What is ET?
Evapotranspiration = Evaporation + Transpiration
As the sun heats the Earth’s surface, water returns to the atmosphere in two ways:
- 1 Evaporation: Water evaporates from the soil, open water, and other surfaces.
- 2 Transpiration: Water is released as vapor from plants through tiny pores in their leaves and stems called stomata.
Together, these processes are called evapotranspiration or ET.
Think of ET as the reverse of precipitation. Instead of water falling from the sky, ET is water rising back into the atmosphere as vapor. Once there, it can travel long distances, condense, and eventually fall again as rain or snow—completing the water cycle.
Why is ET Important?
ET is the second-largest component of the water cycle (after precipitation). ET is a major component of consumptive water use, and quantifies water that is transferred to the atmosphere and cannot be recaptured and reused within a watershed or water system. Understanding ET is also key to both local agricultural irrigation decision-making and large-scale water management and planning.
Empowers strategic irrigation decision-making
ET data help determine how much water crops are using over a given period of time, and supports farmers in making data-driven irrigation decisions to maximize both water use efficiency and crop yields.
Supports water management across local, regional, and basin scales
ET data supports strategic decisions across all scales, helping to balance overall supply and demand for communities and ecosystems.
Measuring the Water Plants Actually Use
Water that is consumed through ET and transferred to the atmosphere is no longer available for reuse with a local watershed.
In contrast, water that runs off, drains, or seeps back into an aquifer is water that can often, though not always, be captured and reused.
By quantifying the amount of water that is consumed through ET and removed from a water system, OpenET provides data that are essential for creating accurate water budgets and making sustainable water management decisions.
A Comprehensive View
Understanding water use is key to developing sustainable water management plans.
For decades, most water management has focused on how much water is diverted from a stream or pumped from an aquifer and applied to a field. This only provides part of the picture. Accurate ET data allows water managers to understand how much of the water applied to a field is consumed through ET.
When consumptive use outpaces supply, aquifers and reservoirs shrink, ecosystems suffer, and communities face water insecurity. With accurate ET data, local water managers can better monitor and plan for mismatches between supply and demand over time.
Monitor & balance supply and demand
Avoid overdrafting shared water resources
Reward conservation & groundwater recharge efforts
Learn More About OpenET Science
Data Methods
Information about the models, input datasets, and compute resources used by OpenET.
Accuracy & Known Issues
Field-scale accuracy assessments and known issues that impact OpenET data.
Literature & Research
Browse published papers about OpenET data and associated research.
Filling the Biggest Data Gap
in Water Management
As we expand globally, our mission remains the same: deliver open, reliable, and actionable water data—developed with users, for users.