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Accurate assessment of global river flows and stores is critical for informing water management practices, but current estimates of global river flows exhibit substantial spread and estimates of river stores remain sparse. Estimates of river flows and stores are hampered by uncertainties in land runoff, an unobserved quantity that provides water input to rivers. In their new research, geoscientists from NASA's Jet Propulsion Laboratory and elsewhere leverage global river flow observations and an ensemble of land surface models to generate a globally gauge-corrected monthly river flow and storage dataset. They estimate a global river storage mean of 2,246 km3 (that's equivalent to half of Lake Michigan's water and about 0.006% of all fresh water, which itself is 2.5% of the global volume) and a global continental flow of 37,411 km3 per year. Collins et al. estimated flow through 3 million river segments, identifying locations around the world marked by intense human water use, including parts of the Colorado, Amazon, Orange, and Murray-Darling river basins, shown as gray here. Image credit: NASA. Rivers are considered the most renewable, most accessible and, hence, most sustainable source of freshwater. Accordingly, several studies have sought to quantify the water in our world's rivers. Yet, surprisingly little is known about the average and temporal variability of global river water storage, as well as the temporal variability of global river flows. "Although researchers have made numerous estimates over the years of how much water flows from rivers into the ocean, estimates of the volume of water rivers collectively hold — known as storage — have been few and more uncertain," said Dr. Cédric David, a researcher at NASA's Jet Propulsion Laboratory. "We don't know how much water is in the account, and population growth and climate change are further complicating matters." "There are many things we can do to manage how we're using it and make sure there is enough water for everyone, but the first question is: How much water is there? That's fundamental to everything else." For the study, Dr. David and colleagues used a novel methodology that combines stream-gauge measurements with computer models of about 3 million river segments around the world. They identified the Amazon basin as the region with the most river storage, holding about 850 km3 of water — roughly 38% of the global estimate. The same basin also discharges the most water to the ocean: 6,789 km3 per year. That's 18% of the global discharge to the ocean, which averaged 37,411 km3 per year from 1980 to 2009. Although it's not possible for a river to have negative discharge — the study's approach doesn't allow for upstream flow — for the sake of accounting, it is possible for less water to come out of some river segments than went in. That's what the researchers found for parts of the Colorado, Amazon, and Orange river basins, as well as the Murray-Darling basin in southeastern Australia. These negative flows mostly indicate intense human water use. "These are locations where we're seeing fingerprints of water management," said Dr. Elyssa Collins, a researcher at the University of North Carolina at Chapel Hill. The study is published in the journal Nature Geoscience. _____ E.L. Collins et al. Global patterns in river water storage dependent on residence time. Nat. Geosci, published online March 15, 2024; doi: 10.1038/s41561-024-01421-5
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