INVASIVE PLANTS AND WATER-RESOURCES IN THE WESTERN-CAPE-PROVINCE, SOUTH-AFRICA - MODELING THE CONSEQUENCES OF A LACK OF MANAGEMENT

1. The invasion of fynbos shrublands by woody weed species can reduce the water yield from catchment areas dramatically. We modelled the con sequences of uncontrolled invasion on water yield using a geographical information system (Arc/Info). 2. Five important processes were recog nized: the occurrence of fire; the spread and establishment of alien p lants after fire; rainfall-to-run-off ratios; growth and changes in bi omass between fires; and effects of these changes on streamflow. 3. Th e simulations of water yield were modelled with the Arc/Info GRID modu le using a 200 x 200-m grid. It was assumed that the interval between fires was 15 years and that proliferation and dispersal of alien plant s took place only after fires. 4. Between fires, the model simulated t he growth of the vegetation and its effects on streamflow, using relat ionships between rainfall and run-off, and run-off and aboveground bio mass. 5. Results for the Kogelberg area in the Western Cape Province s howed that alien plants invaded about 40% of the grid cells within 50 years. Cover of alien plants increased from an initial estimate of 2.4 % to 62.4% after 100 years. 6. Invasion of catchment areas would resul t in an average decrease of 347 m(3) of water per hectare per year ove r 100 years, resulting in average losses of more than 30% of the water supply to the city of Cape Town. In individual years, where large are as would be covered by mature trees, losses would be much greater. 7. In addition, invasion of fynbos by alien plants will cause the extinct ion of many plant species, increase the intensity of fires, destabiliz e catchment areas with resultant erosion and diminished water quality, and decrease the aesthetic appeal of mountain areas. 8. Control of al ien weed species is necessary to avert the above impacts, and the cost s of control operations could be justified by the savings achieved in maintaining adequate water run-off from stable catchments in the long term.