## How much water? Inches of rain vs. gallons

Some quick calculations to relate rainfall to a watering schedule. This is pretty simple for a square-foot garden, the area of a square is one square foot. This makes some of the math really simple, too. These numbers all apply to one square foot. Just multiply by the number of square feet to figure how much water to put down, or ask me if you have trouble.

General formulae & notation
1 gal = 1 gallon (volume)
1 oz = 1 liquid ounce (volume)
1 ft = 1 linear foot (distance)
1 ft^2 = 1 square foot (area)
1 ft^3 = 1 cubic foot (volume)
One cubic foot is 7.48 gallons
One gallon is 1/7.48 = 0.134 ft^3
One gallon is 128 ounces
One foot is 12 inches

Rainfall equivalent to one gallon
One gallon x (1 cu ft / 7.48 gal) = .134 ft^3
.134 ft^3 / 1 ft^2 = .134 ft
.134 ft x (12 in / 1 ft) = 1.6 in
so one gallon of water applied to one square foot is equivalent to 1.6 inches of rain

Gallon equivalent to one inch of rain
1-in x (1-ft / 12-in) x 1 ft^2 = 1/12 ft^3 of water
1/12 ft^3 x (7.48-gal / ft^3) = 7.48/12 = .62 gal
.62 gal x (128-oz / 1 gal) = 80 oz
so one inch of rain over one square foot is equivalent to 5/8 gallon or 80 ounces

A free tomato to anyone who helps me correct my math.

## Drip Irrigation

I put in a drip irrigation system to water all the plants. For the most part I used Dig parts, but I mixed and matched with brands from three stores. Other than the barbed fittings from Lowes that don’t fit any hose (including the Lowes hose) everything works well.

I ran somewhere around 500 feet of 1/2-inch black polypro line in two runs, one to each side of the house. I put their recommended 40psi pressure regulator on at the faucet, but it started leaking at the threads. I put a gage on the faucet and the static pressure there is 40psi so I just took out the regulator. (I have an adjustable regulator on my main line coming into the house. I set it to 50psi when I installed it three or four years ago.)

Most plants get a 1gph (gallon per hour) dripper. The honeysuckle and trumpet vines get 2gph, and the trees get 2gph. I’m planning to water for 1 hour every 2-3 days and adjust from there.

I need to calculate the system load and see if I should run a second supply line to feed the shrubs. Working from memory I think the long line has 6x2gph + 27x1gph drips for a total of 39 gallons per hour. Eventually I’d like to measure the output at each drip and see what my output actually is.