Role and Influence of Cross Section Geometry in the Hydrologic Response through Coupled Frameworks

For this study we employ two separate, yet complementary, coupled modeling frameworks to investigate the role of cross section geometry in the hydrologic response and to assess the potential for improving simulations of the hydrologic response through the routing process alone. The first framework employs a coupled distributed rainfall-runoff and hydraulic model while the second one emphasizes the basin scale by coupling the network instantaneous response with hydraulic geometry. Thus, we began by considering one-dimensional unsteady flow conditions along a river reach under simulation scenarios distinguished by varying cross section detail. In applying the previous to two headwater basins in Oklahoma (~2,000 km^2), we found improvements in moving from a single power law cross section to more complex cross sections with distinct geometry for the channel and floodplain. Results also showed that benefits from utilizing more complex cross sections can be easily masked when ungaged inflows are allowed to exercise an increasing control over flows in the routing reach. In search of a more general explanation of the influence of floodplain geometry on the response, the role of cross section geometry was revisited using a simplified coupled framework. This latter framework is simplified relative to the unsteady modeling in that it deals with steady conditions and average network and cross section geometry boundaries. Nonetheless, it has the clear advantage of explicitly allowing the consideration of network-dependent effects (e.g. geomorphic dispersion) and the basin scale (as opposed to the reach scale) on the response. Using this latter framework we found that the floodplain geometry can have a direct effect on the partitioning of the network dispersion, the nonlinearity of the network response, and the relative effect of hillslope contributions to the basin response. We conclude by highlighting the benefit of explicitly distinguishing between channel and floodplain geometry, given the role of this geometry in the routing dynamics.