Impact of an unstressed canopy conductance on the Bouchet‐Morton complementary relationship

Abstract

Indirect methods of quantifying evapotranspiration (λEa) are sought since regional estimations of λEa require prohibitive instrumentation or highly parameterized and data‐intensive land surface models (e.g., involving temporally and spatially varying soil moisture, soil hydraulic properties, and vegetation properties). Complementary relationship (CR) models, based on Bouchet's hypothesis, are one such method of estimating λEa from routinely measured meteorological variables. Bouchet's CR states that given a change in regional surface moisture availability (MA), changes in λEa are reflected in changes in potential evapotranspiration (λEp), such that λEa + λEp = 2λE0, where λE0 is an assumed equilibrium condition at which λEa = λEp = λE0 given sufficiently large MA. Whereas λEp conceptually includes a transpiration component, the treatment of vegetation in existing CR applications varies from neglecting it to indirectly accounting for it through recalibration of Penman's empirical wind function. We utilize the First International Land Surface Climatology Field Experiment (FIFE) data set to demonstrate that inclusion of a maximum (i.e., unstressed) canopy conductance (gc,max) in a Penman equation with stability‐corrected atmospheric conductance (i.e., replacing the Penman equation with an unstressed Penman–Monteith equation) significantly improves both CR convergence and symmetry. Inclusion of gc,max results in more accurate λEa estimates than are found with the Penman equation (using either Monin–Obukhov atmospheric conductance or using empirical wind functions in the literature). The proposed method also performs better than the 1992 advection‐aridity CR method, modified to include atmospheric stability effects, which attributes noncomplementarity to horizontal advection and corrects for it by adjusting λE0 by the magnitude of the potential sensible heat flux (∣Hp∣), found from the surface energy balance of the λEp calculation. In the proposed method a similar energy balance adjustment occurs naturally because the finite canopy conductance causes an increase of surface temperature and sensible heat flux in the λEp energy balance calculation. The proposed method is consistent with CR explanations that rely on feedbacks with the boundary layer vapor pressure deficit since the impact of such changes on both λEp and λEa would be modulated by stomatal conductance.