Slope Scaling Effect, Transformation And It's Influence To Soil Erosion Assessment

Slope gradient is one of the main factors in soil erosion. In the research of regional soilerosion accessment, slope gradient is generally calculated from lower resolution DEM.However, slope gradient becomes smaller with resolution becomes coarser, thus makes itdifficult to show the relationship between slope gradient and soil erosion and makes themodeling result of regional soil erosion poor. In this research, the digital image process,digital terrain analysis and geostatistical analysis were mixed together. We studied therelationship between slope gradient and resolution, explained the reason of slope reductionwith resolution, build a theoretical slope distribution model, got a mathmetial slopedown-scaling model and the key parameter in the model over China(downscale from90mSRTM to25m) and evaluated the change of regional soil erosion accessment result because ofslope down-scaling.There are60research sites in this paper. Six of them are called analyse-samples which aredused in study of slope reduction with resolution, validation of theoretical slope distributionmodel and mathmetial slope down-scaling model. Fifty three of them are called downscalingvalidation-samples which are used in validation of parameters of mathmetial slopedown-scaling model over China. One of them are called soil erosion accessment-sampleswhich is used in comparation of soil erosion accessment based on coarser and down-scalingslope. The results of this research are as follows.(1)There are some differences between varied terrain types in the change of basic slopestatistics with resolution. Mean slope and slope standard deviation (STD) reduce withresolution becomes coarser. However in different terrin types the change is different. In LoessHilly area (LP sample) and Red Soil Hilly areas (SE sample) the change is most intensive. InRocky Area of Shandong province (SD sample) the change is moderate. In Northeast GentleHilly Black Soil area (NE sample) and Wind Drift Sand area in North Shannxi Province (NSsample) the change is most gentle. In Purple Soil Hilly area in Sichuan province the change is gentle when resolution is finer and is more intensive when resolution is coarser. Frequencyand Cumulate Frequency curves moves to gentle slope with resolution become coarser. Thechange in different terrain types is similar as the change of mean slope and STD of slope.(2) Semi-variogram is a useful tool in analyse of terrain structures. Change of slopeinformation in different scale components with resolution and terrain types can be detected.The overall variance of slope reduces and correlation distance becomes larger with resolutionbecomes coaser. The result of ISM model shows that with resolution becomes coarser thechange of high frequency component of slope is most significant and that of low frequencycomponent is small. In NE and NS sample low frequency component of slope is moreimportant than in other samples. However in LP and SE sample high frequency component ofslope is more important than in other samples. In SC sample moderate frequency componentof slope is more important than in other samples. In SD sample the proportion of high, lowand moderate frequency component in overall variance is moderate in the six samples. Theresult well explained the reason for the difference in slope scale effect with resolution indifferent terrain types.(3) This paper got a theoretical slope distribution model based on slope gradient of DEMsurface and The chi-square distribution. The main parameter in this model is std of slopegradient p and q. The model efficiency is dependent on whether the terrain type is single inthe study area that is whether p and q is normally and independent with zero mean and eaqualvariance. The model has been used in the six analyse-samples at series of resolutions. Theresult shows that the model used in this paper can model slope frequency and cumulatefrequency curves, mean and std of slope. The model efficiency is high in the sixanalyse-samples.(4)Slope down-scaling mathematical model was got based on theoretical slope distributionmodel and histogram matching method and applied in the six analyse-samples. The mostimportant parameter in the slope down-scaling mathematical model in this paper is the std ofp and q at high resolution. The paper offers the std of p and q at resolution of25m based on25m Hc-DEM all over China and the parameter was validated at the53downscalingvalidation-samples.(5) In this paper the method showed in Standards for classification and gradation of soilerosion (SL-190-2007) was used as the method of soil erosion assessment. The study area isthe county of Ansai. In order to know the influence of slope-downing to soil erosionassessment, the slope based on90m SRTM (SRTM slope for short) was downscaled throughthe slope down-scaling mathematical model to25m. Then soil erosion grade was calculatedbased on SRTM slope,25m slope and the downscaled slope. The result shows that the erosion grade result is similar based on25m slope and the downscaled slope. However there isdifference between erosion grade result based on SRTM slope and the other two slopesurfaces, the erosion grade result based on SRTM is relatively gentle. The slope downscalingprocess in regional soil erosion assessment is essential.