Soil frost effects on soil water and runoff dynamics along a boreal foresttransect: 1. Field investigations

Tn determine how soil frost changes flowpaths of runoff water along a hills lope, a transect consisting of four soil profiles directed towards a small stream in a mature forest stand was investigated at Svartberget, near Vinde ln in northern Sweden. Soil temperature, unfrozen water content, groundwate r level and snow depth were investigated along the transect, which started at the riparian peat, and extended 30 m upslope into mineral soils. The two , more organic-rich profiles closest to the stream had higher water retenti on and wetter autumn conditions than the sandy mineral soils further upslop e, The organic content of the soil influenced the variation in frost along the transect. The first winter (1995-96) had abnormally low snow precipitat ion, which gave a deep frost down to 40-80 cm, whereas the two following wi nters had frost depths of 5-20 cm. During winter 1995-96, the two organic p rofiles close to the stream had a shallower frost depth than tile mineral s oil profile higher upslope, but a considerably larger amount of frozen wate r. The fraction of water that did not freeze despite several minus degrees in the soil was 5-7 vol.% in the mineral soil and 10-15 vol.% in the organi c soil. From the measurements there were no signs of perched water tables d uring any of the three snowmelt periods, which would have been strong evide nce for changed water flowpaths due to soil frost. When shallow soil layers became saturated during snowmelt, especially in 1997 and 1998, it was beca use of rising groundwater levels. Several rain on frozen ground events duri ng spring 1996 resulted in little runoff, since most of the rain either fro ze in the soil or filled up the soil water storage. Copyright (C) 2001 John Wiley Sr Sons, Ltd.