Toward Fit-for-Purpose Evapotranspiration Observations

Justin Huntington, Yeonuk Kim , Todd G. Caldwell, Meetpal Kukal, Shawn Naylor, Bruno Comini de Andrade, Christopher Pearson, Richard Jasoni, John Volk, Murphy Gardner, Maria Isabel Zamora Re, Dragomira Zheleva, Robert Lotspeich

Evapotranspiration (ET) underpins water, energy, and carbon cycling, yet remains among the least observed hydrologic fluxes, creating a persistent paradox: hydrology is advancing rapidly through artificial intelligence and data-driven methods while its observational foundation remains sparse and fragmented. Eddy covariance provides robust ET measurements, but high cost and operational demands limit deployment. The scarcity of in situ ET observations for model validation creates a persistent barrier to evaluating, interpreting, and adopting scalable satellite-based ET products. Emerging low-cost sensors offer a pathway to expand observations, but rely on simplifying assumptions, proprietary processing, and limited diagnostics, introducing uncertainty and constraining process interpretation. We present an ET measurement ladder that organizes approaches along levels of affordability, capturing trade-offs in reliability, physical constraint, and analytical capability. A fit-for-purpose strategy is essential to expand ET measurements, strengthen confidence in ET observations and products, and enable credible, scalable monitoring of hydrometeorological processes under increasing resource constraints.