Modelling electrical conductivity variation using a travel time
distribution approach
Abstract
Water quality dynamics depend strongly on hydrologic flow paths and
transit time within catchments. In this paper we use a travel time
tracking method to simulate stream salinity (as measured by electrical
conductivity) in the Duck River catchment, NW Tasmania, Australia. The
approach couples the StorAge transit time modelling approach with two
different approaches to model electrical conductivity. The first assumes
the catchment has a cyclic salt balance (rainfall source, stream flow
sink) that is in dynamic equilibrium and evapoconcentration of salt is
the only process changing concentration. The second assumes that the
salinity of water in catchment storages is a function of water age in
those stores, without explicitly simulating salt mass balance processes.
The paper compares these alternate approaches in terms of salinity
simulation, simulated stream water age distributions, and simulated
storage age distributions. Both salinity simulation approaches reproduce
stream salinity with high fidelity under calibration and perform well
under validation. The simulations using the age-related solute
concentration approach produce less biased results and thus high model
efficiencies for validation periods. This approach also produces more
consistent model parameter estimates between periods. There are
systematic differences in the resultant age distributions between
models, particularly for the solute balance based simulations where
parameters (catchment storage size) changed more between calibration
periods. The effect of time varying versus static storage selection
functions are compared, with clear evidence that time varying storage
selection functions with parameters linked to catchment conditions
(flow) are essential for adequate simulation of event concentration
dynamics.