Lake Powell Water Losses
Attached is a table from the last "Water Year" taken from the Bureau of Reclamation Web Site.
It shows the data including inflows, releases, evaporation (net), and bank storage. The Glen Canyon Institute and other proponents of draining Lake Powell point to a large "hole" in the bathtub, with the water going who knows where.
During the years that the dam was filling there was a large amount of unaccounted water. The water seeped into the sandstone in the area around the lake. This is confirmed by well levels that were in place prior to the filling of the lake. The water table in these wells miles from the lake shore have seen an increase of several hundred feet.
The bank storage flow has become within the accuracy of the water flow measurements into and out of the lake.
The bank storage is calculated by taking the lake elevation accounting for the measured and estimated inflows and releases, subtracting the evaporation (which is calculated based on ambient conditions), and what is unaccounted for is the change in bank storage.
The official bank storage number the Bureau puts out for a Water Year is not based on this however. It is calculated by assuming that 8% of the change in lake "live" capacity is the change in bank storage. (The number used for Lake Mead is 6.5%).
You can see that the "provisional" numbers for the last water year show a net evaporation of 587,000 acre-ft. The bank storage numbers from the accounting (or the unaccounted for water) shows a decrease in bank storage for the Water Year of -120,000 acre-ft.
An important point to make is that the water flow measurements made by the U.S. Geological Service rate the measuring stations as "good" which is defined as +/- 10%. Discussions with the USGS people involved with these stations reveal that they feel that the stations are probably accurate to +/- 3%. Statistically the effect of this accuracy on all the measurements would be cumulative (more that 3%), but for the sake of this discussion 3% of the inflow is 480,000 acre-ft.
In conclusion, any statements about a "hole in the bathtub" are without any factual basis. The bank storage has stabilized, and fluctuates with the lake elevation.
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