John Ruprecht

Understanding how summer low flows in a Mediterranean climate are influenced by climate and land use is critical for managing both water resources and in-stream ecohydrological health. The Eucalyptus forest ecosystems of southwestern Australia are experiencing a drying and warming climate, with a regional step decline in rainfall in the mid-1970s. Reductions in catchment water storage may be exacerbated by the deep rooting habit of key overstorey species (>30 m has been reported), which can buffer against drought during dry years. Root exploitation of deep soil moisture reserves and/or groundwater can accelerate the long term decline in summer low flows, with a trend towards more ephemeral flow regimes. In contrast, conversion of forests to agricultural land in some catchments can lead to counter-trends of increased low flows due to a rise in groundwater pressure. These are invariably associated with an increase in stream salinity as regolith stores of salt are mobilized. There has also been extennsive reforestation of farmland in some catchments. In this study we perform a detailed analysis of changes to annual summer seven day low flow trends in perennial catchments and flow duration curves in ephemeral catchments across 39 catchments in south-western Australia that have long term records of runoff, rainfall and land cover. Results showed that 15% of catchments exhibited increased low flows and 85% decreased flows or decreased flow days since the 1970s. Significant downward step changes in low flows were observed in 17 catchments (44%). The earliest downward step changes occurred in three catchments between 1981-82 (a lag of one decade after the rainfall decline), with the most recent step changes for five catchments occurring in 2001-2004 (three decades after rainfall decline). Eleven catchments were already ephemeral in the 1970s, but exhibited continued declines in the number of annual flow days over subsequent decades. Step changes occur when groundwater becomes disconnected or reconnected to the stream invert, with disconnection associated with rainfall decline and vegetative water use. The statistical methods we used in this study can be applied to any catchment in order to aid land and water managers assess the impact of climate change and land cover manipulation on low flow response.

Forests can contribute to climate change mitigation through (1) protection and enhancement of existing carbon stocks, (2) increasing carbon stocks and (3) substituting forest products for energy production or energy intensive building materials. Payments for forest carbon mitigation are occurring under various arrangements in different jurisdictions and the scale of future activity could be large. The likely impacts of broad-scale forest-mitigation on water yield and quality are not understood. Various approaches to water management using carbon mitigation have been examined in south-western Australia, a region with a drying Mediterranean climate and limited potable water supplies. The impacts on water yield and water quality of (1) deforestation and thinning of natural forests, and (2) reforestation of farmland have been studied. Approaches to reforestation have concentrated on Eucalypts and Pinus spp. and include total reforestation of watersheds, integration of strips of trees with farmland and 3–5 year rotations of trees interspersed with cereal cropping. Mitigation has been through both sequestration and bioenergy production and has occurred on both productive and abandoned land. Forest cover profoundly affects water yield and quality, not only through changes in watershed water balance, but also on the release of dissolved salts into the landscape.