Fig 1.
Soil moisture prediction framework.
The folders are the inputs and outputs and the ovals are methods for data preparation (data bases harmonization), modeling (for prediction) and validation (for assessing the reliability of soil moisture maps). The field data from the North American Soil Moisture Database (NASMD) was only used for validation purposes (i.e., not for training the model).
Fig 2.
Elevation and hydrologically meaningful terrain parameters at 1x1km of spatial resolution derived using the standard SAGA-GIS basic terrain parameters module.
These maps were normalized (between 0–1) and then used as prediction factors to downscale soil moisture across CONUS.
Table 1.
The cross-validation results for each year.
This table shows the correlation, root mean squared error (RMSE) in m3/m3, the number of training data available (n), the optimal kernel function (okf), and the optimal number of neighbors used for predicting to new data (k).
Fig 3.
Annual means of soil moisture (1991–2016) downscaled to 1x1km grids across CONUS using terrain parameters as prediction factors.
Fig 4.
Comparison of the original (27km grids) and the downscaled (1km grids) soil moisture products.
Median (a, b) and standard deviation (c, d, sdev) values of satellite soil moisture and downscaled soil moisture values (1991–2016). Uncertainty reported by the ESA-CCI soil moisture (e) and the explained variance map (r2) between field data and downscaled soil moisture (f).
Fig 5.
Validation of soil moisture gridded estimates (original 27 and 1km grids) against NASMD field observations.
Dashed line represents the relationship of field stations and soil moisture gridded estimates at 27x27km, while black line represents the relationship between field stations and the downscaled 1x1km soil moisture product. In all cases (all sample sizes), the 1x1km product showed higher r2 with the NASMD than the ESA-CCI soil moisture estimates.
Fig 6.
Explained variances computed for each meteorological station of the NASMD and the corresponding pixel of our soil moisture predictions based on geomorphometry.
Fig 7.
Relationships between the first PC of terrain parameters with soil moisture field data (a), with the ESA-CCI satellite product (b), and with the soil moisture predictions based on terrain parameters (c).