Sphagnum is a key ingredient of natural flood management

In 2008 Joe Holden and colleagues published research that showed how water running over Sphagnum on blanket peatlands moved much more slowly (often ten times slower) than water running through sedges or bare peat. This spawned a new body of research which has shown how revegetation of peat, particularly if it is possible to get a dense Sphagnum cover, can slow the flow of water during storm events to reduce the flood peak downstream. The research indicates priority areas in the landscape for dense surface revegetation that will generally maximise flood benefits. Given that research has also recently shown sediment release from bare peat strongly influences peatland stream ecosystems this gives added impetus to revegetating peatlands and enhancing Sphagnum cover to achieve maximum downstream benefits for river habitats and flood risk.

Unlike most soil types where movement of water through the soil attenuates the rate of water loss into rivers, research has shown that water movement in blanket peatlands tends to be dominated by flow very close to the surface or at the surface1. This means large volumes of water move over short periods of time, associated with rainfall or snowmelt, producing very high flow peaks in blanket peatland rivers compared to the flows that occur during dry weather2, 3. The condition of the peatland surface may therefore be crucial in determining the downstream flood peak during storms.

Over a decade ago research was published that showed how water running over Sphagnum on blanket peatlands moved much more slowly than water running through sedges or bare peat4. This spawned a new body of research which tried to establish whether such effects made any difference to riverflow. This work, which included both empirical field demonstrations5, 6 and modelling experiments7-9, has now shown that revegetation of peat, particularly if a dense Sphagnum cover can be achieved, can slow the flow of water during storm events reducing the flood peak downstream. These effects hold (and can be proportionally greater) even for the very largest storm events10. The research indicates priority areas for the densest revegetation in the landscape to maximise flood benefits. These areas include strips of peatland several metres wide that run either side of streams, ditches and other watercourses, and areas of peatland covering other gently sloping parts of the catchment.

Research has also recently shown that sediment release from bare peat strongly influences peatland stream ecosystems11, 12 affecting both their biodiversity and functioning. This shows that we need to do all we can to disconnect sediment sources from the peatland streams. The most effective way to do so is to support revegetation of peatlands, especially near any watercourses. Thus, targeted restoration work that aims to achieve an end-point with a dense Sphagnum understorey will deliver maximum downstream benefits for river habitats and flood risk, while simultaneously adding resilience to the peatland ecosystems in the face of climate change, drought and wildfire.

This article by Prof. Joe Holden was originally published in the IUCN Peatland Programme newsletter July 2019

References

1.         Holden, J. & Burt, T. P. Runoff production in blanket peat covered catchments. Water Resources Research 39, 1191, doi:10.1029/2003WR002067 (2003).

2.         Acreman, M. & Holden, J. How wetlands affect floods. Wetlands 33, 773-786, doi: 10.1007/s13157-013-0473-2 (2013).

3.         Price, J. S. Blanket Bog in Newfoundland 2. Hydrological Processes. Journal of Hydrology 135, 103-119 (1992).

4.         Holden, J. et al. Factors affecting overland flow velocity in peatlands. Water Resources Research 44, W06415, doi: 10.1029/2007WR006052 (2008).

5.         Grayson, R., Holden, J. & Rose, R. Long-term change in storm hydrographs in response to peatland vegetation change. Journal of Hydrology 389, 336-343 (2010).

6.         Shuttleworth, E. L. et al. Restoration of blanket peat moorland delays stormflow from hillslopes and reduces peak discharge. Journal of Hydrology X 2, 100006 (2019).

7.         Gao, J., Holden, J. & Kirkby, M. J. A distributed TOPMODEL for modelling impacts of land-cover change on river flow in upland peatland catchments. Hydrological Processes 29, 2867-2879, doi: 10.1002/hyp.10408 (2015).

8.         Gao, J., Holden, J. & Kirkby, M. J. The impact of land-cover change on flood peaks in peatland basins. Water Resources Research 52, 3477-3492 (2016).

9.         Lane, S. N. & Milledge, D. G. Impacts of upland open drains upon runoff generation: a numerical assessment of catchment-scale impacts. Hydrological Processes 27, 1701-1726 (2012).

10.       Gao, J., Kirkby, M. & Holden, J. The effect of interactions between rainfall patterns and land-cover change on flood peaks in upland peatlands. Journal of Hydrology 567, 549-559 (2018).

11.       Aspray, K. L., Holden, J., Ledger, M. E., Mainstone, C. & Brown, L. E. Organic sediment pulses impact rivers across multiple levels of ecological organisation. Ecohydrology doi: 10.1002/eco.1855 (2017).

12.       Brown, L. E. et al. Sediment deposits from eroding peatlands alter headwater river invertebrate biodiversity. Global Change Biology 25, 602-619 (2019).

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