Partitioning of evapotranspiration in a semi-arid eucalypt woodland in south-western Australia

In semi-arid ecosystems, evapotranspiration (Et) generally represents the greatest flux of water out of an ecosystem and is sensitive to changes in vegetation cover. In southern Australia, removal of native, deep-rooted perennials to make way for annual crops and pastures with shallower roots and less annual transpiration has led to rising, saline groundwater. Steps to redress this hydrological imbalance require knowledge of the ecohydrology of native systems such as eucalypt woodlands, which grow on deeper soils low in the landscape, and, unlike annual crops, sustain growth and water use during summer months. We assessed the partitioning of Et in a common woodland type in south-western Australia (Eucalyptus capillosa subsp. capillosa Brooker and Hopper) and the response of tree transpiration (Esap) to both atmospheric and soil water conditions. Overall, the total woodland evapotranspiration (Etotal) exceeded present-year rainfall (359 mm vs. 265 mm) but was equal to average rainfall of the preceding 10 years, suggesting long-term balance in water supply and Etotal in this system. The contribution of Esap to Etotal was surprisingly small (53 and 22% of Etotal in summer and winter, respectively) while woodland floor evapotranspiration (Efloor) contributed 38 and 64% during the same periods. Rates of Esap increased in response to summer rainfall, due primarily to reduced sensitivity in summer to vapour pressure deficit when soil water potentials increased. Additionally, night-time tree transpiration contributed appreciably to the overall water budget (2–8% of seasonal Etotal), being similar in magnitude to interception losses, and therefore should be included in water budgets. Partitioning of Etotal in this woodland type also demonstrated that trees provide a more stable water flux out of the system than Efloor, most likely because they access soil water from a large volume of soil.