Calculate Gallons Of Rainwater Collection From A Tarp

July 9, 2015, by Ken Jorgustin

rainwater-runoff-from-a-tarp

UPDATED

When it rains, there is a stunning amount of water that falls from the sky, and it adds up very quickly for every square foot that you capture and collect from water runoff (as from a tarp).

For survival and preparedness, or for the purposes of simply collecting rainwater for various uses including drinking or irrigation, having a method of capturing the rain will provide you with a large quantity of free water.

Here’s how many gallons of rainwater you can collect based on some examples of various tarp sizes and the amount of rainfall:


 
One great prep item is an ordinary tarp (lots of uses). Even a small 5×7 foot tarp can collect a-lot of rain! For example, by using a few lengths of cordage, tie up the tarp corners to some tree branches (or use support poles -sticks), slope the tarp downwards, and form a ‘V’ such that any rainwater will channel to one runoff point (into your container). Even from just 1/10 of an inch rain, you could collect nearly 2 gallons of water with that small tarp!

 
Most rain events are less than 1 inch. I’ve calculated several scenarios to illustrate how much rainwater is available for collection based on several different tarp sizes (their square footage) versus the amount of rainfall.

Gallons Of Rainwater Collection From 1/10 Inch Rain

(2) 5×7 Tarp
(3) 6×8 Tarp
(7) 9×12 Tarp
(9) 10×14 Tarp
(12) 12×16 Tarp
(17) 14×20 Tarp

Gallons Of Rainwater Collection From 1/4 Inch Rain

(5) 5×7 Tarp
(7) 6×8 Tarp
(17) 9×12 Tarp
(22) 10×14 Tarp
(30) 12×16 Tarp
(44) 14×20 Tarp

Gallons Of Rainwater Collection From 1 Inch Rain

(22) 5×7 Tarp
(30) 6×8 Tarp
(67) 9×12 Tarp
(87) 10×14 Tarp
(120) 12×16 Tarp
(175) 14×20 Tarp

 
Let’s say you set up a rainwater collection system for the roof of your house (adapting to the gutters), and let’s say that the roof footprint is 28×40 feet. A 1-inch rain event will yield an incredible 700 gallons of water! That’s enough water to (minimally) support two people with drinking water for nearly a year!

The point being that it doesn’t take much to be able to collect lots of rainwater. By keeping an ordinary small tarp in your kit, you will not only have the ability to make a simple shelter, but it will provide the ability to capture valuable water if you need to.

 
TECHNICAL CALCULATIONS

14.44 cubic inches per cup (a fact)
1728 cubic inches in a cubic foot (12 x 12 x 12)
119.7 cups per cubic foot (1728 / 14.44)
7.5 gallons per cubic foot (119.7 / 16)

Example: 1 tenth inch of rain on a 10 by 10 foot surface

CONVERT SQUARE FOOTAGE: 10 x 10 = 100 square feet
CONVERT INCHES TO FEET OF RAIN: (1/10) / 12 = 0.0083 feet
CALCULATE CUBIC FEET OF WATER: 100 x 0.0083 = 0.83 cubic feet
CONVERT TO GALLONS: 0.83 x 7.5 = 6.2 gallons