Where can rewetting of forested peatland reduce extreme flows? Model experiment on the hydrology of Sweden

Abstract. Historical drainage to improve forestry practices has resulted in around 7×105 ha drained forested peatland in Sweden. This has reduced the storage of water in the landscape and may impact greenhouse gas emissions, biodiversity, and the damping of extreme water flows. National restoration actions therefore aim at rewetting 1×105 ha of forested peatland in Sweden, despite the limited and sometimes contradictory evidence of the impacts of rewetting. To clarify the potential impact of rewetting on extreme flows and their cause–effect relationships, we simulated flow under various conditions of climate, local hydrology, and rewetting practices (ditch blocking alone or combined with reduced tree cover). For this, we used the HYPE model setup across Sweden (450 000 km2) with improved calculations of runoff in drained forest and routines for recharge and discharge areas. National evaluation of changes in discharge extremes was combined with a detailed study in south-east Sweden, with the aim to understand rewetting impacts at various scales. We found that the change in discharge extremes from catchments of 10 km2 is small because there is considerable mixing with runoff from various land covers. Hence, at the larger scale, rewetting is not an efficient measure to combat droughts or floods. However, for ecosystems in streams draining only peatlands, rewetting can have an impact if appropriate sites for restoration are selected. The results show that groundwater level prior to rewetting and reduced tree cover govern the effect on water runoff. Wetland allocation and management practices are thus crucial if the purpose is to reduce flow extremes in peatland streams.