Modelling the impact of subcatchment and regional scale drainage in the Blackwood Basin, Western Australia

► Average groundwater levels rise and streamflows and salt loads increase under no drainage. ► Artificial drainage is likely to lower groundwater levels and reduce groundwater salinity. ► Artificial drainage is likely to further increase streamflows and salt loads in streams and creeks. ► Most artificial drainage strategies are projected to further degrade the lower Blackwood River. ► Modified LASCAM proved useful for assessing the feasibility of subcatchment and regional drainage. This study assesses the impacts of several subcatchment and regional scale artificial drainage on on-site groundwater levels and salinity, and off-site streamflows, salt loads and lake discharge rates using a hydrological model LASCAM. The model is applied to the 21,147km2 Blackwood Basin in southwestern Australia, much of which has been experiencing long-term rises in groundwater levels in response to large-scale clearing of native vegetation over the past 160 years. These rises in groundwater levels have led to substantial increases in stream salinity, waterlogging and land salinisation. The model predicts that under the baseline scenario (that is, without artificial drainage or any other salinity management strategy) most of the subcatchments of the Blackwood Basin are likely to experience rises in both future average groundwater levels and groundwater salinity, but at a slower rate than is currently occurring. Streamflows and stream salinity are also projected to increase over time under the baseline scenario. Artificial drainage is likely to lower both the groundwater levels and groundwater salinities. Mean annual and peak streamflows are expected to be considerably larger under the drainage scenarios than under the baseline scenario. All strategies assessed for the management of artificial drainage water are projected to further degrade the lower Blackwood River from higher salinity levels than are expected under the baseline scenario, with the exception of one strategy where drainage discharge is managed via subcatchment scale evaporation basins.