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Impact of an unstressed canopy conductance on the Bouchet‐Morton complementary relationship
Author(s) -
Pettijohn Justin C.,
Salvucci Guido D.
Publication year - 2006
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/2005wr004385
Subject(s) - evapotranspiration , canopy conductance , penman–monteith equation , environmental science , atmospheric sciences , transpiration , advection , wind speed , canopy , stomatal conductance , potential evaporation , latent heat , vegetation (pathology) , meteorology , mathematics , vapour pressure deficit , physics , geography , thermodynamics , medicine , ecology , photosynthesis , botany , archaeology , pathology , biology
Indirect methods of quantifying evapotranspiration ( λE a ) are sought since regional estimations of λE a 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 λE a from routinely measured meteorological variables. Bouchet's CR states that given a change in regional surface moisture availability ( M A ), changes in λE a are reflected in changes in potential evapotranspiration ( λE p ), such that λE a + λE p = 2 λE 0 , where λE 0 is an assumed equilibrium condition at which λE a = λE p = λE 0 given sufficiently large M A . Whereas λE p 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 ( g c, 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 g c, max results in more accurate λE a 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 λE 0 by the magnitude of the potential sensible heat flux (∣ H p ∣), found from the surface energy balance of the λE p 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 λE p 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 λE p and λE a would be modulated by stomatal conductance.