The unsaturated soil hydraulic conductivity as a sum, an average, or a junction of domain conductivities

Abstract

Recent models of the unsaturated hydraulic conductivity curve (UHCC) are the sum of separate UHCCs for domains of capillary water, film water, and water vapor. A new theoretical framework for aggregating domain conductivities to a bulk soil UHCC reveals that this requires parallel domains. The same theory also generates arithmetic, harmonic, and geometric averages of the liquid-water conductivities, which can be arithmetically averaged with the vapor conductivity. However, current models for capillary and film conductivities are intrinsic, i.e., valid within their respective domain. The vapor conductivity is a bulk conductivity, i.e., it gives the conductivity of the gaseous domain as it manifests itself in the soil. Conversion relationships use the domain volume fractions as approximations of the as-yet unknown weighting factors to convert between intrinsic and bulk conductivities. This facilitates consistent averaging of domain conductivities. The fitted curves for the capillary and film water depend on the averaging (or adding) method. Hence, they are not strictly characteristic of their respective domains. Truly intrinsic domain conductivity functions may not exist, or are perhaps merely tools to arrive at a good fit of the UHCC of the bulk soil. Given these complications, a simpler junction model that joins a capillary and a film limb at a junction point offers an attractive alternative.