I think something that is missing from this discussion is the concept of “soil moisture subclasses.” A taxonomic moisture subclass that is not “typic” reflects a taxonomic intergrade or extragrade. Within the aridic areas of western part of the US we have significant areas of Xeric Aridic and Ustic Aridic soil moisture regimes where Mollisols are found.
Ustolls allow either an ustic moisture regime or an aridic regime bordering on ustic. Xerolls allow either a xeric moisture regime or an aridic regime bordering on xeric. If the moisture regime is aridic, the “bordering on” is portrayed by assigning a soil moisture subclass of xeric or ustic rather than typic. In some of these cases you can't rely on formative elements or the subgroup level classification to give you all of the moisture information. The series or component concept is needed.
The guidance and examples for how this is handled is covered in Soil Survey Technical Note 9 “Populating Taxonomic Moisture Class and Subclass in NASIS” (https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/ref/?cid=nrcs142p2_053576)
If we query all of the soil components in SSURGO with taxonomic suborder “Xerolls” we find that 61% (n=20801) are “Typic Xeric”, 25% (n=8331) are “Xeric Aridic” and 10% are “Aridic Xeric”, with bulk of the remainder being “Oxyaquic Xeric” and “Aquic Xeric.”
Similarly, looking at “Ustolls” we find that 52% (n=31156) are “Typic Ustic”, 22% are “Aridic Ustic”, 19% are “Udic Ustic” and 5% are “Ustic Aridic”.
If we take the “Xeric Aridic” and “Ustic Aridic” components these are our Mollisols that have an aridic soil moisture regime.
Some example soil series where these "aridic bordering on X" soil moisture concepts are used include: Sevenmile, Devada, Sumine, Urmafot, Ravendog, Ninemile.
I have attached an older guide used in the state of Nevada for standard soil moisture statements for range in characteristics as an example.
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Climate_statements_NV_1990.pdf
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