How much water?

One of the points of our Watershed Event was to find a way to visualize how much water goes down the strom drain when it rains.  So, how do you figure this out?  (If the arithmatic is confusing, see the simple conversoin table at the end.)

First we need to relate an area - like a square foot(12" by 12") - to a volume - like a gallon. 

To do that, think about this question:  How much water is there if you have a pan that is 1 square foot and there is one inch of rain?   The one inch of rain gives the third dimension that changes the area to a  volume (12' by 12" by 1").  We can call this 1/12 of 1 cubic foot or 144 cubic inches.

If we go to a conversion table or website like http://www.onlineconversion.com/volume.htm, we can look up the relanship of gallons and ounces.  (remember to choose US liquid if there are choices).  So, I cubic inch and ounce (US, Liquid)  and I get:
                          1 cubic inch = 0.554 112 554 1 ounce [US, liquid] 
                                           Let's round this to 0.554 oz.
With 1 inch of rain, there are 144 cubic inches of water per square foot.  That means that there are:
                         144 cubic inches = 144 * 0.554 ounces = 79.776 ounces
                                           Let's round this to 80 ounces

What does 80 ounces look like? 
                         1 pint of milk is 16 ounces - so 80 ounces is the same volume as 5 pints of milk
                                           If you try to drink 5 pints of milk in one sitting, you'll be awfully full.
                         most soda cans are twelve ounces - so 80 ounces is the same volume as 6 2/3 soda cans
                                           If you try to drink 6 2/3 cans of soda in one sitting, you'll burp a lot.
           
 Now let's relate this to a driveway.  At my house, the driveway is 10 feet wide and 40 feet long.
                         10 feet * 40 feet = 400 square feet

So when a 1" rain occurs, my driveway sheds how much water?
                         400 square feet * 144= 17,600 square inches
                         17,600 * 1" rain = 17,600 cubic inches of rain
                         17,600 cubic inches of rain * 80 ounces per 144 cubic inches of rain = 9777 cubic inches
   OR
                         17,600 cubic inches of rain  * 5 pints per 144cubic inches of rain = 611 pints
                                                 611 pints * 1 gallon per 8 pints = 76.4 gallons

So my small driveway would shed 76.4 gallons if it rained 1".  On average, my driveway sheds 305.6 gallons per month, since the avaerage rainfall here is 4" per month.  That means that in a year, my driveway has shed 3,667.2 gallons into the storm sewer.

My property is 40' by 100', so it is 4,000 square feet.  If all of the water is shed off my property when there is a 1" rain,  764 gallons go down the storm drain.  That is a lot of gallons.

Here is a table for you to use for conversion: