Determination of Contributing Area Threshold and Downscaling of Topographic Factors for Small Watersheds in Hilly Areas of Purple Soil

The results of topographic factor computations are highly sensitive to the setting of contributing area thresholds when applied to soil erosion modeling to evaluate soil erosion; however, the existing choice of contributing area thresholds is highly arbitrary. Meanwhile, due to regional-scale limitations, lower-resolution DEM data are usually used to calculate topographic factors, and with the fragmentation of land parcels in hilly areas of purple soil, lower-resolution DEM data respond to very limited topographic information. This study focuses on solving the mentioned issues by selecting the Lizixi watershed in a hilly area of purple soil as the research subject. It establishes a relationship equation between the resolution of DEM data and the optimal contributing area threshold. This is achieved by investigating the change in the contributing area threshold with the resolution of DEM data, determining the optimal contributing area threshold for different resolutions of DEM data, and establishing the relationship equation between the resolution of DEM data and the optimal contributing area threshold. Meanwhile, to solve the key problem of fragmented land parcels in the purple soil area, where the low-resolution and medium-resolution DEM data cannot accurately reflect the topographic information, combined with the principle of histogram matching, the downscaling model between the topographic factors under the low-resolution DEM data and the topographic factors under the high-resolution DEM data is established. This study confirms that the scale transformation model developed has a strong simulation effect, and the findings can offer technical assistance for the precise computation of soil erosion in small watersheds in hilly areas of purple soil.