On the Panday and Huyakorn surface-subsurface hydrology test case: Analysis of internal flow dynamics

A number of previous studies using models of integrated surface-subsurface hydrology have adopted the Panday and Huyakorn (P&H) tilted V-catchment test case (Panday S, Huyakorn PS. 2004. A fully coupled spatially distributed model for evaluating surface/subsurface flow. Advances in Water Resources 27: 361-382) to show inter-code comparability. The P&H test case is used to evaluate models that simulate a broad range of hydrological processes, and yet only the catchment outflow hydrograph has been presented as verification of the consistency between codes. Therefore, a more comprehensive evaluation of the surface-subsurface hydrology of the P&H case is needed. This study explores the internal catchment functioning of the P&H case, using the popular catchment simulator MODHMS. The processes leading to streamflow generation in the model are illustrated, including separation of overland flow (OLF) and groundwater discharge to the stream. The results identify non-physical flow processes due to the problem set-up, and modifications to the P&H case are suggested that include changes to stream roughness and incision of the stream channel to overcome these shortcomings. A modified P&H case produced more plausible transfers between OLF and the stream, and an increased groundwater discharge to the stream (6·5% of streamflow in the modified case compared to 0·5% in the original case). Despite changes to internal flow processes, near-identical outflow hydrographs were obtained, showing the importance of considering and comparing internal flow processes when using surface-subsurface hydrology test cases to evaluate integrated hydrological simulators.