Thesis presents unique study on flood mitigation in rewetted peatlands

Rewetting peatlands has become a focal point in recent years for Swedish policymakers looking for countermeasures to both climate change and extreme weather events. Over time, peatlands can accumulate and store vast amounts of carbon, while also potentially acting as a hydrological buffer, mitigating the effects of both floods and droughts. But will rewetting peatlands achive the same effects as those in pristine condition?

In pursuit of these answers, researchers at SLU established the Trollberget Experimental Area outside Umeå in 2018. As part of the trials a drained peatland area was rewetted in November 2020, and closely monitored in the years since. Two years after rewetting, results on emissions of greenhouse gasses were presented. Now, PhD-student Shirin Karimi can publish results on how rewetting affected the hydrological functions in the unique site.

Promising results - under the right conditions

– We wanted to look at the peatlands role both locally and within the larger network, says Shirin Karimi, PhD-student at the department of Forest Ecology and Management, at the Swedish University of Agicultural Sciences, SLU. As part of her thesis, she monitored the hydrological effects of rainfall events in the newly rewetted area compared to nearby Degerö stormyr, a natural peatland with similar conditions as Trollberget, as well as sites in the larger Krycklan catchment area.

In the years following rewetting, the Trollberget site showed increasingly similar hydrological responses to the control sites. The rewetted site increased its capacity for storing water and showed both lower peak flow and runoff response during heavy rainfall events. Even visually, the difference in a few years was striking.

– Now it's so much wetter. You don't dare to walk over it now, especially in the autumn. It doesn't look like a natural peatland, but now you see surface ponding and high water levels, says Shirin Karimi.

These results indeed look promising for rewetting. But the study shows the success is conditional to specific site characteristics. When looking at these hydrological responses at a larger network scale, other factors have a bigger impact on moderating floods than the peatland itself.

– For example, sorted sediment soil depth and the size of the area are very correlated to the response characteristics. More sorted sediment soil means lower peak flow, discharge increase and higher lag. It was surprising that there is almost no response for peatlands on these characteristics. These other factors play a much more significant role, says Shirin Karimi. 

Important step in facing future weather extremes

Shirin Karimi suspects that the Trollberget Experimental Area being located in a basin downstream and being less intensly degraded than other peatlands contributed to the successful result. In a future where extreme weather, droughts and summer precipitation are expected to increase, rewetting might be an effective tool. These empirical results however show that more research is needed to see if the effects persist, and if they are applicable elsewhere. Similar trials under different conditions might not yeild the same results, she says.

– I would say not every site is suitable. They are different, and a bigger assessment is needed. Otherwise you might waste your time and money, says Karimi.

Dissertation:

Shirin Karimi defends her thesis Peatland hydrology in boreal Sweden: Modelling, long-term data analysis, and experimental rewetting on 3 May 2024, in P-O Bäckströms Sal, at SLU in Umeå. Join the disstertation online: https://slu-se.zoom.us/webinar/register/WN_TTEraaZeQcyRsk8YL2QKgg#/registration 

Read the thesis:

https://publications.slu.se/?file=publ/show&id=128527