Use of Bimodal Hydraulic Property Relationships to Characterize Soil Quality.

Soil hydraulic properties have a predominating impact on soil quality because they directly or indirectly control plant-available water and air capacities, infiltration and drainage, nutrient leaching, microbial activity, greenhouse gas generation, and carbon sequestration.  Soil hydraulic properties are most usefully described using semi-empirical relationships, such as the coupled van Genuchten soil water retention and hydraulic conductivity (θ-K-h) functions.  As many soils contain a “structure domain” of large pores and a “matrix domain” of small pores, their hydraulic properties are often better described using dual or “bimodal” θ-K-h functions, where one function applies to the structure domain and the other applies to the matrix domain.  This study describes how bimodal van Genuchten θ-K-h functions can be fitted to θ-K-h data, and then used to characterize soil quality from the perspective of storage and transmission of water and air in soils containing a structure domain of large pores and a matrix domain of small pores.  Relationships and example calculations are given for estimating the porosities, water and air capacities, air-entry values, and field capacities of the structure and matrix domains.