SMAP (L3·L4)¶

Soil Moisture Active Passive (SMAP) — L3 radiometer soil moisture (SPL3SMP V9, SPL3SMP_E V6) and L4 assimilated soil moisture and carbon (SPL4SMGP V8, SPL4CMDL V8)

Available

At a glance¶

Note on period: All four products run from 2015-03-31 to the present. Safe-mode events interrupted observations on 2019-06-19 to 07-23 and 2022-08-06 to 09-20; L3 has no data then, and L4 continued but without SMAP observations (model only). Files for the geolocation-issue period (2026-05-14 to 07-28) are being replaced with reprocessed versions; on 2026-10-13 the SPL3SMP file for 2026-06-01 already carried a higher file counter (_004) and was updated on 2026-09-23.

Overview¶

NASA's SMAP satellite measures soil moisture in roughly the top 5 cm globally with an L-band (1.4 GHz) microwave radiometer. Its radar failed in July 2015, so all products since then are radiometer-based. The L3 products (SPL3SMP at 36 km, SPL3SMP_E at 9 km) composite a day of half-orbit retrievals into separate morning and evening fields, while L4 assimilates SMAP brightness temperatures into the NASA GEOS land model to give gap-free 3-hourly surface and root-zone soil moisture (SPL4SMGP) and daily net ecosystem carbon exchange (SPL4CMDL).

What changes with the successor¶

SMAP L3 SPL3SMP V8 · SPL3SMP_E V5 → SMAP L3 SPL3SMP V9 · SPL3SMP_E V6 (CRID R19)¶

Transition: 2024-10

A reprocessing that applied an improved method to the input L1B/L1C brightness temperatures; the soil moisture algorithms, file structure and variables are unchanged. Forward processing began in December 2023, reprocessing of the full record from 2015 finished on 30 October 2024, and the previous versions were retired on 16 December 2024.

Sources: NSIDC notice — Data set updates SMAP Level-1, Level-2, and Level-3 (2024-10-30) · SPL3SMP V9 user guide

SMAP L4 Version 7 → SMAP L4 Version 8¶

Transition: 2025-04

The L4 soil moisture and carbon products were reprocessed from 2015 as Version 8, and Version 7 was retired on 30 June 2025. Precipitation forcing, the radiative transfer model and snow albedo changed, input brightness temperatures were standardised on R19 (SPL1CTB V6), and the carbon model was recalibrated against a new flux-tower network.

Sources: NSIDC notice — SMAP Level-4 Version 8 Release (2025-04-15) · NSIDC SPL4SMGP Version 8 page · NSIDC SPL4CMDL Version 8 page

Available variables¶

Main variables per the NSIDC user guides and the L4_SM Product Specification Document. Files are HDF5 and variables sit inside groups (L3: Soil_Moisture_Retrieval_Data_AM/_PM, with ..._Polar_AM/_Polar_PM for the SPL3SMP_E polar grid; SPL4SMGP: Geophysical_Data; SPL4CMDL: NEE, GPP, RH, SOC, QA). The fill value for floating-point fields is -9999.0; for L3 integer (uint16) flags it is 65534.

instantaneous mean accumulated maximum minimum static — instantaneous: value at the time stamp · mean/accumulated/maximum/minimum: over the indicated interval · static: does not change with time

L3 radiometer soil moisture (SPL3SMP 36 km / SPL3SMP_E 9 km, daily)¶

L4 soil moisture and land surface (SPL4SMGP, 3-hourly averages)¶

L4 carbon (SPL4CMDL, daily means)¶

The full variable list is in the official documentation.

How to get the data¶

• NSIDC DAAC — SPL3SMP V9 — Earthdata Search / earthaccess / HTTPS (CMR virtual directory) / NSIDC Data Access Tool / Harmony
  • A free NASA Earthdata Login is required. The legacy on-premises access routes closed on 2025-10-28; NASA Earthdata Cloud is now the primary archive.
• NSIDC DAAC — SPL3SMP_E V6 — Earthdata Search / earthaccess / HTTPS / Harmony
  • The 9 km global and North Polar grids are in the same file.
• NSIDC DAAC — SPL4SMGP V8 — Earthdata Search / earthaccess / HTTPS / Harmony / AppEEARS
  • Eight files per day (3-hourly averages). Analysis updates (SPL4SMAU) and land model constants (SPL4SMLM) are separate collections. AppEEARS can extract points or areas.
• NSIDC DAAC — SPL4CMDL V8 — Earthdata Search / earthaccess / HTTPS / Harmony / AppEEARS / Worldview
  • Each daily file holds NEE, GPP, Rh and SOC, plus per-plant-functional-type (PFT) means.
• NASA Earthdata Cloud (AWS S3, us-west-2) — Direct cloud access (S3, from us-west-2 only)
  • Requires temporary S3 credentials. Useful for bulk processing on compute in the same region.

Example code¶

Uses earthaccess to download a week of SPL3SMP_E V6 in July 2024 and read morning (AM) soil moisture, keeping only recommended-quality values. You will be asked for your Earthdata username and password on first use. EASE-Grid 2.0 files have no 1-D latitude/longitude coordinates, so use the latitude/longitude arrays in the same group (or EASE-Grid 2.0 coordinate files) for mapping.

import earthaccess
import h5py
import numpy as np

earthaccess.login()  # free NASA Earthdata account
results = earthaccess.search_data(
    short_name="SPL3SMP_E", version="006",
    temporal=("2024-07-01", "2024-07-07"),
    bounding_box=(124, 33, 132, 39),   # (west, south, east, north)
)
files = earthaccess.download(results, "smap_l3e")

with h5py.File(files[0], "r") as f:
    g = f["Soil_Moisture_Retrieval_Data_AM"]
    sm = g["soil_moisture"][:]               # (1624, 3856), m3/m3
    qf = g["retrieval_qual_flag"][:]
sm = np.where((sm == -9999.0) | ~np.isin(qf, [0, 8]), np.nan, sm)
print(np.nanmean(sm))

# L4 3-hourly data: short_name="SPL4SMGP", version="008", group "Geophysical_Data", variable "sm_rootzone"

Practical tips¶

• For L3, keep only values whose retrieval_qual_flag is 0 or 8, which the user guide defines as recommended quality. Without this filter, lower-quality retrievals get mixed in.
• The 9 km SPL3SMP_E grid is interpolated from ~36 km observations. The scale actually resolved is still tens of kilometres, so do not treat individual 9 km cells as independent observations.
• During the safe-mode periods (2019-06-19 to 07-23, 2022-08-06 to 09-20) L3 has no data and L4 was filled by the model without observations. A data-quality caution was issued for a period after the 2019 recovery.
• If you downloaded data for 2026-05-14 to 07-28 before reprocessing, those files have the geolocation error. Download them again once the reprocessed files appear (a higher counter at the end of the file name marks the new file).
• L4 values are model estimates constrained by assimilated SMAP brightness temperatures, so they exist at depths (root zone) and on days without observations. Root-zone soil moisture is a GEOS land model estimate, not an observation.
• SPL4SMGP file times are the centre of the 3-hour averaging interval (T013000 = 00–03 UTC). For a Korean-time daily mean, group the eight files using a 15 UTC day boundary.
• Each new version reprocesses the whole record and the old version is retired (L3 V8 on 2024-12-16, L4 V7 on 2025-06-30). In your paper give the short_name and version, plus the CRID (R19…) or science version (Vv80…) from the file name.

Notices and change history¶

References for your paper¶

Dataset citation¶

SPL3SMP V9 (L3 36 km daily) — DOI 10.5067/4XXOGX0OOW1S

@misc{spl3smp_v9,
  author    = {O'Neill, P. E. and Chan, S. and Njoku, E. G. and Jackson, T. and Bindlish, R. and Chaubell, J.},
  title     = {{SMAP} {L3} Radiometer Global Daily 36 km {EASE-Grid} Soil Moisture, Version 9},
  year      = {2023},
  publisher = {NASA National Snow and Ice Data Center Distributed Active Archive Center},
  address   = {Boulder, Colorado USA},
  doi       = {10.5067/4XXOGX0OOW1S}
}

SPL3SMP_E V6 (L3 enhanced 9 km daily) — DOI 10.5067/M20OXIZHY3RJ

@misc{spl3smp_e_v6,
  author    = {O'Neill, P. E. and Chan, S. and Njoku, E. G. and Jackson, T. and Bindlish, R. and Chaubell, J. and Colliander, A.},
  title     = {{SMAP} Enhanced {L3} Radiometer Global and Polar Grid Daily 9 km {EASE-Grid} Soil Moisture, Version 6},
  year      = {2023},
  publisher = {NASA National Snow and Ice Data Center Distributed Active Archive Center},
  address   = {Boulder, Colorado USA},
  doi       = {10.5067/M20OXIZHY3RJ}
}

SPL4SMGP V8 (L4 3-hourly soil moisture) — DOI 10.5067/T5RUATAQREF8

@misc{spl4smgp_v8,
  author    = {Reichle, R. H. and De Lannoy, G. and Koster, R. D. and Crow, W. T. and Kimball, J. S. and Liu, Q. and Bechtold, M.},
  title     = {{SMAP} {L4} Global 3-hourly 9 km {EASE-Grid} Surface and Root Zone Soil Moisture Geophysical Data, Version 8},
  year      = {2025},
  publisher = {NASA National Snow and Ice Data Center Distributed Active Archive Center},
  address   = {Boulder, Colorado USA},
  doi       = {10.5067/T5RUATAQREF8}
}

SPL4CMDL V8 (L4 daily carbon net ecosystem exchange) — DOI 10.5067/U7SN8JDZL0UC

@misc{spl4cmdl_v8,
  author    = {Kimball, J. S. and Endsley, A. and Jones, L. A. and Kundig, T. and Reichle, R.},
  title     = {{SMAP} {L4} Global Daily 9 km {EASE-Grid} Carbon Net Ecosystem Exchange, Version 8},
  year      = {2025},
  publisher = {NASA National Snow and Ice Data Center Distributed Active Archive Center},
  address   = {Boulder, Colorado USA},
  doi       = {10.5067/U7SN8JDZL0UC}
}

Reference papers¶

SMAP mission overview — DOI 10.1109/JPROC.2010.2043918

@article{entekhabi2010smap,
  author  = {Entekhabi, Dara and Njoku, Eni G. and O'Neill, Peggy E. and Kellogg, Kent H. and Crow, Wade T. and Edelstein, Wendy N. and Entin, Jared K. and Goodman, Shawn D. and Jackson, Thomas J. and Johnson, Joel and Kimball, John and Piepmeier, Jeffrey R. and Koster, Randal D. and Martin, Neil and McDonald, Kyle C. and Moghaddam, Mahta and Moran, Susan and Reichle, Rolf and Shi, J. C. and Spencer, Michael W. and Thurman, Samuel W. and Tsang, Leung and Van Zyl, Jakob},
  title   = {The Soil Moisture Active Passive ({SMAP}) Mission},
  journal = {Proceedings of the IEEE},
  volume  = {98},
  number  = {5},
  pages   = {704--716},
  year    = {2010},
  doi     = {10.1109/JPROC.2010.2043918}
}

SMAP L4 soil moisture algorithm (as of Version 4) — DOI 10.1029/2019MS001729

@article{reichle2019l4sm,
  author  = {Reichle, Rolf H. and Liu, Qing and Koster, Randal D. and Crow, Wade T. and De Lannoy, Gabrielle J. M. and Kimball, John S. and Ardizzone, Joseph V. and Bosch, David and Colliander, Andreas and Cosh, Michael and Kolassa, Jana and Mahanama, Sarith P. and Prueger, John and Starks, Patrick and Walker, Jeffrey P.},
  title   = {Version 4 of the {SMAP} Level-4 Soil Moisture Algorithm and Data Product},
  journal = {Journal of Advances in Modeling Earth Systems},
  volume  = {11},
  number  = {10},
  pages   = {3106--3130},
  year    = {2019},
  doi     = {10.1029/2019MS001729}
}

Acknowledgements¶

NSIDC asks users to cite the DOI of each product and version actually used, describe the subset (region and period) and give the access date. Each product has its own DOI, so cite L3 and L4 separately if you used both. The L4_SM analysis update (SPL4SMAU, 10.5067/02LGW4DGJYRX) and land model constants (SPL4SMLM, 10.5067/PXQIBL2ALDZD) have separate DOIs.

Information on the datasets used in this study was compiled with the help of the Korean Climate Data Navigator (KCDN) (Sim, 2026; https://doi.org/10.5281/zenodo.23300923).

Links¶

• NSIDC SMAP data collection
• NSIDC SMAP Data Announcements
• SMAP version history
• SPL3SMP V9 user guide
• SPL3SMP_E V6 user guide
• SPL4SM V8 user guide
• L4_SM Product Specification Document (PSD v17, Version 8)
• SPL4CMDL V8 user guide
• NASA JPL SMAP mission page
• This site — SMAP sea surface salinity (ocean products from the same satellite)