Flow Rate Dependence of Soil Hydraulic Characteristics

The rate dependence of unsaturated hydraulic characteristics was analyzed using both steady state and transient flow analysis. One‐step and multistep outflow experiments, as well as quasi‐static experiments were performed on identical, disturbed samples of a sandy and a loamy soil to evaluate the influence of flow rate on the calculated retention and unsaturated hydraulic conductivity curves. For the sandy soil, a significant influence of the flow rate on both the retention and unsaturated hydraulic conductivity characteristic was observed. At a given matric potential, more water was retained with greater applied pneumatic pressures. Matric potential differences of 10 to 15 cm (for given saturation) and water content differences of up to 7% (for given potential) could be observed between the slowest and the fastest outflow experiments, predominantly at the beginning of drainage. The hydraulic conductivity also increased with increasing flow rate for higher saturations, while a lower hydraulic conductivity was observed near residual saturation for the higher flow rates. We observed a continuously increasing total water potential gradient in the sandy soil as it drained, especially for high‐pressure transient one‐step experiments. This indicates a significant deviation from static equilibrium, as obtained under static or steady‐state conditions. For the finer textured soil, these flow‐rate dependent regimes were not apparent. A number of physical processes can explain the observed phenomena. Water entrapment and pore blockage play a significant role for the high flow rates, as well as lack of air continuity in the sample during the wettest stages of the experiment.