Performance of SMOS soil moisture products over core validation sites

Authors: COLLIANDER, A. - Jet Propulsion Laboratory; KERR, Y. - Centre National De La Recherche Scientifique; WIGNERON, J-P. - Institut National De La Recherche Agronomique (INRA); AL-YAARI, A. - Universite Grenoble Alpes; RODRIGUEZ-FERNANDEZ, N. - Centre National De La Recherche Scientifique; LI, X. - Institut National De La Recherche Agronomique (INRA); CHAUBELL, J. - Jet Propulsion Laboratory; RICHAUME, P - Center For The Study Of The Biosphère From Space(CESBIO); MIALON, A - Center For The Study Of The Biosphère From Space(CESBIO); ASANUMA, J. - University Of Tsukuba; BERG, A - University Of Guelph; Bosch, David; CALDWELL, T - Us Geological Survey; Cosh, Michael; Holifield Collins, Chandra; MARTINEZ-FERNANDEZ, J - University Of Salamanca; MCNAIM, H - Agriculture And Agri-Food Canada; Seyfried, Mark; Starks, Patrick; SU, Z - University Of Twente; THIBEAULT, M - Scientific Research Commission Of Buenos Aires; WALKER, J.P. - Monash University

Submitted to: IEEE Geoscience and Remote Sensing Magazine
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/19/2023
Publication Date: 6/16/2023
Citation: Colliander, A., Kerr, Y., Wigneron, J., Al-Yaari, A., Rodriguez-Fernandez, N., Li, X., Chaubell, J., Richaume, P., Mialon, A., Asanuma, J., Berg, A., Bosch, D.D., Caldwell, T., Cosh, M.H., Holifield Collins, C.D., Martinez-Fernandez, J., Mcnaim, H., Seyfried, M.S., Starks, P.J., Su, Z., Thibeault, M., Walker, J. 2023. Performance of SMOS soil moisture products over core validation sites. IEEE Geoscience and Remote Sensing Magazine. 20:1-5. https://doi.org/10.1109/LGRS.2023.3272878.
DOI: https://doi.org/10.1109/LGRS.2023.3272878

Interpretive Summary: Accurate measurements of surface soil moisture and vegetation condition are valuable for a wide range of agricultural applications including irrigation scheduling, crop yield forecasting, drought assessment, and fertilizer management. The European Space Agency (ESA) launched the SMOS (Soil Moisture Ocean Salinity) satellite in 2009. One of the measurements made by the SMOS satellite is the soil water content near the earth’s surface. These estimates were compared against data collected at several high density partner networks across the world. The study evaluated various methods for estimating soil moisture utilizing the SMOS measurements. The accuracy of the SMOS soil moisture estimates were similar to that obtained from the NASA Soil Moisture Active Passive satellite. These results are useful for further refining SMOS soil moisture estimates and informing applications that use soil moisture remote sensing.

Technical Abstract: The European Space Agency (ESA) launched the SMOS (Soil Moisture Ocean Salinity) mission in 2009; currently, multiple global soil moisture (SM) products are based on the measurements of its L-band radiometer. In this study, four SMOS products are compared with each other: Level 2, Level 3, IC (INRA-CESBIO), and Near Real Time products. The comparisons are focused over core validation sites (CVS), whose spatial representativeness errors allow the estimation of the SM product performance for bias-insensitive metrics (unbiased root mean square error (ubRMSE) and correlation (R), and anomaly R) with negligible uncertainty and also for bias-sensitive metrics (mean difference (MD) and root mean square difference or RMSD) with acceptable uncertainty. When the products were compared with CVS independently, the results showed that the ubRMSE, R, and anomaly R of the IC product were better than those of the other products. At the same time, the MD was larger, indicating that the IC product had the greatest sensitivity to SM changes but still had room for improvement. However, the differences between the performances were smaller when the products were assessed using only the data points when each product had a valid retrieval. This indicates that the algorithms themselves have similar performance and that quality flagging of the retrievals substantially impacts the performance. The NASA SMAP mission also produces an SM product at a similar spatial resolution as SMOS using an L-band radiometer. The closeness of the ubRMSE, R, and anomaly R performance of the IC product and the SMAP product (0.039 m3/m3 vs. 0.041 m3/m3, 0.80 vs. 0.081, and 0.75 vs. 0.75) indicates that the SMOS and SMAP radiometers can achieve similar SM sensitivity. Because of the low uncertainty of the CVS for determining bias-insensitive metrics, this result is particularly important.