It is quite possible to use radar technology from space to measure how 'thirsty' trees and crops are. This is important to better understand the connection between ecosystems, water cycle and carbon cycle. In theory, this would allow us to monitor crops on a global scale and map when it is necessary to intervene. Tim van Emmerik recently obtained his PhD on this subject at TU Delft.
Thirsty
Van Emmerik researched precipitation, evaporation and ways to measure plant thirst. The main focus was on tropical rainforests. After all, these play an essential role in the Earth's water, oxygen and carbon cycles. 'If the Amazon rainforest gets drier and drier,' explains van Emmerik in a press release, 'or keeps getting smaller due to deforestation, that will affect the water cycle and the carbon cycle. That could contribute to global climate change.'
Snakes
A practical stumbling block he encountered in the research was that measuring instruments are expensive or can damage trees. Also, they cannot withstand the extreme conditions of a rainforest. 'As soon as you hang boxes of equipment in a jungle, snakes, spiders and other animals start living in them. Aerial measurement campaigns with satellites or radar are a good measurement option. But you only really know whether your measurements are correct if you can also check on the ground what exactly is happening.'
Sensors
UItually, Van Emmerik ended up with sensors, which have become much cheaper and more robust with the rapid rise of the smartphone industry. With colleague Rolf Hut, the researcher figured out that accelerometers could be used as an affordable alternative to measure tree movements for longer periods of time. By linking a tree's accelerations to other meteorological and hydrological data, he could relate tree movements to external loads (such as wind, evaporation and rainfall) and distinguish them from the physical properties of the tree itself (such as elasticity, mass and water content).
Tomatoes
Van Emmerik has studied all this step by step in tomato plants, in maize fields and in Brazil. To investigate the effect of water stress in tropical rainforests, accelerometers were installed on 19 trees in the Brazilian Amazon to measure their movement. There, he took measurements for four years. The way trees move depends on several properties. Van Emmerik shows that movement is sensitive to tree mass, the amount of water remaining on the canopy and the degree of interaction between the tree and the atmosphere. The data also showed a clear difference between the wet and dry seasons.
Sensitive
The assumption is that this change is the result of changes in tree mass, caused by changes in water content, or the shedding of leaves in response to increasing water shortage. 'I used field data of water deficits in trees combined with radar observations to show that radar reflectance is highly sensitive to increasing water stress. During the transition from the wet to the dry season, a clear decrease was found in radar reflectance, which can be explained by the rapid increase in measured water deficit in trees.'
Observable differences
'The sensitivity of radar reflectivity to water stress in vegetation has been debated for years. Unfortunately, so far there has always been a lack of observations to test this. I have shown that water stress in vegetation leads to significant changes in water content and conductance properties of plants, which then leads to observable differences in radar reflectance.'
By: Kelly Bakker
Source: TU Delft
