Soil and atmospheric drought explain the biophysical conductance
responses in diagnostic and prognostic evaporation models over two
contrasting European forest sites
Abstract
Diagnosing and predicting evaporation through satellite-based surface
energy balance (SEB) and land surface models (LSMs) is challenging due
to the non-linear responses of aerodynamic (ga) and stomatal conductance
(gcs) to the coalition of soil and atmospheric drought. Despite a
soaring popularity in refining gcs formulation in the LSMs by
introducing a link between soil-plant hydraulics and gcs, the utility of
gcs has been surprisingly overlooked in SEB models due to the overriding
emphasis on eliminating ga uncertainties and the lack of coordination
between these two different modeling communities. Therefore, a
persistent challenge is to understand the reasons for divergent
evaporation estimates from different models during strong
soil-atmospheric drought. Here we present a virtual reality experiment
over two contrasting European forest sites to understand the apparent
sensitivity of the two critical conductances and evaporative fluxes to a
water-stress factor (b-factor) in conjunction with land surface
temperature (soil drought proxy) and vapor pressure deficit (atmospheric
drought proxy) by using a non-parametric diagnostic model (Surface
Temperature Initiated Closure, STIC1.2) and a prognostic model
(Community Land Model, CLM5.0). Results revealed the b-factor and
different functional forms of the two conductances to be a significant
predictor of divergent response of the conductances to soil and
atmospheric drought, which subsequently propagated in the evaporative
flux estimates between STIC1.2 and CLM5.0. This analysis reaffirms the
need for consensus on theory and models that capture the sensitivity of
the biophysical conductances to the complex coalition of soil and
atmospheric drought for better evaporation prediction.