How to cut harmful emissions from ditches and canals – new research
Ditches and canals are the underdog of the freshwater world. These human-made waterways are often forgotten, devalued and perceived negatively – think “dull as ditchwater”. But these unsung heroes have a hidden potential for climate change mitigation, if they’re managed correctly.
We know that ditches and canals have a large global extent, covering at least 5.3 million hectares — about 22% of the UK’s total land area. However, no one has yet mapped all global ditch and canal networks robustly, so it’s potentially more.
These waterways are also hotspots of greenhouse gas emissions, which contribute to climate change. We have previously calculated that ditches emit 333 teragrams of carbon dioxide equivalents (a common unit to express the climate impact of all greenhouse gases), which is nearly comparable to the UK’s total greenhouse gas emissions in 2023.
Ditches often contain stagnant waters and are commonly found running through farmland or cities, where they receive high amounts of nutrients from fertilisers, manure and stormwater run-off. This creates the low-oxygen, high-nutrient conditions that are ideal for the production of potent greenhouse gases methane and nitrous oxide – both of which warm the atmosphere considerably more than CO₂.
Read more: Ditches and canals are a big, yet overlooked, source of greenhouse gas emissions – new study
However, ditches and their surrounding landscape can be managed (by farmers and landowners, for example) in ways that reduce nutrient inputs and therefore lower their greenhouse gas emissions. This makes them an untapped solution for reducing the effects of climate change.
Many nature restoration solutions focus on storing atmospheric carbon – by planting trees or mangroves, for example. But there are also immediate wins to be made simply by reducing greenhouse gas emissions. The importance of methane reduction has now been recognised by more than 160 countries, all of which signed the global methane pledge to cut human-caused methane emissions by at least 30% from 2020 levels by the end of the decade.
Our new study outlines the steps needed to reduce emissions from global ditches and canals. First, we need to better understand these systems by mapping their global extent. We also need to collect more measurements of greenhouse gas emissions from underrepresented regions like South America and Africa. Emissions from irrigation ditches in these understudied places could be large.
We also need to improve our understanding of how the potent greenhouse gas methane escapes the sediments in bubbles. This involves using sensors that monitor methane concentrations continuously, in order to capture “hot moments” when weather or human activity (such as fertiliser use on farmland) cause sudden pulses of emissions.
All of these strategies will improve estimates of global greenhouse gas emissions from ditches. From that new baseline, any progress in reducing emissions can be more accurately measured.
New directions for ditches
There are several ways to reduce greenhouse gas emissions from ditches and canals. These include reducing fertiliser application rates on farmland, excluding livestock from areas beside ditches to reduce the amount of manure that ends up in waterways (which has already been shown to be effective for ponds), and managing pollution sources like wastewater treatment plants.
In the Netherlands, researchers have tested the effects of dredging agricultural ditches to remove the nutrient- and organic matter-rich sediments that release greenhouse gases.
They found that dredging resulted in a 35% decline in ditch emissions after one year. However, this method isn’t perfect, as the emissions from the removed sediments still need to be accounted for at a later stage, and dredging disturbs aquatic habitats and organisms.
Planting vegetation alongside ditches helps intercept nutrients and sediments before they reach the ditch. This vegetation also provides shading, which reduces water temperature and rates of greenhouse gas emissions. A study across Denmark, Great Britain and Sweden found that riverside vegetation helped to considerably reduce nutrient inputs to rivers and streams, and improved habitats for stream organisms like bugs and frogs.
Introducing floating vegetation can also trap methane and create the conditions for its removal before it is released into the atmosphere. Current trials in the UK are looking at introducing Sphagnum moss to peatland ditches. Once a floating mat of this moss has been established, it can trap bubbles of methane in an oxygen-rich environment created by the photosynthesising moss.
When methane and oxygen are present together, methane-eating bacteria can convert methane to carbon dioxide, which has a much lower impact on the climate. Initial results showed a decrease in methane of approximately 40% when Sphagnum was present.
Some of these techniques might be too expensive to scale, and many are still at the early stages of research into their use in ditches. Nevertheless, ditches and canals can in future be climate heroes – we just need to give them the chance by managing them in smart and sustainable ways.
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Teresa Silverthorn has received funding for ditch research from from Defra, the Environment Agency, and EPSRC (UK research councils).
Jonathan Ritson has receive funding from the GGR-Peat project (UKRI funding, BB/V011561/1).
Mike Peacock has received funding for ditch research from Defra, the Environment Agency, NERC and EPSRC (UK research councils), and Formas and VR (Swedish research councils).