What is ET

ET is short for evapotranspiration. Simply put, it is the amount of water that transpires through a plant (water used by a plant) and the water that evaporates from the soil surrounding the plant over a period of time.

ET changes as weather changes. It is affected by sunlight, temperature, wind and humidity. ET is an important indicator of how much water plants need for healthy growth.

As ET goes up because of warm temperatures or wind, your individualized outdoor water budget for irrigation goes up. As ET goes down due to rainfall or cooler temperatures, your outdoor water budget goes down.

The California legislature set maximum annual water allowances for landscape sites that were planted before January 1, 2010 to 80% of the local evapotranspiration rate. Landscape sites that were installed after January 1, 2010 are limited to 70% of the local ET rate. Special sites such as public recreational sports parks and historic landmarks receive 100% of the local ET.

Plants and ET

The plant from which ET is measured is tall fescue, a cool-season grass species.  Cool-season grasses have one of the highest water requirements of all landscape plants.  That is why it is used as the reference plant for calculating ET.

  • Trees, shrubs and groundcovers typically require 25-50% less water than cool-season turfgrass.
  • Citrus trees typically require 20% less water than cool-season turfgrass. Avocado trees are high water use trees and require similar amounts of water as cool-season turfgrass.
  • Vegetables and herbs typically require less water than turfgrass, but have higher water needs during establishment and fruiting.
  • California Friendly plants, succulents, and native plants require much less water than cool-season turfgrass

How Much Water Does Cool-season Turfgrass Need in a Year?

  • 1 square foot typically needs about 35 gallons per year
  • 1,000 square feet needs roughly 35,000 gallons per year
  • 10,000 square feet needs about 350,000 gallons per year