Use of Physically Based Models to Evaluate USDA Soil Moisture Classes

Soil Taxonomy differs from other international soil classification systems because it includes the estimation of the soil moisture regime (SMR) and is primarily based on the estimation of dry days in a soil moisture control section (SMCS). Alternatively, the International Committee on Soil Moisture and Temperature Regimes (ICOMMOTR) has proposed a new approach based on biweekly soil water potential values determined at a fixed depth. Currently, SMR estimation is performed through simple models (standard approaches), such as the Newhall and Billaux models, that are not physically based. The aims of this work were to test the appropriateness of both standard Soil Taxonomy and the ICOMMOTR approaches through the use of a hydrologic model based on the Richards equation (SWAP), and to evaluate possible SMR classification alternatives. The SWAP-derived SMR classifications were compared with those derived by standard approaches, as well as the ICOMMOTR proposals, under eight pedoclimatic conditions (southern and northern Italy) where the SWAP model was calibrated and validated. The standard approaches clearly overestimated the dry conditions in the SMCS. Being mainly climate based, however, they were able to separate different pedoclimatic settings. In contrast, the physically based approach showed realistic results in terms ofSMR estimation but did not differentiate pedoclimatic settings for either Soil Taxonomy or ICOMMOTR approaches. Three possible alternatives to SMR classification were investigated: (i) applying the ICOMMOTR classification approach supported by the use of hydrologic Richards-based models; (ii) changing the rules of Soil Taxonomy for classifying SMR and SMCS to better fit "real" soil hydrologic behavior; and (iii) continuing to apply Newhall and Billaux models but clearly clarifying their climate-driven criteria.