Accounting for subgrid scale topographic variations in flood propagation modeling using MODFLOW

To be computationally viable, grid-based spatially distributed hydrological models of large wetlands or floodplains must be set up using relatively large cells (order of hundreds of meters to kilometers). Computational costs are especially high when considering the numerous model runs or model time steps necessary in calibration and long-term simulation. To parameterize such models, upscaling procedures are required in furnishing equivalent hydrological model parameters on the grid scale. Here we investigate the effect of a new upscaling technique for hydrological flow parameters dependent upon the small-scale terrain elevation distribution, namely, resistance to flow, infiltration, and depression storage volume. In the new procedure, these hydrological model parameters on the grid scale become functions of the water table elevation. These functions are established by preparatory, nonrecurring, simulations using the highest available topographic data as input. In this way, the variability and spatial correlation of elevation data on the subscale is preserved on the model scale, at least to some extent. The MODFLOW-based hydrological model of the Okavango Wetlands is used as a study case. The effectiveness of the new upscaling technique is judged on the basis of comparison of computed flooding patterns with and without implementation of the new technique. It is shown that model results are considerably influenced by this new more flexible parameterization. The partitioning of flows toward the different distributary systems of the Okavango Wetlands is improved by the new technique.