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Area-wide Measurements of Soil Water Improve Management of Scarce Water Resources in Agriculture

Training course participants learning about soil moisture monitoring in a field in Grabenegg, Austria, in collaboration with the Institute for Land and Water Management Research, Federal Agency of Water Management, Petzenkirchen, Austria.

Global climate change and population growth are increasing pressure on agricultural systems and water resources. Agricultural communities around the world must increasingly maximise yields to feed an expanding population and minimise the use of scarce freshwater resources. To achieve these goals, assessing available soil moisture is crucial.  

The cosmic ray neutron sensor (CRNS) is a device that provides a novel way of quantifying the water content in soils over large areas without the disruptive effect of traditional measurement systems. This information is vital to determine irrigation needs and hence to help manage valuable water resources in agriculture.

The CRNS is available both as a stationary sensor and as a mobile sensor, in the form of a backpack or a vehicle attachment, both of which have been tested by the IAEA in its Soil and Water Management & Crop Nutrition Laboratory (SWMCNL) for the past four years. The sensor counts neutrons, small subatomic particles, near the soil surface that indicate the amount of water present in the soil of the surveyed area, which can be about 20 hectares. The results are immediately available and provide essential information for the sustainable use of agricultural water: “Looking at different scenarios supports decision making, for instance which crops to plant to better manage scarce water resources”, explains Ms Ameerah Hanoon Atiyah from the Ministry of Science and Technology in Baghdad, Iraq. In July, she participated in a two-week training course at the SWMCNL in Seibersdorf, Austria, part of the Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture. The training focused on measuring the water content of soils with the CRNS and translating the obtained results into management decisions, as well as making use of the AquaCrop simulation model for agricultural water management developed by the FAO.

“I could see the wheels in the participants’ minds spinning on how the CRNS could be applied in the local settings of their home countries,” says Mr Trenton Franz, hydrogeophysicist at the University of Nebraska-Lincoln, USA, who lectured at the training course. Training course participant Mr Imad Eldin Ahmed Ali Babiker from the Ministry of Agriculture and Forestry, Agricultural Research Corporation in Khartoum, Sudan, says: “My home country is affected by climate change and drought. The training has opened a new window for us to manage soil water content.” 

Traditional methods of monitoring the water content of soils at field-scale use in situ measurements that only capture information in a few centimetres surrounding the probe. To obtain information via traditional point-based methods on a large and naturally heterogeneous area would be time and labour intensive, and require a network of measurements. “Traditional methods involve taking several soil samples, drying them in an oven for 48 hours and measuring the weight difference between the original and the dried samples,” explains Franz. Another traditional device is the time-domain reflectometer. “Especially in areas where the soil is hard it can be difficult to stick this device into the ground,” he says, giving insight into some of the everyday challenges the CRNS helps soil scientists avoid.

A total of 20 participants took part in the training course. It was supported by the IAEA’s technical cooperation regional project on ‘Enhancing Crop Nutrition and Soil and Water Management and Technology Transfer in Irrigated Systems for Increased Food Production and Income Generation’. Recently, some Member States have already obtained CRNS devices through the IAEA’s Technical Cooperation mechanism.

I could see the wheels in the participants’ minds spinning on how the CRNS could be applied in the local settings of their home countries.
Trenton Franz, hydro-geophysicist at the University of Nebraska-Lincoln, USA

The Science

CRNS relies on the detection of neutrons in the soil and close to the surface. These neutrons are produced when incoming high-energy cosmic rays (mainly protons) that originate outside the solar system collide with atomic nuclei (mainly nitrogen and oxygen) in the upper atmosphere. These collisions shatter the nucleus of the atmospheric atom, breaking it into protons, neutrons and other sub-atomic particles that cascade through the atmosphere where subsequent collisions continue. By the time they reach the Earth’s surface, they have become fast-moving neutrons. More so than any other element, hydrogen present in the environment absorbs the energy of these fast neutrons on collision, thereby slowing them to a lower energy state. Because fast neutrons mainly interact with environmental hydrogen, and the majority of hydrogen in a terrestrial environment is present in the form of water in the soil, a relationship can be established between the number of fast neutrons and the amount of water in the soil of any given area. Drier soils contain a higher number of free neutrons because they have smaller amounts of soil moisture (i.e. hydrogen), causing higher counts. Wetter soils display lower counts because they have greater amounts of hydrogen. A calibration process included in the CRNS methodology helps to remove the signal of environmental hydrogen that is not contained in the soil water.

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