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

The groundwater system is dynamic in time, in particular in high productivity aquifers such as the heterogeneous and fractured Chalk aquifer. Unlike river catchments, which are generally topographically controlled and therefore stable, groundwater catchments vary seasonally. The location and magnitude of intercatchment groundwater flow (IGF) can therefore also vary seasonally. This can pose a significant challenge for hydrological conceptual models. Building on work previously conducted by the authors on perceptualising the spatial variability of IGF and incorporating this into the DECIPHeR conceptual rainfall-runoff model, we have followed the same data-led approach in an investigation of the temporal variability of IGF. An evidence-based perceptual model of the River Kennet, UK (a tributary of the River Thames) was first developed and then used to inform the design of model edits that capture seasonal IGF in-line with the perceptual understanding. From review of recorded data, it was found that a strong sinusoidal climate signal propagates through to the groundwater table, river flow and catchment water balance annual profiles, but that this signal is highly variable between years. The temporal variability observed in the test sub-catchments was applied to an IGF flux within the DECIPHeR model, and the results compared to both the baseline model and the spatial IGF model developed in previous work. Four model structures were developed and tested, show-casing an increasing level of hydrogeological information and seasonal analysis. Model calibration at the annual scale was no better than the spatially-variable IGF model, but there was a marked improvement in the representation of the monthly flow profile when an additional IGF flux sinusoidal amplitude parameter was introduced. The timings of the autumnal increase in river flow, plus the slow spring recession, are now able to be replicated. The findings prompted a discussion on the challenges that remain when representing intercatchment groundwater flow in conceptual hydrological models. Most notably, these include a lack of representation of long-term catchment storage limiting a model’s ability to replicate the inter-annually variable groundwater catchment areas that are so characteristic of Chalk catchments.