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Location: National Soil Erosion Research

Title: Analytical method for determining rill detachment of purple soil as compared with that of loess soil

item CHEN, XIAO-YAN - Southwest University
item HUANG, YU-HAN - Southwest University
item ZHAO, UY - Southwest University
item MO, BIN - Southwest University
item MI, HONG-XING - Southwest University
item Huang, Chi Hua

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/27/2017
Publication Date: 4/1/2017
Citation: Chen, X., Huang, Y., Zhao, U., Mo, B., Mi, H., Huang, C. 2017. Analytical method for determining rill detachment of purple soil as compared with that of loess soil. Journal of Hydrology. 549:236-243.

Interpretive Summary: Purple and Loess soils are main sources of sediments in two largest rivers in China, i.e., the Yangtze River in the south and the Yellow River in the north. Despite many past research efforts in quantifying the erosion potential of these two soils, there has not been an experiment that these two soils are compared together in the same laboratory using the same research procedure. In this research, we are interested in finding out how these two soils erode in rill channels under concentrated water flow. We found both soils followed similar trends, i.e., detachment was decreased as sediment concentration and rill length were increased. Nevertheless, the Purple soil was less erodible and reached a maximum sediment level much quicker than the Loess soil. The important message from this study is that different soils may have similar erosion trends, but have different specific responses depending on properties of the particular soil. It is also important that identical erosion research techniques are used when assessing erosion potential of different soils.

Technical Abstract: Rills are commonly found on sloping farmlands in both the loess and purple soil regions of China. Rill erosion is an important component of slope water erosion, and primary sediment sources in small catchments in the areas. A comparative study on rill erosion on loess and purple soils is important to compare research results between these soils. The data used in this study were obtained from a series of laboratory rill-erosion experiments on loess and purple soils, using the volume-replacement method. The experiments involved a combination of five slope gradients (5°, 10°, 15°, 20°, and 25°) and three flow rates (2, 4, and 8 L min-1) to produce sediment concentration distribution processes along the rill 0.5, 1, 2, 3,..., 8, 10, and 12 m from the rill outlet. Sediment-distribution data were plotted against the rill distances, before they were used to estimate rill-detachment rates, with the numerical method. Results showed that rill-detachment rates on the two soils decreased exponentially with rill length and linearly with sediment concentration based on computations using the numerical method. These two relationships indicated that rill detachment rate was negatively and linearly related to sediment concentration in the water flow and exponentially decreased with sediment concentration. Therefore, when sediment concentration increased exponentially with rill length, the deficit and detachment rate decreased exponentially with rill length. The high coefficients of determination for both exponential and linear functions indicated high significances of the functions to relate detachment rate with rill length and sediment concentration, respectively. The results were good for both purple and loess soil rills. However, crucial differences in rill-detachment rates were observed between the two soils. Purple soil was clay loam with higher clay and aggregate contents, whereas loess soil was sandy loam with low clay content and almost no aggregates. Purple soil was initially detached at relatively higher applied erosive forces than loess soil, indicating greater resistance of former to water erosion. The maximum detachment rate in purple soil rills was significantly lower than that in loess soil at the same flow rate and slope gradient. The increase in detachment rate of purple soil was 4% faster than that of loess soil, indicating that rill flow in purple soil may reach the sediment transport capacity more rapidly than rill flow in loess soil. The results from this study indicated that research methods and data on rill erosion can be interchanged between loess and purple soils.