THE ROLE OF SURFACE AND SUBSURFACE RUNOFF PROCESSES IN CONTROLLING CATION EXPORT FROM A WETLAND WATERSHED

Water and cation budgets were calculated for two sub-basins within a s mall low relief watershed in South-Central Ontario during a period of ephemeral runoff which was initiated by spring snow melt. The hydrolog y of one (upland) sub-basin was strongly influenced by seasonal fluctu ations in the level of regional ground water. Saturated contributing a reas formed in low lying regions adjacent to the stream channel where the water table rose to the surface, and stream discharge was a mixtur e of ground water and saturation overland flow. In the second sub-basi n a wetland provided a large and spatially less variable saturated con tributing area. Clay soils underlying the wetland resulted in a shallo w perched water table, poorly drained and highly organic soils, and gr eatly reduced inputs of regional ground water. Stream discharge was la rgely the result of surface runoff from the wetland and adjacent areas of saturated soil. Inter-basin variations in water export were by far greater than variations in stream chemistry. As a result, inter-basin variations in cation export strongly reflected variations in water ex port over the time interval in which the majority of a given ion was l ost from the watershed. Spatial differences in water export were least at the onset of runoff when basin saturation was greatest and overlan d flow made large contributions to the discharge from both sub-basins. Potassium and hydrogen had high concentrations at this time which cau sed these ions to show only small spatial differences in export. With decreases in the areal extent of soil saturation, and increases in the storage capacity of the wetland, the hydrologic contrast between sub- basins increased. Greater water loss from the upland area resulted fro m a greater discharge of regional ground water, and a more rapid expan sion of the saturated contributing areas during storm events. Calcium, magnesium, and sodium concentrations increased steadily during the fi rst 3 weeks of runoff, so that the peak export of these cations occurr ed later in the runoff period at times of higher concentration, but lo wer and spatially more variable discharges. Consequently, spatial diff erences in the loss of these ions was great and favoured the upland su b-basin, since the majority of export occurred when the hydrologic con trast between sub-basins was largest.