What Is Evapotranspiration (ET)?

Evapotranspiration — usually shortened to ET — is the combination of two processes: evaporation, water leaving the soil surface directly, and transpiration, water leaving plants through their leaves. Together they are the main way an irrigated landscape loses water to the atmosphere. Every day a property either banks water as rain or spends it as ET, and the difference decides how much irrigation is actually needed.

To make ET comparable across places and seasons, scientists use a standardized reference evapotranspiration, called ET-zero or ET₀. It estimates the water use of a standardized reference surface — a well-watered, uniform grass reference crop — from weather alone. The worldwide standard method, published in the FAO's Irrigation and Drainage Paper 56, is the Penman-Monteith equation, which combines solar radiation, air temperature, humidity, and wind speed.

Reference ET describes the standard surface, not any particular plant. Crop or landscape coefficients scale it: a coefficient near 1.0 means the planting uses water at about the reference rate, while lower coefficients apply to slower, dormant, or shaded plantings. Combined with local weather, this produces a water-use estimate for a specific site and season.

The scheduling view is a running balance. Water in — rain and irrigation — minus water out — evapotranspiration — leaves a deficit or surplus in the root zone. When the accumulated deficit reaches a threshold appropriate to the soil and planting, it is time to irrigate. Watching the balance over days, rather than reacting to each hot afternoon, is what turns ET from a weather statistic into an operating tool.

Local conditions matter more than most people expect. Two properties can record the same maximum temperature on the same day, yet wind and humidity can push one day's ET substantially higher than the other's. That is also why ET pairs naturally with soil moisture measurements: the balance estimates what the landscape should have lost, and in-ground readings confirm what the root zone actually kept.