RUNOFF GENERATION IN ZERO-ORDER PRECAMBRIAN SHIELD CATCHMENTS - THE STORMFLOW RESPONSE OF A HETEROGENEOUS LANDSCAPE

Hydrometric and isotopic (oxygen-18) observations were used to delinea te the runoff processes operating in several headwater catchments on t he Precambrian Shield of Canada. The catchments comprise patches of co nifer forest situated on thin soils among areas of lichen-covered gran itic bedrock. Horton overland flow occurred from the lichen-bedrock ar eas in all precipitation events that exceeded 4- 6 mm. Runoff from the forest stands occurred mainly as subsurface stormflow, but in some in stances saturation overland flow was observed. The occurrence of satur ation overland flow was controlled by the topography of the bedrock be neath the forest soils. The area contributing runoff and the pathway b y which water was conveyed to the catchment outflow switched from the open lichen-bedrock areas producing overland flow on the rising limb o f the storm hydrograph to the forest stands contributing subsurface st ormflow on the recession limb of the hydrograph. The areal extent and position of the landscape units in the basin were important to the rat e and magnitude of stormflow production. Runoff was generated from the catchments only during and immediately after snowmelt and/or rainfall events. The catchments were dry and/or frozen for about 70% of the ye ar.