Comparison of Isothermal and Isobaric Water Retention Paths in Nonswelling Porous Materials

Water in porous media near the Earth's surface is subject to large fluctuations in pore water pressure and temperature, often causing significant changes in the degree of saturation. Quantitative comparisons of isothermal versus isobaric water retention are necessary to accurately predict changes in saturation due to the coupled influence of changes in the pore water matric potential ψ and temperature T. Yet there is a lack of experimental measurements of isobaric (i.e., constant ψ) water retention, inhibiting comparisons between isobaric and isothermal processes. In the present study the influence of the chronological sequence of changes in ψ and T is examined to determine whether the volumetric water content θ for any given final ψ‐T condition is independent of the ψ‐T sequence, when θ changes monotonically. To obtain the necessary data for these comparisons, isothermal water retention experiments were performed over a range in ψ from 0 to −100 kPa, and isobaric water retention experiments were performed at 20° and 80°C on core samples of a sandy soil and a nonwelded tuff. Results provide further evidence that the effect of T on ψ is considerably greater than estimates based on pore water capillary theory. For these materials the thermal enhancement of ψ was 4–12 times greater than capillary theory would predict. The effect of T on θ during isobaric water retention was several times greater for drying (warming) conditions than for wetting (cooling) conditions at a given ψ, resulting in net losses in θ ranging from 3 to 30%. For a given final ψ at 80°C, virtually identical θ values were obtained regardless of the chronological sequence of isothermal and isobaric drainage paths for both materials. This confirms the validity of unique θ (ψ, T) surfaces describing monotonic changes in θ as functions of both ψ and T in nonswelling porous materials. Determination of these θ (ψ, T) response surfaces for drying and wetting should yield water retention envelopes, useful in modeling water retention in near‐surface environments where both ψ and T vary.