Abstract:
The post-launch Cal/Val phase of the Soil Moisture Active Passive (SMAP) satellite mission is guided by two primary objectives for each science product team: 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 6) SMAP Level 4 Carbon (L4_C) product. The L4_C Version 6 (v6) global record now spans approximately seven years (March 2015 – present) of SMAP science operations and has benefited from six major reprocessing updates to the operational product. These reprocessing events and L4_C product release updates have incorporated various algorithm refinements and calibration adjustments to account for similar refinements to upstream SMAP brightness temperature retrievals, the GEOS land model assimilation system, and SMAP Level 4 Soil Moisture (L4_SM) inputs used for L4_C processing.

The SMAP L4_C algorithm utilizes a Terrestrial Carbon Flux (TCF) model informed by daily surface and root zone soil moisture information from the SMAP L4_SM product, along with optical remote sensing-based (i.e., from the Moderate Resolution Imaging Spectroradiometer, MODIS, or the Visible Infrared Imaging Radiometer Suite, VIIRS) land cover and canopy fractional photosynthetic active radiation (fPAR), 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 (~0-5 cm depth) soil organic carbon (SOC) stocks and associated environmental constraints to these processes, including soil moisture-related controls on GPP and ecosystem respiration (Kimball et al. 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 to these processes, 2) documenting primary connections between terrestrial water, energy and carbon cycles, and 3) improving understanding of terrestrial carbon sink activity.

The SMAP L4_C algorithm and operational product are mature and at a CEOS Validation Stage 4 level (Jackson et al. 2012) based on extensive validation of the multi-year record against a diverse array of independent benchmarks, well-characterized global performance, and systematic refinements gained from six major reprocessing events. There are no Level-1 mission science requirements for the L4_C product; however, self-imposed requirements have been established focusing on NEE as the primary product field for validation, and on demonstrating L4_C accuracy and success in meeting product science requirements (Jackson et al. 2012). The other L4_C product fields also have strong utility for carbon science applications (e.g., Liu et al. 2019, Endsley et al. 2020); however, analysis of these other fields is considered secondary relative to primary validation activities focusing on NEE. The L4_C targeted accuracy requirements are to meet or exceed 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 30 g C m-2 yr-1, emphasizing northern (≥45°N) boreal and arctic ecosystems; this accuracy is similar to that of tower eddy covariance measurement-based observations (Baldocchi 2008).

Methods used for the L4_C v6 product performance and validation assessment have been established from the SMAP Cal/Val plan and previous studies (Jackson et al. 2012, Jones et al. 2017; Endsley et al. 2020) and include: 1) consistency evaluations of the product fields against earlier product releases (version 5 or earlier); 2) comparisons of daily carbon flux estimates with independent tower eddy covariance measurement-based daily carbon (CO2) flux observations from core tower validation sites (CVS); and 3) consistency checks against other global carbon products, including soil carbon inventory records, global GPP records derived from tower observation upscaling methods, and satellite-based observations of canopy solar induced chlorophyll fluorescence (SIF) as a surrogate for GPP. Metrics used to evaluate agreement between L4_C product fields and observational benchmarks include correlation (r-value), RMS differences, bias, and model sensitivity diagnostics. Following these validation criteria, the present report provides a validation assessment of the L4_C v6 product. Detailed descriptions of the L4_C algorithm and additional global product accuracy and performance results are given elsewhere (Jones et al. 2017, Endsley et al. 2020).

The L4_C v6 product replaces earlier product versions and continues to show: (i) accuracy and performance levels meeting or exceeding SMAP L4_C science requirements; (ii) continuing improvements over earlier product versions (v5 and earlier); and (iii) suitability for a diversity of science applications. Example L4_C applications from the recent literature include clarifying environmental trends and controls on the northern terrestrial carbon sink (Liu et al. 2019, Endsley et al. 2022); diagnosing ecosystem productivity behavior in response to extreme climatic events including droughts and heatwaves (Li et al. 2020, Kwon et al. 2021); and regional monitoring of cropland conditions for projecting annual crop yields (Wurster et al. 2020).