Soil moisture and vegetation optical depth from cross-calibrated SMOS brightness temperatures using SMAP algorithms

Authors: CHAUBELL, JULIAN - Jet Propulsion Laboratory; YUEH, SIMON - Jet Propulsion Laboratory; HAYASHI, AKIKO - Jet Propulsion Laboratory; BINDLISH, RAJAT - Goddard Space Flight Center; COLLIANDER, ANDREAS - Jet Propulsion Laboratory; DUNBAR, SCOTT - Jet Propulsion Laboratory; ENTEKHABI, DARA - Massachusetts Institute Of Technology; ASANUMA, JUN - University Of Tsukuba; BERG, AARON - University Of Geulph; Coffin, Alisa; Cosh, Michael; Flerchinger, Gerald; Fortuna, Ann-Marie; Holifield Collins, Chandra; LYCHUK, TARAS - Agriculture And Agri-Food Canada; MARTINEZ-FERNANDEZ, JOSE - University Of Salamanca; MOORE, KAYLA - Agriculture And Agri-Food Canada; ZHONGBO, SU - University Of Twente; WALKER, JEFFREY - Monash University

Submitted to: Remote Sensing of Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/13/2026
Publication Date: N/A
Citation: N/A

Interpretive Summary: National Aeronautics and Space Administration’s (NASA) Soil Moisture Active Passive (SMAP, since 2015) and European Space Agency’s (ESA) Soil Moisture and Ocean Salinity (SMOS, since 2009) missions use radiometers to measure polarimetric brightness temperature at L-band (1.4 GHz) and provide estimates of surface soil moisture and L-band vegetation optical depth. These estimates are provided approximately every 2-3 days at the equator, with a spatial resolution of ~40 km and with a local overpass time of 6 AM/PM. Integrating the brightness temperature observations (both AM and PM) from both L-band satellites (SMAP and SMOS) can reduce the equatorial revisit time to about 1 day. This helps to address fast-response hydrologic processes that cannot be addressed with the 2-3 days revisit, and will allow for capturing the minimum and maximum soil moisture conditions. By doing so, we will have a better understanding of events related to hydrologic conditions such as drainage and recharge to groundwater, early dry down after storms, and pre-storm soil moisture for runoff determination. This paper details integrating the SMAP and SMOS observations, leading to a product that combines soil moisture and vegetation optical depth. The SMOS brightness temperature was first adjusted based on the SMAP brightness temperature, making the combined records consistent. Then, the SMAP baseline soil moisture and L-band vegetation optical depth algorithm was applied to the SMOS brightness temperature record. The comparison of the retrieved soil moisture to core validation sites showed unbiased root-mean-square-difference of 0.039 m3/m3 and the retrieved vegetation optical depth demonstrated consistency with independent biomass and tree height estimates.

Technical Abstract: National Aeronautics and Space Administration’s (NASA) Soil Moisture Active Passive (SMAP, since 2015) and European Space Agency’s (ESA) Soil Moisture and Ocean Salinity (SMOS, since 2009) missions measure polarimetric brightness temperature (TB) at L-band (1.4 GHz) and provide estimates of surface soil moisture (SM) and L-band vegetation optical depth (L-VOD) estimates approximately every 2-3 days at equator, with a spatial resolution of ~40 km and with a local overpass time of 6 AM/PM. Integrating the TB observations (both AM and PM) from both L-band satellites (SMAP and SMOS) can reduce the equatorial revisit time to about 1 day, thus helping address fast-response hydrologic processes that cannot be addressed with the 2-3 days revisit. This will allow for capturing the minimum and maximum SM conditions and a better understanding of events such as drainage and recharge to groundwater, early dry down after storms, and pre-storm SM for runoff determination. This paper details integrating the SMAP and SMOS observations, leading to a combined SM and L-VOD product. The SMOS TB interpolated to 40° incidence angle was first adjusted based on the SMAP TB, making the combined TB record consistent. Then, the SMAP baseline SM and L-VOD retrieval algorithm was applied to the SMOS TB record. Finally, the comparison of the retrieved SM to core validation sites showed unbiased root-mean-square-difference of 0.039 m3/m3 and the retrieved L-VOD demonstrated consistency with independent biomass and tree height estimates.