What is Evapotranspiration?

What is Evapotranspiration?

Learn more about what evapotranspiration is and why it is important.

Evapotranspiration = Evaporation + Transpiration

As the sun heats the surface of the Earth, water evaporates from land and water surfaces and transpires, or is released, from plants and re-enters the atmosphere. This combined process is called evapotranspiration, or ET. 

ET can be thought of as the opposite of precipitation: Rather than water falling from the atmosphere to the land, ET is water that leaves the land surface and goes back to the atmosphere as water vapor. In the atmosphere, the water from evapotranspiration eventually condenses, forms clouds and precipitates back to the Earth elsewhere. 

As the second-largest component of the water cycle (after precipitation), ET is a critical piece of information both for determining how much irrigation water to apply to fields and for managing water at larger scales. 

ET and Consumptive Water Use

Evapotranspiration often approximates consumptive water use, which refers to all of the water within a system that cannot be recovered or reused, and includes water that is consumed by plants or humans, evaporated, or contaminated.

Because water that evaporates and transpires re-enters the atmosphere and travels away from the immediate area, this water is effectively “consumed” and cannot be reused within a watershed. Consequently, ET is a measure of water that is removed from a local system after it has been applied to, or fallen on, the landscape. This is in contrast to water that flows into local drainage or stream systems or recharges groundwater basins.

Why do ET and consumptive use matter?

ET data are essential to creating an accurate water budget, which is often the first step toward proactive water management at the farm, water district and watershed scale. Similar to a household budget, a water budget is a simple accounting of how much water is flowing into (supply) and out of (demand) an area of interest. Managing water without ET data is like trying to manage your household budget without knowing how much money you are spending.

But a water budget is more complicated than a household budget because of reuse. Water applied to an area can come from precipitation, diversion from streams or rivers, or groundwater extraction. Some of that applied water is consumed via ET, while the rest flows back into streams, rivers or aquifers and is usually available for reuse. Only the part that leaves as ET is actually removed from the system.

To date, much of the public’s understanding of water use, water law, and water management has been based primarily on diversion data, such as groundwater pumping. However, such data gives an incomplete picture of water use because it reflects only the total quantity of water diverted from the original supply. It excludes critical information about how much of the diverted water is actually used by plants to grow – the consumptive water use – and how much water is reusable.

Over time, if outflows exceed inflows (consumptive use is greater than supply)  water storage within aquifers or reservoirs starts to decline and a system can go into “overdraft,” which has long-term consequences for people, plants, and wildlife. Quantifying ET and consumptive water use can help local communities with planning processes and strategies to balance water supply and water demand, and proactively prevent “overdrafting” precious water resources. These data can also be used to credit farmers and landowners when they conserve water or recharge depleted groundwater basins. 

Learn more about how OpenET measures ET: