Calibration of Noah soil hydraulic property parameters using surface soil moisture from SMOS and basin-wide in situ observations

Soil hydraulic properties (SHPs) control infiltration and redistribution of moisture in a soil column. The Noah land surface model (LSM) default simulation uses SHPs selected according to a location's mapped soil texture class. SHPs are instead estimated at seven sites in North America through calibration. A single-objective algorithm minimizes the root mean squared difference (RMSD) between simulated surface soil moisture and observations from: (1) a dense network of in situ soil probes, (2) SMOS (Soil Moisture Ocean Salinity) satellite retrievals, and (3) SMOS retrievals adjusted such that their mean equals that of the in situ network. Parameters are optimized in 2012 and validated in 2013 against the in situ network. RMSD and unbiased RMSD (ubRMSD) assess the resulting surface soil moisture behavior. At all sites, assigning SHP parameters from a different soil texture than the one that is mapped decreases the RMSD, by an average of 0.029 cm3 cm-3. Similar improvements result from calibrating parameters using in situ network data (0.031 cm3 cm-3). Calibrations using remotely-sensed data show comparable success (0.029 cm3 cm-3) if the SMOS product has no bias. Calibrated simulations are superior to texture-based simulations in their ability to decrease ubRMSD at times of year when the default simulation is worst. Changes to both RMSD and ubRMSD are small when the default simulation is already good. Most calibrated simulations have higher runoff ratios than do texture-based simulations, a change that warrants further evaluation. Overall, parameter selection using SMOS data shows good potential where biases are low.