Characterization of Runoff and Sediment Yield in Farmlands on Loess Slopes Based on R/S and Wavelet Analysis

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the National Natural Science Foundation of China (No.41371273)

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    【Objective】On the Loess Plateau of China, water erosion in slope farmlands is the major type of soil erosion. Knowledge about changes in runoff and sediment yield therein will sure contribute to understanding nature of the erosion on a microscopic scale. The objective of this study was to investigate temporal variation of runoff and sediment yield on microrelief like farmland on loess slope under rainfall. An in-lab experiment was designed to have slopes, 5°, 10°, 15°, 20°, and 25° in gradient, and four commonly adopted tillage patterns (flat slope, artificial digging plowing, artificial backhoe plowing, contour tillage) paired in combination. 【Method】For the in-laboratory experiment, a steel trough (2.0 m long, 1.0 m wide and 1.0 m deep ) filled with soil was set up with slope gradient adjustable in the range of 0~30°. Rainfall was simulated with downward sprinklers, 18 m high above the ground, which allows all the artificial raindrops to reach targeted terminal velocity and size of raindrops in natural rainfall and to distribute like a natural rainfall. Each artificial rainfall event lasted 90 minutes. Samples of runoff were collected every two minutes after the initiation of runoff for. fractal-theory-based rescaled range analysis (R/S) to predict trend of the variation of temporal sequence of runoff and sediment and for Morlet wavelet analysis to identify principal cycles of the variation of each runoff and sediment yield sequence.【Result】Results show that Hurst index of the runoff sequence lies in the section between 0.567 and 0.798, while that of the sediment sequence in the section between 0.632 and 0.861, both being higher than 0.5, displaying a long-range positive correlation between the two. Hence the temporal sequence of runoff and sediment yield on loess slope farmlands in future will remain the same as that in the past. In addition, the volume of runoff and sediment yield displayed a positive and negative logarithmic relationship, respectively, with duration of rainfall, expressed by the equation of S=a+ bln t (R2>0.5, a and b are constants). Generally, runoff peaked in volume 28 to 29 minutes after its initiation on the slope farmlands regardless of tillage pattern, while, sediment yield might have a long cycle of over 30 minutes in sequence on slope farmlands under certain tillage, indicating that the sediment yield on these slope farmlands might vary periodically every 30 minutes, which, however, needs to be validated by more detailed data of a longer time sequence.【Conclusion】All the findings in the study demonstrate that it is reliable to use R/S analysis combined with Morlet wavelet analysis in predicting temporal variation of runoff and sediment yield. This study also enriches the theory of run-off and sediment transfer, and lays a foundation for in-depth understanding of the runoff and sediment, which is of great significance to further researches on relationship between soil erosion and time sequence on a microtopographic scale.

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ZHANG Huihui, ZHANG Hui, DONG Yan, ZHANG Qingfeng. Characterization of Runoff and Sediment Yield in Farmlands on Loess Slopes Based on R/S and Wavelet Analysis[J]. Acta Pedologica Sinica,2017,54(6):1345-1356.

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  • Received:January 24,2017
  • Revised:July 08,2017
  • Adopted:July 11,2017
  • Online: September 04,2017
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