Emergent Simplicity of Continental Evapotranspiration Mediated by Land–Atmosphere Coupling

Evapotranspiration (ET) plays a central role in the terrestrial water, energy and carbon cycles. However, ET is extraordinarily difficult to model because it depends, in part, on heterogeneous land surface features -- such as soil moisture, land cover type and plant physiology -- resulting in substantial disagreement between models. Here, we show that the evaporative fraction (ET as a proportion of available energy at the surface) can be estimated across a broad range of water- and energy-limited conditions using a simple, closed-form equation with no free parameters and no land surface information; only air temperature and specific humidity observations are required. The method is based on a recent theory of land-atmosphere coupling under idealized steady conditions (McColl et al., 2019), which we here extend to real-world cases with temporal variation. The equation performs well when compared to eddy covariance observations at 84 inland continental sites, with overall prediction errors indistinguishable from errors in the observations themselves, despite substantial variability in surface conditions across sites. This reveals an emergent simplicity to continental ET that has not been previously recognized, in which land-atmosphere coupling efficiently embeds land surface information in the near-surface atmospheric state on daily to monthly time scales. This result allows ET to be studied globally using weather data, which have much greater spatial and temporal coverage compared to current ET observation networks. Since lack of observations has been a major limitation in studying ET, this work opens up substantial new opportunities for constraining global water, energy and carbon budgets.

Reference

McColl, K.A., Salvucci, G.D., Gentine, P., 2019. Surface Flux Equilibrium Theory Explains an Empirical Estimate of Water-Limited Daily Evapotranspiration. Journal of Advances in Modeling Earth Systems. https://doi.org/10.1029/2019MS001685