Optimizing seasonally variable photosynthetic parameters based on joint carbon and water flux constraints

Terrestrial biosphere models (TBMs) often adopt the Farquhar biochemical model coupled with the Ball-Berry stomatal conductance ( g s ) model to simulate ecosystem carbon and water fluxes. The parameters m , representing the sensitivity of g s to the photosynthetic rate, and V c m a x 25 , representing the leaf photosynthetic capacity, are two pivotal parameters but the two main sources of uncertainties in TBM simulations. The temporal variations of m in TBMs are still elusive, due to the lack of direct observations. It also remains unclear how accurate estimates of m and V c m a x 25 can improve the simulations of carbon and water fluxes. In this study, we used a Bayesian parameter optimization approach to estimate seasonally varying m and V c m a x 25 from eddy covariance observations in a mixed forest stand at the Borden Forest Research Station located in southern Ontario, Canada and used in-situ observations of m and V c m a x 25 for validation. Three strategies were tested for optimizing m and V c m a x 25 , including the carbon, water, and carbon-water coupling scenarios. m and V c m a x 25 optimized from carbon-water coupling constraints shows best correlations with the measured m (R2 = 0.70) and V c m a x 25 (R2 = 0.70). By incorporating optimized m and V c m a x 25 with seasonal variations, we found considerable improvements in the estimated gross primary productivity (GPP) and evapotranspiration (ET) compared with constant m and V c m a x 25 , with R2 increasing from 0.78 to 0.85 for GPP, from 0.65 to 0.71 for ET and RMSE reducing from 2.579 g C m −2 d −1 to 2.038 g C m −2 d −1 for GPP, from 1.151 mm d −1 to 0.137 mm d −1 for ET. This study proposes an effective approach to retrieve m and V c m a x 25 for TBMs and demonstrates the efficacy of incorporating seasonally variable m and V c m a x 25 for reducing the uncertainties in GPP and ET simulations, which supports accurate quantifications of land-atmosphere exchanges.