Abstract
Satellite sensor systems for soil moisture measurements have been continuously evolving. The Soil Moisture Active Passive (SMAP) mission represents one of the latest advances in this regard. Thus far, much of our knowledge of the accuracy of SMAP soil moisture over the Great Lakes region of North America has originated from evaluation studies using in situ data from the U.S. Department of Agriculture (USDA) Natural Resources Conservation Service Soil Climate Analysis Network and/or the U.S. Climate Reference Network, which provide only several in situ sensor stations for this region. As such, these results typically underrepresent the accuracy of SMAP soil moisture in this region, which is characterized by a relatively large soil moisture variability and is one of the least studied regions. In this work, SMAP Level 2‒4 soil moisture products: SMAP/Sentinel-1 L2 Radiometer/Radar Soil Moisture (SPL2SMAP_S), SMAP Enhanced L3 Radiometer Soil Moisture (SPL3SMP_E), and SMAP L4 Surface and Root-Zone Soil Moisture Analysis Update (SPL4SMAU) are evaluated over the southern portion of the Great Lakes region using in situ measurements from Michigan State University’s Enviro-weather Automated Weather Station Network. The unbiased root-mean-square error (ubRMSE) values for both SPL4SMAU surface and root zone soil moisture estimates are below 0.04 m3 m−3 at the 36-km scale, with an average ubRMSE of 0.045 m3 m−3 (0.037 m3 m−3) for the surface (root-zone) soil moisture against the sparse network. The ubRMSE values for SPL3SMP_E a.m. (i.e., descending overpasses) soil moisture retrievals are close to or below 0.04 m3 m−3 at the 36-km scale, with an average ubRMSE of ~0.06 m3 m−3 against the sparse network. The average ubRMSE values are ~0.05‒0.06 m3 m−3 for high-resolution SPL2SMAP_S soil moisture retrievals against the sparse network, with the skill of the baseline algorithm-based soil moisture retrievals exceeding that of the optional algorithm-based counterparts. Clearly, the skill of SPL4SMAU surface soil moisture exceeds that of the SPL3SMP_E and SPL2SMAP_S soil moisture retrievals.
Highlights
Soil moisture, as a key state variable linking the land surface and the atmosphere, plays a crucial role in the Earth’s energy and water budgets
The Soil Moisture Active Passive (SMAP) carries an L-band radiometer and an L-band high-resolution radar to estimate near-surface soil moisture and freeze-thaw state, the radar failed in July 2015
We evaluated three sets of SMAP soil moisture products: SMAP Level 4 (L4) Global 3-h 9-km Equal-Area Scalable Earth (EASE)-Grid Surface and Root-Zone Soil Moisture Analysis Update (SPL4SMAU), version 4 [29]; SMAP Enhanced Level 3 (L3) Radiometer Global Daily 9-km EASE-Grid Soil Moisture (SPL3SMP_E), version 3 [30]; and SMAP/Sentinel-1 Level 2 (L2) Radiometer/Radar 30-Second Scene 3-km EASE-Grid Soil Moisture (SPL2SMAP_S), version 2 [31]
Summary
As a key state variable linking the land surface and the atmosphere, plays a crucial role in the Earth’s energy and water budgets. Especially satellite microwave remote sensing, is able to provide large-scale spatially distributed near-surface soil moisture estimates (e.g., [1,2,3,4,5,6,7,8]). The SMAP carries an L-band radiometer and an L-band high-resolution radar to estimate near-surface soil moisture and freeze-thaw state, the radar failed in July 2015. Except for the raw and/or calibrated instrument measurements (e.g., radiometer brightness temperature and radar backscatter data) as provided in the Level 1 products, the SMAP-derived soil moisture estimates are delivered through its Level 2, 3, and 4 products
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