The Effect on Storm Runoff Response of Seasonal Variations in Contributing Zones in Small Watersheds

Storm runoff production by saturation overland flow has been studied in two small watersheds in south-central Ontario. One is in rural land use and contains a wetland. The total saturated zone is at its maximum extent during spring snowmelt, when the overwhelming majority of total runoff and quickflow is produced. The stream seldom flows during the summer and winter seasons but does so occasionally in the fall if the wetland is recharged sufficiently. Simple calculations show that storm-to-storm variations in quickflow response are controlled by the extent of surface soil saturation (measured by field mapping and correlated with antecedent wetness variables) and precipitation amount. The other watershed has been undergoing suburban development since 1973. Quickflow response during summer and fall seasons has not changed noticeably since then, although 30% of the watershed surface has been disturbed. This is because in those seasons quickflow is still produced as saturation overland flow from the original contributing zones alongside the stream. In contrast, during spring snowmelt the very wet soil conditions and a high water table allow the disturbed surfaces with low infiltration capacities to be integrated into the contributing area via normally dry drainage lines. As a result spring quickflow has increased by three to four times over the original values.