| Soil erosion has an important impact on the ecological degradation of the Three Gorges Reservoir Area(TGRA)and the reservoir operations.Several previous studies have basically investigated soil erosion in the TGRA,but little attention has been considered towards the following questions:(a)low-resolution images used to evaluate the soil erosion;(b)lack of long-term and multi-periodical analysis of soil erosion change;(c)lack of research on soil erosion caused by rainfall in the water-level fluctuating zone(WLFZ).Therefore,the present study used different resolution images and CSLE models to analyze the impact of different resolutions of land use and vegetation coverage to evaluate soil erosion change in the reservoir area.In addition,this research used the method of histogram matching to rescale the vegetation coverage within the study period(from 1990 to 2015)as an effective method for evaluating the changes in soil erosion based on rescaling data.We further used Google Earth’s multi-period images to assess soil erosion caused by rainfall in the WLFZ.In the line with the findings of the present study,the main results and conclusions were reported as follows:1.The impact of remote sensing images with different resolutions on soil erosion evaluation in the TGRA.The difference between soil erosion factors extracted from 250 m,30 m,15 m,8 m and 2 m resolution images in 2015 were analyzed in this study.The results showed that there was an obvious change in the area of land uses and the B factor,especially for dry cropland.The maximum differences in area of land use and B factor for dry cropland were 26.58% and 35.20%,respectively.Considering the results of soil erosion,the improved accuracy of paddy field extraction by using the MNDWI factor and land use with different resolutions presented a little effect on soil erosion evaluation in the TGRA.Due to low-resolution images,higher estimate of vegetation coverage and B factor with different resolutions revealed a greater impact on the evaluation of soil erosion in the TGRA,with 22.68% of the maximum difference of soil erosion area.The comprehensive consideration of the land use area and the soil erosion evaluation combined with the analysis of soil erosion in small watershed suggested that the 8 m image was more appropriate for the evaluation of soil erosion in the TGRA.2.Rescaling transformation of vegetation coverage with low resolution.The rescaling transformation models of vegetation coverage was performed from 30 m resolution to 8 m resolution.After the scale transformation,the distribution of vegetation coverage between 30 m resolution and 8 m resolution was approximately close.The evaluation of soil erosion with the transformed vegetation coverage of 30 m resolution indicated that the accuracy was obviously better than non-transformed.Regarding the evaluation area of soil erosion,the difference between after scale transformation of 30 m resolution and 8 m resolution was 1.68%,which was quite different with 12.91% before transformation.In terms of soil erosion modulus,the difference between the calculated value and the measured value among two small watersheds were 9.59% and 9.51% after the scale transformation of vegetation coverage while the values before scale transformation were 18.75% and 22.20%.3.Dynamic changes of soil erosion before and after reservoir operation.The CSLE model and the scale transformed vegetation coverage were used to evaluate the soil erosion from 1990 to 2015 in the TGRA.The soil erosion area varied from 26811.87 km2 to 24068.12 km2 for 25-years of the study period,accounting for 41.55% to 46.28% of the TGR area.The mild erosion area was the largest,accounting for 26.49% to 27.99% of the total soil erosion area.The soil erosion area of the TGRA were reduced by 10.23% after the TGR operation in 1990,and it was continually declining since 1990 to 2015,with a particular significant rate of 226.77 km2/yr from 2000 to 2010.From the perspective of the soil erosion spatial change,after the TGR operation,soil erosion area in TGRA reduced from 5.67% to 40.05% along different elevations.The soil erosion area at <200 m decreased by 40.05% while the areas experiencing human activities(200～900 m)reduced by 7.71%.After the TRG operation,the soil erosion area was increased in the WLFZ,and this was contrary to the trend of soil erosion area change in the TGRA.The soil erosion area in the WLFZ increased by 49.30% after the TGR operation,and the area of strong and above erosion intensity was 4.78 times than before the TGR operation.The intensive monthly rainfall erosivity,low initial vegetation coverage,and high soil erodibility are the main reasons explaining the increase of erosion area in the WLFZ.4.Human activities effect on soil erosion change in the TGRA.The area of paddy field and dry cropland were continually decreasing in the TGRA from 1990 to 2015,with the reduction rates of 13.44% and 5.82%,respectively.Moreover,the area of paddy fields and dry cropland decreased significantly after 2000.Contrary to the variation trend of dry land and paddy field,the area of forest,residential land and water body were increased at the rates of 53.85%,224.11 and 1.41%,respectively.The TGR operation,the implementation of ecological protection policies in the middle and upper reaches of the Yangtze River in the early 1990 s,and the implementation of agricultural policies were the main driving forces for soil erosion changes in the TGRA,and consequently reduced soil erosion area by 1016.11 km2.From the time and space of soil erosion change,the TGR operation and the agricultural policies influenced the erosion area from 2000 to 2015 below 1300 m and while the ecological protection policies in the middle and upper reaches of the Yangtze River in the early 1990 s affected the variation of the erosion area from 1990 to 2000,at 200 m to 1300 m. |
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