulated rainfall. The main results are listed as follows:(1) With the rainfall, the combined effects of the detachment by raindrop impact and the transport of runoff lead to lower surface roughness. The range of surface roughness decrease is most, and there are significant differences for all treatments during the first rainfall. The roughness of HS and HQ1 decrease greater than HQ2 due to the higher aggregates stability of HQ2. Under 1mm/min rainfall intensity, the roughness of three soils decrease rapidly during the first rainfall, and the change is little during the second and third rainfall. However, under 2mm/min rainfall intensity, roughness of the three soils increase with different degrees during the second and third rainfall. The crust of HS is not easy to be destroyed, thus its roughness increase least. Soil sealing tends to reduce the roughness, and rills tends to increase roughness, changes in surface roughness depends on the balance between the two factors. The changes of the roughness in different slope positions are different during continuous rainfall. For the upslope, roughness decrease most, for the midslope, roughness decrease in a certain degree, and the roughness of downslope decrease least. The main reason for this phenomenon is: the three subprocesses of soil erosion—erosion, transport and sediment, have different performance for different positions of the slope.(2) Depression storage capacity is found to be highly related to soil surface roughness. The change of depression storage capacity is similar to roughness, which is decreased during continuous rainfall, specially for the first rainfall. The trends of three soils are different during the second and third rainfall, this is related to stability of aggregate and soil crust. When rainfall intensity is 1mm/min, depression storage capacity is more sensitive to roughness.(3) We use drainage density, stream frequency and fractal dimension to represent characteristics of drainage networks. Drainage density and stream frequency represent the effectiveness of runoff drainage at a different angle. With the rainfall, drainage density and stream frequency increase, specially for the first rainfall. The trends of three soils are different during the second and third rainfall, this is related to rainfall intensity and soil properties. Soil surface roughness is found to be highly related to drainage density and stream frequency. Basically, as soil surface roughness increased, the drainage density and stream frequency decreased. Fractal characteristics of the drainage networks are determined to describe the degree of network organization. For the study of soil erosion, fractal dimension D_S can distinguish characteristics of drainage networks before and after rainfall well. When rainfall intensity is 1mm/min, D_S increase first, then decrease and is close to 2 at the end of the experiments, indicating that a high degree of network organization.(4) For each order stream of drainage networks, the sinuosity, gradient and orientation are calculated respectively. These three parameters describe runoff patterns and changes in runoff path characteristics from different angles. During continuous rainfall, by the analysis of the changes of three parameters, the different trends of the characteristics of runoff path for three red soils are described. The average of sinuosity and orientation trend to decrease as a whole, and they trend to increase during the second and third rainfall, when rainfall intensity is 2mm/min. With the cumulative rainfall increasing, the average of gradient also increase and trend to the slope of the plot. Most of the changes appear in the course of the first rainfall. The effect of continuous rainfall result in the different characteristics of runoff path for three soils increase at the end of the experiment. Different from HS and HQ1, the runoff path characteristics of HQ2 change uniformly with the rainfall. Soil surface roughness is found to be highly related to the sinuosity, gradient and orientation of runoff path. For different rainfall, the change of runoff path characteristics with the magnitude of roughness are also different, the range is less for 2mm/min rainfall intensity.(5) When rainfall intensity is 1mm/min, microrelief parameters was found to be highly related to the time to start runoff. But when rainfall intensity is 2mm/min, depression storage capacity and roughness just have some influence on the time to start runoff. There is a a certain interaction between runoff coefficient, runoff rate and roughness. Basically, as soil surface roughness increase, the runoff coefficient and runoff rate decrease. When rainfall intensity is 1mm/min, the effect of roughness on runoff is more evident.(6) Initially similar network configurations yielded different erosion values but resulted in different network characteristics at the end of the rainstorm experiments. Raindrop detachment, clod destruction and microrelief changes were identified as important mechanisms of network configuration and stream property changes during the rainstorm and erosion events. Because of different aggregate stability and soil surface property, erosion responses are different for three soils. This result in difference of drainage networks and runoff path properties is observed at the end. Besides, the change of networks configuration contribution to continuous decrease of soil erosion. Networks yielding fractal dimensions near 2 are the result of network self organization at the end of the rainstorm experiments. The results support the idea of optimization in drainage network development.