## 3 Results and Discussion
### Important Features
\begin{table}
\begin{tabular}{l l l l l l l l} \hline \hline \multirow{2}{*}{[PERSON] et al. [60]} & \multirow{2}{*}{Global} & \multirow{2}{*}{RF} & \multicolumn{2}{l}{CYGNSS, MODS} & \multicolumn{2}{l}{CYGNSS reflectivity, trailing edge slope, specular point} & \multirow{2}{*}{-} & \multirow{2}{*}{SSM} & \multirow{2}{*}{RF} \\  & & & MODS & & & incidence angle, NDVI, vegetation water content, elevation, & \\  & & & & day, silt & & day, silt & & \\ \hline \multirow{2}{*}{[PERSON] et al. [181]} & \multirow{2}{*}{Global} & \multirow{2}{*}{RF} & \multicolumn{2}{l}{CYGNSS, SMAP} & \multicolumn{2}{l}{Relecttivity, TES, incidence angle, elevation, NDVI, VWC,} & \multirow{2}{*}{-} & \multirow{2}{*}{10} & \multirow{2}{*}{} \\  & & & & slit, day & & & \\ \hline \multirow{2}{*}{[PERSON] et al. [182]} & \multirow{2}{*}{Global} & \multirow{2}{*}{CNN} & \multicolumn{2}{l}{CYGNSS, SMAP} & \multicolumn{2}{l}{Analog power, effective scattering area, BRCS VWC, NDVI,} & \multirow{2}{*}{-} & \multirow{2}{*}{10} & \multirow{2}{*}{} \\  & & MODS & & elevation, slope, water percentage, day, silt & & & \\ \hline \multirow{2}{*}{[PERSON] et al. [183]} & \multirow{2}{*}{Global} & \multirow{2}{*}{CNN} & \multicolumn{2}{l}{CYGNSS, SMAP} & \multicolumn{2}{l}{DOLV, VWC, elevation, surface water occurrence} & \multirow{2}{*}{-} & \multirow{2}{*}{SSM} & \multirow{2}{*}{-} \\  & & & & & & \\ \hline \multirow{2}{*}{[PERSON] et al. [184]} & \multirow{2}{*}{Global} & \multirow{2}{*}{ANN} & \multirow{2}{*}{Fengwu, SMAP} & \multicolumn{2}{l}{Brightness temperature, microwave vegetation index} & \multirow{2}{*}{-} & \multirow{2}{*}{5} & \multirow{2}{*}{-} \\  & & & & & & \\ \hline \multirow{2}{*}{[PERSON] et al. [185]} & \multirow{2}{*}{Global} & \multirow{2}{*}{RF} & \multirow{2}{*}{MODS} & \multicolumn{2}{l}{LST \({}^{*}\), LST difference, NDVI, EVI, API \({}^{*}\), clay, sand, silt} & \multirow{2}{*}{-} & \multirow{2}{*}{SSM} & \multirow{2}{*}{} \\  & & & latitude \(\downarrow\), longitude \(\uparrow\) & & & & \\ \hline \multirow{2}{*}{[PERSON] et al. [186]} & \multirow{2}{*}{Global} & \multirow{2}{*}{Transfer Learning} & SMAP, Sentinel-1, & V, VH, incident angles, sand, clay, BD, elevation, slope, aspect, & \multirow{2}{*}{-} & \multirow{2}{*}{5} & \multirow{2}{*}{-} \\  & & & MODIS & NDVI, Ta, precipitation, day of year & & & \\ \hline \multirow{2}{*}{[PERSON] et al. [187]} & \multirow{2}{*}{China and USA} & \multirow{2}{*}{Deep and parallel} & \multirow{2}{*}{Sentine-1, \(\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{ \text{\text{\text{\text{ \text{ \text{ \texttexttexttexttexttexttexttexttexttext      {  \text{ }}}}}}}}}}}}}}}} 1 & \text{V,V+VH, incident angle, NDVI, elevation, land covers,} & \multirow{2}{*}{-} & \multirow{2}{*}{SSM} & \multirow{2}{*}{-} \\  & & model & Sentinel-2 & & soil properties & & \\ \hline \multirow{2}{*}{[PERSON] et al. [188]} & \multirow{2}{*}{United States and Mexico} & \multirow{2}{*}{Methyl CNN} & \multirow{2}{*}{MODS, SMAP} & \multicolumn{2}{l}{NDVL, LST, SSM} & \multirow{2}{*}{-} & \multirow{2}{*}{10} & \multirow{2}{*}{} \\  & & & & & & & \\ \hline \multirow{2}{*}{[PERSON] et al. [81]} & Australia and \multirow{2}{*}{Austria} & \multirow{2}{*}{CNN} & \multirow{2}{*}{Sentine-1} & \multirow{2}{*}{Sigma0, VH and Sigma0, VV or Gamma0,VH and Gamma0,VV} & [PERSON] & \multirow{2}{*}{20} & \multirow{2}{*}{-} \\  & & & or Beta0,VH and Beta0,VV & & & & \\ \hline \multirow{2}{*}{[PERSON] et al. [65]} & Australia and \multirow{2}{*}{Austria} & \multirow{2}{*}{CNN} & \multirow{2}{*}{Sentine-2} & B1–B12 bands (B1—Causal aerosol, B2—Blue, B3–Green, B4—Red, BS—Vegation red edge, B6—Vegation red edge, B7—Vegation red edge, B8—Nik, B8—Narrow NIR, B9—Water vapor, B10—SWIR–Cirrus, B11—SWIR *, B12—SWIR *, and TDR & \multirow{2}{*}{30} & \multirow{2}{*}{-} \\  & & & B12–W12 bands (B1—Causal aerosol, B2—Blue, B3–Green, B4—Red, BS—Vegation red edge, B6—Vegation red edge, B8—Vegation red edge, B8—Vegation red edge, B8—Nik, B8—Narrow NIR, B9—Water vapor, B10—SWIR–Cirrus, B11—SWIR *, and TDR & \multirow{2}{*}{30} & \multirow{2}{*}{-} \\  & & & B9–Water vapor, B10–SWIR–Cirrus, B11—SWIR *, and TDR & & & \\ \hline \multirow{2}{*}{[PERSON] et al. [93]} & USA and \multirow{2}{*}{Australia} & \multirow{2}{*}{ANN} & \multirow{2}{*}{CYGNSS} & Surface reflectivity, TES, and SP incidence angle, NDVI, VWC, elevation, slope, b-parameters. & \multirow{2}{*}{-} & \multirow{2}{*}{SSM} & \multirow{2}{*}{-} \\  & & & elevation, slope, b-parameters. & & & \\ \hline \multirow{2}{*}{[PERSON] et al. [189]} & USA and \multirow{2}{*}{Switzerland} & \multirow{2}{*}{CNN} & \multirow{2}{*}{SMAP} & \multirow{2}{*}{SSM} & \multirow{2}{*}{-} & \multirow{2}{*}{SSM} & \multirow{2}{*}{-} \\  & & & & & & \\ \hline \hline \end{tabular}  
* In the input feature column, asterisk (\({}^{*}\)) indicates the most important features for surface soil moisture estimation. All the abbreviations can be found in the Supplementary Table S3.  
\end{table}