Dynamics of catchment-scale chloride transport: A study of time-variant travel time distribution at the Kringlan catchment (Sweden)

Abstract:

We present a complete catchment-scale transport analysis of the Kringlan sub-catchment that is part of the Norrström basin in mid-East Sweden, by means of simulating transient flow, transport and the time-variant travel time distribution (TTD). The time-variant TTD at the outlet captures the dynamic nature of the catchment-scale hydrological transport, originating from temporal variability of flow velocity. Transient three-dimensional surface-groundwater flow is first calculated as an integrated hydrologic system. The calibrated and validated flow rate simulations at the outlet show good accuracy relative to measured data, the Mean Error (ME) being approximately (0.2 m³/day). We interpret decadal daily hydro-chemical data, from the Kringlan sub-catchment by simulating chloride concentrations at the catchment outlet, with a view toward quantifying physical processes of dry and wet deposition of chloride. We show that the chloride transport load is mainly dominated by flow in terms of magnitude and temporal fluctuations. However, only 29% of the annually-averaged total amount of chloride originates from wet atmospheric deposition. The numerical simulation of flow and transport is performed with a three-dimensional model MIKE SHE. The numerical approach complements the recently advanced analytical formulation of transient TTD which requires input/output fluxes, storage volume of different water compartments in addition to relying on mixing assumption. Our results show that the fast response of the catchment to the climatic forcing in the event-scale of a year is reflected in the TTDs of water particles originating from topsoil (root zone), whereas decadal scale window show that water stored within saturated zone is older.