Identification of processes affecting stream chloride response in the Hafren catchment, mid-Wales

Early hydrochemical catchment models, assuming chloride to be hydrologically and chemically inert, failed to reproduce observed damped stream chloride responses to highly variable rainfall inputs. To explain this, the presence of a large volume hydrological store within the catchment has been inferred, although this is generally unobservable, and unidentifiable from hydrological modelling studies. However, there is increasing awareness of a range of physical, chemical and biological processes which can affect chloride transport. These include the biogeochemical cycling of chloride in soil, a significant chloride adsorption capacity of some organic soils and diffusive transfers between mobile and immobile soil waters which can give rise to a similar ‘adsorption’ response. There is therefore a need to evaluate the importance of non-conservative adsorption processes in observed stream chloride response. This issue is addressed here via the calibration and structural assessment of a specially developed model using stream chloride data from the Hafren catchment, mid-Wales. The major conclusions are that the non-conservative model produces improved simulation of stream chloride in comparison with a conservative model. This is not an artefact of the increased complexity of the non-conservative model; the assumption of a large chloride reservoir is structurally invalid for long-term prediction. Adsorption, or adsorption-like processes thus appear to play a dominant role in influencing stream chloride. These findings have important and wide-ranging consequences for hydrological process studies.