Future water availability is affected directly by the effects of climate change on 16 water loss through evapotranspiration (ET) GCMs representing a period centred on 2050s was projected and then downscaled to the 27 study catchments. The future CO 2 concentration (i.e., eCO 2 ) at 2050 was projected to be 28 550ppm. Results from this study show eCO 2 increases canopy leaf area index (LAI) in all 29 four catchments and increases ET and decreases runoff in the water limited forest catchment 30 and the two grassland catchments, but reduces ET and increases runoff in the energy-limited 31 forest catchment. The effects of future climate on canopy LAI, ET and total runoff were 32 opposing in sign to those of eCO 2 for all four catchments. Our results also suggest that the 33 interactions between the direct and indirect effects on ET are relatively strong in the two 34 grassland catchments but relatively weak in the two forest catchments, possibly because the 35 deep-rooted forest system can utilize more available soil water than grasslands. Interactions 36 on runoff were relatively strong in the two water-limited catchments but weak in the two 37 energy-limited catchments, possibly because ET in the water-limited ecosystems are mainly 38 constrained by water and arid ecosystems have higher water use efficiency. This study 39 highlights that failure to account for impacts of eCO 2