Tar Heels first to check satellite data on small rivers

On a rapidly warming day in October 2023, Sophia Harvey waded into the muddy water of California’s Owens River to anchor a GPS receiver on a tripod. She and three Carolina classmates were gathering data on the river’s water level to compare with data from NASA’s new Surface Water and Ocean Topography satellite.

Since its launch 10 months earlier, the SWOT satellite has orbited Earth at an altitude of over 500 miles to measure water surfaces worldwide. It yielded data on oceans, rivers and streams in precise detail never seen before. NASA and its partners at France’s space agency designed the satellite to survey rivers and streams that are wider than 50 to 100 meters, showing details such as the slope of a stream’s flow and the height of the water in it.

Researchers were surprised by the satellite’s ability to register smaller rivers, but no one had compared SWOT data for the smaller rivers to on-the-ground measurements. The opportunity to do so came when students in a first-year seminar taught by Drew Coleman, professor in the College of Arts and Sciences’ Earth, marine and environmental sciences department, prepared for a research trip to California’s Owens Valley near the Nevada border.

Tamlin Pavelsky, a freshwater researcher and professor in the same department who leads the hydrology science team for NASA’s SWOT mission, knew that the satellite would pass over the valley when the class was there. Pavelsky asked Coleman if the students could perform some of the first ground-truthing of satellite data by measuring water levels along the Owens River, which ranges from 6 to 15 meters in width.

Coleman agreed. Pavelsky’s lab staff trained Harvey and classmates Talia Benjamin, Kate Merrin and Jackqueline (Jack) Wilson on setting up global navigation satellite equipment. On one day they visited three locations, setting up a receiver to measure water surface elevation for an hour. The device received signals from the same GPS satellite constellation that smartphones use, but with much higher accuracy. The students used these GPS signals to capture the precise height of the water surface above sea level with an accuracy of a few inches.

Then the students compared their findings to SWOT data. “For two of the ground-truthing locations, the average height of the water surface elevation data points SWOT picked up had a discrepancy of less than 1 meter compared to our data and a discrepancy of 3 meters at the other,” their report reads. “Additionally, SWOT could accurately approximate the river’s slope, yielding a 0.1 m/km discrepancy. While more data must be collected, these findings suggest SWOT may be capable of surveying narrower rivers than initially expected.”

“It’s hard to comment on precision and accuracy when we visited so few spots along the river,” said Harvey. “But SWOT may have the potential to tell us important things about discharge and water availability on narrow rivers that impact ecosystems or human use.”

Pavelsky shared the students’ findings with NASA, which acknowledged that the data suggested that SWOT can provide useful water surface elevation data on relatively narrow rivers. “To my knowledge, their work was the first time anyone had compared SWOT elevations on such a narrow river to on-the-ground measurements of water surface elevation,” he said.

Coleman’s class produced four SWOT-related projects. “All of the projects were ground-breaking, and this one stood out. It is truly UNC undergraduates contributing to international research. And they were first-year students, no less,” Coleman said.