Assessment of the SMAP Level-4 Surface and Root-Zone Soil Moisture Product Using In Situ Measurements

Rolf H. Reichle , Gabrielle J. M. de Lannoy , Qing Liu , Joseph V. Ardizzone , Andreas Colliander , Austin Conaty , Wade Crow , Thomas J. Jackson , Lucas A. Jones , John S. Kimball , Randal D. Koster , Sarith P. Mahanama , Edmond B. Smith , Aaron Berg , Simone Bircher (1) , David Bosch , Todd G. Caldwell , Michael Cosh , Ángel González-Zamora , Chandra D. Holifield Collins , Karsten H. Jensen , Stan Livingston , Ernesto Lopez-Baeza , José Martínez-Fernández , Heather Mcnairn , Mahta Moghaddam , Anna Pacheco , Thierry Pellarin (2) , John Prueger , Tracy Rowlandson , Mark Seyfried , Patrick Starks , Zhongbo Su , Marc Thibeault , Rogier van Der Velde , Jeffrey Walker , Xiaoling Wu , Yijian Zeng

1 CESBIO - Centre d'études spatiales de la biosphère
2 IGE - Institut des Géosciences de l’Environnement

The Soil Moisture Active Passive (SMAP) mission Level-4 Surface and Root-Zone Soil Moisture (L4_SM) data product is generated by assimilating SMAP L-band brightness temperature observations into the NASA Catchment land surface model. The L4_SM product is available from 31 March 2015 to present (within 3 days from real time) and provides 3-hourly, global, 9-km resolution estimates of surface (0-5 cm) and root-zone (0-100 cm) soil moisture and land surface conditions. This study presents an overview of the L4_SM algorithm, validation approach, and product assessment versus in situ measurements. Core validation sites provide spatially averaged surface (root zone) soil moisture measurements for 43 (17) "reference pixels" at 9- and 36-km gridcell scales located in 17 (7) distinct watersheds. Sparse networks provide point-scale measurements of surface (root zone) soil moisture at 406 (311) locations. Core validation site results indicate that the L4_SM product meets its soil moisture accuracy requirement, specified as an unbiased RMSE (ubRMSE, or standard deviation of the error) of 0.04 m³ m^-3 or better. The ubRMSE for L4_SM surface (root zone) soil moisture is 0.038 m³ m^-3 (0.030 m³ m^-3) at the 9-km scale and 0.035 m³ m^-3 (0.026 m³ m^-3) at the 36-km scale. The L4_SM estimates improve (significantly at the 5% level for surface soil moisture) over model-only estimates, which do not benefit from the assimilation of SMAP brightness temperature observations and have a 9-km surface (root zone) ubRMSE of 0.042 m³ m^-3 (0.032 m³ m^-3). Time series correlations exhibit similar relative performance. The sparse network results corroborate these findings over a greater variety of climate and land cover conditions.

Domains: Sciences of the Universe [physics]; Earth Sciences

Rolf H. Reichle, Gabrielle J. M. de Lannoy, Qing Liu, Joseph V. Ardizzone, Andreas Colliander, et al.. Assessment of the SMAP Level-4 Surface and Root-Zone Soil Moisture Product Using In Situ Measurements. Journal of Hydrometeorology, 2017, 18, pp.2621-2645. ⟨10.1175/JHM-D-17-0063.1⟩. ⟨insu-03668266⟩