A novel study involving researchers in India and the United Kingdom has succeeded in using satellite images to detect water hyacinth, an invasive aquatic weed, in Vembanad Lake in Kuttanad.
The results of the study, which used the Synthetic Aperture Radar (SAR) data provided by the European Space Agency satellite Sentinel-1 for monitoring the weed, show significantly greater positive detection ratings compared to more traditional detectors.
The researchers, who have published their study in the journal Remote Sensing, say they have shown for the first time that SAR could be used to detect water hyacinth in the lake with a high level of accuracy. They also prepared a heatmap, a first of its kind, showing the water hyacinth presence/coverage over a two-year time frame, which could be used to aid the weed management practices within the area.
"Considering its speed of propagation on the water surface, constant monitoring is needed to aid local bodies, governments and policymakers involved in remedial measures. The synoptic coverage provided by satellite imaging and other remote-sensing practices make it convenient to find a solution using this type of data," reads the study.
The team collected satellite images of an area near the Thanneermukkam Bund in Vembanad Lake for a period between January 2019 and April 2020 and studied the spread of water hyacinth.
SAR is a type of active data collection where a satellite sensor produces radiation (energy rays) and then records the amount of that energy reflected back after interacting with objects on the Earth. Unlike the normal optical imagery, in SAR, the signals are responsive to surface characteristics such as the structure and moisture of the objects on which the radiation falls. Water hyacinth increases the surface roughness of the lake and therefore introduces a difference in backscattering in the image.
"A clean water surface without water hyacinth causes the electromagnetic wave to bounce back in a particular direction, making those areas of the lake appear dark. However, the presence of a mat of water hyacinth causes the signal to scatter in different directions," says G. Nagendra Prabhu, principal investigator, Centre for Research on Aquatic Resources (CRAR), Sanatana Dharma (SD) College, Alappuzha, who is a collaborator for the project.
Mr. Prabhu says that at present, there was no data on the scale of water hyacinth infestation in India to allow local self-government bodies and policymakers to make informed decisions on its control or management. "Since SAR microwaves penetrate most weather conditions, the technique can produce images of the study area and monitor changes in the growth of hyacinth throughout the seasons," he says.
The study was carried out as part of a project titled 'Multimodal data analysis for monitoring invasive aquatic weeds in India'. Savitri Maharaj of the University of Stirling, Scotland, is the principal investigator of the project.
Other team members include researchers from the International Crops Research Institute for the Semi-Arid Tropics, Hyderabad, National Institute of Plant Health Management, Hyderabad, Central Scientific Instruments Organisation, Chandigarh, SD College, Alappuzha and University of Strathclyde, Scotland.
