Millions of people are predicted to continue moving to the desert southwest, but the water available to serve them seems to be declining.

Water in the Colorado River Basin, which includes Colorado's West Slope, was divvied up among four Upper Basin and three Lower Basin states in 1922, based on supply figures that were above the long-term average. Now with climate change, supplies are below that long-term average.

This has been a repeated topic at the annual Water Seminar in Durango, and was again at this year's April 3 event, sponsored by the Southwest Water Conservation District.

The river basin has around 35 million people now and is projected to grow to 80 million, according to Eric Kuhn, general manager of the Colorado River Water Conservation District.

The river basin has had drought conditions since the 1980s. In the last two years, a pressure ridge has sent snow to the Northeast, he said. Some parts of Colorado have received decent snowpack during that time, but not Southwest Colorado, Utah, Arizona, or California.

"The closer you are to California, the worse you are for water supply," Kuhn said. "We think dry periods could last 50 years. Don't assume next year will be wet."

Lee's Ferry in the Grand Canyon below the Glen Canyon dam is considered the dividing line between the upper and lower basin states. The 1922 compact allocated 7.5 million acre feet of water per year to the Upper Basin states and 7.5 million to the Lower Basin states. This is an average over 10 years that has to be met.

Lake Powell is the water bank for the Upper Basin states to meet deliveries to Lake Mead for the Lower Basin states. Other reservoirs in Colorado, including Navajo, are a water bank for Lake Powell.

Kuhn said Lower Basin states "hope for big flows from the Upper Basin to fill Lake Powell, so they have to spill downstream. In the last 16 years, there was only one year of spill in 2011, so Lake Mead keeps going down. It's a structural deficit that's nothing new."

Since 2000, there's been a supply versus use deficit of around 2 million acre feet per year at Lake Mead, he said. Lake Powell releases are controlled by interim guidelines approved by the states in 2007.

"Lake Powell is our water bank. We're drawing it down, but as long as there's water there, we can meet our compact needs, Kuhn said, but he warned that this year is shaping up to be like the drought year of 2002.

The latest estimate was 3.5 million AF for inflow to Powell this year, versus an average of 7 million. The critical low water level in Powell is when hydro power can no longer be generated at the dam. That could happen in 2016 or 2017, Kuhn said.

Ted Kowalski from the Colorado Water Conservation Board added, "This has been the driest 15 years on record (in the basin) except for 2011. Drought contingency planning is important because if levels in Mead and Powell go below critical levels, there are threats to water supply for drinking, irrigation, power generation, and environmental resource preservation" such as salinity control. Power generation provides funding for programs like that.

One goal for contingency planning is "to provide additional security for extended drought so the Secretary of Interior doesn't dictate what will be done to protect power generation," Kowalski said, meaning the 2007 guidelines.

Normal water releases from Powell have continued despite many years of below average inflow, resulting in falling water level, he said.

"We see shortages every year. In the Lower Basin, they ask for water and it shows up. If we see continued drought, how do we deal with that up here? What happens at Powell is tied to what happens at Mead in the 2007 guidelines. Currently, Powell is 45 percent full, and Mead is 40 percent."

In an effort to keep Powell and/ or Mead from dropping to critical levels, contingency planning started last summer for the entire Colorado River Basin and within the Upper Basin.

The contingency plan includes expanded cloud seeding, including in the Navajo Reservoir area, since Navajo (along with Blue Mesa and Flaming Gorge reservoirs) could be tapped to keep Powell from dropping below its critical level, Kowalski said.

Demand management and water banking also are being explored.

"Are there ways to reduce consumptive uses, and can you see benefit to the streams and reservoirs?" Kowalski asked. This includes less irrigation and leaving test plots fallow, municipal conservation, interruptible supply agreements, and system efficiencies.

He cited the contingency planning agreement signed last July, including the Denver Water Board, to reduce consumptive uses. "We are close to an agreement on how we implement that and how it will be compensated. The devil will be in the details as we negotiate these contingency plans."

Attorney Greg Walch spoke for the Southern Nevada Water Authority and the Las Vegas Valley Water Authority. They are spending $1.4 billion to construct a third water intake from Mead as the water level drops below previous intakes. The level has dropped 130 feet, he said.

It has continued going down despite significant conservation efforts that have meant less water used even with population growth, Walch said.

He cited a river basin-wide contingency planning agreement. "It's a measure of progress that we can recognize there's a structural deficit," he said.