Abstract
The Soil Moisture Active Passive (SMAP) mission with high-precision soil moisture (SM) retrieval products provides global daily composites of SM at 3, 9, and 36 km earth grids measured by L-band active and passive microwave sensors. The capability of passive microwave remote sensing has been recognized for the estimation of SM variations. The purpose of this work was to establish an interaction between the highly variable SM spatial distribution on the ground and the SMAP’s coarse resolution radiometer-based SM retrievals. In this work, SMAP Level 3 (L3) and Level 4 (L4) SM products are validated with in situ datasets observed from the different locations of the Soil Moisture Network within the ShanDian River (SMN-SDR) Basin over the period of January 2018 to December 2019. The values of the unbiased root mean square error (ubRMSE) for L3 (SPL3SMP_E) SM retrievals are close to the standard SMAP mission SM accuracy requirement of 0.04 m3/m3 at the 9-km scale, with an averaged ubRMSE value of 0.041 m3/m3 (0.050 m3/m3) for descending (ascending) SM with the correlation (R) values of 0.62 (0.42) against the sparse network sites. The L4 (SPL4SMGP) Surface and Root-zone SM (RZSM) estimates show less error (ubRMSE < 0.04) and high correlation (R > 0.60) values, and are consistent with the previous SMAP-based SM estimations. The SMAP L4 SM products (SPL4SMGP) performed well compared to the L3 SM retrieval products (SPL3SMP_E). In vegetated land, the variability and compatibility of the SMAP SM estimates with the evaluation metrics for both products (L3 and L4) showed a good performance in the grassland, then in the farmland, and worst in the woodlands. Finally, SMAP algorithm parameters sensitivity analysis of the satellite products was conducted to produce time-series and highly precise SM datasets in China.
Highlights
Soil moisture, a most active key variable, interrelates the processes of Earth’s energy and water between the atmosphere and land surface, and plays a vital role in hydrological processes [1,2], bio-geochemical processes, bio-ecological processes, and crop yield estimation [3]
This study focused on achieving the following purposes: (1) to validate the Soil Moisture Active Passive (SMAP) soil moisture (SM) products across the humid area (SMNSDR) of the North China Plain, over the period of 1 January, 2018 to 31 December, 2019, to provide practical implementation assistance in the region; (2) to evaluate the Level 3 (L3) and Level 4 (L4)
These metrics were calculated from the 3-hourly SPL4SMGP surface soil moisture (SSM)
Summary
A most active key variable, interrelates the processes of Earth’s energy and water between the atmosphere and land surface, and plays a vital role in hydrological processes [1,2], bio-geochemical processes, bio-ecological processes, and crop yield estimation [3]. The traditional method for determining the SM content requires reasonably precise ground assessments at a point scale utilizing site networks [7,8], such as the United States Department of Agriculture (USDA) and Soil Climate Analysis. The distribution of surface soil moisture (SSM) on a regional or global scale is difficult to identify due to the lack of sufficient ground measurement sites in many places. At a large spatial scale with a high temporal resolution, microwave remote sensing provides near-SSM datasets with relatively high accuracy. The SMAP [9], developed by National Aeronautics Space Agency (NASA), launched on 31 January 2015, providing high-resolution soil SM moisture and freeze/thaw datasets at 2–3-day intervals with a 36, 9, and 3-km earth grid estimated by active and passive microwave sensors.
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