Fresh water divined in Easter Island seas using drones
Image credit: Thomas Griggs | Unsplash
Researchers at Binghamton University, State University of New York, have demonstrated the effectiveness of using drones to locate freshwater sources in the seas around remote Easter Island.
Although surrounded by the vast Pacific ocean, fresh water is a scarce commodity on Rapa Nui, as the island is known to its inhabitants. No rivers or streams cross its surface and there are only three small crater lakes, which regularly dry up during periodic droughts.
However, as first reported by European explorers in the late 1700s, the people of Easter Island appeared to drink directly from the sea. Today, animals - most famously horses - can be observed doing the same thing.
Due to a quirk of geology, rainwater immediately sinks down through the porous bedrock, where it feeds an underground aquifer. That freshwater emerges at spots on the coastline known as coastal seeps.
Robert DiNapoli, a postdoctoral research associate in environmental studies and anthropology at Binghamton University, explained the phenomenon: “At some of these locations on the shoreline, there is so much water coming out [from the seeps] that it’s basically fresh. It’s somewhat salty, but not unpalatably salty. It’s just not the best tasting water, basically.”
DiNapoli, along with Carl Lipo (professor of Anthropology and Environmental Studies); Timothy De Smet (director of the Geophysics and Remote Sensing Laboratory), and Terry Hunt (from the University of Arizona), demonstrated the effectiveness of using thermal-imaging drones to detect the location of these coastal seeps.
It is not possible to use satellite imagery to identify freshwater sources on Rapa Nui because the pixel sizes are too large to register the coastal seeps in any detail, DiNapoli said. A drone, on the other hand, flies at lower altitudes and can gather high-resolution images to reveal greater detail.
Thermal-imaging drones have previously been used in Hawai‘i to investigate coastal seeps there. However, Lipo and DiNapoli were unsure if this technology would also work in Rapa Nui, where the freshwater plumes were much smaller in scale. The results of their pilot project have proved encouraging.
Before thermal-imaging cameras, researchers literally needed to wade out into the water and measure the temperature and salinity by hand. “That’s really labour-intensive because you need to walk and then every 10 metres or so, you need to take a measurement,” DiNapoli said.
Not only is the drone system faster, but it provides a more substantial picture of the larger water feature, allowing researchers to see its characteristics as a whole. One downside observed was that the heat reflecting off metal roofs and a local bonfire skewed the temperature readings on one of the flights.
Certain objects have characteristics that confuse the thermal camera and thus their actual temperature isn’t recorded, DiNapoli explained. A metal roof in the sun, for example, tends to be quite hot, but the thermal cameras register it as cold because of the reflected radiation.
When the drone takes off, it begins calibrating and determining the area’s high and low temperatures. Anomalies such as fires throw off that calibration. DiNapoli said: “It still gets us usable images in the sense that you can see relative differences in temperature, but the absolute temperature it’s giving us is wildly off, so it says things are much colder or much hotter than they actually are”.
In their research, Lipo and DiNapoli explore the way that ancient communities used these seeps, on an island where droughts are common. In addition to collecting rainwater, the Rapa Nui people constructed basin-type wells called 'puna' that intercept the stream from the aquifer before it reaches the sea. They also built underwater dams in the ocean to prevent the seawater from mixing with the fresh water at the seep sites, although these structures have long since crumbled away.
“It again provides an interesting example of how the people there were responding to the constraints of the island,” DiNapoli said. “They were faced with a very difficult place to live and they came up with these interesting strategies for survival.”
As the first European visitors noted, the people also drank directly from the ocean at the seeps. The island’s current inhabitants still pump water from these spots in the sea to water crops and provide water for their livestock.
In their previous research, Lipo and DiNapoli noted that Easter Island’s famous archaeological features, such as the moai platforms, correlate closely to the location of freshwater resources. “This is where they’re doing their ceremonial activities, this is where they’re building their villages,” DiNapoli said.
Nor did topography prove a significant obstacle to the resilient islanders. Rapa Nui’s western side is rimmed by steep cliffs, which would have made it difficult for people to access the shoreline seeps. Archaeological evidence and analyses of human bones in this area, however, suggests that people were indeed making it down to the sea at these sites. In their research paper, DiNapoli and Lipo investigated an area called Te Peu where coastal seeps are located directly adjacent to an ancient village.
The study’s results are the jumping off point for another research project, funded by a National Geographic grant, to identify the location of coastal seeps throughout the island. The thermal-imaging drone will play a major role in collecting the data.
One of the questions they hope to answer is how coastal seeps respond to lengthy droughts. During their last research trip, the island was experiencing a multi-year dry spell, which had dried up two of the crater lakes and sunk the third to low levels.
“We identified these coastal seeps all around the island. That suggests that when the island experiences these drought events, the seeps are one of the last water sources to be affected by it,” DiNapoli said.
One potential explanation is that when the water goes underground after heavy rain, it likely stays in the aquifer for several years before discharging into the ocean. In their forthcoming field work, DiNapoli and Lipo hope to confirm this and determine exactly how long it takes rainwater to make its underground journey to the sea.
“We don’t know that for sure, but that’s one potential explanation for why these seeps are more resilient,” DiNapoli said.
The results of the pilot project, 'Thermal Imaging Shows Submarine Groundwater Discharge Plumes Associated with Ancient Settlements on Rapa Nui (Easter Island, Chile)', are published in the journal Remote Sensing.
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