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 Dehn h. plantation shown to be using shallow saline groundwater. Annual wat er use, obtained from two years of heat pulse measurements of daily wa ter use, was approximately 300 mm for both species with a winter minim um and a weak maximum in spring. Canopy conductance obtained from the ratio of daily water use to daily mean vapor pressure deficit (VPD) wa s low during summer, but rose to high values in winter. Diurnal variat ion in stomatal conductance was recorded in late summer and spring. St omatal conductance increased with radiation but showed no evidence of a relationship with VPD. Canopy conductance was generally less than ha lf the observed stomatal conductance on a leaf area basis, and usually declined steadily through the day, showing a strong inverse relations hip 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 con ductances. The difference in Omega values is interpreted as demonstrat ing the inclusion of a soil to leaf conductance within the canopy cond uctance estimates. Soil to leaf conductance is smaller than both stoma tal 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 a s the soil in the vicinity of the active roots dries as a result of wa ter uptake during the day. Plantations may be useful for transpiring s hallow groundwater to control rising water tables and salinity, but th eir effectiveness as water users will be reduced as the water table is drawn down in soils of low hydraulic conductivity. Plantations irriga ted with pumped groundwater or drainage water may provide a more effic ient disposal system where these water sources are available.