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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
,
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
,
John Prueger
,
Tracy Rowlandson
,
Mark Seyfried
,
Patrick Starks
,
Zhongbo Su
,
Marc Thibeault
,
Rogier van der Velde
,
Jeffrey Walker
,
Xiaoling Wu
, and
Yijian Zeng

Abstract

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.

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