Response of streamflow to afforestation and thinning at Red Hill, Murray Darling Basin, Australia

Competition for water resources in Australia’s Murray Darling Basin has resulted in plans to account for water used by various land use change activities including plantation forests. To date generalised forest conversion models have been used to assess the likely impacts of future afforestation. These models are a useful starting point but do not account for forest age or silvicultural intervention such as thinning. A twenty year record from the Red Hill paired catchment study was analysed to show that forest age is a significant factor in determining the streamflow response to afforestation. Compared to the Kileys Run pasture catchment, streamflow in the Red Hill catchment afforested with Pinus radiata plantations steadily declined, particularly after age 6 years when stand basal area rapidly increased. Mixed effect model analysis indicated that over the first 20 years of the rotation the mean annual impact of afforestation with pines was 155 mm. The modelled impact peaked at 211 mm in year 14 prior to thinning. Thinning at age 14 years had a significant positive effect on streamflow that has persisted for at least 6 years. Drought conditions coupled with a process of recharging the catchment soils contributed to a delayed response to thinning. Factors such as forest age and thinning should be incorporated into models used in water resources planning to more accurately predict the hydrological effects of afforestation.