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Measurements of water vapour flux from semi-arid perennial woodland (mallee) were made for 3years using eddy covariance instrumentation. There have been no previous long-term, detailed measures of water use in this ecosystem. Latent energy flux (LE) on a half hourly basis was the measure of the combined soil and plant evaporation, evapotranspiration’ (E-LE) of the site. Aggregation over 3years of the site measured rain (1136mm) and the estimated evaporation (794mm) suggests that 342mm or 30% of rain had moved into or past the root zone of the vegetation. Above average rainfall during 2011 and the first quarter of 2012 (633mm, 15months) would likely have been the period during which significant groundwater recharge occurred. At times immediately after rainfall, E-LE rates were the same or exceeded estimates of potential E calculated from a suitably parameterized Penman-Monteith (E-PMo) equation. Apparent free water E from plant interception and soil evaporation was about 2.3mm and lasted for 1.3days following rainfall in summer, while in autumn, E was 5.1mm that lasted over 5.4days. The leaf area index (LAI) needed to adjust a wind function calibrated Penman equation (E-PMe) to match the E-LE values could be back calculated to generate seasonal change in LAI from 0.12 to 0.46 and compared well with normalized difference vegetation index; r=0.38 and p=0.0213* and LAI calculated from digital cover photography. The apparently conservative response of perennial vegetation evaporation to available water in these semi-arid environments reinforces the conclusion that these ecosystems use this mechanism to survive the reasonably common dry periods. Plant response to soil water availability is primarily through gradual changes in leaf area. Copyright (c) 2015 John Wiley & Sons, Ltd.