Responses of evapotranspiration at different topographic positions and catchment water balance following a pronounced drought in a mixed species eucalypt forest, Australia

Across southern Australia, a large proportion of urban water supply is sourced from mountainous catchments forested with native eucalypts. Mixed species eucalypt forest (MSEF) is the most common forest type in this region and occurs on relatively dry, fire prone sites, yet factors controlling forest water use and stream flow in response to topography, disturbance and drought are poorly understood. This study investigated the patterns and drivers of water balance over a 4 year period in a 1.36 km², MSEF catchment by: quantifying spatial and temporal variability in evapotranspiration (E_t) and its components; evaluating the abiotic, structural and physiological factors controlling water use across the catchment; and testing the effects of antecedent soil water conditions on water fluxes after drought. This was done using a 'bottom up' measurement approach that included stream flow and E_t (sap flow, interception troughs and evaporation dome) and a simple empirical model of Er to track catchment response to drought. Spatial variability was considerable, with 40% lower rates of Er at an up slope plot compared to mid and bottom slope plots. Tree transpiration was the dominant flux annually and was correlated to reference E_t (r² = 0.35-0.80), implying strong limitation by atmospheric demand across the catchment. Annual E, totals were relatively consistent between years (841 +/- 34 mm) despite large variation in rainfall (463-1179 mm y^-1). Annual stream flow represented a very small proportion of the water budget (< 2% of rainfall) and showed little recovery from the drought period. The modelling showed that the change in soil water storage following drought was large (up to -330 mm) and was responsible for decreased rates of stream flow. These findings show that E_t in the MSEF is sensitive to topography and demand limitation and suggests that water yield in this forest type may be particularly sensitive to future climatic change as shown by the sustained effect of drought on catchment water balance.