The Effect of Afforestation of Indigenous Scrub Forest withEucalyptuson Streamflow from a Small Catchment in the Transvaal, South Africa

Abstract

A small catchment under indigenous scrub forest on the Westfalia Estate near Tzaneen in the northern Transvaal was treated by first cutting the riverine (riparian) vegetation and subsequently clearing the remainder of the catchment and establishing a Eucalyptus plantation. The effects of these treatments on streamflow response were investigated by means of a catchment experiment. Clearing the indigenous rpiarian vegetation (10% of the area) had a negligible effect on mean monthly streamflow. This lack of influence is ascribed to the rapid recovery of riparian vegetation under the subtropical climatic conditions at Westfalia. Subsequent clearing of the indigenous scrub forest on 83% of the remaining area resulted in a small increase in streamflow of about 20 mm per year. Establishment of a plantation of exotic Eucalyptus grandis on the total available area caused a decrease in streamflow of 200 mm per year (48% of mean annual streamflow before treatment) within three years after planting. The large water use by Eucalyptus is assumed to be related to differences in the transpiration of Eucalyptus trees and indigenous trees, rather than to differences in interception.