SMAP Level 4 Surface and Root Zone Soil Moisture (L4_SM)
The primary SMAP measurements of land surface microwave emission at 1.41 GHz are directly related to surface soil moisture (in the top 5 cm of the soil column). Many applications, however, require knowledge of root zone soil moisture (defined here as soil moisture in the top 1 m of the soil column), which is not directly linked to SMAP observations. The foremost objective of the SMAP Level 4 Surface and Root Zone Soil Moisture (L4_SM) product is to fill this gap and provide estimates of root zone soil moisture that are informed by and consistent with SMAP observations. Such estimates are obtained by merging SMAP observations with estimates from a land surface model in a soil moisture data assimilation system.
Fig 1: SMAP L4_SM root zone soil moisture on Jun 1, 2015. Inset: Rainfall percentage of mean in May 2015 (Source: Bureau of Meteorology).
Table 1: Unbiased RMSE of SMAP L4_SM surface and root zone soil moisture vs in situ measurements from SMAP core validation site reference pixels
The L4_SM algorithm is a variant of the NASA GEOS-5 land data assimilation system and includes the following components:
- Distributed ensemble Kalman filter (EnKF)
- NASA GEOS-5 Catchment land surface model
- Surface meteorological forcing from NASA GEOS-5 Forward Processing system
- Precipitation corrections with NOAA Climate Prediction Center “Unified” global, 0.5 degree gauge-based product
- SMAP L1C_TB observations
- Seasonal Tb bias correction
References
De Lannoy, G. J. M., and R. H. Reichle (2015), Global Assimilation of Multi-Angular SMOS Brightness Temperature Observations into the GEOS-5 Catchment Land Surface Model for Soil Moisture Estimation, Journal of Hydrometeorology, conditionally accepted.
Entekhabi, D. et al. 2014. SMAP Handbook–Soil Moisture Active Passive: Mapping Soil Moisture and Freeze/Thaw from Space. Pasadena, CA USA: SMAP Project, JPL CL#14-2285, Jet Propulsion Laboratory.
Reichle, R. H., and Q. Liu. 2014. Observation-Corrected Precipitation Estimates in GEOS-5. NASA/TM2014-104606, Vol. 35. Global Modeling and Assimilation Office, Goddard Space Flight Center, Greenbelt, MD. http://gmao.gsfc.nasa.gov/pubs/docs/Reichle734.pdf, 495 KB.
Reichle, R. H., R. Koster, G. De Lannoy, W. Crow and J. Kimball, 2014. SMAP Algorithm Theoretical Basis Document: L4 Surface and Root-Zone Soil Moisture Product. SMAP Project, JPL D-66483, Jet Propulsion Laboratory, Pasadena, CA. http://nsidc.org/sites/nsidc.org/files/files/272_L4_SM_RevA_web.pdf, 1.4 MB
Reichle, R. H., R. A. Lucchesi, J. V. Ardizzone, G.-K. Kim, E. B. Smith, and B. H. Weiss, 2015: Soil Moisture Active Passive (SMAP) Mission Level 4 Surface and Root Zone Soil Moisture (L4_SM) Product Specification Document. GMAO Office Note No. 10 (Version 1.4), 82 pp, NASA Goddard Space Flight Center, Greenbelt, MD, USA. Available from http://gmao.gsfc.nasa.gov/pubs/office_notes/
Reichle, R. H. et al. 2015. Soil Moisture Active Passive (SMAP) Project Assessment Report for the Beta-Release L4_SM Data Product. NASA/TM2015-104606, Vol. 40. Global Modeling and Assimilation Office, Goddard Space Flight Center, Greenbelt, MD. http://gmao.gsfc.nasa.gov/pubs/tm/, 5812 KB.
Reichle, R., G. De Lannoy, R. Koster, W. Crow, and J. Kimball. 2015. SMAP L4 9 km EASE-Grid Surface and Root Zone Soil Moisture Analysis Update. Version 1. [indicate subset used]. Boulder, Colorado USA: NASA National Snow and Ice Data Center Distributed Active Archive Center. http://dx.doi.org/10.5067/SHSINYSFEV8L.
Reichle, R., G. De Lannoy, R. Koster, W. Crow, and J. Kimball. 2015. SMAP L4 9 km EASE-Grid Surface and Root Zone Soil Moisture Geophysical Data. Version 1. [indicate subset used]. Boulder, Colorado USA: NASA National Snow and Ice Data Center Distributed Active Archive Center. http://dx.doi.org/10.5067/HJK4FUNIML52.
Reichle, R., G. De Lannoy, R. Koster, W. Crow, and J. Kimball. 2015. SMAP L4 9 km EASE-Grid Surface and Root Zone Soil Moisture Land Model Constants. Version 1. [indicate subset used]. Boulder, Colorado USA: NASA National Snow and Ice Data Center Distributed Active Archive Center. http://dx.doi.org/10.5067/ORO7MNBKY525.
