Preserving water with particle physics

A project supported by the STFC Food Network+ (SFN) is using nuclear and particle physics to develop better methods to help conserve freshwater supplies.

Climate change, the growing global population and pollution are just some of the challenges putting pressure on natural resources. A particular concern is that fresh water could become scarce, leading to unrest and food insecurity. Agriculture consumes over 70% of the world’s freshwater supplies (FAO), mostly for irrigation, yet much of this is wasted due to excess application. Accurate and timely monitoring of soil moisture levels could help farmers to optimise their irrigation practices, preserving this precious resource as much as possible.

But current methods to do this have limitations, as Dr Patrick Stowell (University of Durham) explains. “Typically soil moisture sensors are point probes that are placed in the ground. These can give very biased measurements as they only give a reading from a single point. Readings can vary considerably across a field, particularly if some parts are more prone to flooding.”

 At the other end of the scale is remote-sensing data collected by satellites. These typically give an average over a large area, usually at the kilometre level. A technique is needed to bridge the gap between these two extremes. “Our aim is to produce a soil moisture detector in the middle ground between what is currently available, one that measures at field scale” says Patrick.

To achieve this, Patrick is part of an SFN-funded project that is adapting a technology originally developed for the nuclear industry for a new use in agriculture. Although the goal is to develop a simple product that farmers can immediately use without training, the process involves some heavy particle physics theory. The detectors sense cosmic ray neutrons: fast, high-energy particles produced when incoming cosmic rays interact with elements in the earth’s atmosphere. The neutrons travel downwards and penetrate the soil where most will be scattered back upwards unchanged. But some collide with hydrogen atoms (mostly from water molecules) and lose energy, before being absorbed in the soil.

“The number of neutrons just above ground level is inversely correlated with the level of soil moisture” Patrick says. “Therefore, a soil moisture detector that measures these particles doesn’t even need to be placed in the ground. Instead, it can be mounted on a pole in a field and give continuous, real-time measurements.”

The Centre for Hydrology and Ecology (CEH) have already established a nation-wide network of neutron-based soil moisture detectors in the UK (COSMOS-UK). The major problem is that these neutron soil moisture probes tend to use chemicals that are either highly toxic (Boron-Trifluoride) or very expensive (Helium-3). Patrick is currently being supported by the Royal Commission for the Exhibition of 1851 on a research fellowship which aims to use STFC capabilities to reduce the overall cost of these detectors.

The SFN scoping project support was used to assemble two prototype neutron detector systems in partnership with the team’s STFC-based university spin-out company, Geoptic Infrastructure Investigations Limited. “Having these industrial links from the outset was great, as it provided us an opportunity to robustly test the detector’s design in an industrial setting” Patrick says.

The alternative detector developed by Patrick and his colleagues, Lee Thompson (University of Sheffield), and Chris Steer (Geoptic Infrastructure Investigations Limited) contains a lithium-based scintillator: a material that produces light when struck by a neutron. “The emitted light is picked up by what is effectively an extremely sensitive single-pixel ‘camera’ called a photomultiplier. This magnifies the signal into a strong electrical pulse we can detect” says Patrick. Because the low-energy neutrons are scattered in the air, the detector can measure variations in soil moisture content over a radius of 200 metres, ideal for field-scale applications.

Through the National Physical Laboratory (NPL)’s Measurement for Recovery program, the team were able to test the response of one of the neutron detector systems at the NPL neutron facility. This was followed by a rigorous round of field tests which checked the sensitivity to temperature and humidity, besides the prototype’s ability to withstand the extremes of the British weather.

The group are now working with the CEH and Newcastle University to compare the sensitivity of their prototype in the field against an existing Boron-Trifluoride station based at Cockle Park, Newcastle. If successful, the team intend to test whether low-energy neutrons can distinguish between areas with poor or good drainage. “This could inform flood management strategies and provide quantifiable data on how less destructive farming methods improve the soil” says Patrick. “Many farmers are interested in using more environmentally-friendly practices, such as no-till farming, but need some demonstration of the benefits to persuade them.”

High-energy neutron-based detectors would be equally applicable in drought management, and could be especially useful for producers of high-value, water-sensitive crops, such as salad crops and wine grapes. “In wine production, keeping the vines at just the point of water stress makes the grapes sweeter – it can be detrimental if the ground is saturated” Patrick says.

Patrick also hopes to investigate the possibility of combining neutron-based soil detectors with machine learning software that could automatically adjust irrigation patterns.

“This has been a real learning exercise, working in a very different environment to what I’m used to in particle and nuclear physics, with a focus on making a product tailored to end-user needs” Patrick says.

With water so visibly fundamental to life on earth, it seems oddly fitting that a particle we cannot even see could help us to make the most of every drop.

You can learn more about how high-energy neutrons can be used to measure soil moisture levels through watching this short video produced by the FAO.