Rewetting refers to the process of restoring natural hydrological conditions in wetlands, peatlands, or other waterlogged ecosystems that have been drained or altered. This process can involve different measures that contribute to retain water within the area. 

The most important measures that can be implemented to rewet freshwater ecosystems are: 

Rewetting can be performed in various types of ecosystems that have been drained or hydrologically altered often for agricultural production. Rewetting involves a restoration of the near-natural or natural hydrology of wetlands, like swamps, bogs and fens, peatlands, that are areas with accumulated peat, riparian zones and floodplains.  

Why  

Different measures can be used to rewet an area (listed above). Rewetting addresses many different societal challenges, including climate change mitigation and adaptation, disaster risk and preparedness, water management and biodiversity enhancement but the efficiency will depend on the degree to which the natural processes are restored, the size of the area that is rewetted, the spatial location of the site relative to flood prone areas as well as other features of the site including the level of the terrain, soil characteristics.  

Rewetting can contribute to a reduction in climate gas emissions if applied in low-lying areas with intermediate to high contents of organic carbon in the soil by reducing CO₂ emissions from the area. This NbS can also enhance carbon dioxide sequestration from the atmosphere thereby restoring natural carbon sinks. 

Rewetting may also protect downstream areas from flooding in periods with high levels of precipitation, because rewetted areas can absorb and store excess water. The significance will depend on the size of the rewetted areas and terrain conditions.  

Rewetting can also stimulate denitrification in rewetted areas and help reduce the transport of nitrate to streams and downstream coastal areas. Overall, denitrification acts as a natural filter, removing excess nitrates from drainage water entering the area thereby reducing the risk of nutrient pollution of downstream ecosystems. Nitrate is reduced through denitrification, which is a natural process by which bacteria convert nitrates (NO_3-) and nitrites (NO_2-) into nitrogen gas (N₂), which is released into the atmosphere. This process only occurs under anaerobic conditions, meaning in the absence of oxygen, and water saturation is therefore a prerequisite for this process to occur. Organic matter should also be present in the soil to serve as an energy source for the denitrifying bacteria.  Rewetting can also reduce soil erosion, provided that it involves restoring features of a natural stream/river (raise stream/riverbed level and remeander the course of the river).  

In agricultural landscapes, rewetting drained areas can enhance water retention, which can be highly advantageous in periods of drought and contribute to a more sustainable agricultural practice. 

The most important measures that can be implemented to rewet freshwater are explained below. 

Ditch and drain blocking and filling: This measure can be implemented by either excavating the entire drainpipe or by cutting/crushing the pipes at suitable intervals to prevent water flow. When drainage pipes from systems outside the project area are encountered, it's essential to ensure that the drainage water percolates deeper than the root zone and that it doesn't trickle onto the terrain surface, although the latter has been the normal procedure. Existing ditches within the area should be covered along the entire stretch or at appropriate intervals to rewet the area. 

Disconnecting functional drainpipes: The disconnecting can be achieved by either excavating the entire drainpipe or by cutting/crushing the pipes at suitable intervals to prevent water flow. When drainage pipes from systems outside the project area are encountered, it's essential to ensure that the drainage water percolates deeper than the root zone and that it doesn't trickle onto the terrain surface, although the latter has been the normal procedure. 

Raise stream/riverbed levels: When raising the riverbed, considerations must be made regarding the relationship between the natural width and depth of the river to ensure that this NbS does not result in overly wide rivers with shallow depths. The material used to raise the riverbed level should consist of sand, gravel and stone in mixture to resemble the natural substrate composition of the river where the intervention is planned. 

Remeander the course of the stream/river: Remeandering of a channelized stream course should follow general guidelines in fluvial geomorphology. The distance between two riffles should be approximately 5-7 times the width of the undisturbed stream channel to promote natural stream dynamics, sediment transport, and habitat diversity. However, this is a general rule and the specific distance should be planned, depending on local conditions such as the width and slope of the river, sediment transport dynamics, and local geomorphological conditions.  

Reduced nitrogen pollution of aquatic ecosystems for better water management: Nitrate polluted water from disconnected drainage pipes can, provided that anaerobic conditions are generated, stimulate nitrogen removal by denitrification thereby reducing the transport of nitrogen to downstream river reaches, lakes and coastal areas. Factors such as temperature, pH, soil moisture, and substrate availability influence the rate and efficiency of denitrification in rewetted areas. Warmer temperatures generally promote higher rates of denitrification, while acidic conditions can inhibit the process.  

Reduced climate gas emissions: The reduction in CO₂ emissions will be highest in areas where organic soil contents are high (>6%) and where the water level in the river is as close as possible to the surface of the terrain over a large part of the project area. This creates oxygen-free conditions that slow down the decomposition of the organic matter in the soil thereby mediating the largest reduction in CO₂ emissions. 

Protect downstream areas from flooding: The discharge through the river is reduced, when the surrounding land is flooded. Consequently, water is retained that would otherwise be transported to downstream areas. This can be highly beneficial if downstream areas should be protected from flooding like urban or cultivated areas. The efficiency of raising the riverbed level for flood protection depends on the length of the river reach where the riverbed is raised, the discharge of the river and the characteristics of the surrounding land, since these parameters will all affect the amount of water that can be retained. The efficiency will be highest in low-laying project areas that are sufficiently large to retain large quantities of water.

Potential side effects 

Methane emission: There is a high risk of methane emissions in areas with standing water. Anaerobic conditions creates favourable conditions for the formation of methane gas through anaerobic decomposition and, as methane is a greenhouse gas, just like CO₂, methane emission may counteract the positive effect of less CO₂ emission. Therefore, it is very important to maintain a water level just below the surface to minimize this risk.  

Phosphorus mobilization: When former agricultural land with high contents of phosphorus is flooded there is a high risk of phosphorus mobilization from the soil that can enter the river and cause eutrophication of downstream river reaches, lakes and coastal areas. Therefore, mitigation measures to reduce this risk should be considering before the intervention. This could be harvesting to remove nutrients in the biomass, topsoil removal or other measures.  

Altered hydrology outside the project area: When the groundwater level is raised in a river reach there can be a risk of affecting water level in upstream reaches, drainage pipes and ditches that discharge into the river within the project area. Therefore, the project boundary should be defined so only low-lying areas are included in the project, while higher-lying areas are excluded. This will diminish the risk of negatively affecting drainage conditions outside the project area. 

ATTENTION 

To ensure biodiversity net gains within the project area it is important to be aware that high inputs of nitrate can be critical for many plant species and therefore that biodiversity net gain may not respond positively within the area if nitrate-rich drainage water percolates into the root zone of the plants. This will affect interspecific competition and favour species that compete effectively at high levels of nutrients. These species are not in general species that are associated with biodiversity net gains.  Instead, disconnected drainpipes should be placed below the root zone to ensure that the outflowing nutrient-rich water does not come into contact with the root zone, but instead into the layers below where denitrification can occur. 

Costs

Implementation (manpower, technology, costs of buying land etc.), operational costs, maintenance and monitoring costs. This text should be qualitative rather than quantitative.