Sulphate deposition to a small upland catchment at Suikerbosrand, South Africa

Abstract

In 1992, a study was initiated by the Water Research Commission of South Africa, to investigate the relationship between atmospheric deposition and water quality in a small upland catchment. The selected catchment, which had a seasonal stream, was a pristine site at the Suikerbosrand Nature Reserve, which is 80 km south-east of Johannesburg. The catchment is 32.5 ha in extent and is characterised by having a quartz geology with sandy soils. Fifty-four percent of the catchment area is bare rock and the average soil depth is 15 cm. The climate is relatively arid when compared to other catchment studies in the northern hemisphere (Birkeness and Hubbard Brook) with long dry periods in the winter months and a low annual runoff (8.4 - 8.9% of mean annual precipitation). The measured inputs to the catchment included rainfall, rainwater chemistry, ambient SO, concentrations, rock runoff and bulk or particulate deposition. Outputs from the catchment included the measurement of runoff using a V-Notch weir and intensive sampling of a range of chemical water quality variables. During the wet summer months the dry deposition was estimated to be between 39 and 62% of the total atmospheric sulphate inputs into the catchment, whereas in the dry winter months this was estimated to be 90% of inputs. Over a complete annual cycle the net accumulation of sulphate on the catchment surface was estimated to be between 83 and 91% of inputs.