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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. Precipitation rates from the IMERG-Final half-hourly product for a given month are used only where they are consistent with the rates of the IMERG-Final monthly product. In this case, the (hourly) L4_SM precipitation forcing is corrected to match the IMERG-Final daily total precipitation. When the IMERG-Final half-hourly data are inconsistent with the corresponding IMERG-Final monthly data, which is typically the case for solid precipitation (snowfall), the L4_SM precipitation forcing is corrected to match only the IMERG-Final monthly total precipitation.
  • The L-band microwave radiative transfer model (mwRTM) was updated as follows: the mwRTM uses the Mironov soil mixing approach, replacing the Wang & Schmugge model used in earlier L4_SM versions; the mwRTM uses updated values of the L-band scattering albedo, soil roughness, and vegetation opacity climatology obtained from Version 6 of the SMAP Level-2 dual-channel soil moisture retrieval product (SPL2SMP_E); the mwRTM uses updated values of the polarization mixing parameters of the rough surface reflectivity, matching those of the SPL2SMP_E product.
  • 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).
  • Two bug fixes were implemented. The first corrects excessive precipitation rates that occasionally appeared along certain longitudes in North America in earlier versions of L4_SM. The second fixes an error in the ancillary soil data that misclassified a small region in Argentina as peat.

For the full major and minor version history, go to <a href="https://nsidc.org/data/smap/version-history">https://nsidc.org/data/smap/version-history</a>

Product Summary

Platforms
GEOS-5 ,
SMAP
Instruments
NOT APPLICABLE ,
SMAP L-BAND RADIOMETER
Spatial Extent

N: 85.044 S: -85.044 E: 180 W: -180

Spatial Resolution
9 Kilometers x 9 Kilometers
Geographic Region
GLOBAL LAND
Coordinate System
CARTESIAN
Granule Spatial Representation
CARTESIAN
Temporal Extent
2015-03-31 to Present
Concept ID
C3480440639-NSIDC_CPRD
Data State
ACTIVE
Number of Files/Granules
31480
Processing Level
4
Published
Science Keywords
Root Zone Soil Moisture ,
Surface Soil Moisture

Citation

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.

Copy Citation

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 Analysis Update, Version 8 [Data set]. NASA National Snow and Ice Data Center Distributed Active Archive Center. https://doi.org/10.5067/02LGW4DGJYRX Date Accessed: 2026-01-11

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