Development of a salt and water balance model for a large partially cleared catchment

A salt and water balance model was developed to represent stream flow and salinity generation processes following land use changes. At first a fundamental building-block model was developed based on the 'downward approach'. The building-block model was tested and validated with data from six experimental sub-catchments within the Collie River basin in Western Australia. The approach requires specification of five physically meaningful key parameters, most of which can be obtained a priori or easily calibrated. Stream flow and salinity from the Collie River catchment, with an area of 2,545 km2, has increased significantly due to clearing of 26% of the area during 1940-70s. For this study the catchment was divided into 91 sub-catchments and the building-block model was applied to each of the sub catchments. Most of the known catchment attributes such as stream length, average slope, soil type, profile thickness and salt storage were incorporated into the model. Parameter values obtained from experimental sub-catchments were appropriate for representing the daily stream flow generation processes of the whole catchment. However, the prediction of stream salinity and salt loads was improved by running the model a number of times and taking the final values of the transient stream zone stores as an initial condition. The modelled daily stream flow, salinity and salt load hydro graphs matched very well for all gauged sub-catchments.