Emergent Simplicity of Continental Evapotranspiration

Evapotranspiration (ET) is challenging to model because it depends on heterogeneous land surface features-such as soil moisture, land cover type, and plant physiology-resulting in rising model complexity and substantial disagreement between models. We show that the evaporative fraction (ET as a proportion of available energy at the surface) can be estimated accurately across a broad range of conditions using a simple equation with no free parameters and no land surface information; only near-surface air temperature and specific humidity observations are required. The equation performs well when compared to eddy covariance measurements at 76 inland continental sites, with prediction errors comparable to errors in the eddy covariance measurements 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.