Abstract. It is now well established that forested catchments have higher evapotranspiration than grassed catchments. Thus land use management and rehabilitation strategies will have an impact on catchment water balance and hence water yield and groundwater recharge. The key controls on evapotranspiration are rainfall interception, net radiation, advection, turbulent transport, leaf area, and plant-available water capacity. The relative importance of these factors depends on climate, soil, and vegetation conditions. Results from over 250 catchments worldwide show that for a given forest cover, there is a good relationship between long-term average evapotranspiration and rainfall. From these observations and on the basis of previous theoretical work a simple two-parameter model was developed that relates mean annual evapotranspiration to rainfall, potential evapotranspiration, and plant-available water capacity. The mean absolute error between modeled and measured evapotranspiration was 42 mm or 6.0%; the least squares line through the origin had a slope of 1.00 and a correlation coefficient of 0.96. The model showed potential for a variety of applications including water yield modeling and recharge estimation. The model is a practical tool that can be readily used for assessing the long-term average effect of vegetation changes on catchment evapotranspiration and is scientifically justifiable. IntroductionThe massive land use change in Australia associated with agricultural development has caused an imbalance in catchment hydrological regime, leading to increased land and water salinization over large areas. It is estimated that each year the total cost of salinization to the nation is about $270 million including cost of lost production, damaged infrastructure, and degraded environmental assets (Prime Minister's Science, Engineering and Innovation Council, Dryland salinity and its impact on rural industries and the landscape, Canberra, ACT, Australia, 1999, available at http://www.dist.gov.au/science/ pmseic/2ndmeeting.html). A number of land rehabilitation programs have been established by the commonwealth and state governments to control the degradation. According to Forest Plantations 2020 Vision, a major initiative of the commonwealth and state governments, the area of tree plantations by the year 2020 will treble [Department of Primary Industries and Energy, 1997], with part of this increase justified by environmental benefits. If the 2020 Vision is accurate, the plantation area in Australia will increase to over 3 million ha and will have significant impacts on catchment water yield and salinity. The impacts of such plantations on the trade-offs between economic viability, environmental sustainability, and water resource security will depend on the spatial distribution of the plantations. It is important to be able to predict the water balance-vegetation relationships at regional scales to determine these trade-offs. For the relationships to be useful, they must be dependent only on data that is generally...

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Water sources of Eucalyptus camaldulensis Dehn. trees were investigated on a semiarid floodplain in south-eastern Australia. The trees investigated ranged in distance from 0.5 to 40 m from a stream, with electrical conductivity 0.8 dSm, and grew over groundwater with electrical conductivity ranging from 30 to 50 dSm. The sources of water being used by the trees were investigated using the naturally occurring stable isotopes of water and measurements of soil water potential. Xylem water potential and leaf conductance were also examined to identify the trees' response to using these sources of water. Trees at distances greater than about 15 m from the stream used no stream water. The trees used groundwater in summer and a combination of groundwater and rain-derived surface-soil water (0.05-0.15 m depth) in winter. In doing so they suffered water stress at electrical conductivities higher than approximately 40 dSm (equivalent to approximately -1.4 MPa). Trees adjacent to the stream used stream water directly in summer, but may have used stream water from the soil profile in winter, after the stream had risen and recharged the soil water. E. camaldulensis appeared to be partially opportunistic in the sources of water they used.