About freeET

Most watering is set once and left alone, so a landscape gets the same water in a cool, cloudy week as in a hot, windy one. The fix is to water back what the weather took, and that number, ET, has been hard to get: scattered across agency sites, behind logins, or quoted without saying where it came from. freeET gives anyone in the USA or Canada a plain, free ET number for their ZIP or postal code, with the weather and the sources behind it on the screen, so a contractor can trust it and show it to a customer.

ET, short for evapotranspiration, is how much water left the ground and the plants in one day: evaporation from the soil plus what the plants breathed out. It is a depth of water, like rain, and it is what a landscape needs put back.

freeET runs the ASCE Standardized Penman-Monteith equation, the standard irrigation science has used for 30 years, for every day and every place, from five weather readings: sun, high and low temperature, humidity and wind, plus the place's elevation and latitude. Those are the readings on every place screen. The code reproduces the FAO-56 handbook's own worked example to within one percent.

A day's ET exists only once the day is over. Today's number is computed from forecast weather and wears a FORECAST tag until midnight; yesterday's is computed from observed weather. The tag always tells you which you are looking at. “So far today” is this morning's hourly ET added up to the current hour.

Reference ET is for a short, well-watered lawn. Shrubs, trees and native plants need less, a sedum roof roughly half to two thirds, so the sprinkler step multiplies by a plant factor the way the EPA WaterSense water budget does. Agriculture usually quotes the tall (alfalfa) reference, which runs 1.1 to 1.4 times higher; it is the next number we add.