Quantifying water requirements of riparian river red gum (Eucalyptus camaldulensis) in the Murray–Darling Basin, Australia – implications for the management of environmental flows Tanya M. Doody, 1 * Matthew J. Colloff, 2 Micah Davies, 2 Vijay Koul, 2 Richard G. Benyon 3 and Pamela L. Nagler 4 1 CSIRO Land and Water, PMB 2, Glen Osmond, South Australia 5064, Australia 2 CSIRO Land and Water, GPO Box 1700, Canberra, Australian Capital Territory 2601, Australia 3 Department of Forest and Ecosystem Science, University of Melbourne, 221 Bouverie Street, Parkville, Victoria 3010, Australia 4 US Geological Survey, Southwest Biological Science Centre, Sonoran Desert Research Station, 1110 E. South Campus Drive, Room 123, Tucson, AZ 85721, USA ABSTRACT Water resource development and drought have altered river flow regimes, increasing average flood return intervals across floodplains in the Murray–Darling Basin, Australia, causing health declines in riparian river red gum (Eucalyptus camaldulensis) forests and woodlands. Environmental flow allocations helped to alleviate water stress during the recent Millennium Drought (1997–2010); however, quantification of the flood frequency required to support healthy E. camaldulensis communities is still needed. We quantified water requirements of E. camaldulensis for 2 years across a flood gradient (trees inundated at frequencies of 1:2, 1:5 and 1:10 years) at Yanga National Park, New South Wales, to help inform management decision-making and design of environmental flows. Sap flow, evaporative losses and soil moisture measurements were used to determine transpiration, evapotranspiration and plant-available soil water before and after flooding. A formula was developed using plant-available soil water post-flooding and average annual rainfall, to estimate maintenance time of soil water reserves in each flood frequency zone. Results indicated that soil water reserves could sustain 1:2 and 1:5 trees for 15 months and 6 years, respectively. Trees regulated their transpiration rates, allowing them to persist within their flood frequency zone, and showed reduction in active sapwood area and transpiration rates when flood frequencies exceeded 1:2 years. A leaf area index of 0·5 was identified as a potential threshold indicator of severe drought stress. Our results suggest that environmental water managers may have greater flexibility to adaptively manage floodplains in order to sustain E. camaldulensis forests and woodlands than has been appreciated hitherto. Copyright © 2015 John Wiley & Sons, Ltd. KEY WORDS flood frequency; wetland connectivity; inundation; drought; floodplain ecosystem function; water resource management; soil water; floodplain vegetation Received 25 August 2014; Revised 21 December 2014; Accepted 24 December 2014 INTRODUCTION Floodplains and wetlands worldwide are of immense ecological and economic importance because, relative to their small extent (~1·2 million km 2 ), they support dispro- portionately high levels of biodiversity and supply ecosystem services upon which people depend for their well-being (Millennium Ecosystem Assessment, 2005; Russi et al., 2013). Their very importance means they are threatened by population pressures and demands for water for consumptive use. Floodplains and wetlands are highly vulnerable to climate change (Capon et al., 2013), and environmental degradation from water diversions is increasing globally (Vörösmarty et al., 2010; Grafton et al., 2012). The combined stresses of historical and current water resource development and climate change have resulted in alteration of flow regimes in many river basins (Nilsson and Berggren, 2000; Grafton et al., 2014). Addressing competing water demands for consumptive and environ- mental uses requires policies and management that conserve floodplain and wetland ecosystems and ensure the supply of ecosystem services. In regulated river systems, this means allocation of environmental flows (Poff et al., 2010; Arthington, 2012), and the setting of clear ecological objectives within an adaptive framework *Correspondence to: Tanya M. Doody, CSIRO Land and Water, PMB 2, Glen Osmond, South Australia 5064, Australia. E-mail: tanya.doody@csiro.au ECOHYDROLOGY Ecohydrol. (2015) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/eco.1598 Copyright © 2015 John Wiley & Sons, Ltd.