Epsilon Archive for Student Projects

Abstract

Forestry is one of the main industries in Scandinavia but also timber harvesting can have a large impact on the ecology and hydrology of boreal forests. The removal of the forest canopy alters many variables in the catchments such a snow accumulation, timing of the snow melt, evapotranspiration, interception loses and soil characteristics. All these factors result in changes in the dynamic of streams draining operational forests.

This thesis reports on the changes in the stream regimes during a three year period after forest harvesting, performed in March 2006, at the Balsjö Catchment study in Northern Sweden. To quantify the effect of harvesting the different variable of the water balance equation were investigated by comparing the harvested and the reference catchments. The experiment is set up as a paired catchment study so that the observed changes can be reported relative to the reference area and to the pre-harvesting period which includes 18 months before forest harvesting (September 2004 - March 2006).

After the forest harvesting the runoff increased approximately 30% in average at the two harvested sites relative to the reference. The number of days with low flows (<1mm/day) decreased about 20% after harvest for all harvested studied sites, the days with moderate (1-5mm/day) flow increased more than 60%, relative to the reference site. The peak events responded the strongest at the reference catchment before clear cutting. After forest removal almost all peak flows were observed to be highest at the harvested sites. The annual evapotranspiration quantified by the water balance approach decreased from 286 mm in 2005 to 167 mm in 2006 and 151 mm in 2007 at the harvested site, whereas it increased in 2008 again, possibly due to the re-growth ground vegetation. The snow accumulation was higher at the open areas in comparison to the forest resulting in a difference in snow depth of about 8-18 cm which accounts for a difference in the snow water equivalent (SWE) between 30 and 80 mm.

These results suggest that forest harvesting strongly affects the water balance of a catchment. The higher runoff is mainly caused by lower evapotranspiration during the growing season as well as higher snow accumulation and therefore melted water contributions during the winter season.