Toward improved understanding of the quantitative linkage between ecological processes and hydrologic dynamics in south-east Australian catchments

The high country catchments in south-east Australia are probably the most important of all ecosystems in terms of water supply for millions of urban people in the major cities. These mountainous catchments are predominantly forested with mixed-species native eucalypts that are relatively unknown hydrologically. In the context of climate change, rising temperature and increasing frequency of high intensity bushfires, the questions we are trying to answer are threefold: 1) How does plant structure and physiology control water use; 2) How does spatial variation in water use affect water yield; and 3) do physiological controls or biophysical constraints determine variation in water yields. This information is necessary to assess the consequences of climate change on the terrestrial water cycle, and guiding hydrological models for managing catchments in south-east Australia. In this study, water relations of high country forests in response to the environment were studied at the leaf, tree, and stand scale, using a range of measurements and modelling frame works. A large proportion of the analyses in this study rely on sap flow measurements collected using the Heat Ratio Method. Eucalypt water use in the high country was largely governed by the atmospheric environment, mainly vapour pressure deficit and radiation, compared to soil moisture and wind speed, with species-specific sensitivity to atmospheric drought that were supported by species distribution patterns within the landscape. A generic model is developed using data-driven techniques to estimate tree water use from atmospheric demand and potential incoming radiation derived from digital elevation model. According to modelled sap flow tree water use is lowest on higher elevations, and is greatest on steep southern aspects. Upscaling evapotranspiration (ET) to a catchment scale was subject to fundamental issues. The accuracy of ET derived from stream flow integration was limited to wet conditions when the catchment was connected. Biophysical constraints of ET also explained variations in streamflow better during wet conditions. Locally controlled ET was underestimated with soil water balances during dry conditions because overstorey vegetation sourced water from deeper in the soil profile, and vegetation water use was more strongly coupled to the local soil moisture availability. Practical implication of such information is in estimating the potential impact of climate change on water yield from forested catchments and informing hydrological models for managing water resources in south-east Australia.