Predicting the hydraulic properties of compacted soils: model validation

Abstract. This paper reports on the validation of two numerical approaches that were developed to determine the effect of compaction on the soil water retention curve (WRC). The proposed approaches satisfactorily expanded the applicability of the van Genuchten (1980) model. In Approach 1, an optimization problem was solved to enable the van Genuchten model parameters α and η to be estimated for a range of soil bulk densities, based on the WRC of the corresponding non-compacted soil and the estimated saturation, residual, and permanent wilting-point (-1500 kPa) water contents of the compacted soil. In Approach 2, the parameter η was assumed to be unaffected by changes in soil bulk density. Subsequently, the parameter α was determined using an equation especially developed by this study. Compared with measured data, Approach 2 yielded slightly better predictions of the WRC than Approach 1. However, both numerical approaches may be used with confidence in a wider range of scenarios than those presented in this study. Across all soils, our analyses showed that relatively small increments in soil bulk density, due to compaction, can result in significant reductions in the available water storage capacity (AWSC) of soils; the effect being dependent on soil type and the relative increase in soil bulk density. Mechanization systems that mitigate, or where possible avoid, soil damage due to compaction (coupled with management practices that reduce loss of soil organic carbon) are encouraged. Quantification of the benefits of compaction avoidance, in terms of improved infiltration, soil water retention and water-use efficiency, as well as better predictions of the hydrology of compacted soils, may be possible through the application of the models reported in this study.