A generalized resource-constrained framework for quantifying coupled ecosystem water and carbon fluxes

Photosynthesis, quantified as gross primary productivity (GPP), and evapotranspiration (ET) are two fundamental processes in coupled water and carbon cycles. The strong regulation of ecosystem on carbon and water fluxes by stomata is well understood at the leaf level. However, the coupling is complex at regional or ecosystem scales. The objective of this study is to understand key environmental factors that control both water and carbon fluxes at regional scales and develop a robust resource-constrained framework (RCF) for estimating climatology of coupled ecosystem carbon and water fluxes. Water balance data from 1927 catchments were obtained to parameterize the model and independent observations from 107 flux stations were used to validate the method. Results demonstrated robust model performance with Nash–Sutcliffe efficiency (NSE) of 0.65 for GPP and NSE of 0.55 for ET against independent flux observations. The RCF approach estimated global mean GPP and ET at 1141 g C m⁻² a⁻¹ and 530 mm a⁻¹, respectively, corresponding to an annual terrestrial carbon uptake of 142.4 Pg C a⁻¹. Further analysis identified the ecosystem energy and water limited regimes, with about 40% land areas energy-limited, 40% water limited, and 20% co-limited for both GPP and ET across globe. This study reveals consist estimates of GPP and ET by disentangling the spatial interplay of energy and water constraints. The RCF approach provides a transparent and scalable approach to jointly estimate and attribute carbon and water fluxes, offering new insights into ecosystem functioning and a pathway to improve coupled ecosystem modeling.