Why do the soil moisture values in the SMAP Level-4 data vary from what I expect in a particular region?

There are a few reasons that the soil moisture data values in SMAP Level-4 data products may vary from what you expect in a particular region. The first step a data user should take in investigating apparently anomalous values is to look at the rich quality information and other data flags contained in SMAP data files and associated QA files. A user should also consider all of the ancillary data that are used in generating SMAP products. For the SMAP Level-4 data in particular, the soil parameter inputs are based on texture composites from the Harmonized World Soil Database version 1.21 (HWSD1.21) and the State Soil Geographic (STATSGO2) project. These inputs and the soil hydraulic parameters derived from them may not be perfect and regional expertise might well reveal the SMAP soil moisture values as a poor reflection of conditions on the ground. While the model may have difficulty estimating absolute values of soil moisture in some cases, the SMAP Level-4 soil moisture data should still display representative temporal variations. If a data user has a good reason to doubt the magnitude of soil moisture variations, he or she can rescale the product based on local knowledge.