Water reservoirs are shrinking and becoming less reliable

A recent study has revealed that major water reservoirs across the continental United States are facing longer, more severe, and increasingly variable periods of low storage compared to several decades ago. 

The most significant issues are occurring in the western and central regions, though reservoirs in the eastern and southeastern U.S. are also affected. According to the experts, these reservoirs are now less reliable and more vulnerable to the impacts of climate change than they were in the past.

Insights into water storage trends

The findings, which offer updated insights into water storage trends, are expected to enhance water forecasting capabilities. This improvement could help water managers at all levels – national, regional, and local – make better-informed decisions about the timing and volume of water release. 

As natural water storage sources like snowpack, rivers, and groundwater become less dependable, reservoirs are becoming increasingly critical. 

However, disruptions to their normal operations can lead to widespread water availability problems. For example, low storage levels in Lakes Mead and Powell from 2000 to 2021 – during the region’s driest 22-year period in 1,200 years – led to widespread water-use restrictions across the southwestern United States.

Storage capacity in water reservoirs 

Reservoir storage capacity is influenced by drought, water withdrawals, and sediment buildup behind dams. Each of these factors has been changing, often pushing reservoirs beyond the conditions for which they were originally designed.

“Reservoirs are a key component of the modern water cycle, and they’re a part that water managers can influence,” said Caelan Simeone, a hydrologist at the U.S. Geological Survey’s Oregon Water Science Center, who led the study. 

“We know that reservoirs are changing, and that reservoirs were designed for historical water conditions. So now there’s uncertainty about how or whether reservoirs will be able to adapt.”

Nationwide perspective on water reservoirs 

Much of the existing research on reservoirs is local or regional, limiting the broader understanding of how climate and human activities are affecting water storage on a national scale.

“Water reservoir managers could benefit from having that knowledge,” Simeone said. “It would enable them to consider larger, national trends in water as well as more local patterns.”

To gain a nationwide perspective, Simeone and colleagues analyzed water levels in 250 large reservoirs from 1981 to 2020, focusing on changes in baseline, maximum, and minimum water levels. 

The researchers also compared these water levels to management practices and climate, seeking patterns that could explain any observed changes. The study did not include data from the U.S. Northeast due to a lack of available information.

Reservoirs across various regions

Reservoirs in the more arid western and central United States experienced longer, more severe, and more variable periods of low storage. This was somewhat expected, as these reservoirs are designed to handle variable annual runoff and drought conditions. However, current drought conditions are pushing these reservoirs to their limits.

Surprisingly, the study found that reservoirs in wetter regions like the Southeast and Pacific Northwest, as well as arid areas, showed a decline in annual maximum storage levels. 

Out of the 250 reservoirs studied, 169 experienced declining maximum storage, with 89 showing significant drops. Across all reservoirs, the median decline in maximum water storage relative to the mean was 2.2%; for those with significant declines, the median decrease was 8.1%.

Declining maximum water levels

Simeone did not anticipate such widespread reductions. “The reduction of maximum annual storage was widespread, which really surprised us,” he said. 

“Many reservoirs just aren’t filling to the levels they once did. Overall, we’re getting this picture of declining maximum water levels across the United States. This was the case even in places that were not seeing more low storage periods.”

Increased sediment and changing hydroclimatic conditions are likely driving the observed variability in water storage and overall decreases in water levels.

Reservoir managers are attempting to adapt to these changing conditions, but this is challenging given that many reservoirs were designed decades ago with the assumption that climate and societal conditions would remain relatively stable. Most of the reservoirs and dams in the study were constructed between roughly 1930 and 1970.

“There was an assumption that conditions would be more or less stationary. Climate change interrupted that. Now, managers need to try to mitigate the hydrologic shifts we’re seeing,” Simeone concluded.

The study is published in the journal Geophysical Research Letters.

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