Theory

Main formula for watering: source

(Eto x PF x SF x 0.62 ) / IE  =  Gallons of Water per day

where:

Eto: Get this from http://www.rainmaster.com/historicET.aspx
PF: This is the plant factor. Different plants need different amounts of water. Use a value of 1.0 for lawn. For water loving shrubs use .80, for average water use shrubs use 0.5, for low water use shrubs use 0.3.
SF: This is the area to be irrigated in square feet. So for a 30 foot x 50 foot lawn you would use 1500.
0.62: A constant value used for conversion.
IE: Irrigation efficiency. Some irrigation water never gets used by the plant, this value compensates for that. I suggest using 0.75 as the value for this. Very well designed sprinkler systems with little run-off that using efficent sprinklers can have efficiencies of 80% (use 0.80). Drip irrigation systems typically have efficiencies of 90% (use 0.90).

Know How Much Water You Use

Installation

npm install watering

Usage

Make sure you change the values in config/development.json first.

watering
.calculate()
.then(function(data){
	// watering volume and time 
})
.catch(function(err){
	// handle errors
})