Compared to several decades ago, severe, prolonged and unpredictable periods of low water levels are more common in water reservoirs in the Americas, a new study shows.

The problem is most pronounced in the west and center of the country, but reservoirs in the east and southeast of the United States are also showing signs of being at risk.

Published in Geophysical Research LettersThe study provides insights into the current state of reservoirs in the United States, which are becoming increasingly important as natural water sources such as groundwater reserves, snowpack and rivers become increasingly unreliable.

“We know that reservoirs change and that they were designed for historical water conditions, so there is now uncertainty about how and if reservoirs can adapt,” Caelan Simeone, a hydrologist at the U.S. Geological Survey’s Oregon Water Science Center who led the study, said in a statement.

The findings are intended to help water managers at all levels make more informed decisions regarding water release and storage, and potentially improve water forecasting.

Reservoirs play a crucial role in mitigating the effects of droughts by regulating the flow of water to downstream areas.

However, operational disruptions can lead to widespread water availability problems, as demonstrated by the historically low water levels in Lakes Mead and Powell between 2000 and 2021.

This period was, according to a study in Nature Climate Changeled to extensive restrictions on water use throughout the southwestern United States.

The study examined water levels in 250 large reservoirs from 1981 to 2020. The aim of the research was to identify patterns in base, maximum and minimum water levels and to understand how these levels correlate with management practices and climate conditions.

The results showed that reservoirs in the arid western and central United States were more frequently affected by periods of severe and fluctuating water stress. This trend was partly expected due to the natural variations in annual discharge and drought conditions in these regions.

“Declining water levels in our major reservoirs in the West are causing water supply problems for cities and farmers,” said Robert Glennon, a water policy and law expert and professor emeritus at the University of Arizona who was not involved in the study. News week.

“Reservoirs have traditionally served as buffers to compensate for years with little rain and snow. But our reservoirs in the West are located in canyons that are V-shaped or martini glass-shaped. The deeper the surface level goes, the faster it falls.

“At some point the water levels will drop so low that the dams can no longer generate hydroelectric power. And at some point the reservoirs will reach the ‘dead point’, which means that no more water will be released from the dams.”

But it’s not just the western and central reservoirs that are at risk. The study also found significant declines in the maximum storage capacity of reservoirs in wetter regions such as the Southeast and Pacific Northwest. Of the 250 reservoirs studied, 169 showed a decline in maximum storage capacity, and 89 showed a significant decline.

“The reduction in the maximum annual storage capacity was far-reaching and really surprised us,” said Simeone.

“Many reservoirs are simply not filling up as they once did. Overall, we are seeing a decline in maximum water levels across the United States.”

The study suggests that increased sediment deposition and altered hydroclimatic conditions likely contribute to these observed declines and increased variability in water storage.

These findings pose a challenge for reservoir operators, who must now adapt to conditions that are significantly different from those expected when most of these reservoirs were built between 1930 and 1970.

“It was assumed that conditions would remain more or less unchanged,” Simeone noted. “Climate change has thwarted that. Now those responsible must try to mitigate the hydrological shifts we are seeing.”

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References

Simeone, CE, Hammond, JC, Archfield, SA, Broman, D., Condon, LE, Eldardiry, H., Olson, CG, & Steyaert, JC (2024). Decreasing reservoir reliability and increasing reservoir vulnerability: Long-term observations reveal longer and more severe periods of low reservoir levels in major U.S. reservoirs. Geophysical Research Letters, 51(16). https://doi.org/10.1029/2024GL109476

Williams, AP, Cook, BI, & Smerdon, JE (2022). Rapid intensification of the emerging megadrought in southwestern North America during 2020–2021. Nature Climate Change, 12(3), 232-234. https://doi.org/10.1038/s41558-022-01290-z