Wilting Point is the Key Link between the Water and Carbon Cycles in a Remote Sensing-Constrained Global Terrestrial Biosphere Model

Abstract:

To understand the relationships between soil moisture and carbon cycling, we integrate soil moisture from the Soil Moisture & Ocean Salinity (SMOS) mission, the MPI-BGC global gross primary productivity (GPP) product from the FLUXNET-based MTE model, together with the Simplified Simple Biosphere (SSiB) land surface model to evaluate global impact on carbon budgets with the improved simulated soil moisture. SMOS data provide the best currently available multi-year global soil moisture observations. The MPI-BGC product provides an observation-based benchmark for global GPP. This is an opportunity for terrestrial ecosystem science to bridge water and carbon cycles. SMOS data is used as a constraint to improve the model performance of soil moisture simulation. After the B parameter and hydraulic conductivity at saturation is modified in SSiB model, although it produces a better simulated soil moisture matching against the SMOS data, it results in a worse GPP match against MPI-BGC GPP product, which shows the changes in soil moisture have big impacts on vegetation processes, especially in semi-arid regions. To overcome the deterioration, we further adjust wilting points to adjust the soil moisture climatology. After the modification of wilting points, the simulated global soil moisture and GPP are both improved in the model. This study confirms the close relationship between soil moisture and terrestrial carbon processes and it is important to adequately include both processes to have a comprehensive understanding of water and carbon cycles. Forthcoming work will integrate SMAP soil moisture and fluorescence from GOSAT and OCO-2 as further constraints and coupling between the water and carbon cycles.