Again we ask — when will Lanier run out?
We’re going to dig a bit deeper into the numbers we posted a few days ago. I’ll give the starting numbers so that those of you mathematically inclined and try your hand at it and maybe improve what we post.
The depth of the lake right now is about 137.35 feet (currently at 1055.35 ft AMSL, and will be completely dry at 919 ft AMSL). Buford dam will no longer function at 116 feet, so we’ve got about 21.35 feet to go.
The volume of water in the lake right now is about 1,413,000 acre feet, which is about 460 billion gallons, or about 61 billion cubic feet.
As we discussed the other day, the lake is losing about 800 million gallons per day. This does not account for evaporation or any rainfall. We’ll assume that with nominal rainfall, it will cancel out. If you would like to try your hand at calculating the daily evaporation, be my guest. 
Hopefully starting on November 16, the lake will only lose 650 million gallons/day.
As of November 15, after losing 800 million gallons/day, the lake will be at a height of around 136.4 feet. The next day starts the loss of only 650 million gallons per day. 268 days later (August 9, 2008), we go below the dam and we’re in trouble.
Had they not decided to lower the flow and it stayed at 800 million gallons/day, we would have gone below the dam about two months earlier — June 8, 2008.
Again, those numbers assume a bit of rain to offset the evaporation. If we don’t get any rain (or don’t get enough to balance out the evaporation), those numbers would change. If you can handle the math to factor in evaporation, go for it. Post your results below or e-mail them to me and we’ll sort it out.
November 4th, 2007 at 2:56 pm
Someone is watching. Good stuff - keep up the good work.
November 4th, 2007 at 8:32 pm
So for simplicity let’s call it 136 feet on 11/15, and 650 million cf/day on 11/15. That’s 20 feet to the point where the dam stops operating.
How much of the 460 billion cubic feet of total volume, does the top 20 feet represent? You say 268 days which would be 174 billion cubic feet, or about 38% How accurate is that (or where did that number come from)?
November 4th, 2007 at 8:44 pm
First, you transposed a few numbers — it’s 460 billion GALLONS or 61 billion cubic feet.
My number came from building a cone model in Excel. If the height is 136.4 feet and the volume is 60,275,712,471 cubic feet, the radius would be 20,542 ft. I then computed the current slope of the cone from those numbers (height divided by radius = roughly 0.00664) and made sure the slope remained constant as the lake dropped.
When I removed the 650M gallons of water for each day, I was able to compute the new radius based on the new total volume (old volume minus 650M gallons) and the slope constant (0.00664). Once I had that new radius, I could determine the new height. It seemed to work pretty well.
I really need to factor in evaporation, but I’ve not figured that out. There must be a “typical winter evaporation rate per acre” or something, but I can’t find anything yet. I’ll keep looking.
November 4th, 2007 at 10:25 pm
Whoops. Yeah, 460 billion cubic feet would put Atlanta sitting pretty for quite a while yet. My bad. Thank you for explaining.
November 5th, 2007 at 1:55 am
A detailed e-mail I received suggests that Lanier loses about 25 million gallons/day due to evaporation. Obviously, this number can vary wildly based on sunlight, temperature, humidity, wind speed, etc, but they felt 25M/day was a good average.
Based on that, it costs us about 10 days, putting the magic date at July 31. If they don’t lower the release rate and keep it at a loss of 800M gallons/day, the new date becomes June 1, 2008 (instead of June 8, 2008).
November 5th, 2007 at 1:56 am
I should also mention that the rate of evaporation will slow down as the lake gets lower (less surface area and less total volume), and that hasn’t been factored in.
November 5th, 2007 at 1:34 pm
^ Get yourself a college level text on hydrology or check out wikipedia to learn more.
November 5th, 2007 at 5:09 pm
[...] few days ago we once again tried to compute how many days worth of water were left in Lake Lanier, based on the new information that the flow through Buford Dam would be cut by 16% [...]
December 3rd, 2007 at 2:26 pm
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