Streamflow responses to the clearing of alien invasive trees from riparian zones at three sites in the Western Cape Province

Abstract

The changes in streamflow following the removal of invasive wattle (Acacia mearnsii and A. longifolia) from riparian zones during the dry summer months in three small catchments in the Western Cape Province are described. Portable steel box weirs, with a 30° compound V-notch and equipped with Belfort water level recorders, were used to measure streamflow. Each of the three streams had a control catchment against which it was correlated during a pre-treatment period. The riparian zones of the treatment catchments were cleared after this period of calibration, and the response of stream-flow after treatment was recorded.

In the three catchments there was a marked increase in streamflow after clearing of the riparian invasive vegetation. The streamflow increases in the three catchments were 8,8, 10,4 and 12 m³/day per ha cleared. The responses measured in these experiments are the result of changing from tall vegetation to minimal cover and represent a maximum response. Streamflow is expected to decrease again as vegetation regrows, but not to the levels that characterised the invaded site. The riparian areas should be kept under short indigenous vegetation, such as grass or fynbos, to sustain the long-term increases in streamflow.