Flow rate dependence of soil hydraulic characteristics

The rate dependence of unsaturated hydraulic characteristics was analyzed u sing 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 in fluence of flow rate on the calculated retention and unsaturated hydraulic conductivity curves. For the sandy soil, a significant influence of the flo w rate on both the retention and unsaturated hydraulic conductivity charact eristic was observed. At a given matric potential, more water was retained with greater applied pneumatic pressures. Matric potential differences of 1 0 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 fastes t outflow experiments, predominantly at the beginning of drainage. The hydr aulic conductivity also increased with increasing flow rate for higher satu rations, while a lower hydraulic conductivity was observed near residual sa turation for the higher flow rates, We observed a continuously increasing t otal water potential gradient in the sandy soil as it drained, especially f or high pressure transient one-step experiments. This indicates a significa nt deviation from static equilibrium, as obtained under static or steady-st ate conditions, For the finer textured soil, these flow-rate dependent regi mes were not apparent. A number of physical processes can explain the obser ved phenomena. Water entrapment and pore blockage play a significant role f or the high flow rates, as well as lack of air continuity in the sample dur ing the wettest stages of the experiment.