The impact of flooding on modelling salt transport processes to streams

The development of many of the worldʹs arid and semi-arid regions has resulted in the salinisation of land and water resources. In these areas, soils and groundwaters are often naturally saline and any disturbance of the delicate hydrological balance results in mobilisation of the stored salt. The salt transport mechanisms are often highly complex, the understanding of which necessitates the use of computer modelling in combination with field studies. In this paper the transport of salt between groundwater and streams on the Chowilla floodplain in south-eastern Australia was modelled and compared with available field data. The large salinity contrast between the fresh stream and floodwater and the saline groundwater results in density-dependent flow behaviour, and hence necessitated the use of a variable density flow and solute transport model (SUTRA). The model was applied in cross-section over a 6.1-km-long transect across the floodplain. Time varying boundary conditions were employed at the locations of three streams on the transect to simulate the interaction between the rising and falling streams and the adjacent aquifer during and after floods. The model was used to assess the importance of overbank floods in the transport of salt to floodplain streams by carrying out simulations under various recharge scenarios. The simulations showed that the mixing of floodwater and groundwater within the bank storage adjacent to the streams could predict the observed short-term (<12 months) salt load recessions. In order to predict the observed long-term (12–24 months) salt load recessions, the inclusion of localised recharge during overbank floods is required, as hypothesised by previous field-based studies.