Effects of model complexity on karst catchment runoff modeling for flood warning systems

Severe flood events are deemed more frequent in the near future with a changing climate. Headwater catchments, especially when karstified, exhibit a pronounced susceptibility to swift and substantial responses to precipitation events, leading to flooding. In this study, a karstified headwater catchment in SW Germany is investigated, focusing on gaining insights into the key processes controlling its discharge behavior. Intensive fieldwork was conducted and a variety of field data were collected and analyzed to determine the general system behavior during low flow and flood events. Field insights reveal a groundwater borne streamflow generation with a subsurface catchment largely differing from the surface catchment. Episodic and sporadic springs were identified as crucial contributors to stream flow generation.

The study was undertaken to evaluate the viability of simulating streamflow for flood warning using a lumped modeling approach at a sub-daily temporal scale, since lumped models are widely used for karst spring discharge modeling. Based on field data observations, a comparative analysis of different model structures was undertaken, aiming at assessing the required degree of model complexity for representing catchment runoff generation as well as the relevant system features and properties. In order to find an adequate model structure, a total of 21 models with varying degree of complexity were set up and run. Both, subsurface and surface catchment limits were considered. Results show that the hydrograph of the whole catchment can be represented by a rather simple lumped model in the present case under two prerequisites: (1) input needs to represent the groundwater catchment emphasizing the groundwater borne nature of flow and (2) the models need to allow for direct runoff, as the sporadic springs observed in the field contribute significant discharge to streamflow during flood events. It is revealed that it seems valid to start modeling with a relatively simple storage model as long as key processes in the catchment are represented. The general feasibility of such a simple modeling approach in this complex catchment encourages its feasibility in other headwater catchments.