Techniques for assessing the effects of afforestation on catchment hydrology: the South African experience

Abstract

South Africa is a largely semi-arid country, where areas of high rainfall are limited to relatively small regions in the east and south of the country that benefit from coastal rain-bearing weather systems, or from the orographic effect associated with mountains and escarpments. These areas are of great significance in maintaining river flows which support a great diversity of downstream economic activity. The original vegetation in these areas was mainly seasonally–dormant natural grassland (summer rainfall region) or Machia-type shrubland (winter-rainfall region). Much of this vegetation has now been replaced by plantations of eucalypts, pines and acacias, which are relatively deep-rooted, evergreen, and characterised by higher rates of total annual evapotranspiration. These forest plantations now cover nearly 1.5 million ha. Catchment water yields have consistently declined in areas of new afforestation, causing widespread concern amongst other water users. This problem led to the early initiation of intensive research into the effects of forest plantations on catchment hydrology. This paper provides a brief overview of some of the techniques employed by South African hydrological researchers to understand the link between afforestation and catchment water yields.