Abstract:
The NASA Soil Moisture Active Passive (SMAP) mission has a projected launch in 2014 and will provide global mapping of surface soil moisture and landscape freeze/thaw (F/T) status using L-band (1.26 GHz) active/passive microwave remote sensing. SMAP science objectives include reducing uncertainty regarding terrestrial carbon (CO2) uptake and release and the purported missing carbon sink on land. An operational level 4 carbon (L4_C) product is planned for quantifying surface soil organic carbon (SOC) stocks, soil moisture and temperature controls for heterotrophic respiration and the net ecosystem CO2 exchange (NEE) using model assimilated soil moisture, temperature and F/T measurements from SMAP with ancillary land cover and vegetation productivity inputs. We conducted an initial global implementation and evaluation of the SMAP L4_C algorithms using MODIS productivity inputs and MERRA reanalysis daily air temperature and soil moisture fields. The resulting model simulations are generally consistent with the distribution and magnitude of SOC stocks available from global soil inventories, while estimated carbon fluxes correspond (R2 > 0.6; RMSE < 1.5 g C/m2/day) with CO2 flux measurements from the global tower network (FLUXNET). A model uncertainty analysis indicates an anticipated L4_C product accuracy for NEE within 30 gC/m2/yr (1.6 g C/m2/day), and similar to accuracies attained from tower eddy covariance measurements.