We present a new model for the soil-water retention curve, (h m ), which, in contrast to earlier models, anchors the curve at zero water content and does away with the unspecified residual water content. The proposed equation covers the complete retention curve, with the pressure head, h m , stretching over approximately seven orders of magnitude. We review the concept of pF from its origin in the papers of Schofield and discuss what Schofield meant by the 'free energy, F'. We deal with (historical) criticisms regarding the use of the log scale of the pressure head, which, unfortunately, led to the apparent demise of the pF. We espouse the advantages of using the log scale in a model for which the pF is the independent variable, and we present a method to deal with the problem of the saturated water content on the semi-log graph being located at a pF of minus infinity. Where a smaller range of the water retention is being considered, the model also gives an excellent fit on a linear scale using the pressure head, h m , itself as the independent variable. We applied the model to pF curves found in the literature for a great variety of soil textures ranging from dune-sand to river-basin clay. We found the equation for the model to be capable of fitting the pF curves with remarkable success over the complete range from saturation to oven dryness. However, because interest generally lies in the plant-available water range (i.e. saturation, y s , to wilting point, y wp ), the following relation, which can be plotted on a linear scale, is sufficient for most purposes: ðh m Þ ¼ wp þ k 1 fexpðÀk 0 =15 000 n Þ À expðÀk 0 =h n m Þg, where k 0 , k 1 and n are adjustable fitting parameters.

No abstract

No abstract

Summary The structural condition of swelling soils can be assessed from their shrinkage curves. We re‐evaluated data on six British swelling soils using modern methods to model the void ratio, e, as a function of the moisture ratio, ϑ. The points on the e–ϑ curve were fitted with a constitutive shrinkage equation using an unbiased least‐squares, curve‐fitting program. The shrinkage curves were then differentiated to obtain their slopes, σ(ϑ), which were used to calculate the overburden potentials, Ω. The slope functions were subsequently differentiated to obtain the curvatures, κ(ϑ), from which the maximum curvature at the wet end was used to separate the structural shrinkage, Sc, from the proportional (unsaturated) shrinkage. At the point of maximum curvature, Sc and the volumetric air content, θac, were calculated and found to correspond closely to those reported previously. Water retention curves were constructed and fitted using the van Genuchten equation, from which the α coefficient appears an important structural parameter. The structural condition of a swelling soil appears to be well described by its air content at the point of maximum curvature, its van Genuchten α coefficient, and a parameter describing the effect of the overburden potential.