Evapotranspiration Everywhere, All the Time: Towards a Unified View From Earth Observation

Area of Science:

• Earth Science
• Hydrology
• Remote Sensing

Background:

• Evapotranspiration (ET) is crucial for understanding the water cycle.
• Historically, ET was inferred as a residual component in water budgets.
• Measurement techniques have advanced from ground-based methods to large-scale remote sensing.

Purpose of the Study:

• To discuss the evolution of Earth observation techniques for measuring evapotranspiration (ET).
• To highlight the progression towards achieving ET measurements that are both spatially and temporally comprehensive.
• To explore the role of technological advancements in improving ET estimation.

Main Methods:

• Review of historical ET measurement techniques (pans, lysimeters, sap flow).
• Analysis of scaling-up methods: eddy-covariance flux towers, proximal sensing (drones, aircraft).
• Examination of satellite-based observations and the development of ET models.

Main Results:

• No single method has achieved ET measurement 'everywhere, all the time'.
• Flux towers provide continuous data but lack spatial coverage.
• Satellites offer broad coverage but often lack temporal resolution; smallsats are emerging to bridge this gap.

Conclusions:

• Significant progress has been made in Earth observation for ET estimation.
• Advancements in satellite technology, cloud computing, and machine learning are key.
• The goal of ubiquitous, continuous ET monitoring is approaching, aiding water management and planetary sustainability.