Does peatland rewetting mitigate flooding from extreme rainfall events?

Abstract. Pristine peatlands are believed to play an important role in regulating hydrological extremes because they can act as reservoirs for rainwater and release it gradually during dry periods. Rewetting of drained peatlands has therefore been considered an important strategy to reduce the catastrophic effects of flooding. With the anticipation of more frequent extreme rainfall events in the future due to a changing global climate, the importance of peatland rewetting in flood mitigation becomes even more important. To date, however, empirical data showing that rewetting of drained peatlands actually restores their hydrological function similar to pristine peatlands are largely lacking, particularly for boreal fens. To assess whether peatland rewetting can mitigate flooding from extreme rainfall events and ensure water security in a future climate, we measured event-based runoff responses before and after rewetting using a BACI approach (before–after and control–impact) within a replicated, catchment-scale study at the Trollberget Experimental Area in northern Sweden. High-resolution hydrological field observations, including groundwater level (GWL), discharge, and rainfall data, were collected over 4 years, allowing us to detect and analyze 17 rainfall-runoff events before and 30 events after rewetting. We found that the rewetted sites experienced an increase in the GWL following rewetting and that this was consistently observed across all distances from the blocked ditch within the peatland. Our rainfall-runoff analysis revealed that rewetting significantly decreased peak flow and the runoff coefficient and reduced the overall flashiness of hydrographs, making the rewetted sites function more like the pristine control peatland. However, “lag time”, which was already similar to pristine conditions, was pushed farther away from pristine conditions following rewetting. Yet, our results showed that the effectiveness of ditch blocking in flood moderation was strongly influenced by the initial condition and the catchment percentage of restoration, as one of our two rewetted peatlands did not show significant change, attributed to it being already similar to the pristine site, suggesting less treatment effect, and the other catchment, with higher restoration percentage, had a better response to treatment. In summary, our findings suggest that peatland rewetting has the potential to mitigate flood responses; however, further research over a longer time period is needed, as peat properties and the peatland vegetation will develop and change over time.