In South Africa, the invasion of riparian forests by alien trees has the potential to affect the country’s limited water resources. Tree water-use measurements have therefore become an important component of recent hydrological studies. It is difficult for South African government initiatives, such as the Working for Water (WfW) alien clearing program, to justify alien tree removal and implement rehabilitation unless hydrological benefits are known. Consequently, water use within a riparian forest along the Buffeljags River in the Western Cape of South Africa was monitored over a 3-year period. The site consisted of an indigenous stand of Western Cape afrotemperate forest adjacent to a large stand of introduced Acacia mearnsii. The heat ratio method of the heat pulse velocity sap flow technique was used to measure the sap flow of a selection of indigenous species in the indigenous stand, a selection of A. mearnsii trees in the alien stand and two clusters of indigenous species within the alien stand. The indigenous trees in the alien stand at Buffeljags River showed significant intraspecific differences in the daily sap flow rates varying from 15 to 32 L day 1 in summer (sap flow being directly proportional to tree size). In winter (June), this was reduced to only 7 L day 1 when limited energy was available to drive the transpiration process. The water use in the A. mearnsii trees showed peaks in transpiration during the months of March 2012, September 2012 and February 2013. These periods had high average temperatures, rainfall and high daily vapor pressure deficits (VPDs – average of 1.26 kPa). The average daily sap flow ranged from 25 to 35 L in summer and approximately 10 L in the winter. The combined accumulated daily sap flow per year for the three Vepris lanceolata and three A. mearnsii trees was 5700 and 9200 L, respectively, clearly demonstrating the higher water use of the introduced Acacia trees during the winter months. After spatially upscaling the findings, it was concluded that, annually, the alien stand used nearly 6 times more water per unit area than the indigenous stand (585mma 1 compared to 101mma 1/. This finding indicates that there would be a gain in groundwater recharge and/or streamflow if the alien species are removed from riparian forests and rehabilitated back to their natural state.