The combined effect of geology, phosphate sources and runoff on phosphate export from drainage basins

A large degree of uncertainty is usually associated with phosphate loads estimated by means of export coefficients. Methods of reducing this uncertainty were investigated. Soluble and total phosphate export coefficients were calculated for 7 South African catchments over periods covering 3–5 years. Spatial variance in the export coefficients could largely be accounted for if catchments were grouped according to their geological characteristics; those in the upper Limpopo River drainage basin having igneous and those in the Vaal River drainage basin having sedimentary geological formations. A secondary, but nonetheless important factor contributing to the spatial variance of export coefficients was whether or not a particular catchment contained mainly point or mainly non-point sources. After catchments were grouped according to these two factors, 74–99% of temporal variance of export coefficients could be explained by fitting linear regression equations to phosphate export vs runoff. The possible link between the bio-availability of particulate phosphorus and catchment geology is discussed. The significance of such a link, with regard to phosphate load estimates and making projections of future trophic status, is emphasised.