Validation of remotely sensed and modelled soil moisture at forested and unforested NEON sites

Authors: AYRES, E - National Ecological Observatory Network (NEON); REICHLE, R - National Aeronautics And Space Administration (NASA); COLLIANDER, A - California Institute Of Technology; Cosh, Michael; SMITH, L - National Ecological Observatory Network (NEON)

Submitted to: IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/11/2024
Publication Date: 7/19/2024
Citation: Ayres, E., Reichle, R., Colliander, A., Cosh, M.H., Smith, L. 2024. Validation of remotely sensed and modelled soil moisture at forested and unforested NEON sites. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing. 17:14248-14264. https://doi.org/10.1109/JSTARS.2024.3430928.
DOI: https://doi.org/10.1109/JSTARS.2024.3430928

Interpretive Summary: Long term monitoring of soil moisture is valuable for ecological and hydrological applications, but observational platforms are difficult to operate for long time scales. A comparison between a long term ecological network, an assimilation modeled product and a satellite soil moisture product was conducted to understand performance differences between these products. Forested site products were not as well correlated as the unforested sites, because of the high biomass at these sites, which is a confounding nature of the water within the vegetation. This work will have an impact on scientific interpretations of these datasets and how they are used in operations and decision support tools.

Technical Abstract: Soil moisture (SM) is an important driver for forest ecosystems, creating a need for globally extensive SM information that only satellite-based sensors or models can achieve. However, the reliability of remotely sensed or modeled SM data in forests is poorly understood due to a lack of suitable validation sites and interference with remote sensing caused by vegetation water content. Here we examine three multi-year SM products: (i) remotely sensed surface SM from a combination of Soil Moisture Active Passive (SMAP) and Sentinel-1 observations (SMAP/Sentinel), (ii) the SMAP Level-4 surface and root-zone (0-1 m) SM data assimilation product (SMAP-L4), and (iii) simulated surface and root-zone SM from the North American Land Data Assimilation System (NLDAS). These estimates were compared with in situ measurements from 39 National Ecological Observatory Network sites throughout the contiguous US. At 21 unforested sites, the performance of the three data products was similar for surface SM, and all three were able to track temporal changes in surface SM. The performance of the three products declined at the 18 forested sites; however, while the difference in performance was modest for SMAP-L4 and NLDAS, the performance of SMAP/Sentinel declined so much that it was largely unable to track changes in surface SM. The SMAP-L4 and NLDAS products also reliably captured temporal changes in root-zone SM at both forested and unforested sites. Our findings indicate that both the SMAP-L4 and NLDAS products can be used to track surface and root-zone SM changes in forests (unbiased RMSD: 0.03-0.06 m3 m-3 ).