Modeling the soil shrinkage and water retention curves with the same equations

Recent studies underlined the likeness of soil water retention (WRC) and shrinkage curves (ShC) with respect to their shapes. This paper aims at experimentally discussing the possible use of the same equations to fit them. The WRC (on the tensiometric range) and ShC were simultaneously determined on a series made of 28 undisturbed soil cores collected in surface horizons from a wide variety of soil types, with clay content ranging from 8.5 to 65% and of 30 repacked soil samples of various clay contents and mineralogies. The van Genuchten (VG) closed-form equation of WRC and the VG modified equation of ShC, with five and three parameters, respectively, were found to fit well to both curves, but they did not properly reproduce the observed linear parts and sloping ends of both curves, and the dissymmetric shapes of the ShC as well. The exponential shrinkage model XP fitted significantly better to both the WRC with five parameters and the ShC with eight parameters. It is shown that the transition points of the XP equations independently fitted on the ShC and WRC curves occur at the same gravimetric water content, thus illustrating the likeness of the curves with respect to their shape. The WRC was estimated with a reasonable accuracy from the water content of the ShC transition points plus some measured suction values.