SMAP Validation Experiment 2019-2022 (SMAPVEX19-22): Field campaign to improve soil moisture and vegetation optical depth retrievals in temperate forests

Authors: COLLIANDER, A - Jet Propulsion Laboratory; Cosh, Michael; BOURGEAU-CHAVEZ, L - Michigan Technological University; KELLY, V - Cary Institute Of Ecosystem Studies; Kraatz, Simon; SIQUEIRA, P - University Of Massachusetts, Amherst; WALKER, V - Oak Ridge Institute For Science And Education (ORISE); CHEN, X - University Of Massachusetts, Amherst; ROY, A - University Of Quebec; LAKHANKAR, T - City University Of New York; MCDONALD, K - City University Of New York; STEINER, N - City University Of New York; KURUM, M - University Of Georgia

Submitted to: IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/27/2025
Publication Date: 3/3/2025
Citation: Colliander, A., Cosh, M.H., Bourgeau-Chavez, L., Kelly, V., Kraatz, S.G., Siqueira, P., Walker, V.A., Chen, X., Roy, A., Lakhankar, T., Mcdonald, K., Steiner, N., Kurum, M. 2025. SMAP Validation Experiment 2019-2022 (SMAPVEX19-22): Field campaign to improve soil moisture and vegetation optical depth retrievals in temperate forests. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing. 18:10749-10771. https://doi.org/10.1109/JSTARS.2025.3553085.
DOI: https://doi.org/10.1109/JSTARS.2025.3553085

Interpretive Summary: Monitoring soil moisture status within forested environments is of interest to civil agencies who deal with ecosystem health as well as fire risk. It is difficult to physically monitor large expanses of forests. Remote sensing provides an alternative with high temporal repeat and reasonable accuracy if algorithms can be developed to accurately estimate soil moisture through the forest canopy. An experiment was established to compare data from the Soil Moisture Active Passive experiment for temperate eastern forests in the United States to begin the algorithm development. For four years, data was collected from in situ stations and compared to the satellite product to determine accuracy and identify needs for future development of algorithms. A key issues is to determine what the ground reference is for soil moisture at the forest floor and what is visible to the satellite sensor. This research is of value to sensor developers and forest managers.

Technical Abstract: Validated retrieval of forest soil moisture (SM) and vegetation optical depth (VOD) is one of the long-standing unresolved issues hindering the advances in hydrology, ecology, and understanding of the Earth system. The fundamental reason for the lack of progress is the lack of adequate reference data in forested areas; to close this gap, NASA’s Soil Moisture Active Passive (SMAP) mission, with its partners, conducted a field experiment in temperate forests of northeast USA, named the SMAP Validation Experiment 2019-2022 (SMAPVEX19-22). The primary objective of SMAPVEX19-22 was to improve the SM and VOD retrievals in forested areas. It incorporated a comprehensive measurement regime to characterize SM in the top layers of soil, surface conditions, and vegetation. The scope and scale of the campaign exceeded anything that has been done thus far for the development of forest SM and VOD retrieval algorithms. The first analysis of the measurements showed the value of all measurements and deployed instruments. The campaign showed that the experimental deployment of tree sensors with SM sensors and the destructive sampling of the vegetation water content of branches and leaves were successful; they should be significantly expanded in similar future endeavors. The low-cost zenith-pointing cameras incorporated into the SM network provided the coverage and temporal fidelity to track the phenology at the sites, providing an invaluable reference for brightness temperature (TB) interpretation. Vegetation structure was sampled manually and with a mobile terrestrial laser scanning (MLS), demonstrating the capability of MLS to characterize the three-dimensional forest structure needed for electromagnetic modeling and interpretation of microwave measurements. A foreseen challenge was the characterization of SM in the organic soils typical for forests. The three-sensor design of the deployed network and the two-depth manual measurements are crucial for understanding the evolution of SM and its effect on TB. However, determining volumetric SM from the soil samples requires further development because of the difficulty characterizing the spatially highly variable soil bulk density. The comparisons of the experiment measurements to the SMAP TB observations showed SMAP’s ability to retrieve the soil RDC simultaneously with VOD, including the frozen season with reduced RDC and VOD, for footprints partially and fully forested in the temperature forests of northeastern USA. The results also indicated that the L-band scattering albedo is temporally variable, and the L-band TB is sensitive to leaves of the typical deciduous forests in the northeastern USA, which will impact the future design of SM and VOD retrieval algorithms.