Comments on: Again we ask — when will Lanier run out?

By: Atlanta Water Shortage » Georgia Drought » Blog Archive » Some predictions about when the water might run out

Mon, 03 Dec 2007 19:26:36 +0000

[...] posed the question before — when will the water run out? The answer is usually another question: who [...]

By: Atlanta Water Shortage » Georgia Drought » Blog Archive » Lanier should stay above 1035 feet until sometime next summer

Mon, 05 Nov 2007 17:09:20 +0000

[...] 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% [...]

By: Billy

Billy — Mon, 05 Nov 2007 13:34:31 +0000

^ Get yourself a college level text on hydrology or check out wikipedia to learn more.

By: mickey

mickey — Mon, 05 Nov 2007 01:56:11 +0000

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.

By: mickey

mickey — Mon, 05 Nov 2007 01:55:31 +0000

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).

By: rkolter

rkolter — Sun, 04 Nov 2007 22:25:37 +0000

Whoops. Yeah, 460 billion cubic feet would put Atlanta sitting pretty for quite a while yet. My bad. Thank you for explaining.

By: mickey

mickey — Sun, 04 Nov 2007 20:44:42 +0000

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.

By: rkolter

rkolter — Sun, 04 Nov 2007 20:32:44 +0000

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)?

By: John Shadows

John Shadows — Sun, 04 Nov 2007 14:56:23 +0000

Someone is watching. Good stuff - keep up the good work.