Reforestation, water availability and stream salinity: A multi-scale analysis in the Murray-Darling Basin, Australia

There is much debate about the role of plantation forestry in southeast Australia in helping control stream salinity and in exacerbating stream flow reductions. We reviewed modelling tools developed to plan forestry for surface water and salinity outcomes and tested three of these in a multi-scale study within the Murray-Darling Basin, to assess the impact of plantation forestry on water availability; whether these impacts are counterbalanced by stream salinity benefits; and to what extent spatial planning can optimise this trade-off. Comparison with stream flow data suggested that in medium rainfall areas (600–1000 mm year−1) runoff generation from agricultural land is very similar to that of forested land, implying that reforestation in these regions would have a limited stream flow impact. The estimated reduction in total surface water resources across the basin due to future plantation expansion was estimated at <0.3% by 2020. Plantation expansion was not predicted to alter flow regime in large catchments, but may do so in catchments smaller than ∼2000 km2. Spatial planning is likely to increase the effectiveness of reforestation against stream salinity by more than seven times. The limited availability of suitable areas and the relatively small impact per hectare planted appear to rule reforestation out as an effective way to reduce stream salinity in the Murray River. Reforestation can help to reduce stream salt exports from smaller catchments, but reductions in both average stream salinity and high salinity events may be difficult to achieve. The available models generally allow us to arrive at these conclusions with a reasonable degree of confidence, but the cause(s) of the lower than expected runoff generation on agricultural land remains an outstanding question.