Flow routing in large-scale models using vector addition

Large-scale distributed and semi-distributed models sub-divide the basin at a coarse resolution to improve computational efficiency. Recently, there has been a strong research effort directed at up-scaling small-scale hydrological processes to large-scale for use in large-scale hydrological models and atmospheric models. However, up-scaling methods that do not incorporate the hydraulic structure of the basin have proven unsatisfactory. Significant limitations of this approach have been identified in earlier work by the author and his colleagues. That work clearly demonstrates that a new method that incorporates sub-grid scale variability in flow directions is required for routing flow between large-scale model sub-grids. We propose a simple, automated method that determines flow routing between sub-grids, at a pixel scale, using flow vectors. Processing a digital elevation model with a landscape analysis model provides flow vector information for the basin. Flow vectors for each sub-grid are summed using vector addition and the resultant vector is used to assign flow direction. The methodology also incorporates weighting schemes for the vectors. Results indicate that using flow vectors, weighted with upstream catchment area values, provide a good representation of flow patterns within the basin. The flow pattern resolved by the flow vector method compares well against the flow pattern derived by more complex ‘expert system’ software previously developed by the authors.