Forest fire and impact on water supplies

Forest fires create direct visible changes in the landscape. But how does a forest fire affect the water supply? Following the major forest fire in the county of Västmanland four years ago, researchers at SMHI investigated the water supply in the area and downstream. They reached the conclusion that forest fires primarily affect the seasonal distribution of water, but that the changes are local and other changes have a bigger impact.

The Hälleskogsbrännan nature reserve, end of June 2015
The Hälleskogsbrännan nature reserve at the end of June 2015, almost one year after the major forest fire. First, the vegetation returns closest to the watercourses. Enlarge Image

Following the major forest fire in Västmanland in 2014, the Formas research council carried out a strategic initiative to investigate various aspects of the impact of a forest fire. SMHI and the Swedish University of Agricultural Sciences (SLU) have conducted measurements of water supply and water flows both in and downstream of the fire area, to find out how they are changed by a forest fire.

“After the fire we had a different seasonal distribution of the water flows. In the autumn we had increased flows as the vegetation didn’t absorb as much water. In the spring the snow melted more quickly, as there wasn’t as much vegetation to shade and protect the snow,” says Berit Arheimer, Head of the Unit for Hydrological Research at SMHI.

Mostly local effects

The researchers were also able to confirm that the effects are most evident close to the fire area in the drainage areas, where more than half of the area had burnt. The effects were quickly diluted downstream.

“When we compare measurements in the fire area and downstream, we see that a forest fire has a pronounced local impact. If you compare with other changes, the changes in land use and vegetation have very little impact on water flows and water balance. Water regulation and climate variation have much more of an impact,” confirms Berit Arheimer.

The Hälleskogsbrännan nature reserve, June 2018
The Hälleskogsbrännan nature reserve at the end of June 2018, almost four years after the major forest fire, which started on 31 July 2014. Larger plants are now turning green at the site. Enlarge Image

Knowledge improves base data for decisions

The study improves the understanding of how a forest fire affects the water supply and water flows. The study involved a comparison of both affected and unaffected areas. The effects of the fire were also compared with other factors such as trees falling in storms, climate fluctuations and various human activities, for example urban growth and water regulation. The knowledge is used by SMHI’s researchers in the calculation models used for hydrological forecasts and future scenarios, in order to provide base data for decisions on how we can reduce our vulnerability to changes in the future.