Výpočet erozní ohroženosti půdy s využitím gridu

Soil Erosion Estimation Using Grid-based Computation Soil erosion estimation is an important part of a land consolidation process. Universal soil loss equation (USLE) was presented by Wischmeier and Smith. USLE computation uses several factors, namely R – rainfall factor, K – soil erodability, L – slope length factor, S – slope gradient factor, C – cropping management factor, and P – erosion control management factor. L and S factors are usually combined to one LS factor – Topographic factor. The single factors are determined from several sources, such as DTM (Digital Terrain Model), BPEJ – soil type map, aerial and satellite images, etc. A conventional approach to the USLE computation, which is widely used in the Czech Republic, is based on the selection of characteristic profiles for which all above-mentioned factors must be determined. The result (G – annual soil loss) of such computation is then applied for a whole area (slope) of interest. Another approach to the USLE computation uses grids as a main data-structure. A prerequisite for a grid-based USLE computation is that each of the above-mentioned factors exists as a separate grid layer. The crucial step in this computation is a selection of appropriate grid resolution (grid cell size). A large cell size can cause an undesirable precision degradation. Too small cell size can noticeably slow down the whole computation. Provided that the cell size is derived from the source’s precision, the appropriate cell size for the Czech Republic varies from 30m to 50m. In some cases, especially when new surveying was done, grid computations can be performed with higher accuracy, i.e. with a smaller grid cell size. In such case, we have proposed a new method using the two-step computation. The first step computation uses a bigger cell size and is designed to identify higher erosion spots. The second step then uses a smaller cell size but it make the computation only the area identified in the previous step. This decomposition allows a quick computation while the precision degradation is smaller then in the one-step computation. The above-described two-step method is suitable mainly for a combination of larger areas (several cadastral areas) and high precision sources at the same time. It is evident that in present time such a combination is rather unique, which means that in most cases the one-step computation with 30m–50m cell sizes is adequate. It is expected that in the future all sources used for the determination of the factors will have a higher accuracy and, therefore, the two-step computation will be needed.