Response of streamflow to afforestation and thinning at Red Hill, Murray Darling Basin, Australia Ashley A. Webb a,⇑ , Amrit Kathuria b a Forests NSW, PO Box 4019, Coffs Harbour Jetty NSW 2450, Australia b Forest Science Centre, Industry and Investment NSW, PO Box 100, Beecroft NSW 2119, Australia article info Article history: Available online 26 May 2011 This manuscript was handled by G. Syme, Editor-in-Chief Keywords: Forest hydrology Radiata pine Plantation water use Southeastern Australia Mixed effect model Thinning summary 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 affores- tation 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. Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved. 1. Introduction The hydrological impacts of afforestation and other forms of vegetation management and land use change are recognised glob- ally (Zhang et al., 2001; Wilcox, 2002; Andreassian, 2004; Oudin et al., 2008; Zhao et al., 2010). More specifically, a number of paired catchment studies from regions of the world including Australia (Bren and Turner, 1980; Major et al., 1998; Putuhena and Cordery, 2000), New Zealand (Fahey and Watson, 1991; Fahey and Jackson, 1997) and South Africa (e.g. Lesch and Scott, 1997; Scott and Prinsloo, 2008) have demonstrated impacts of afforestation with Pinus species on catchment streamflows. However, to date compar- atively few studies have reported the hydrological effects of thin- ning and/or harvesting (Baker, 1986; Lesch and Scott, 1997; Bubb and Croton, 2002; Waterloo et al., 2007; Webb, 2009). In the Murray Darling Basin (MDB), Australia water scarcity is a significant issue for water resource management which, combined with high levels of river regulation and catchment development, has contributed to environmental problems that include, inter alia, loss of native vegetation (Walker et al., 1993), elevated stream salinity (Jolly et al., 2001), toxic cyanobacterial blooms (Baker and Humpage, 1994), threats to native fish communities (Gehrke et al., 1995) and impacts upon the breeding of waterbirds (Leslie, 2001). Recent water reforms such as Australia’s National Water Ini- tiative (NWI) and proposed MDB Basin Plan have recognised many risks to sustainable water resource allocation that need consider- ation and, if appropriate, regulation and management. Under the NWI large-scale plantation forestry has been identified as a future land use change activity with the ‘‘potential to intercept significant volumes of surface and/or ground water now and in the future’’ (COAG, 2004). Consequently, each jurisdiction in Australia is to ap- ply ‘‘appropriate planning, management and/or regulatory mea- sures where necessary to protect the integrity of the water access entitlements system and the achievement of environmental objectives’’ (COAG, 2004). The proposed Basin Plan endorses this approach. It seeks to identify current diversion limits and to set fu- ture sustainable diversion limits for plantation water use within each major river basin in the MDB (MDBA, 2010). These policies potentially have implications for commercial plantation forestry managers including Forests NSW, which owns and manages 280,000 ha of plantations mostly within the MDB. 0022-1694/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2011.05.033 Abbreviations: AHD, Australian Height Datum; COAG, Council of Australian Governments; DBHOB, diameter at breast height over bark; MDB, Murray Darling Basin; MDBA, Murray Darling Basin Authority; NWI, National Water Initiative. ⇑ Corresponding author. Tel.: +61 2 6650 5707; fax: +61 2 6651 5027. E-mail addresses: ashleyw@sf.nsw.gov.au (A.A. Webb), amrit.kathuria@sf.nsw. gov.au (A. Kathuria). Journal of Hydrology 412–413 (2012) 133–140 Contents lists available at ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol