While cities and farms across the Colorado River basin are likely to conserve much more water over the coming decades, there’s no “silver bullet” solution for the river’s major water problems, according to a new federal report.

A great deal of progress has already been made across the basin in making urban and agricultural water use more efficient in recent decades, said the report, released Tuesday and produced by working groups organized by the U.S. Bureau of Reclamation.

But population growth and improved farm productivity may eat up a significant amount of those savings, meaning that they won’t result in less diversion of water from the over-tapped river, the report said.

Also, widely varying conditions across the seven-state basin mean that “efforts that are effective and relevant in one location may not be as effective or acceptable in another,” the report said.

The Colorado River is the main source of drinking water for Tucson.

The report is the product of a two-year study commissioned by the bureau to try to stave off an expected shortfall of about 3.2 million acre-feet of water on the river by 2060. Three study groups representing a wide range of interests produced separate sections on urban conservation, agricultural conservation and environmental and recreational uses of river water.

The lower Colorado basin states of Arizona, California and Nevada face a more immediate task — to eliminate a current 1.2 million acre-foot shortfall between their water supply and use by the three states and Mexico. If that isn’t done, the three states could face major water cutbacks sometime after 2020 if Lake Mead drops below 1,000 feet in elevation.

Specifically, the new report said:

  • Two million acre-feet of water has been saved in the past two decades by cities including those in Arizona through water conservation and effluent reuse such as what’s occurred on a large scale here in Tucson. Per capita water use has dropped by a range of 10 to 26 percent just since 2000 in the cities of the seven-state river basin.
  • Agricultural water use has been relatively constant over the past two decades. The growing efficiency of farming operations has increased their productivity significantly. That means they can grow the same amount of crops with less water or more crops with the same amount of water. Across the basin, agricultural productivity has risen about 25 percent since 1980.
  • Cities and other urban users plan to conserve more than 700,000 acre-feet of additional water and to reuse 400,000 more acre-feet of water by 2030.
  • Water conservation and reuse may not reduce diversions from the river because the efforts will be needed to offset population growth and offset or delay the need for additional water supplies.
  • Similarly, increased water efficiency in farming will likely continue the trend of increased productivity as opposed to simply reducing use. While water managers will continue to expand conservation programs in response to changing river conditions, future water use efficiency measures will become increasingly more expensive and difficult.
  • Environmental, recreational and hydropower resources across the basin will be increasingly vulnerable. The river and its tributaries support many native species, including several threatened and endangered species. The nine National Park Service units on and near the river received 20 million visits in 2012 and 2013, and visitors spent $1.2 billion and $1.5 billion, respectively.

Contact reporter Tony Davis at tdavis@tucson.com or 806-7746. On Twitter: tonydavis987