A Report on Evaporation and Groundwater Seepage

by Dave Wegner, Science Director

A recent study commissioned by Glen Canyon Institute, conducted by Dr. Tom Myers, has confirmed that the amount of water lost from evaporation and bank seepage from reservoir Powell is much larger than what would occur if the river flowed freely through Glen Canyon. Myers also discovered that the Bureau of Reclamation has underestimated gross evaporation from the reservoir by more than 5,000,000 acre feet (an acre foot of water would cover one acre of land one foot deep). Myers’ results were based upon analysis of data during the 34 year period between the 1963 completion of Glen Canyon Dam and 1997.

Prior to construction of Glen Canyon Dam, annual evaporative loss was approximately 102,000 acre feet. At this rate, 3,468,000 acre feet of water would have evaporated naturally had Glen Canyon Dam not been built. Actual evaporation from the reservoir has been 23,500,000 acre feet. The difference, approximately 20,032,000 acre feet of water, has evaporated as a direct result of Glen Canyon Dam. That’s 590,000 acre feet per year.

The reservoir filled for the first time in 1980, seventeen years after the gates of Glen Canyon Dam closed. The cumulative amount of water that had seeped into the surrounding Navajo sandstone and other rock layers peaked at 11,000,000 acre feet in 1983 (when the reservoir overfilled). Today the total amount varies between 10,000,000 and 10,500,000 acre feet. Limited data reduces the reliability of estimates of total groundwater seepage.

A groundwater model utilized in the research suggests that over half of the seepage loss will occur in the initial 37 years as the available groundwater storage spaces become filled and the gradient from the reservoir to the bank decreases. Groundwater equilibrium would not occur for about 1400 years, with approximately 21,600,000 acre feet of water lost. Due to limited data it is unclear how much of this lost water will drain back into the reservoir basin as the reservoir level is lowered.

Over the 34 years of data available for analysis, the cumulative loss from the reservoir has been approximately 30,000,000 acre feet, an average of 882,000 acre feet per year. This represents approximately 6.3% of the average annual flow, or 3.3% of the full reservoir.

Seepage and evaporation represent a substantial loss of water from the Colorado River system. As the demands on the river's flow increase, the loss due to evaporation and groundwater seepage will become more critical.

This initial study of the evaporation and groundwater seepage utilized the available data and models. The results indicate:

1 The annual loss of 6.3% is substantial and will become more important as more stress is placed upon the lower river system.

2 The present models and assumptions utilized by Reclamation are limited. A more detailed analysis and development of refined evaporation and seepage models are required.

3 The loss of water could be reduced, and water could be applied more efficiently downstream if allowed to flow freely through Glen Canyon.

Glen Canyon Institute is committed to evaluating the historic assumptions used to support the existence of reservoir Powell and apply sound scientific logic in the evaluation of the restoration of Glen Canyon.

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