The Åstdalen catchment (373 km2) represents largely a natural ecosystem, typical for the higher central and southern parts of the Scandinavian Peninsula. It is underlain by fractured sedimentary rocks with a thin cover of unconsolidated Quaternary deposits.
The runoff process in Åstdalen is mainly determined by: Large accumulation of precipitation as snow, large areas covered by peatlands, and a high waterstorage capacity in the bedrocks. Groundwater discharge occurs in river beds, in springs and as flow towards the soil surface, especially toward peatland areas.
The discharges of the main river Åsta and of investigated springs rise rapidly following snowmelt and rainstorms during the two water-years of study, 1 October 1980 to 30 September 1982. This shows that the catchment has a high infiltration capacity, and that water moves rapidly downslope in the saturated zone, following macropores in soils, peatlands and bedrock fractures. Groundwater, especially shallow groundwater, plays an important role of streamflow peak generation, even during snowmelt. About 60-70 % of the river discharges during the melting periods in 1981 and 1982 were baseflow.
There is a net gain of protons, ammonium, potassium, nitrate and chloride to the catchment from the atmosphere during the year, and a net loss of calcium, magnesium sodium and sulfate. Constituents showing a net loss have an internal geologic and/or organic source in addition to the atmosphere.
The concentration of most substances in river Åsta and in the investigated springs are lowered during peak flows. The concentration of protons is on the other hand typically increased during snowmelt. This indicates that fresh rain and/or snowmelt water dominates such flow periods and not older prestorm water.
About 56 % of the protons needed to supply the weathering-derived ions to river Åsta come from precipitation.
- Received January 10, 1986.
- © IWA Publishing 1986