From perceptualisation to modelling: Improving the representation of spatially variable intercatchment groundwater flow in hydrological models

Lumped and semi-distributed hydrological models commonly do not include representation of intercatchment groundwater flow (IGF) fluxes. However, IGF can be a significant component of a catchment’s water balance and have important water resources implications for rivers. In models that have added a water flux to represent IGF, developments have often been made with limited prior justification based on any evidence of perceived subsurface processes. Here, we follow a perceptualisation pathway, underlined by increasing levels of hydrogeological knowledge, to showcase model structure changes to the DECIPHeR hydrological model to incorporate IGF fluxes. The River Kennet, UK (a tributary of the River Thames), was selected as our test catchment. A perceptual model was first developed, utilising available national data on meteorology, hydrogeology and geology, and reviewing water balance calculations and statistics. Four model structural development scenarios were then selected to provide increasing spatial variability in IGF, whereby IGF is modelled with disconnected, semi-routed, or fully routed connectivity between sub-catchments. We demonstrate the decisions that could be taken by a modeller in the incorporation of IGF fluxes to existing models, given an increasing level of hydrogeological perceptualisation. The inclusion of this missing flux in the DECIPHeR model improves calibrations in heavily groundwater dominated sub-catchments. We also show, however, how a lack of prior hydrogeological perceptualisation could lead to a model structure selection that is at odds with the physical reality of the catchment, whereby modelled IGF losses/gains could readily be used as a proxy for other calibration issues e.g. input data errors etc. Discussion is provided on the balance between improved calibration and realism, the importance of transparent and justifiable structural decisions, and the uncertainties associated with this. The perceptual model and the modelling results also highlight the potential of seasonal variations in IGF flux, which necessitates further investigation.