The environmental isotope oxygen-18 was used as a tracer to separate stream discharge into flows originating from groundwater and fresh meltwater. Two, mainly forested, watersheds (areas 6.6 and 4.0 km2) in southern Sweden were studied during snowmelt 1979. The seasonal variation of δ18O in precipitation makes the δ18O of meltwater different from that of groundwater, hence making the separation possible.

The major part of the streamflow this spring originated from groundwater. Of the total water volumes that left the areas, during the two periods of snowmelt that occurred this year, the fraction of groundwater ranged between 0.7 ± 0.2 and 0.9(±) 0.2 The volume of meltwater in the streams corresponded to the volume of snowmelt and precipitation over 10-15% of the catchments. A field survey indicated that these percentages reasonably well describe the extension of the saturated areas, i.e. the effluent areas for groundwater where no infiltration can take place.