Validation of SMAP soil moisture at terrestrial National Ecological Observatory Network (NEON) sites show potential for soil moisture retrieval in forested areas

Authors: AYRES, E. - Collaborator; COLLIANDER, A. - Jet Propulsion Laboratory; Cosh, Michael; ROBERTI, J.A. - Collaborator; SIMKIN, S. - Collaborator; GENAZZIO, M. - Collaborator

Submitted to: IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/7/2021
Publication Date: 10/19/2021
Citation: Ayres, E., Colliander, A., Cosh, M.H., Roberti, J., Simkin, S., Genazzio, M. 2021. Validation of SMAP soil moisture at terrestrial National Ecological Observatory Network (NEON) sites show potential for soil moisture retrieval in forested areas. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing. 14:10903-10918. https://doi.org/10.1109/JSTARS.2021.3121206.
DOI: https://doi.org/10.1109/JSTARS.2021.3121206

Interpretive Summary: The National Ecological Observatory Network (NEON) was established to provide a source for long time series ecological parameter datasets. Soil moisture data is one such parameter that plays a critical part of the evolution of an ecosystem. NEON operates a set of five soil moisture profiles across each of their sites. There were forty sites with sufficient data and homogenous landscape features that we used for validation of the Soil Moisture Active Passive (SMAP) mission’s 9 km soil moisture products, providing a new set of sites with high vegetation water content. The remote sensing products from SMAP had a reasonable accuracy for these forested regions which are usually beyond the assumed capability of the satellite instrument. This is an important step in the expansion of the soil moisture remote sensing product into more densely vegetation landscapes.

Technical Abstract: Soil moisture influences forest health, fire occurrence and extent, and insect and pathogen impacts, creating a need for regular, globally extensive soil moisture measurements that can only be achieved by satellite-based sensors, such as NASA’s Soil Moisture Active Passive (SMAP). However, SMAP data for forested regions, which account for ~20% of land cover globally, are flagged as unreliable due to interference from vegetation water content, and forests were underrepresented in previous validation efforts, preventing an assessment of measurement accuracy in these biomes. Here we compare over twelve thousand SMAP soil moisture measurements, representing 88 site years, to in-situ soil moisture measurements from forty National Ecological Observatory Network (NEON) sites throughout the US, half of which are forested. At unforested NEON sites, agreement with SMAP soil moisture (unbiased RMSD: 0.046 m3 m-3) was similar to previous sparse network validations (which include inflation of the metric due to the representativeness errors between the point measurements and area-average soil moisture). For the forested sites, SMAP achieved a reasonable level of accuracy (unbiased RMSD: 0.06 m3 m-3) indicating SMAP is sensitive to changes in soil moisture in forest ecosystems. Moreover, we identified that both an index of vegetation water content and canopy height were related to mean difference, which incorporates measurement bias and representativeness bias, and suggests a potential approach to improve SMAP algorithm parameterization for forested regions. In addition, expanding the number and extent of soil moisture measurements at forested validation sites would likely further reduce mean difference by minimizing representativeness errors.