Too many 'straws' sucking water out of the Colorado River

The team notes that activities such as building dams and expanding irrigation make it hard to use these data for estimating the effects of global warming on river flow. Dam building is perhaps the least of these influences on long-term trends because water still gets released during the course of a year – a factor that tends to get smoothed out when looking at longer-term flow trends.

In the end, climatic events such as the shift from El Ninos to La Ninas and back exert far more influence on runoff than do direct human activities, because of the changes they bring to regional rain and snowfall patterns, the team explains.

Still, the team notes, rivers with reduced flows snake through regions of the globe that have experienced the most pronounced drying over the past several decades. These drying patterns are consistent with projections from global climate models. Meanwhile, regions seeing increased flow generally fall into areas where climate models have projected rising rain and snowfall with global warming.

Rising populations in these areas, combined with the projected effects of global warming on key river systems, are likely to increase the pressure on already scarce water resources in several low- and mid-latitude regions, the team concludes.

How it might play along the Colorado

Along the Colorado, if climate change trims runoff by 10 percent, the bureau will be unable to fulfill delivery commitments 58 percent of the time by 2050, the Scripps scientists estimate. If runoff  falls by 20 percent, the bureau will be unable to meet commitments 88 percent of the time.

Ordinarily, even those shortfalls might be offset with water-conservation measures if a drought isn't already under way, the researchers suggest. But if those shortfalls coincide with drought, it will be harder to deal with them.

"All water-use planning is based on the idea that the next 100 years will be like the last 100," notes Dr. Barnett. But studies of the ups and downs of flows along the Colorado River over hundreds of years indicate that the 20th century was an unusually wet one. So even without climate change, current allocations aren't sustainable, the team writes.

The team fired a significant shot across the bows of water managers last year with a study showing that if no changes are made to water allocations along the Colorado, there is a 50 percent chance that Lake Mead, behind Hoover Dam, would be functionally dry by 2021 (if the Bureau of Reclamation didn't intervene). You can find a PDF of that study here.

They acknowledged at the time that water managers would be expected to do everything they could to prevent that from happening. For their current study, they assume that the Bureau of Reclamation will run Hoover Dam in such a way that the lake level never falls below a level that leaves Las Vegas's water intake pipe sucking air.

As Monitor colleagues Dan Wood and Gloria Goodale write, conflicts between farmers and southern California's cities over water allocations are building – and lapping at the steps of the state Capitol. You can read more about that here.

The issue also touches on efforts to preserve ecologically important areas that rely on the river. You can read more about one of these areas here.  And it touches on a often underappreciated aspect of water use in energy production. You can read more about that here and here.

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