Contents lists available at ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol Research papers Using reforestation to reverse salinisation in a large watershed John Ruprecht a, ⁎ , Tim Sparks b , Ning Liu a , Bernard Dell a , Richard Harper a a Murdoch University, Perth, Australia b Department of Water and Environmental Regulation, Western Australia, Australia ARTICLE INFO This manuscript was handled by G. Syme, Editor-in-Chief Keywords: Salinity Hydrology Reforestation ABSTRACT Restoration of water quality in deforested watersheds is a major environmental and economic challenge in many parts of the world. In south-western Australia water quality issues manifest as salinisation, where reactivation of groundwater systems has occurred post-deforestation with the consequent discharge of salts stored in deep regolith into rivers. Prior to deforestation the stream salinity of the Denmark River (a forested watershed of 502 km 2 ) was between 150 and 350 mg L -1 TDS (Total Dissolved Solids) and was developed as a small water supply with potential for a much larger development. By the 1970s, 20% of deep rooted vegetation in the watershed was removed resulting in annual flow-weighted stream salinity of 1500 mg L -1 TDS making the river unsuitable as a water supply. Two main policy approaches were used to restore this watershed: (1) the control of further deforestation on private land through regulation; and (2) a program to encourage private reforestation with eucalypt pulp-wood plantations. By 2010, 14.5% of the watershed was reforested leaving only 5.5% still deforested, with a strong relationship between streamflow and stream salinity and the amount of reforestation. River salinity had fallen to 500 mg L -1 TDS by 2017. Although streamflow had fallen from a mean 28.6 GL yr -1 in 1985–1990 to 13.6 GL yr -1 in 2012–2017 this was with water that was potable. The challenge into the future is to ensure the lower stream salinity is maintained through maintenance of forest cover. Importantly, this paper demonstrates that stream salinity can be reversed following deforestation if an appropriate scale of reforestation is deployed. 1. Introduction Damage to land and water resources as a result of deforestation is a major environmental and economic problem facing many parts of the world (Millar and Stephenson, 2015; Ruprecht and Dogramaci, 2005). As one major example of this, in many areas salinisation of rivers and streams is considered a growing threat that poses a risk of causing se- vere biodiversity losses and compromising the ecosystem goods and services that rivers, wetlands and lakes provide (Cañedo-Argüelles et al., 2013; Vengosh, 2003), with climate change potentially further exacerbating the threat. The causes of river salinisation are varied and include irrigated agriculture, mining, de-icing of roads, wastewater and industrial effluent, and deforestation for agriculture (Cañedo-Argüelles et al., 2013). Globally, the management responses to increasing river salinity have included setting water quality targets, salt interception schemes, improved farming systems, and improved irrigation efficiency (Cañedo-Argüelles et al., 2013; Connor, 2003; Williams, 2001). Stream salinity from secondary salinisation is a major water quality issue in southwestern Western Australia (SWWA). Over 55% of the rivers have salinities which make them unsuitable for public water supply (Mayer et al., 2005). In addition to the high stream salinities, more than a million hectares of farmland is salt-affected with many more millions considered at risk (National Land and Water Resources Audit, 2001). Consequently, and combined with broadscale deforesta- tion, the region is considered to be a global diversity hotspot (Myers et al., 2000) with 450 plant species across SWWA at risk of extinction (Environmental Protection Authority, 2007). The Denmark River (Fig. 1), with a watershed area of 502 km 2 and a mean flow of around 13.6 GLyr -1 , was once considered a major po- tential water supply for SWWA, with a major reservoir site proposed at Mt Lindesay (Bari et al., 2004). The first signs of rising salinity in the Denmark River came from stream gauging records on the main stream and tributaries. The stream salinity of the Denmark River is estimated to have been between 150 and 350 mg L -1 TDS (Total Dissolved Solids) before European settlement in the 1830s (Collins and Fowlie, 1981). The rising salinity trend was also reported by Ruprecht et al. (1985), Moulds and Bari (1995) and Bari et al. (2004). Stream salinity increased as a consequence of replacing high-water- use native forest with lower-water land uses with annual pasture and crop species. This land use change altered components of the water https://doi.org/10.1016/j.jhydrol.2019.123976 Received 23 May 2019; Received in revised form 9 July 2019; Accepted 20 July 2019 ⁎ Corresponding author at: Murdoch University, 90 South Street, Murdoch WA 6150, Australia. E-mail address: john.ruprecht@murdoch.edu.au (J. Ruprecht). Journal of Hydrology 577 (2019) 123976 Available online 22 July 2019 0022-1694/ © 2019 Elsevier B.V. All rights reserved. T