Calculating Canopy Stomatal Conductance at Eddy Covariance Sites without Energy Budget Closure

Abstract:

For over 20 years, the standard method to estimate canopy-scale stomatal conductance has been from eddy covariance (EC) measurements of evapotranspiration via the inverted Penman-Monteith (PM) equation. But the EC method and the PM equation make strange bedfellows. The latter was designed to estimate transpiration without knowing the sensible heat flux (H), which it implicitly reconstructs as the residual of the (almost always incomplete) site energy budget. EC towers, on the other hand, always measure H along with evapotranspiration (and no less accurately). Using theoretical analysis and real data, we show that the PM equation’s unnecessary assumption of energy budget closure leads to errors of up to 35% in canopy stomatal conductance at EC sites (see Figure 1). Because these substantial errors vary systematically with time of day and with site characteristics, they skew apparent relationships between stomatal conductance and its drivers. We argue that a more accurate and straightforward method to estimate canopy stomatal conductance is via the flux-gradient equations that underlie the Penman-Monteith equation, using sensible and latent heat flux measurements that have been adjusted to close the long-term site energy budget. Switching to this alternate approach would enable future studies to provide a clearer picture of stomatal behavior at the ecosystem scale, and to better constrain ecosystem models and predictions of transpiration.