The Characterization of the Vertical Distribution of Surface Soil Moisture Using ISMN Multilayer In Situ Data and Their Comparison with SMOS and SMAP Soil Moisture Products

In this paper, we investigated the vertical distribution characteristics of surface soil moisture based on ISMN (International Soil Moisture Network) multilayer in situ data (5, 10, and 20 cm; 2, 4, and 8 in) and performed comparisons between the in situ data and four microwave satellite remote sensing products (SMOS L2, SMOS-IC, SMAP L2, and SMAP L4). The results showed that the mean soil moisture difference between layers can be -0.042 similar to -0.024 (for the centimeter group)/ -0.067 similar to -0.044 (for the inch group) m(3)/m(3) in negative terms and 0.020 similar to 0.028 (for the centimeter group)/0.036 similar to 0.040 (for the inch group) m(3)/m(3) in positive terms. The surface soil moisture was found to have very significant stratification characteristics, and the interlayer difference was close to or beyond the SMOS and SMAP 0.04 m(3)/m(3) nominal retrieval accuracy. Comparisons revealed that the satellite retrievals had a higher correlation with the field measurements of 5 cm/2 in, and SMAP L4 had the smallest difference with the in situ data. The mean difference caused by using 10 cm/4 in and 20 cm/8 in in situ data instead of the 5 cm/2 in data could be about 0.019 similar to 0.018/ 0.18 similar to 0.015 m(3)/m(3) and 0.026 similar to 0.023/ 0.043 similar to 0.039 m(3)/m(3), respectively, meaning that there would be a potential depth mismatch in the data validation.