Eddy covariance towers still miss the surface energy budget after four decades of use

Closure is a comparison between two independently instrumented sides of one equation. The covariance of vertical wind speed with air temperature gives sensible heat; the covariance of wind speed with water vapor concentration gives latent heat ^[5]. Available energy, the other side, is normally simplified as net radiation minus the heat conducted through the soil, and net radiation is itself the difference between incoming and outgoing shortwave and longwave radiation ^[8]. In principle the two sides are equal ^[7].

Each flux value is a large statistic. Sensors sample vertical wind, air temperature and water vapor at 10 to 20 hertz ^[4], and the covariances are usually evaluated over 30-minute or 1-hour periods ^[6]. That is 18,000 to 36,000 observations behind a half-hourly number, and 36,000 to 72,000 behind an hourly one ^[17]. The available-energy term it gets compared against is assembled from radiation and soil heat measurements instead ^[8].

Complaints about measuring evaporation predate the tower. Until the 1980s, technologies for measuring evapotranspiration lagged those for other meteorological measurements ^[14]. In his 1979 presidential address to the Royal Meteorological Society, John Monteith recalled that the 19th century British meteorologist G. J. Symons had called evaporation "the most desperate branch of this desperate science" of meteorology ^[15]. "One of the main reasons for despair was the lack of techniques for measuring evaporation over natural surfaces," Monteith said ^[16].

The techniques arrived. Eos reports that researchers have been plagued by the closure problem despite theoretical and technical advances in recent decades ^[20]. What the account does not give is a size. It reports no magnitude for the residual, no bias figure for evapotranspiration, and no finding on whether practitioners carry the imbalance into the models they feed ^[19].

Adoption is what has changed. Eos reports that the accelerating use of eddy covariance for scientific and regulatory purposes is what is compelling scientists to address the problem collectively ^[12], and the 2025 meeting was called to spur clear, coordinated best practices for collecting, correcting, analyzing and applying the measurements and datasets ^[21]. About four decades separate that meeting from the 1980s, when towers became the preferred instrument ^[18].