Delineation of topographic process entities using SRTM for hydrological modelling

The concept of distributed hydrological models is based on the integration of landscape components to delineate distributed process entities. These Hydrological Response Units (HRU) are topologically connected model entities and represent areas of homogeneous topographic and physiographic environment and therewith the hydrological system response. The HRU regionalisation concept is realised by the intersection of landscape parameters such as topography, land use, soils and geology. In many river catchments of the world most of these required data are only available on a coarse spatial resolution or poor quality. Furthermore, there is often a lack of data for discharge and precipitation. The demand for involving these catchments in planning of water management has determined the goals of the research program Prediction in Ungauged Basins (PUB) by the International Association of Hydrological Sciences (IAHS). The availability of new remote sensing products provides the opportunity to by-pass these data gaps and offers a GIS-database for regionalisation of river catchments based on HRUs. Topography is one of the most important factors for runoff and it is influencing the evolution of other landscape components such as soil catena or land cover patterns. The assumption of a strong, process-driven feedback between the topography and further landscape components as well as runoff dynamics lead us to a modified delineation of process entities by a topographic oriented HRU approach on the base of the globally and freely available SRTM elevation data. The methodology is based on the expectation that the water balance of watershed catchments can be estimated using SRTM-based delineations of process-oriented model entities to get a suitable prediction of runoff dynamics with disposable landscape components in spite of an insufficient data base. To make the approach applicable in Ungauged Basins a methodology is presented that is depend on the differentiation of landscape classes to allow a transfer to catchments with comparable relief characteristics. The transferability is essential for both the delineation of landscape dependent process entities (HRUs) and the parameterisation of model parameters for the distributed hydrological model J2000. The derivation of the HRUs is realised by a cluster analysis of the SRTM-derived relief indices under comprehension of other free global or trans-regional GIS data of the landscape components land use, soils and geology. Suitable relief indices combinations were selected with respect to process sensitivity and significance for the predominant landscape classes. The derived hydrological process units were joined topologically (1:n) for runoff routing and used as model entities for a comparative catchment modelling with J2000 in investigation areas of different relief characteristics in Germany and South Africa. Due to the fact that a model calibration and validation in Ungauged Basins because of missing runoff time series is hardly possible, the range of the J2000 model parameters had to be reduced that the relevant parameters can be estimated a priori for each landscape type. A main goal of the developed method is the determination of model parameter selections that are especially significant and sensitive regarding to different landscape classes and relief characteristics in the catchments. The parameter sets are then transferred to catchments with comparable terrain roughness coefficient. This approach does not claim to provide an optimised configuration of the hydrological model. It offers a suitable basis parameterisation for the hydrological model run with J2000 in catchments with poor data availability on the base of available information of the relief and other landscape components.