Richard's equation:

Under what circumstances, the displacement of the resident air will significantly impede the advance of water into a pore during transient unsaturated flow, where the application of Richard’s equation may not do justice?

Under what circumstances, the amount of storage change resulting from the (unsat) aquifer compaction and water expansion under the conditions of negative pressure head in the water phase becomes significant with reference to the amount of storage change resulting from changes in water content for steady-state unsaturated conditions?

While the dependence of hydraulic conductivity on the water content leads to a relatively slow migration of water under a given energy gradient, under what circumstances, ‘the changes in the volumetric water content’ associated with ‘the changes in water storage’ leading to the formation of a sharp wetting front – become significant?

Whether the application of static steady state initial condition for an unsaturated flow model would remain sensible for humid zone conditions (with significant tensions in the upper part of the profile)?

Will it be feasible to replicate a dynamic steady-state profile that is completely wet at field capacity?

Will there be significant error associated with – if the hysteresis effect (whether the soil is draining or wetting) is ignored, when the relationship between ‘the unsaturated hydraulic conductivity and pressure-head’; and the relationship between ‘the moisture content and the pressure-head’ is considered?

Will the simplified coupling between one-dimensional vertical flow in unsaturated zone and the one-dimensional horizontal flow in the saturated zone yield better results, given the fact that the vadose zone model just requires few 10s and 100s of seconds towards total simulation time, while the saturated model will require a total simulated time on the order of years?

Whether Richard’s equation can do justice, when the water mobility is associated with the horizontal soil column?

Whether the pressure gradient required to drive the flow of the gas phase in an unsaturated zone can ever become significant?