Rainfall Calculator

First: Some Questions

How much rain falls on your property? In gallons per storm? In gallons per year? 
How much rain falls on your roof during a rainstorm? 
On your driveway? 
Are those gallons being absorbed into the ground or are they running straight into the pond carrying with it the cyanobacteria encouraging nutrient phosphorus? 
How much of that water are you managing today? Can you do more to reduce the amount of rain going into the pond from your property? 

These are some of the questions being answered by the LakeSmart team when you invite them in to provide a LakeSmart evaluation of your property. The team will offer suggestions on how all that runoff could be managed more effectively. And often with some very inexpensive solutions. 

Amount of Rain Falling on Your Property

Lets take a look at some of the volumes we are talking about here. Did you know that in New Hampshire we get an average of about 40 inches of rain per year? If you own a single acre of property, that 40 inches of rain means your property receives over 1 million gallons of rain per year. That's right. On average 1,086,171 gallons of water falls on just 1 single acre of land in New Hampshire every year. So if you have 1/2 acre, that's still over 500,000 gallons of water per year. Does it drain into the pond carrying pollutants with it or does it stay on your property and get absorbed into the soil where it's then naturally filtered before it enters the pond through springs? 

Some facts that go into the calculations:
  • 1 square foot = 144 square inches (12 x 12)
  • 1 acre = 43,560 square feet or 6,272,640 square inches (43,560 x 144) 
  • 1 inch of rain falling on 1 square foot = 144 cubic inches (12 x 12 x 1) 
  • 40 inches rain on 1 acre (6,272,640 square inches) of land = 250,905,600 cubic inches of water (6,272,640 x 40)
  • 1 gallon of water = 231 cubic inches if water
  • 250,905,600 cubic inches of water divided by 231 cubic inches per gallon yields 1,086,171.43 gallons (250,905,600 / 231) 
Therefore: 1 acre gets over 1,000,000 gallons of water per year. 

Calculations Done for You

A couple of charts and an online calculator:
  • Gallons of Water per Acre per Year, showing some calculations on the number of gallons per year that fall on some common property sizes. 

  • Gallons of Water per Acre per Inch of Rain, showing how much rain falls on some common property sizes during a 1 inch rainfall. 

  • USGS Online Calculator Want to do some calculations on your own? Maybe you want to calculate how much rain falls on your roof so you get a sense of how much water you could be managing by installing a Dripline Infiltration Trench next to your house or downspouts feeding either a Dry Well or a Rain Barrel. All you need to do is measure your building size, width and length, and enter those numbers in the calculator provided by the folks at the U.S. Geological Survey. You can even enter varying inches of rain, say 1 inch or even the yearly average in New Hampshire of 40 inches.

    Or using that same calculator you can enter the number of acres you own and it will calculate the gallons falling on your entire property.