HIGH INPUT-SENSITIVITY OF NONCAPILLARY FLOW IN THE UNSATURATED ZONE

In classic unsaturated flow theory, capillarity plays two major roles. (1) It determines the geometric configuration of the water phase, hence the water retention relation and the hydraulic conductivity. (2) In addition to gravity, it generates a major driving force.

In most modes of preferential flow the influence of capillarity is greatly diminished. Field and lab experiments of recent decades provide considerable evidence concerning what does control the flow. Flow may be pulsed, taking the form of sliding drops or falling slugs of water. In these modes capillarity contributes minimally to driving force, and though it may influence the shape of air-water interfaces of the drops or slugs, the flux is not modulated by a geometrically-determined conductivity. Instead there is great importance of the rate at which water is supplied to where the preferential flow commences; this rate determines the frequency of pulse generation and hence the flux. Preferential flow may also occur in films or rivulets, in which the water-supply rate controls the water-phase thickness, hence also its conductance and travel time, as well as the flux.

Because of these distinctive physical characteristics, mathematical formulas appropriate to flow in drops, slugs, films, and rivulets differ from the Darcy-Buckingham and Richards formulas traditionally used for unsaturated flow. In general, formulas that relate more directly are simpler as they do not depend on highly nonlinear and extremely state-sensitive hydraulic properties. Another achievable advantage is greater sensitivity to quantities that can be known relatively well, in particular rainfall or infiltration rates. These potential advantages point out that many of the difficulties associated with preferential flow modeling are artificial, arising from the lack of standardized approaches and unfamiliarity with the dominant physical processes.

In unsaturated-zone dynamics, when conditions become more favorable to preferential flow, the water-input rate gains in influence while capillarity declines. Much can be gained from developments utilizing this fact, incorporating variables like rainfall rate not merely into a boundary condition, but directly into the modeled phenomenology of unsaturated flow in the subsurface.