The Bureau of Reclamation has tuned up its estimates of the chances of release of extra water to help Lake Mead, now putting them at one in four this year, according to an update published today (Wed. 2/10). The jargon here is “equalization,” which happens when there is enough extra water upstream in Lake Powell to release additional water beyond the nominal 8.23 million acre feet to, in effect, “equalize” the water levels in the two giant reservoirs. From today’s Lake Powell status report:
Given the current conditions of Lake Powell and Lake Mead, it is possible, if hydrologic conditions become wetter than what is currently projected, that an April adjustment to Equalization could occur. If this adjustment were to occur in April, the projected water year release from Glen Canyon Dam could be greater than 10.5 maf. As of early February, given the hydrologic conditions within the Colorado River Basin and the range of possible inflow scenarios that could occur in 2010, Reclamation estimates that there is about a 25% probability that an April adjustment to Equalization will occur.
Last Friday’s Colorado Basin River Forecast Center update on upper basin flows showed a 400,000 acre foot drop in the median runoff forecast, but the range of possible outcomes is still large. The situation is particularly grim in the Upper Green, with flows at Green River, Utah, with a median forecast of just 65 percent of normal. That’s based on Feb. 1 snowpack measurements. Since then, snowpack in the Upper Green has continued to lag, dropping farther behind.
Sometimes extra water for Lake Mead doesn’t equate to water that is actually entered into Lake Mead from an upstream source.
Look at it this way: Releases from Lake Mead is considered to be the normal supply for users down river. However, heavy rains in Arizona and New Mexico supply the Bill Williams (AZ only) and Gila Rivers (AZ & NM) that feed the Colorado River below Lake Mead.
Any inflows from these rivers can mean reduced releases from Hoover Dam (Mead’s outlet) to maintain the required flows to users down river.
Of the two rivers, the Bill Williams has the most impact. Although its capture area (Basin=4730 square miles) is small, any excess water is impounded by Lake Havasu. Havasu is the last reservoir on the Colorado (of any real storage capacity).
The Gila River (Basin=82,000 square mils) discharges a few miles North of Yuma. No storage of any excess flows…
In both cases of water entering the Colorado River from either the Bill Williams or Gila Rivers (above their normal flows) means that less water will be released from Mead to meet demands.
Of course, reduced demands from irrigators down river (due to local rainfall) contributes to lower releases from Hoover. It all adds up.