Hight Resolution DEM Resampling And It's Effects On Soil Erosion Simulation

DEM is one of the important basic data sources in the field of soil erosion. DEMs have been used to calculate many erosion factors, such as slope,slope length, run-off intensity index and so on. It has been a long time that the DEMs are used in the field of soil erosion.Many examples have been introduced, such as used DEM in the loss simulated watershed,in runoff plot modelling,in small watershed or in regional scale soil erosion,as long as the different scale DEM data constantly enriched.Resampling is widely used as one of the commonly methods about multi-scale DEM.The Nearest Neighbor Interpolation (Near), Bilinear Interpolation (Bil) and Cubic Convolution Interpolation (Cub) are used very widely to produce multiscale DEMs.However,current resampling research mainly focused on the data precision,such as resampling elevation error and its distribution characteristics.For different shape terrain surface with different slope,how to choose the most suitable methods among Near, Bil and Cub?And is there any difference when Near, Bil and Cub resampling DEM are used to modelling soil erosion?If the pattern of soil erosion is similar,which pair is the most similar one,Cub and Bil?Which one is the most similar with the original results of soil erosion?With the aim to clear those problems,this study will compare the effect brought by Near,Bil and Cub when used in nine different shape of loess simulation watersheds.The Root Mean Standard Error (RMSE) of elevation was calculated after the original 10×10mm-resolution DEM was resampled.The grid sizes of resampling DEM are as followed, 15mm×15mm,20mm×20mm,25mm×25mm,30mm×30mm,35mm×35mm,40m m×40mm,45mm×45mm,50mm×50mm,55mm×55mm,60mm×60mm,70mm×70mm,80mmx80mm,90mmx90mm.According to the values of RMSE,therecommended method is Bil when use in resampling DEM.When the grid sizes become bigger,the RMSE also increases.The relationship between RMSE and the grid size can be described as a function of y = k×x + ,where y is the RMSE and x is the grid size.Also there is a quantitative relationship among the RMSE,the average values of terrain slope and grid size,which can be described as a function of RMSE = (a×s-b)×g ± (c×s+d)Where s is the average value of slope and g is the grid size,while a, b, c and d are constants changed due to the which sampling method used.Take the Bil resampling method as an example,the resampling can be represented as RMSE = 0.0116 * g * s -0.0756 * g + 0.0676 * s-0.7307.The formula describes the quantitative relationship among the RMSE,surface morphology of topography and grid size.Which means once the average value of slope and the grid size are given,the RMSE can be calculated immediately.It is of great significance when try to determinate the grid size in the resampling process.Based on DEM of the loess simulated watershed,USPED model was used to estimate the erosion and deposition,either in rill erosion or in sheet erosion.First the original DEM with grid size of 10x10 mm were used without resampling.The results show that the maximum value of erosion happened in the 3rd, 5th and 7th watershed.Then the 1-9 watersheds can be regarded as three different pattern,roughly corresponding to the initial period, the active period and the stable period of the landscape development.When the original DEM simulation results are used as the reference value,there is a probability of 20% that the potential erosion and deposition are mistakenly simulated.As the grid size increases,the maximum value of erosion and deposition decreases either in rill erosion or in sheet erosion.For the most case, the maximum values of erosion and deposition in sheet erosion decreases faster than that of rill erosion.If the simulated results of 10mm×10 mm DEM is used as the reference value,when the grid size of 30mmx30 mm is used in the resampling process,the maximum value of erosion and deposition decreases as 50 percent as the reference value.There is a quantitative relationship among the ratio between the resampling maximum value and the reference value (y),the grid size (g) and the average slope or the maximum slope.This formula can be used to estimate the maximum value of erosion and deposition when the average slope and grid size is given,which is very useful in the study of the multiscale soil erosion prediction.The X/Y scatterplot and difference coefficient are used to measure the similarity between the original results without resampling and resampling ones.The results show that the degree of similarity often changes with the grid size.Once the grid size is as small as 10mm×10mm,they are the same.When the grid size increases,the degree of similarity decreases.Among the three methods of Near,Bil and Cub,the results produced by Bil and Cub is more similar to the original one than that of Near.At the same time,the Bil and Cub is the most similar pair among other pairs .When the grid increases,the degree of similarity is increased among Near,Bil and Cub,while they are more and more dissimilar with the true value.