PHOSPHORUS EXPORT FROM NONPOINT SOURCES IN THE BERG RIVER, WESTERN CAPE PROVINCE, SOUTH-AFRICA

A suite of semi-empirical models has been developed to quantify the transport of phosphorus in the Berg River. This paper describes the results of the model simulations to quantify (1) the input of phosphorus from point and nonpoint sources, and (2) the transport of phosphorus in the water prism along the main river channel. Export of phosphorus from nonpoint sources is influenced by the drainage flow during storm events. Mass export under the rising limb of the drainage hydrograph is higher than under the falling limb giving rise to a hysteresis relationship between concentration and flow. Implementation of the 1 mg P/l effluent for point sources is shown to have a minor influence on the phosphorus budget of the river - nonpoint sources contribute over 80 percent of the annual load. In the Berg River, 80 percent of the phosphorus export takes place during storm events which occupy less than 3 percent of the annual hydrological cycle.