Topography modelling and simulation of soil erosion by water, in a small agro-forestry basin

To simulate the impacts of agricultural activities on diffuse pollution at the basin scale, it is important to use adequate topographic data and understand its hydrological behaviour. In this study, the FlowNet Generator module, component of the AnnAGNPS (Annualized Agriculture Nonpoint Source) model, was used to evaluate the influence of the vertical resolution of the Digital Elevation Model (DEM) on the topographic and hydrological configuration, and on runoff-erosion process, of a small agro-forestry basin (190 ha) located in the municipality of Idanha-a-Nova. A georeferenced survey of the basin’s stable surface drainage network was carried out, and the results were compared with output network simulated by FlowNet Generator module for two DEMs with vertical resolutions of 1 m and 5 m. It was observed that the DEMs with a vertical resolution of 5m did not lead to satisfactory results, given that the simulated natural drainage network was significantly different from the one observed in the field. On the other hand, the natural drainage network generated with the 1m resolution DEM was very close to the observed network. The influence of the different topographic configuration of the basin in the simulation of soil erosion by water, using the RUSLE model, resulted in a significant difference considering the values of 5.85 and 4.17 ton/ha.year, respectively for the DEM with 1 m and 5 m vertical resolution.