Does peatland rewetting mitigate 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. Therefore, rewetting of drained peatlands is considered an important strategy to reduce the catastrophic effects of flooding. With the anticipation of more frequent extreme rainfall events due to a changing global climate, the importance of peatland rewetting in flood mitigation becomes even more important. To date, empirical data showing that rewetting actually restores the hydrological function of drained peatlands is largely lacking, particularly in Sweden. To assess whether rewetting peatlands can mitigate 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 table level, discharge, and rainfall data were collected over four years, allowing us to detect and analyze 17 rainfall-runoff events before and 30 events after rewetting. Our rainfall-runoff analysis revealed that rewetting significantly decreased peak flow, runoff coefficient, and reduced the overall flashiness of hydrographs, making the rewetted site 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. We found that the rewetted site experienced an increase in the groundwater table level following rewetting and this was consistently observed across all distances from the blocked ditch within the peatland, providing complementary data for our event-based analysis. 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.