Abstract
This study evaluated the Advanced Microwave Scanning Radiometer 2 (AMSR2) L2 soil moisture product (ver. 3) using in situ hydrological observational data, acquired over 7 years (2012–2018), from a 50 × 50 km flat area of the Mongolian Plateau covered with bare soil, pasture and shrubs. Although AMSR2 slightly underestimated soil moisture content at 3-cm depth, satisfactory timing was observed in both the response patterns and the in situ soil moisture data, and the differences between these factors were not large. In terms of the relationship between AMSR2 soil moisture from descending orbits and in situ measured soil moisture at 3-cm depth, the values of the RMSE (m3/m3) and the bias (m3/m3) varied from 0.028 to 0.063 and from 0.011 to − 0.001 m3/m3, respectively. The values of the RMSE and bias depended on rainfall condition. The mean value of the RMSE for the 7-year period was 0.042 m3/m3, i.e., lower than the target accuracy 0.050 m3/m3. The validation results for descending orbits were found slightly better than for ascending orbits. Comparison of the Soil Moisture and Ocean Salinity (SMOS) soil moisture product with the AMSR2 L2 soil moisture product showed that AMSR2 could observe surface soil moisture with nearly same accuracy and stability. However, the bias of the AMSR2 soil moisture measurement was slightly negative and poorer than that of SMOS with deeper soil moisture measurement. It means that AMSR2 cannot effectively measure soil moisture at 3-cm depth. In situ soil temperature at 3-cm depth and surface vegetation (normalized difference vegetation index) did not influence the underestimation of AMSR2 soil moisture measurements. These results suggest that a possible cause of the underestimation of AMSR2 soil moisture measurements is the difference between the depth of the AMSR2 observations and in situ soil moisture measurements. Overall, this study proved the AMSR2 L2 soil moisture product has been useful for monitoring daily surface soil moisture over large grassland areas and it clearly demonstrated the high-performance capability of AMSR2 since 2012.
Highlights
It is exceedingly important to study soil moisture behavior within the context of the hydrological cycle, and with respect to climate change and global warming [1]
Advanced Microwave Scanning Radiometer 2 (AMSR2) has been successful in monitoring soil moisture globally on a daily basis since its launch and data from the AMSR2 L2 soil moisture products are available from the Japan Aerospace Exploration Agency (JAXA) Global Change Observation Mission (GCOM)-W Web site [5]
AMSR2 observations were largely unaffected by radio frequency interference (RFI) during the observation period despite Soil Moisture and Ocean Salinity (SMOS) having known RFI problems [23]
Summary
It is exceedingly important to study soil moisture behavior within the context of the hydrological cycle, and with respect to climate change and global warming [1]. GCOM-W1 has remained in orbit at an altitude of approximately 700 km, during which time it has measured daily global soil moisture on the surface of the Earth with target accuracy of 0.05 m3/ m3 [5] giving spatial resolution better than 50 km. This orbit was adopted to maintain consistency with AMSR-E, and GCOM-W1 has been participating in the A-Train satellite constellation [9]. AMSR2 has been successful in monitoring soil moisture globally on a daily basis since its launch and data from the AMSR2 L2 soil moisture products are available from the JAXA GCOM-W Web site [5]
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