Modelling of instream salt transport processes

Salinity is a major threat to rivers in the Murray-Darling Basin System and the reduction of river salinity forms a major component of the NSW State Salinity Strategy and the Murray-Darling Basin Commission's (MDBC) Basin Salinity Management Strategy. High salinity levels associated with land salinisation make water unsuitable for drinking, stock watering and irrigation, as well as having a detrimental effect on riverine ecosystems. Computer modelling is being used by the NSW Department of Land and Water Conservation (DLWC) to understand how salt is transported within river systems. The DLWC's-Integrated Quantity-Quality Model (IQQM) is a hydrologic model that can be applied to any river system and operates on a time step of one day or less. The salt modelling capability of IQQM has recently undergone a systematic process of improvement. This began with a revision of the code in conjunction with quality assurance tests using a simplified and the representative systems under static and dynamic conditions. The model was then calibrated using continuous salinity data from the Barwon-Darling system between Louth and Tilpa. This paper presents results that demonstrate IQQM's capability in modelling the processes controlling flow and salinity in river systems. The findings also confirm that the limitations of the model, due to assumptions that transport processes are significant only in the main direction of flow and constituent mixing is complete and instantaneous within each computational reach, can be overcome by the use of a short routing time step. This work is an important step in modelling instream water quality. The model also provides a link to the development of surface runoff/pollutant export generation and groundwater quantity and quality components within IQQM.