Transpiration and canopy conductance in a eucalypt plantation using shallow saline groundwater

Tree water use and canopy conductance were monitored in a 20-year-old Eucalyptus grandis W. Hill ex Maiden and Eucalyptus camaldulensis Dehnh. plantation shown to be using shallow saline groundwater. Annual water use, obtained from two years of heat pulse measurements of daily water use, was approximately 300 mm for both species with a winter minimum and a weak maximum in spring. Canopy conductance obtained from the ratio of daily water use to daily mean vapor pressure deficit (VPD) was low during summer, but rose to high values in winter. Diurnal variation in stomatal conductance was recorded in late summer and spring. Stomatal conductance increased with radiation but showed no evidence of a relationship with VPD. Canopy conductance was generally less than half the observed stomatal conductance on a leaf area basis, and usually declined steadily through the day, showing a strong inverse relationship with VPD. For both species, a decoupling coefficient (Omega) of 0.1 to 0.3 was obtained from canopy conductance and climate observations, and a higher value of Omega was derived from canopy and stomatal conductances. The difference in Omega values is interpreted as demonstrating the inclusion of a soil to leaf conductance within the canopy conductance estimates. Soil to leaf conductance is smaller than both stomatal and aerodynamic conductances, and effectively limits water use by the plantation. The observed variation in soil to leaf conductance is consistent with resistance to water movement to the roots increasing as the soil in the vicinity of the active roots dries as a result of water uptake during the day. Plantations may be useful for transpiring shallow ground-water to control rising water tables and salinity, but their effectiveness as water users will be reduced as the water table is drawn down in soils of low hydraulic conductivity. Plantations irrigated with pumped groundwater or drainage water may provide a more efficient disposal system where these water sources are available.