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Robert Brakenridge is a Senior Research Associate at the Institute of Arctic and Alpine Research (INSTAAR).

After the Mekong River descends from the highlands in China, it flows into Cambodia, Laos, Thailand and Vietnam providing freshwater for farming, fishing, hydropower and transportation. 52 million people rely on the lower Mekong River Basin for their livelihoods, according to the��.��

In the past decade, the Lower Mekong’s flows have dwindled, putting pressure on downstream communities. Until recently, a lack of information on the river’s hydrology has hindered efforts to understand these changes.

from an international group of researchers, including��INSTAAR scientist�� Robert Brakenridge, takes aim at this knowledge gap. They present a suite of tools capable of precisely monitoring river flows.��

The author's techniques rely on satellite-mounted sensors that map “passive microwave radiation” across the Earth’s surface.��

“You’re listening to the radiation actually emitted by the Earth,” Brakenridge said. “It turns out that water bodies are much lower emitters than land bodies… we use that difference as a way to track water changes.”

The researchers also pulled data from an extensive network of physical river monitoring stations in the Lower Mekong to double-check the satellite observations and calibrate their model. The alignment was excellent. The satellite-powered data proved accurate even during periods of drought and flood — river stages that often prove challenging for remote sensing.

“These sensors have very precise dynamic range,” Brakenridge said. “Basically you can use each pixel as a gauging station.”

Trouble on the Mekong

The new investigation is more than just a proof of concept. It also uncovered a nuanced understanding of how the Lower Mekong Basin is changing in the 21st century.��

In 2015 and 2016, the Lower Mekong Basin experienced a severe drought.��. According to the new investigation, river flows have yet to recover from this historic drought.

The Lower Mekong’s slow recovery after drought may be due, in part, to an increase in damming upstream.��, a Chinese electricity company, completed two large dams on the upper Mekong in 2010 and 2012 respectively. In addition, two new water diversion projects are planned for the near future in Cambodia and Thailand.

The analysis reveals that the new Chinese dams likely decreased annual flows on the Lower Mekong overall. As the river struggles to recover from the 2015-2016 drought, the diversion projects in the lower basin could further exacerbate the issue.

While new construction proliferates on the Mekong, climate change is causing precipitation to become more erratic in the region. As such, it is hard to parse out which factor — climate change or human infrastructure — is the driving force behind recent water shortages.

Brakenridge is careful to hedge on this point. There are conflicting results in the literature, and more research is needed to resolve the issue.

“It becomes very political very fast,” he said. “Our conclusions are pretty conservative. They mainly describe the observations we made.”

A new wavelength

A map of the Lower Mekong Basin shows "satellite gauging reaches" — focus sites of Brakenridge and his collaborator's recent investigation. (Courtesy)

While the new investigation stops short of naming a leading culprit of the Mekong’s dwindling water supply, it does suggest a way forward. The results show the power of passive microwave radiation to monitor the river into the past, present, and future.

Brakenridge has honed these techniques for decades. In the early 2000s, he was one of the first to demonstrate the ability of “Ka band” radiation for river monitoring. In recent years, he has joined a new generation of researchers leveraging “L band” radiation data from NASA’s Soil Moisture Active Passive mission, colloquially known as SMAP, for the same purpose.

“With the L band microwave you see through the vegetation much better,” he said. “So if you’re looking at a floodplain with a forest canopy, it doesn’t matter so much.”

The new investigation employs both types of radiation.

“The two seem to counterbalance each other,” he explained.

Though L band data is only available for the past decade, Ka records extend to the late 1990s and other passive microwave records extend back to the late 1970s. By combining all three, the scientists compiled a record of river flows spanning nearly half a century, adding weight to the trends they observed.��

Planned NASA missions will also extend L band data into the future — at least to the 2040s. Brakenridge hopes that one day water managers may use this data to monitor the river in near real time. But for this to happen, NASA or another entity would need to build a system to translate and transmit data to end-users on a daily basis.��

“All that needs to happen now is a daily refresh of the data that’s already being gathered,” Brakenridge said. “It may sound easy, but it’s not yet clear who might have the capacity to take on such a role. It could be an in-country agency, or an international organization such as the Mekong River Commission.”

For now, data from the new investigation is already being put to good use. Several years ago, Brakenridge attended a meeting of the Mekong River Commission in Laos and shared preliminary data from the project. Now that it’s completed, the researchers have shared their results with the commission.

“The short term goal is to improve their technical capabilities,” Brakenridge said. “Long-term, they may be able to avoid bad outcomes and be more prosperous and water-resilient overall.”