Evapotranspiration everywhere, all the time: Towards a unified view from earth observation

Authors: FISHER, J - Chapman University; Anderson, Martha; MIRALLES, D - Ghent University; MALLICK, K - Luxembourg Institute Of Science & Technology; STOY, P - University Of Wisconsin; RYU, Y - Seoul National University; BASTIAANSSEN, W.G.M. - Delft University Of Technology

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/22/2026
Publication Date: 5/10/2026
Citation: Fisher, J.B., Anderson, M.C., Miralles, D.G., Mallick, K., Stoy, P., Ryu, Y., Bastiaanssen, W. 2026. Evapotranspiration everywhere, all the time: Towards a unified view from earth observation. Global Change Biology. 32(5). Article e70898. https://doi.org/10.1111/gcb.70898.
DOI: https://doi.org/10.1111/gcb.70898

Interpretive Summary: Evapotranspiration (ET) describes the exchange of water between the land and the atmosphere and, over croplands, quantifies the water used to grow crops. Accurate satellite-based maps of ET are of great benefit to agriculture: guiding irrigation scheduling, determining crop water requirements, and providing early warning of drought impacts on crop development. Ideally, these maps would be produced at daily timesteps and sub-field spatial scales; however, no single satellite can provide the imagery needed to support these requirements. This paper describes the historical development of satellite ET models and the satellite imagery that has been traditionally used. We also discuss new satellite sources from the U.S. and international partners that can be combined with commercial imagery to map ET “everywhere and all the time”. These ET products will benefit U.S. crop growers and ranchers, providing timely decision support information on vegetation health and water requirements.

Technical Abstract: Scientists want to know everything, everywhere, and all the time. This is particularly true in Earth science, where we seek to understand processes that span from the molecular to the planetary scale in how the world works, how it affects us, and how we impact it—especially the water cycle. Evapotranspiration (ET) was the last component to be measured in closing the water cycle: for decades, closing the water budget meant adding up all the measurable components, then inferring ET as the residual. Early measurements relied on water loss from pans and weighing lysimeters, followed by sensors inserted into plants to monitor sap flow and leaf chambers capturing transpiration. Scaling up to ecosystems became possible through eddy-covariance flux towers and further across landscapes through proximal sensing with drones, aircraft, and, ultimately, with satellites. While enormous progress has been made to measure or estimate ET everywhere and all the time, no single approach has yet achieved both simultaneously. Flux towers help with all the time, but not everywhere. Satellites can do everywhere, but not all the time (except, in part, for geostationary satellites, though with insufficient spatial coverage and resolution). A new advent of smallsat constellations is moving us to everywhere and all the time in detail, though we are only in the beginning of that era. This paper discusses the evolution and revolution of Earth observation for ET, as we advanced from the first Landsat and development of ET models through the progression of increasingly higher spatiotemporal resolution across international space agencies and commercial industry with increasing ET model sophistication, cloud computing, and machine learning. We continue to march ahead towards ET everywhere, all the time, and use that knowledge to better manage water and sustain our planet.