The SMAP mission combined active-passive soil moisture product at 9km and 3km spatial resolutions

Authors: DAS, N. - Jet Propulsion Laboratory; ENTEKHABI, D. - Mitsubishi Chemical Usa, Inc; DUNBAR, R.S. - Jet Propulsion Laboratory; COLLIANDER, A. - Jet Propulsion Laboratory; CHEN, F. - Collaborator; Crow, Wade; Jackson, Thomas; BERG, A. - University Of Guelph; Bosch, David - Dave; CALDWELL, T. - University Of Texas; Cosh, Michael; Holifield Collins, Chandra; LOPEZ-BAEZA, E. - University Of Valencia

Submitted to: Remote Sensing of Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/5/2018
Publication Date: 6/1/2018
Citation: Das, N., Entekhabi, D., Dunbar, R., Colliander, A., Chen, F., Crow, W.T., Jackson, T.J., Berg, A., Bosch, D.D., Caldwell, T., Cosh, M.H., Holifield Collins, C.D., Lopez-Baeza, E. 2018. The SMAP mission combined active-passive soil moisture product at 9 km and 3km spatial resolutions. Remote Sensing of Environment. 211:204-217. https://doi.org/10.1016/j.rse.2018.04.011.
DOI: https://doi.org/10.1016/j.rse.2018.04.011

Interpretive Summary: Accurate measurements of soil moisture are valuable for a wide range of agricultural applications including: irrigation scheduling, crop yield forecasting, drought assessment, and fertilizer application management. In January 2015, NASA launched the Soil Moisture Active/Passive (SMAP) satellite mission with the goal of improving our ability to globally measure soil moisture from space. A shortcoming in past satellite-based soil moisture products has been their coarse spatial resolution (typically > 30 km) which limits their value for some agricultural applications. Therefore, a critical part of the SMAP mission is the development and production of higher-resolution (i.e., 3 and 9 km) soil moisture products via the merger of high-resolution (active) radar observations with lower-resolution (passive) microwave radiometry. This paper describes the production and validation of these products against available ground-based soil moisture observations. The remote sensing techniques developed (and validated) in this paper will eventually be utilized to produce high-quality soil moisture data products at spatial resolution which are more suitable for agricultural applications.

Technical Abstract: The NASA Soil Moisture Active Passive (SMAP) mission with onboard L-band radiometer and radar was launched on January 31st, 2015. The spacecraft provided high-resolution (3 km and 9 km) global soil moisture estimates at regular intervals by combining radiometer and radar observations for ~2.5 months. On July 7th, 2016, SMAP detected an anomaly in its radar, and consequently, the radar ceased operation. Thus, the SMAP capability to produce high-resolution soil moisture beyond the July date was no longer possible. However, the available ~2.5 months worth of the SMAP high-resolution Active-Passive soil moisture data (L2SMAP) falls during the Northern Hemisphere vegetation green-up and crop growth season. Thus the high-resolution data set, upon evaluation and documentation of its performance against in situ measurements in this study, is a valuable resource for scientific and applied science analyses. In this study we evaluate the global product with in situ data from core calibration and validation sites (CVS) and sparse networks. Out of many implementable options of the SMAP Active-Passive algorithm, the selected baseline algorithm and its associated soil moisture retrievals at 9 km spatial resolution has the best comparison statistics against the CVS and sparse networks. The overall unbiased root-mean-square-difference is close to 0.04 m3/m3 that satisfies the SMAP mission requirement. Another promising soil moisture product from the SMAP Active-Passive algorithm option is at 3 km spatial resolution. The analysis of this soil moisture retrievals at 3 km against CVS and sparse networks show an unbiased root-mean-square-difference of ~0.055 m3/m3. The SMAP L2SMAP product for ~2.5 months data is now validated and available to public through the NASA Distributed Active Archive Center (DAAC) at National Snow and Ice data Center (NSIDC). The L2SMAP product is packaged with the geo-coordinates, acquisition times, and all requisite ancillary information. The SMAP validated L2SMAP product is now good for use in geophysical applications and research.