The soil moisture active passive experiments: Towards calibration and validation of the SMAP mission

Authors: YE, N. - Monash University; WALKER, J. - Monash University; WU, X. - Monash University; DE JEU, R.A.M. - Bennekom, The Netherlands; GAO, Y. - Monash University; Jackson, Thomas; JONARD, F. - Universite Catholique; ENTEKHABI, D. - Cambridge University; KIM, E. - Goddard Space Flight Center; MERLIN, O. - University Of Toulouse; PAUWELS, V. - Monash University; RENZUILLO, L. - Csiro European Laboratory; RUDIGER, C. - Monash University; SADAGHY, S. - Monash University; VON HEBEL, C. - Bennekom, The Netherlands; YUEH, S. - Jet Propulsion Laboratory; ZHU, L. - Monash University

Submitted to: Remote Sensing of Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/22/2018
Publication Date: 11/28/2018
Citation: Ye, N., Walker, J., Wu, X., De Jeu, R., Gao, Y., Jackson, T.J., Jonard, F., Entekhabi, D., Kim, E., Merlin, O., Pauwels, V., Renzuillo, L., Rudiger, C., Sadaghy, S., Von Hebel, C., Yueh, S., Zhu, L. 2018. The soil moisture active passive experiments: Towards calibration and validation of the SMAP mission. Remote Sensing of Environment. 221:363-372. https://doi.org/10.1016/j.rse.2018.11.033.
DOI: https://doi.org/10.1016/j.rse.2018.11.033

Interpretive Summary: Soil moisture products from the Soil Moisture Active Passive (SMAP) satellite were validated by a series of field experiments conducted in Australia. Following the launch of the SMAP satellite it was critical that the products be validated for a wide range of conditions. The field experiments involved both ground-based and aircraft observations. The SMAP validation results show a good agreement to field experiment brightness temperature observations as well as the passive microwave-based soil moisture product. Inter-comparison of radiometer observations and retrieved soil moisture between SMAP and another soil moisture satellite (Soil Moisture Ocean Salinity-SMOS) showed very good agreement. This result suggests that SMAP and SMOS can be easily combined to generate a consistent long term soil moisture record that will benefit agricultural hydrology.

Technical Abstract: The Soil Moisture Active and Passive (SMAP) mission was launched by the National Aeronautics and Space Administration (NASA) on 31st January 2015, with the objective to measure global top ~5 cm soil moisture every 2 to 3 days at 9 km resolution using a combination of L-band radiometer observations at 36 km resolution and radar observations at 3 km resolution. To evaluate the SMAP microwave observations and derived soil moisture products, the fourth and fifth Soil Moisture Active Passive Experiments (SMAPEx-4 & -5) were conducted at the beginning of SMAP optional phase being in May and September 2015. The experiments were conducted over the Murrumbidgee River Catchment in south east of Australia, being where the OzNet soil moisture monitoring network has been established and maintained since 2001. During the three weeks period of each campaign, airborne radar and radiometer observations at the same microwave frequencies as SMAP were collected over SMAP 3-dB footprints concurrent with its overpass. In addition, intensive ground sampling of soil moisture, vegetation water content, and surface roughness was carried out primarily for validation of airborne soil moisture retrieval over six ~3 km by ~3 km focus areas, each of which covers a SMAP radar EASE-2 3-km pixel. Airborne radar and radiometer observations, retrieved soil moisture, and upscaled ground soil moisture measurements were taken as independent reference data when evaluating the SMAP products. The SMAP radar, radiometer and derived soil moisture showed a high agreement with the SMAPEx-4 and -5 dataset, with a correlation coefficient (R) of better than 0.9 and a Root-Mean-Squared Error (RMSE) of better than 5 K for radiometer brightness temperature, and better than 0.04 m3/m3 for the radiometer only soil moisture product. The SMAP radar backscatter and retrieved soil moisture had RMSEs of 3.4 dB and 0.11 m3/m3 respectively, when compared with SMAPEx-4 data set. Moreover, the SMAP radar and radiometer downscaled soil moisture showed a R of 0.77 and RMSE of 0.07 m3/m3. Comparisons were also made with other satellite radiometers operating at L-band (i.e. the Soil Moisture and Ocean Salinity (SMOS) and Aquarius).