What would one inch of rain do for Lake Lanier?
It’s obvious that we need more rain to help fight this drought. The question is, how much would a little bit of rain help?
First, we need to use a decent number. The 3/100 of an inch we got last week didn’t do squat. Let’s just use one inch.
The watershed for Lake Lanier is around 1,040 square miles (the “watershed” is the land area that drains back into the lake).
First we need to figure out how much water we’re talking about if 1″ of rain fell. There are 27,878,400 square feet in a mile. Dividing that by 12 would tell us how many cubic feet of rain fall per square mile: 2,323,200. Converting that into gallons (2,323,200 * 7.48 gal/cubic ft) gives us 17,377,536 gallons of water per square miles. So, if we have 1,040 square miles of area, each receiving 17.38 million gallons, that’s a total of about 18 billion gallons that could potentially find its way into Lake Lanier.
From here it gets much tricker. How much of that rain will make it to Lanier? There’s no way to tell. All of it will make it other than what evaporates or is used by a human or animal. For the sake of argument, let’s say that 25% of the water makes it to the lake — that’s about 4.5 billion gallons.
Ok, so 4.5 billion gallons goes into the lake. What does that do for us? Well, we know that the lake loses just over a billion gallons a day, so we can say that 4.5 billion gallons would give us about 5 days worth of water.
If anyone is more knowledgeable about this than me, please leave revised numbers in the comments below. That aside, from the numbers we have right now I’d have to say that one inch of rain across the entire Lake Lanier watershed would add enough water for just under a week.
A couple heavy rainstorms could push the “xx days until we’re out of water” into the spring. The further we can push that number out, the more likely we’ll get some substantial rain at some point.
(update: The above numbers were revised slightly on Oct 26 after some helpful comments)
October 22nd, 2007 at 1:41 am
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October 22nd, 2007 at 1:19 pm
Unfortunately, I’m afraid that far less than 75% of a 1 inch rainfall would make it to Lake Lanier. In a natural, undeveloped watershed, the majority of rainfall is either evapotranspirated by trees and plants or soaked into the ground. Depending on the type of soils in the area, the amount of rainwater that would actually runoff into the lake would be around 4%. In a developed watershed, with impervious surfaces such as roads and houses (such as a city), the amount of runoff could increase to 30-50%. I am not familiar with the unique hydrology of Lake Lanier, but I would guess that some groundwater may also feed the lake. It could be weeks, months or even years, however, before this water reaches the lake after a 1 inch rainfall.
October 22nd, 2007 at 1:22 pm
Angie — thanks for that info. What would be a reasonable number to estimate for Lanier? It’s a rather developed area. 20%?
October 23rd, 2007 at 11:31 am
[...] The weather forecast is looking much better over the next few days. Accuweather is predicting 1.64 inches over the the next five days, which would certainly help. [...]
October 26th, 2007 at 7:53 pm
I have to take issue with your statement that: “One inch of rain will dump a total of 27,878,400 gallons per square mile.” The 27.9 million number is actually the number of square feet in a square mile. (5,280 squared). The number of cubic feet of water for one inch of rainfall over a square mile would be 27,878,400/12 or 2,323,200. Converting to gallons at 7.48 gal/cubic ft would result in 17,377,536 gal of water per square mile.
February 23rd, 2008 at 4:11 pm
[...] Lake Lanier, which serves Atlanta and north Georgia. Listen to these statistics from the website,Atlanta Water Shortage: The watershed for Lake Lanier is around 1,040 square miles (the “watershed” is the land area [...]
February 23rd, 2008 at 4:20 pm
[...] Lake Lanier, which serves Atlanta and north Georgia. Listen to these statistics from the website,Atlanta Water Shortage: The watershed for Lake Lanier is around 1,040 square miles (the “watershed” is the land area [...]