Assessment of Surface Fractional Water Impacts on SMAP Soil Moisture Retrieval

Fractional water (FW) correction of satellite microwave brightness temperature (Tb) observations is a prerequisite for accurate soil moisture (SM) mapping over mixed land and water areas. Failure to adequately account for the influence of FW within a satellite footprint can bias the Tb signals over land, resulting in SM retrieval error. Here, we evaluated the FW impacts on NASA Soil Moisture Active Passive (SMAP) Lband (1.4 GHz) SM retrievals using two water masks including (a) the NASA Terra Moderate Resolution Imaging Spectroradiometer (MODIS) Land Water Mask version 6 (MOD44W) multi-year (2015-2019) water record and (b) the Ocean Discipline Processing System (ODPS) water mask previously used for SMAP global operational Tb and SM processing. The MOD44W and ODPS data were first compared with the European Commission's Joint Research Centre (JRC) Landsat-based water record averaged for non-frozen months from 2015 to 2019 over a 1-km global grid, excluding ocean and permanent snow/ice areas. MOD44W showed major improvements in land/water classifications relative to the ODPS, with producer accuracy increasing from 50.02% to 95.02%, and user accuracy from 53.93% to 91.73% for water pixels. For assessing the FW impacts on SM retrievals, the same single channel V-polarization (SCA-V) algorithm was applied to enhanced resolution (9-km) SMAP Tb datasets corrected using ODPS and MOD44W water masks separately. MOD44W showed overall greater FW values (mean FW increase of 0.006 for landdominated grid cells) relative to the ODPS, leading to relatively drier SM retrievals (mean decrease: -0.012 m3 /m3 ). However, the majority (~77%) of SMAP grid cells showed minimal FW coverage (FW ≤ 0.01), where associated impacts on SM retrievals are negligible (Root Mean Square Difference or RMSD 0.009 m3 /m3 ). Additional comparisons with globally distributed SM measurements confirmed consistently lower SM retrieval biases (mean decrease 0.04 m3 /m3 ) and higher correlations (mean increase 0.06) of the MOD44W-based results relative to those based on the ODPS over five station groups with absolute FW difference higher than 0.01. Our results revealed non-negligible SM retrieval uncertainty introduced from the underlying ancillary FW data for areas with substantial water presence (e.g. FW>0.01). More accurate representation of FW conditions over the SMAP era (post-2015) is expected to further improve the SMAP SM retrieval accuracy.