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Description
SMAP Level-4 (L4) surface and root zone soil moisture (L4_SM) data are provided in three products.
The SMAP L4 Global 3-hourly 9 km EASE-Grid Surface and Root Zone Soil Moisture Geophysical Data (SPL4SMGP, DOI: 10.5067/T5RUATAQREF8) product is a series of 3-hourly time average geophysical land surface fields that are output by the L4_SM algorithm. It is likely of primary interest to most users.
The SMAP L4 Global 3-hourly 9 km EASE-Grid Surface and Root Zone Soil Moisture Analysis Update (SPL4SMAU, DOI: 10.5067/02LGW4DGJYRX) product provides diagnostics from the land surface analysis updates. It consists of a series of 3-hourly instantaneous (or snapshot) files that contain the assimilated SMAP observations, the corresponding land model predictions and analysis estimates, and additional data assimilation diagnostics.
Lastly, the SMAP L4 Global 9 km EASE-Grid Surface and Root Zone Soil Moisture Land Model Constants (SPL4SMLM, DOI: 10.5067/PXQIBL2ALDZD) product provides static (time-invariant) land surface model constants that will be needed by some users for further interpretation of the geophysical land surface fields. This product consists of only one granule (file) per L4_SM data product version (as defined by a distinct Science Version ID).
For each product, SMAP L-band brightness temperature data from descending and ascending half-orbit satellite passes (approximately 6:00 a.m. and 6:00 p.m. local solar time, respectively) are assimilated into a land surface model that is gridded using an Earth-fixed, global cylindrical 9 km Equal-Area Scalable Earth Grid, Version 2.0 (EASE-Grid 2.0) projection.
Version Description
Changes to this version include: The precipitation observations used outside of North America and the high latitudes are from Version 07 of the NASA Integrated Multi-satellitE Retrievals for the Global Precipitation Measurement mission (IMERG) products; The L-band microwave radiative transfer model (mwRTM) was updated; The Catchment land surface model now uses climatological snow albedo values based on observations from the NASA Moderate Resolution Imaging Spectroradiometer, replacing the look-up table parameterization of earlier L4_SM versions; The assimilated brightness temperatures are now taken only from Version 6 (“R19”) of the SPL1CTB product; The brightness temperature scaling parameters in the L4_SM algorithm are now based on nine years of SMAP observations and model simulations (April 2015 – March 2024); Bug fixes.
For the full major and minor version history, go to https://nsidc.org/data/smap/version-history
Citation is critically important for dataset documentation and discovery. This dataset is openly shared, without restriction, in accordance with the EOSDIS Data Use and Citation Guidance.
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Reichle, R., De Lannoy, G., Koster, R., Crow, W., Kimball, J., Liu, Q., & Bechtold, M. (2025). SMAP L4 Global 3-hourly 9 km EASE-Grid Surface and Root Zone Soil Moisture Geophysical Data, Version 8 [Dataset]. NASA National Snow and Ice Data Center Distributed Active Archive Center. https://doi.org/10.5067/T5RUATAQREF8 Date Accessed: 2026-08-04
Reichle, R., De Lannoy, G., Koster, R., Crow, W., Kimball, J., Liu, Q., & Bechtold, M. (2025). SMAP L4 Global 3-hourly 9 km EASE-Grid Surface and Root Zone Soil Moisture Geophysical Data, Version 8 [Dataset]. NASA National Snow and Ice Data Center Distributed Active Archive Center. https://doi.org/10.5067/T5RUATAQREF8 Date Accessed: 2026-08-04
Reichle, Rolf, et al. “SMAP L4 Global 3-Hourly 9 Km EASE-Grid Surface and Root Zone Soil Moisture Geophysical Data, Version 8.” NASA National Snow and Ice Data Center Distributed Active Archive Center, 2025, https://doi.org/10.5067/T5RUATAQREF8. Date Accessed: 2026-08-04
Hettinga, E. Kathryn, Rezanezhad, Fereidoun, Persaud, Bhaleka D., Slowinski, Stephanie, Strack, Maria, Liu, Haojie, Lennartz, Bernd, Humphreys, Elyn, Wu, Jianghua, Skeeter, June, Knox, Sara H., Alcock, Haley, Sonnentag, Oliver, Detto, Matteo, Van Cappellen, Philippe
Land Use/Land Cover Classification, Fire Occurrence, Land Surface Temperature, THERMAL ANOMALIES, Root Zone Soil Moisture, Surface Soil Moisture, Gross Primary Production (gpp), Heterotrophic Respiration (rh), NET ECOSYSTEM CO2 EXCHANGE (NEE), Soil Organic Carbon (SOC)
Endsley, K. Arthur, Kimball, John S., Reichle, Rolf H., Ardizzone, Joseph V., Kundig, Tobias, Colligan, Thomas, Heinke, Jens, Kato, Etsushi, Knauer, Jurgen, Ma, Lei, Nutzel, Tobias, Sun, Qing, Yuan, Wenping
Leaf Characteristics, Photosynthetically Active Radiation, Leaf Area Index (LAI), Fraction Of Absorbed Photosynthetically Active Radiation (fapar), Root Zone Soil Moisture, Surface Soil Moisture, Gross Primary Production (gpp), Heterotrophic Respiration (rh), NET ECOSYSTEM CO2 EXCHANGE (NEE), Soil Organic Carbon (SOC), Atmospheric Carbon Dioxide, Respiration Rate, Air Temperature, Surface Winds, Net Radiation, Heat Flux