Measuring soil water content – Times of Malta


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Soil water content (SWC) has a significant impact on several biophysical processes. It influences the germination of seeds, plant growth and nutrition, microbial decomposition of the soil organic matter and nutrient transformations in the root zone. Besides affecting heat and water transfer at the land-atmosphere interface, SWC also affects water infiltration, redistribution, pollutants, and transport of chemicals in plants.
This is why reliable quantification of SWC is necessary for different applications, ranging from large-scale calibration of global-scale climate models to field monitoring in agricultural and horticultural systems. These include optimisation of irrigation volumes and scheduling, fertilisation, and soil-water gains and losses.
Over decades, various methods were used to measure SWC at different scales. Measurements are conducted either directly or indirectly. Direct measurements refer to destructive techniques that alter the sample irrevocably by changing its water content and physical characteristics. Such methods require separation of water achieved by heating (water vaporisation), by water replacement with a solvent (water absorption) or by chemical reaction (water dissociation). Then the amount of water removed is determined by measuring the change in soil mass.
On the other hand, indirect methods measure some physical or chemical property of the soil that depends on its SWC. These include the complex permittivity and electrical conductivity, heat capacity, and hydrogen content. The complex permittivity characterises the interaction of electric fields with matter and gives a measure of how much energy is stored and/or dissipated in the material.
All these parameters depend on SWC and through the use of well-defined relationships the SWC, or the concentration of a substance can be determined. The indirect methods usually are minimally invasive given that the water content and physical characteristics of the soil is not altered by the measurement.
Recent studies have shown that the SWC can also be quantified directly in the field using multiple probes positioned at different locations across the field. The technology uses probes inserted in the soil to measure the electromagnetic fields (EM) and their response in the presence of other materials. This is based on the knowledge that the interaction between EM fields and materials in particular frequency ranges is highly dependent on the water content. Therefore, by designing the appropriate probes operating at the optimal frequency, we can easily determine the SWC.
This technology is currently being developed by a team of researchers at the Electromagnetics Research Group, Department of Physics, University of Malta, and obtained funding by the Energy and Water Agency, Malta.
Lourdes Farrugia is a researcher and senior lecturer and Iman Farhat is researcher and antenna designer at the University of Malta.
Sound Bites
•        The prospects of magnetic fusion research, proceeding worldwide, aim to simulate a ‘sun’ on the earth for a clean and safe long-term energy solution. Simulating fusion power in laboratories as occurring in the hot interior of the sun, requires plasma, a highly ionised form of high-temperature gas, to be heated to fusion reaction temperature. This reaction typically exceeds 80 million degrees Celsius. To contain such high-temperature plasmas and develop it in labs, several magnetic bottles have been conceived and tested. Nowadays, the main line magnetic bottle configurations are Tokamaks and Stellarators.
•        Magnetically confined plasma experimental research worldwide produces plasma bursts lasting just fractions of seconds. Hence, achieving and sustaining these fusion reactions for prolonged periods remains a major challenge.  Only by developing a steady and reliable way of producing fusion power will it become commercially viable energy.
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There are six types of soil: clay, sandy, silty, peaty, chalky and loamy.
•        Field capacity is the amount of water a soil continues to hold a few days after rain.
•        Clay holds soil water so strongly that about half of the water cannot be extracted by plants.
•        Advanced irrigation management systems are crucial to avoid over- or under-irrigation, leading to several negative environmental and economic impacts.
•        Using today’s irrigation methods, 2,000 more cubic kilometers of water will be needed per year in 2030 to keep everyone fed.
•        Most people may drink only two litres of water a day, but they consume about 3,000 if the water that goes into their food is taken into account.
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