| Gully erosion is the most threatening and destructive soil water erosion with the feature of developing rapidly. Compared with slope erosion, gully erosion shows hysteresis because of spasmodic nature in space and stage of gully development, and the monitoring technologies developed relatively slowly. The purpose of this research are to access the rate of gully erosion in typical Loess Plateau region, analyze theprocesses of gully erosion and factors and provide theoretical basis for comprehensive assessing the benefit of returning farmland to forest project and reference for controlling gully erosion.The research was carried out in Suide County and Wuqi County of Shaanxi province, based on two QuickBird images with pixel of 0.61 m and DEM with pixel of 5 m. The rates of gully erosion were calculated with visual interpretation and GIS technology, the dominant factors of gully erosion were analyzed with correlation analysis, the optimal vegetation coverage on the upslope drainage area for gully erosion control was determined with regression analysis. DEM of gullies was constructed based on multi-period 3-D laser scanning data to calculate the change rate of area, perimeter, volume and gully head retreat, analyze the differences of the gully erosion rate and erosion process in the two study areas with variance analysis and quantified difference. The main results are as followings:(1)The annually average of the proportion of changes in gully area was 0.46% in Suide from 2004 to 2013. Among the three factors related to gully growth rate investigated in this study, the terrace proportion in upslope drainage area was the most important; 32 small watersheds were classified into 5 groups according to terrace proportion and vegetation types. Gully area growth rate in 5 groups of small watersheds increased in the following order:woodland on narrow terrace> grassland on narrow terrace > cropland on terrace> multiple vegetation on terrace> multiple vegetation without terrace. It indicates that the combination of engineering measures such as terrace and conservation vegetation achieves better effect than the single one.(2) Annual average growth rate of gully area in 30 study catchments was less than 0.5%with average is 0.13% in Wuqi from 2007 to 2013. The result of Pearson correlation analysis showed that vegetation coverage on inter-valley was the most important with r=0.619 at significance level of 0.01; The non-linear regression curve was fitted between growth rate of gully area and vegetation coverage in 30 catchments, with R2 was 0.713, indicating that vegetation coverage larger than 60% in inter-valley could significantly inhibit the gully erosion.(3)The results of the rate of gully erosion in two study areas based on 3-D laser scanning data showed that the annual proportion change of area, perimeter, volume and gully head retreat were respective 1.73%,1.08%,8.70% and 0.29 m/a in Wuqi and the result in Suide were 1.79%,1.28%, 12.31% and 0.26 m/a, which indicated the gully erosion in Suide was larger than that in Wuqi. The average proportion of head retreat in gully erosion in Wuqi was 69.90%, while it was 57.69% in Suide, which showed that compared with Suide, gully erosion in Wuqi emphasized particularly on gully head retreat. The results of gully head retreat rate and annual rate of gully area change in three kinds of gullies(Grassland, Grassland & Shurbland and Terrance & Shurbland) were respective 0.33,0.23,0.20 m/a and 2.25,1.94,1.18%, which indicated the site preparation measures and biological measures of returning farmland to forest project could control gully erosion effectively. |
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