The Ring Cycle By Thair Peterson

hen water agencies throughout the West want to tell how much water they might get in the coming year, many measure snow pack in the Rockies or consult gauges placed along the Colorado River.

But what if they want the long view of water supplies?

In that case, scientists head out for dry and rocky land in Colorado, Wyoming and Utah. It is there they sample long cores drawn from centuries-old ponderosa pine, pinyon pine and Douglas fir. Polishing the cores to a fine finish, they look for tree rings —thick ones mean a wet year, while thin ones could mean a drought, one that lasted for decades.

The latest studies of tree rings confirm at least one thing—it has been a good idea for Metropolitan to diversify its sources of water supply.

The National Academies’ National Research Council recently warned that coping with water shortages on the Colorado would become increasingly difficult. This finding is based on tree-ring based reconstructions of the Colorado River’s flow over hundreds of years, coupled with regional warming trends and rapid population growth.
The report noted that much of the earlier understandings and determinations of Colorado River flow was based on gauging stations put in place beginning in the 1890s. It was the data from these stations that determined the legal division of use of Colorado River water among seven states. But as modern-day research has shown, the gauging station results taken more than a century ago came during unusually wet decades.

Tree ring data is painting a different picture of river flows.

The recent findings were “not all that surprising,” said Roger Patterson, Metropolitan’s Assistant General Manager/Strategic Water Initiatives. “In general, they confirmed what we know.”

New research is coming at a time when the Colorado River Basin is experiencing one of its longest recorded dry spells—seven below-normal runoff years out of eight. California state officials are examining potential long-range impacts to water supplies from global warming, California’s Sierra mountain snow pack is at its lowest levels in nearly two decades and Southern California is recording one of its driest years on record.

Tree-ring studies have been around since the 1970s. It can take years to analyze tree rings. A tree ring can show how much water a tree has absorbed, but calculating how much came from the Colorado River has to be extrapolated, based on mathematical formulas.

Tree rings also won’t show when there has been more moisture than trees can absorb, said Jan Matusak, a principal engineer in Metropolitan’s Water Resource Management group and who consulted on the NRC study. That’s why earlier tree ring studies may have underestimated the amount of runoff the Colorado generates in a very wet year.

The science is still evolving, and the scenarios can vary, depending on tree-ring data, and the methods that are used to reconstruct runoff, Matusak said. As a case in point, the most recent tree-ring analysis included in the NRC report actually indicates a greater average flow of water than what was concluded in an earlier tree-ring research project.
“Tree ring data may not be as bleak as previously thought,” he said.

Nevertheless, both sets of analyses indicate Colorado River flows are indeed lower on average than what was assumed more than 80 years ago.

The most recent study covered 62 trees, mostly in Colorado, but also in southwestern Wyoming, and northeastern Utah that can provide details for a period from 1600 to 1997. One major previous study had covered the period up to 1962.

Earlier wake-up calls
Long before the NRC issued its report, Metropolitan had received wakeup calls that demonstrated the need for diversification—notably the shortages and water rate pressures triggered by the drought of the late ‘80s and early ‘90s.

That spurred a diversification strategy under which Metropolitan and its member agencies built Diamond Valley Lake, adopted its Integrated Water Resources Plan, increased local storage, intensified investments in conservation and water recycling, and entered into water storage and transfer agreements with agricultural partners around the state.

For the Colorado River Basin, the real first eye-opener wasn’t the NRC report—it was the drought that hit the Colorado from 2000 to 2004. The extended dry period had exceeded the worst-case scenario reported in the Severe Sustained Drought Study of the mid-1990s.

(It should be noted that even with low Colorado River water deliveries during the last two years of the recent drought, Metropolitan—because of its recent diversification of supply and increased storage—managed to meet all delivery demands in Southern California.)

The NRC report now notes that warmer future temperatures could further reduce future Colorado River supplies and that future droughts may exceed the severity of what occurred during the early 2000s.

Those developments—along with the ongoing uncertainties regarding the impact of warmer temperatures on Sierra snow pack, sea levels and the like—is why Metropolitan supports further research into the effects of climate change, Matusak said.

The developments further strengthen the resolve of users of Colorado River water to jointly work on plans that prepare for worst-case scenarios.

In February 2006, the seven states that draw water from the Colorado or its tributaries delivered to the federal Bureau of Reclamation a proposed plan on how to best allocate supplies during drier years.

At the end of February 2007, the Bureau released a draft Environmental Impact Statement that analyzes four plans, including the seven-state plan as well as another proposal submitted by a coalition of environmental groups. The bureau took into account recent conditions, which include the river’s two main reservoirs being at only 60 percent of capacity. The Bureau predicted it would be unlikely that there would be a shortage greater than 600,000 acre-feet in any one year through 2026 with implementation of the seven-state plan. At the time the Colorado River Compact was negotiated, the natural annual flow of the river at Lee Ferry, Ariz., was estimated to be 18.1 million acre-feet based on the 1903-1920 period. Reclamation estimates the natural flow of the river in its draft EIS to be 15 million acre-feet for the period 1906 to 2004. Natural flow represents the flow that would exist without upstream storage or withdrawal of water.

Cooperative approaches
Patterson said he welcomed the NRC report’s suggestion that the Lower Basin states (California, Arizona and Nevada) conduct joint studies on urban water management practices. Such practices can include conservation, water recycling, minimizing water imports, groundwater programs and transfer/exchange opportunities. Such studies would focus on achieving long-term supply reliability, while dealing with supply interruptions, severe one-year droughts and multiple-dry year scenarios.

One thing is certain—there will be more studies. Whether it’s old tree rings, recent droughts or the newest water flow models, the Colorado Basin state agencies that quench the thirst of millions will continue to examine how much water the Colorado can deliver, and adjust their plans accordingly.

“We’ve learned a lot over the past several years, and we’ve been confronting a lot of issues that have been bedeviling us for decades,” Patterson said. “Water planning never ends, but we’re much closer to knowing what needs to be done.”