| The LS factor extracted from low and medium resolution DEM can't fully reflect the relationship between topography and soil erosion,so it is an important problem to reduce the scale of LS factor extracted from low and medium resolution DEM for regional soil erosion evaluation and efficient utilization of DEM data resources.In this study,six regions are selected from different erosion zones of the Loess Plateau as the sample areas,and 20 DEM?2.5m-90m?with different resolutions are generated according to the ANUDEM.By using the technology of GIS?including digital image processing,digital terrain analysis and digital cartography?,the changes of slope,slope length and LS with resolutions are analyzed from the statistical distribution and spatial pattern,and then downscaling the terrain factors extracted from the lower resolution combing the principle of histogram matching,and the downscaling model is applied to the sampling survey unit to analyze the impact of LS factor before and after downscaling transformation on soil erosion evaluation.The main results are as follows:?1?Scale effect of terrain factor:with the decrease of DEM resolution?2.5m-90m?,the mean slope decreases gradually,the peak value of its frequency moves to the range of low slope,the range of slope decreases and concentrates on the small slope,and the slope reduction mainly occurs on the ditch slope.The average value and standard deviation of slope length increase gradually,and the frequency and cumulative frequency curve move to the range of long slope length,and the range of slope length increases but more scattered,and the expansion mainly occurs on the slope surface of small gully.LS average value and standard deviation first increase and then decrease,reaching the maximum at the resolution of 10m-20m.LS mainly changes on the slope surface,first increases and then decreases on the small gully and decreases on the surface of main gully,LS of the bottom of trench doesn't change significantly.On the whole,the slope reduction is stronger in the steep slope sample area,the slope length expansion is stronger in the gentle slope sample area,and the LS factor changes more in the steep slope sample area.?2?Downscaling calculation of terrain factors and availability of downscaling model:From the statistical characteristic values,the range,average and standard deviation of slope,slope length and LS after scale transformation are close to the values of high-resolution data,the frequency curve and cumulative frequency curve of the transformed data are basically similar to the corresponding high-resolution data,the histogram similarity index?HS?of the slope,slope length and LS after downscaling are higher than those before downscaling,which shows that the slope,slope length and LS factors after downscaling are close to the high resolution data.From the spatial pattern,it is concluded that the slope,slope length and LS macro pattern of the six sample areas after transformation are more similar to the high-resolution data,and the distribution similarity of the steep slope sample area is better than that of the gentle slope sample area,but the micro texture features are similar to the low-resolution data.The slope downscaling model generated by six sample areas is better,followed by the slope length.The scale transformation effect of LS is not as good as that of slope and slope length,mainly because the change of LS is affected by the slope and slope length,and the slope length is affected by the slope.?3?Applicability of LS scale transformation:according to Chinese soil loss equation and 30m resolution rescaling model of slope and slope length,we can obtain that the sampling units of steep slope sample area 1 and moderate gentle slope sample area after scale transformation,the soil erosion rate increased,and the increased value range was 91.3 t·km-2·a-11024.5 t·km-2·a-1,the area weighted average value of sampling unit after scale transformation increased 323.6 t·km-2·a-1,the change is obvious,that is to say,the coarse resolution LS factor in the steep slope area makes the soil erosion rate underestimated.For the sampling units in the gentle steep slope sample area,gentle slope sample area 1 and the gentle slope sample area 2,the soil erosion rate decreases after the scale transformation,and the reduced value range was5.3 t·km-2·a-1320.9 t·km-2·a-1,the area weighted average value of sampling units before and after transformation reduced 76.3 t·km-2·a-1,the change is relatively small,that is to say,the coarse-resolution LS factor makes the soil erosion rate overestimated.Through the comparison of the classification mapping and statistical spatial distribution of LS and soil water erosion rate before and after the downscaling transformation of typical sampling units,it shows that LS mainly changes on the slope surface of the gully and the corresponding soil water erosion rate changes after downscaling transformation,which affects the distribution and classification of soil erosion modulus intensity of different grades,and has a greater impact on soil erosion intensity and soil erosion evaluation.This study can support the soil erosion evaluation of the soil erosion sampling survey in the large area,and lay the foundation for forming a complete and high-precision practical system to calculate soil water erosion rate. |
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