Kite Science: Assessing Water Pollution from the Sky
(This article is derived from a European Researchers' Night 2021 lecture. Theoretical parts and fundamental research by Maša Mramor, Gimnazija Vič; kite flying, kite aerial photography, and photo processing by KAP Jasa kite society.)
As we have said many times before, kites are spectacular, affordable and versatile aerial science platforms. A large Rokkaku can easily lift a couple of kilograms of scientific equipment, it can fly for hours, and flying kites is less restricted than flying other aerial devices. We have employed kites for science before, mostly for archaeological research - discovering a Roman villa rustica, surveying medieval field partitions and the ridge-and-burrow remains at Castle Kalc while searching for an Iron Age settlement, researching the prehistoric landscape near Divača, finding a "Roman" road that doesn't lead to Rome., and exploring a real Roman road - using our two infrared cameras, a modified Pentax WG-10 that was gifted to us by the inimitable Dr Wells, and an AgroCam Geo NDVI.
EUROPEAN RESEARCHERS' NIGHT 2021
We were invited to participate in the largest scientific event of the year, the European Researchers' Night 2021 in Ljubljana (we did ERN twice before, in Banja Luka). This year's ERN theme in Slovenia was water, and while we have a ton of very pretty kite aerial photos of pretty lakes, dreamy rivers, mysterious swamps and the like, we felt showing pretty pictures is not sciency enough - we've got to do some real stuff.
We were thinking - can we use our near-infrared cameras in the sky and the power of the NDVI index to see how much (and why) a body of water is polluted?
THE NDVI INDEX
The NDVI index is a powerful tool used mainly to assess the health of plants. Healthy plants absorb most of blue and red parts of the sunlight (well, the chlorophyll in their chloroplasts does), and reflect most of green and near-infrared. By calculating the ratio of near-infrared (that is mostly reflected) and red light (that is mostly absorbed) we can see how well a plant is doing. In healthy plants the NDVI index approaches +1, stressed or diseased plants reflect less NIR light so the NDVI value drops towards zero, and dead plants reflect almost none of the incoming NIR light, so the index drops down to -1.
By taking a multispectral photo of say a field of crops one can make a nice NDVI map out of it, calculating the index for every pixel and representing the values with nice colours; red for +1, then yellow and green and cyan and deep blue down to -1, thus revealing places where the soil is poor, or where some pest started causing mayhem among the plants, or possibly the buried remains of a Roman villa.
The differences in plants' NIR reflectance are extremely subtle - can't see them with your eyes; the NDVI enhances them and makes them visible. For example, you can't see anything in these two photos of a wheat field near Kašelj - NIR on the left, multispectral on the right. But make a NDVI map from them and ... look!
The buried remains of a Roman villa rustica are clearly visible.
NDVI INDEX AND WATER POLLUTION
Photosynthesising plants living in water are mostly algae and cyanobacteria, and they too do this thing reflecting near-infrared light according to their health, enabling us to do the NDVI map of a body of water. A particular NDVI value can show how well the algae are doing, and in turn can show what kind of pollutants mess with their well-being.
Heavy metals - cadmium, mercury, lead, chromium and the like - wreak havoc upon them, so if a river is getting polluted by them the NDVI values will decrease, and keep decreasing downstream. On the other hand, nitrogen compounds and phosphorus are pollutants as well, but the algae love them, so they thrive if the influx of nitrogen compounds increases - this is the cause of algae and cyanobacterial blooms in lakes and the sea. In this case, the NDVI values increase when these pollutants are present in the water. The third type of water pollution that can be detected with a NDVI map is mechanical, usually associated with illegal removing of river or lake bed, for sand and pebbles used in construction. The disturbed, muddy water reflects less near-infrared light and can also be made visible with the NDVI index.
TESTING: RIVER LJUBLJANICA ON LJUBLJANA MARSHES LANDSCAPE PARK
Armed with a kite and a near-infrared camera we headed to Ljubljana Marshes landscape park to test a hypothesis. We were looking at the River Ljubljanica and chose two points to take NIR kite aerial photos of - one near the village of Bevke, just before the confluence with Borovniščica stream, and the other downstream of the town of Podpeč.
The hypothesis was simple: Borovniščica runs through an agricultural land and by a couple of cattle ranches, while in Podpeč and the villages near it there are mostly individual houses with no connection to a sewer system - therefore the influx of nitrogen (fertilizers and cattle ranches are notorious nitrogen polluters, and even the best septic tanks do leak) into Ljubljanica should be substantial, and as the algae love additional nitrogen they should thrive more at point 2 (downriver of Podpeč) than at point 1 (near Bevke). The NDVI index value at point 1 should therefore be lower than at point 2.
Multispectral image of Ljubljanica near Podpeč; AgroCam Goe NDVI on The Spark Rokkaku kite
We lifted the camera at point 1, got the multispectral photo, cut a part of it showing the water surface, averaged the colour values so things like floating detritus and shadows wouldn't mess with calculations, and computed the NDVI index.
Note: the NDVI index is an index. It doesn't show absolute values of pollutants in the water. It could - if the camera could be totally calibrated and all the photos could be taken at exactly the same condition (time of day, cloudiness, wind disturbing the water surface etc.). The NDVI index shows relative differences in pollution of two or more spots, nothing more.
We repeated the proces at point 2 - fly the camera, take the picture, cut, average, do the NDVI - and we got this:
See the difference? The NDVI value is higher downstream, where Ljubljanica got the stuff from cattle ranches and villages. But is the higher NDVI value at Podpeč really due to algae enjoying themselves in all the excess nitrogen? Are the nitrogen compounds from fertilizers and cattle and humans responsible for this?
Luckily the House of Experiments of Ljubjana, a co-organiser of the European Researchers' Night 2021, gave us a "water research package" with a couple of water pollution measuring strips, so we could do a control test for our kite flying NDVI experiment. These strips have some little active squares on them that are sensitive to different pollutants and water properties, and change colour accordingly (similar to the pH measuring litmus test).