Evapotranspiration Explained
Knowing how much water a crop actually needs is one of the fundamentals of good irrigation management, and evapotranspiration, usually shortened to ET, is one of the key measurements used to work that out. ET describes the combined process of water evaporating from the soil surface and being released by plants through transpiration, together representing the total water lost from the soil-plant system to the atmosphere. Measuring it lets farmers estimate how much water a crop is actually using and how much irrigation is needed to keep growing conditions optimal.
Key takeaways
ET combines two separate processes: evaporation from the soil and transpiration through the plant.
A standard weather station provides everything needed to estimate ET on a given day.
ET data paired with soil moisture monitoring gives a highly accurate read on crop water demand.
Understanding ET moves irrigation from reactive to genuinely data-driven.
The two processes behind evapotranspiration
Evaporation happens when water from soil, irrigation or rainfall turns to vapour and enters the atmosphere, driven mainly by temperature, solar radiation, wind speed and humidity, with higher temperatures and stronger wind generally speeding it up. Transpiration is the plant's side of the equation: as roots draw up water, some goes toward growth and the rest is released as vapour through tiny openings in the leaves called stomata. Together, evaporation and transpiration determine the total water loss from a crop system, which is exactly what ET measures.
How weather stations measure it
A weather station provides the environmental readings ET calculations depend on: air temperature, humidity, wind speed and solar radiation. Feeding these into an ET model gives a daily estimate of how much water a crop is losing, which in turn tells farmers how much needs to be replaced through irrigation.
Using ET to schedule irrigation
ET data can guide irrigation scheduling directly by estimating daily crop water use. If a crop loses 5 millimetres of water to evapotranspiration in a day, for instance, irrigation can be adjusted to replace roughly that amount, keeping soil moisture in an optimal range without overwatering. Paired with soil moisture monitoring, ET data gives a genuinely accurate picture of crop water demand, closing the gap between estimation and what's actually happening in the root zone.
Why it matters for precision agriculture
ET measurements are a real building block of modern precision agriculture. Monitoring environmental conditions and crop water demand together helps farmers improve irrigation efficiency, cut water consumption, optimise crop growth and improve yield consistency season over season.
The bottom line
Evapotranspiration gives farmers a practical way to understand crop water demand, turning irrigation from a reactive guess into a data-driven decision. Combined with a weather station and soil moisture monitoring, ET data supports genuinely more efficient water management and more consistent crop outcomes.