FORESTRY AND STREAMFLOW REDUCTIONS IN SOUTH-AFRICA - A REFERENCE SYSTEM FOR ASSESSING EXTENT AND DISTRIBUTION

Forestry is an important sector of industry in South Africa but the gr owing of timber places significant demands on the available water reso urces. Yet, a ready source of information on the extent, and probable hydrological impacts, of afforestation in specific locations in South Africa has not been available. This paper reports on the modelling exe rcise conducted to produce an easy-to-use, handy catalogue to fill thi s need, and presents some of the notable results of this exercise. Dat abases of quarternary catchment and magisterial district boundaries, r ainfall, streamflow, forestry areas by tree genus, timber rotation len gths (years) and forestry growth potential were combined by means of a geographical information system to delineate uniform blocks of forest ry. On these blocks a robust empirical model predicted total and low-f low reductions as a function of rotation, length, tree genus, water av ailability, growth potential and plantation age distribution. The resu lts were summarised by quaternary, tertiary, secondary and primary cat chment, and by magisterial district and province. The area of commerci al timber plantations in South Africa is estimated at 1.5 million ha ( 57% pine, 35% eucalypts and 8% wattle), covering just 1.2% of South Af rica. But the regions in which forestry is concentrated receive higher rainfall and yield a disproportionately large share of the streamflow , particularly low flow (dry-season flow). The commercial plantations are estimated to reduce mean annual streamflow by 3.2% (1417 x 10(6) m (3)) and low flows by 7.8% (101 x 10(6) m(3)/yr). Our estimate of an a verage reduction of 98.6 mm/yr per unit of planted area is 13% lower t han the previous nation-wide estimate (113.6 mm/yr) of the net effect of forestry on total water resources (DWA, 1986). Mpumalanga Province with the highest concentration of forestry (7.2% of land area) experie nces the largest reductions in flow - almost 10% of total flow and 18% of low flows. However, the largest relative impacts on low flow are s een in Northern Province where small areas of forestry are confined to humid upper catchments that are the principal source of dry-season fl ow in otherwise dry secondary catchments.( )