Abstract
A study was conducted at Stavely Research Station, Alberta, to determine the quantity and quality of surface runoff from small grassland watersheds under three grazing intensities, viz. ungrazed, heavy grazing (2.4 animal unit months per hectare, AUM ha-1) and very heavy grazing (4.8 AUM ha-1). The volume of surface runoff varied each year (1998, 1999 and 2000) and also differed across watersheds, with lower runoff in the ungrazed compared with the heavy and very heavy grazed watersheds. Total dissolved solids in surface runoff water ranged between 34 to 360 mg L-1, and that for runoff from the very heavy grazed watershed was greater than that from other watersheds. Electrical conductivity increased with increased grazing intensity on the watershed. In two of three years the very heavy grazed watershed had greater nitrate concentrations than the other two watersheds. In all three years the levels of nitrate were lower than the maximum acceptable level for drinking water (10 mg L-1 as nitrogen). Levels of orthophosphate (PO43-) in surface runoff from all three watersheds and the three years of study were less than 1 mg L-1, and mostly within the range considered typical for rivers and streams. Total carbon (up to 500 mg L-1) was greater than the amounts considered typical for streams and rivers, and most of it was organic carbon. Nuisance organisms such as algae, nematodes, Giardia spp., Cryptosporidium spp. and rotifers were detected in some surface runoff samples. However, no crustaceans were detected. The results of a canonical correlation analysis indicated that the dominant external forcing factors (meteorological and management) in influencing water quality were year of study, water temperature and grazing. Surface runoff discharge did not influence water quality measurements. The dominant water quality parameters were found to be total carbon, organic carbon, total dissolved solids and electrical conductivity. Overall, this study indicated that during the three years, the surface runoff volumes from the watersheds were small and grazing of these watersheds posed little risk of nutrient (e.g., nitrate, ammonia and orthophosphate) contamination of adjacent streams, but organic carbon loading and dissolved solids may be of concern. The presence of parasites was detected in two or less runoff water samples each year, and thus pose little risk of contamination of adjacent streams. However, it may be necessary to monitor parasites especially in areas under cow-calf operations.
Changes in the water quality of the Wilderness and Swartvlei lake systems, South Africa