Implementing a plant hydraulics parameterization in the Canadian Land
Surface Scheme Including biogeochemical Cycles (CLASSIC) v.1.4
Abstract
Drought conditions caused by soil moisture stress and/or high vapour
pressure deficit pose a challenge to many terrestrial ecosystem models
(TEMs). The Canadian LAnd Surface Scheme Including biogeochemical Cycles
(CLASSIC) employs an empirical approach to link soil moisture stress
with stomatal conductance. Such soil moisture-based empirical approaches
typically perform poorly during drought. Here, we implemented an
explicit plant hydraulics parameterization, i.e., Stomatal Optimization
based on Xylem hydraulics (SOX), in CLASSIC, thereby connecting the
soil-plant-atmosphere continuum through plant hydraulic traits.
Performance of the resulting CLASSIC$_{SOX}$ was evaluated against
carbon and water fluxes measured with eddy covariance at eight boreal
forest flux tower sites in North America. Compared to CLASSIC,
CLASSIC$_{SOX}$ better simulated gross primary productivity (GPP)
across all sites, i.e., coefficient of determination (R$^{2}$)
increased (0.51 to 0.59), root mean square error (RMSE) and bias
decreased (1.85 to 1.54 g C m$^{-2}$ d$^{-1}$) and (-0.99
to -0.58 g C m$^{-2}$ d$^{-1}$), respectively. Under
drought conditions, identified using the Palmer drought severity index,
GPP simulated with CLASSIC$_{SOX}$ improved compared to CLASSIC,
i.e., R$^{2}$ increased (0.51 to 0.60), and RMSE and bias
decreased (1.79 to 1.46 g C m$^{-2}$ d$^{-1}$) and (-0.97
to -0.53 g C m$^{-2}$ d$^{-1}$), respectively. In
contrast, CLASSIC$_{SOX}$ simulated evapotranspiration worsened,
i.e., R$^{2}$ decreased (0.61 to 0.42), RMSE increased (0.54 to
0.62 mm d$^{-1}$), and bias changed direction (0.09 to -0.09 mm
d$^{-1}$). As evaporation is a highly parameterized process in
CLASSIC, it likely needs to be re-parameterized to account for the SOX
transpiration behaviour.