Researcher Susan Steele-Dunne (39) received a Vidi grant last year for her work on mapping the water requirements of crops and other plants using radar measurements. The new technology is said to be able to aid us to use water more sparingly in the future as well as predict droughts better.

Steele-Dunne’s scientific career revolves around water. She became interested in hydrology whilst studying civil engineering in her native Ireland. During her PhD at MIT in the US she studied the quantity of water present in soils using radiometers and radar. "Knowledge concerning soil moisture is essential as the latter is the link between hydrology and atmospheric processes," she explains. "Water only spends a short amount of time in the soil. Either it sinks deeper into the ground or it returns to the atmosphere through evaporation. In other words, the quantity of moisture in a soil changes rapidly. Soil moisture is also the main source of water for vegetation."

Noise

Plants and trees get in the way of measuring soil moisture. "Vegetation influences the radar signal during the measuring soil moisture content as the signal is also sensitive to the water present in plants. Radar can’t make the distinction. Normally researchers try to filter out the noise caused by vegetation, however for my research in Delft I conversely wish to study it as doing so allows me to determine the moisture content of crops."

The study could also generate interesting results for scientists who study soil moisture. Steele-Dunne: "Satellites that use microwaves to measure soil moisture only observe the top few centimetres of the soil. These measurements therefore always have to be combined with computer models to be able to make statements about the rest of the soil. If we can determine how much water trees and other plants contain, then we can also possibly determine what is going on deeper in the ground."

Another drawback is that satellites only take a new measurement every few days. "The quantity of water in plants and soils changes rapidly however. This is why I wish to observe the interaction between soil and vegetation inside a day. For the field research, I want to use a radar system that takes a measurement every hour."

Stress

The research not only involves physics, knowledge of plants is also very important. "Physics and biology come together in this research which is what makes it so fun," says Steele-Dunne. "Ultimately, I want to know which radar frequencies are most suitable for measuring the water content of vegetation. This also requires knowledge on plant physiology." Steele-Dunne intends to invest the Vidi grant in two PhDs. "One will specifically study plants as an electromagnetic medium over the coming years. The second will look at this as a physiologist. In other words, the same problem will be examined from two very different perspectives."

Steele-Dunne is particularly interested in whether plants demonstrate recognisable warning signs if they are short of water. Plants maintain various strategies when soil moisture becomes scarce, she says. Some, like basil and mint, waste water without keeping any in reserve for drier times. "So if you forget to water your basil plant and the water runs out, it will immediately show signs of this as its leaves and stems will wilt. However, if the plant is watered again on time, it will recover. Most varieties of maize are, on the contrary, much more strategic and retain water, creating a reserve. They therefore function differently during as drought. I am curious whether radar signals can be used to measure when plants are stressed due to lack of water."

Applications

This is the first time that satellites and radar have been used to study moisture concentrations in plants so specifically thinks Steele-Dunne. "This technology will, in the future, have all manner of agricultural applications. Think, for instance, of water management. If it were possible to determine when the soil is drying out more accurately, you wouldn’t have to irrigate land as much. In areas where water is scarce, people prefer to only water plants when it is really necessary."

At a higher level, the technology could be used to predict droughts in areas such as the Amazon or the Sahel in Africa. Steele-Dunne: "Measurements of forests or savannahs could be used to this end which could then be combined with land based radar measurements. Perhaps the signs of drought could be determined or improved insight could be gained into how droughts develop over larger areas."

Research farm

It has proven pretty complex to study plants that lack water in the Netherlands. Steele-Dunne’s scientific team will therefore conduct its measurements in the US where they will collaborate with colleagues in Florida. "They have been studying the moisture content of plants there for some time, but only do so once a week. The scientists there have a research farm with a range of crops at their disposal. Florida has the best weather conditions for thoroughly studying a lack of moisture in plants. It’s dry and plants require a lot of watering there."

Basically it is good news that the Netherlands isn’t suitable for research into thirsty plants, concludes Steele-Dunne. "We tried to conduct the research here, but it just rains too much. As far as that is concerned, the Netherlands are blessed with abundant water. The question is whether the country will always be this lucky. If its luck runs out, people will have to make more intelligent choices."

Published: March 2016

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