Abstract:
Since the launch of the Soil Moisture Active Passive (SMAP) satellite mission in 2015, post-launch calibration and validation (Cal/Val) activities have been guided by two primary objectives: 1) To calibrate, verify, and improve the performance of the science algorithms; and 2) to validate the accuracy of the science data products as specified in the SMAP Level 1 mission science requirements. This report provides an assessment of the latest, Version 7 (V7) SMAP Level 4 Carbon (L4_C) product. The L4_C global record now spans eight years (March 2015 – present) of SMAP science operations, including seven major reprocessing updates to the operational product. These updates include various L4_C algorithm refinements and calibration adjustments to account for changes in ancillary inputs.

The SMAP L4_C algorithm estimates a global, daily terrestrial carbon budget that is informed by daily surface and root-zone soil moisture information from the SMAP Level 4 Soil Moisture (L4_SM) product and by land cover from the Moderate Resolution Imaging Spectroradiometer (MODIS), canopy fractional photosynthetic active radiation (fPAR) from the Visible Infrared Imaging Radiometer Suite (VIIRS), and other ancillary biophysical data. The L4_C product provides estimates of global, daily net ecosystem CO2 exchange (NEE) and the component carbon fluxes, namely, vegetation gross primary production (GPP) and soil heterotrophic respiration (RH). Other L4_C product elements include surface (ca. 0-5 cm depth) soil organic carbon (SOC) stocks and associated environmental constraints, including soil moisture-related controls on GPP and RH ecosystem respiration (Kimball, Jones, and Glassy 2014; Jones et al. 2017). The L4_C product addresses SMAP carbon cycle science objectives by: 1) Providing a direct link between terrestrial carbon fluxes and underlying freeze/thaw and soil moisture-related constraints; 2) Documenting primary connections between terrestrial water, energy and carbon cycles; and 3) Improving understanding of terrestrial carbon sink activity.

L4_C is calibrated against eddy covariance (EC) tower CO2 flux measurements, which are a proxy for terrestrial ecosystem NEE. The L4_C product has self-imposed performance requirements related to NEE, the primary product field for validation, although the other L4_C product fields (namely GPP, RH, and SOC) have demonstrated utility for carbon science applications (Liu et al. 2019; Endsley et al. 2020; Wurster et al. 2021). The L4_C targeted accuracy requirement is to stay below a mean unbiased root-mean-square (RMS) error (ubRMSE, or standard deviation of the error) for NEE of 1.6 g C m-2 d-1 (or, equivalently, 30 g C m-2 yr-1), emphasizing northern (≥45°N) boreal and arctic ecosystems; this accuracy is similar to that of EC tower CO2 flux observations (Baldocchi 2008). The methods used for L4_C performance and validation assessment have been established from the SMAP Cal/Val plan and previous studies (Jones et al. 2017; Endsley et al. 2020) and are reported here for L4_C V7.

Our primary validation compares L4_C V7 estimates of NEE, GPP, and ecosystem respiration (RECO) to EC tower flux measurements at 26 globally distributed SMAP Core Validation Sites. We also compared the L4_C V7 mean annual fluxes, interannual variability, and short-term trends to the recent literature and to independent reference datasets, including: solar-induced fluorescence data from the Orbiting Carbon Observatory-2 mission; global, up-scaled EC tower fluxes from an ensemble of machine-learning models; global soil carbon inventory records; an ensemble of dynamic global vegetation models; and two indices of recent (2015-2022) climate oscillations.

V7 shows a slight, statistically insignificant increase in RMSE compared to the previous version, largely owing to the unavoidable change from MODIS to VIIRS fPAR inputs. The L4_C product continues to exceed the target NEE accuracy and continues to show favorable accuracy for GPP and RECO. GPP and RECO interannual variability also show good agreement with independent estimates, particularly in the northern hemisphere. A comparison of recent L4_C flux variability with the literature and with El Niño Southern Oscillation indices demonstrates that L4_C can represent the response of the terrestrial carbon-cycle to moisture and temperature variability, particularly in southern, semi-arid regions. Similarly, L4_C surface SOC anomalies in the southern hemisphere show variability that closely corresponds to recent drying and re-wetting trends.

These assessments underscore the utility of L4_C for diverse science applications; indeed, L4_C surface SOC was recently used in a NASA DEVELOP project, sponsored by Conservation International, for assessing the spatial and temporal variability of irrecoverable carbon reserves (Noon et al. 2021) in Peru and Bolivia. Other recent examples include the use of L4_C for constraining the magnitude and timing of the northern hemisphere land carbon sink (Endsley et al. 2022; Watts et al. 2023); estimating the impact of the COVID-19 pandemic on global carbon emissions (Ray et al. 2022); evaluating the impact of changes to a land surface model (Huang et al. 2022); diagnosing the response of ecosystem productivity to extreme climatic events, including droughts, heatwaves, and ice storms (Li and Wei 2020; Kwon et al. 2021; Dannenberg et al. 2022; Yang and Liu 2023); and regional monitoring of cropland conditions for projecting annual crop yields (Wurster et al. 2021).