Subsurface Initiation of Film Flow In Rock Fractures and Soil Macropores

Experimental and observational evidence from recent decades shows that water films, typically tens of microns thick, can flow down the walls of macropores or fractures at speeds comparable to other preferential flow modes. If the matric pressure is negative, the water in the flowing film is often conceptualized as becoming gradually absorbed into the unsaturated soil or rock matrix. Experiments have shown also that for some media, within a limited range of negative pressures, water in the unsaturated medium may undergo the opposite sort of domain change, from matrix to film. This makes possible an initiation or augmentation of macropore flow at depth in an unsaturated medium. Hydrologically this process of film-flow initiation is important because it could enable:

· Isolated macropores or fractures (i.e. those for which any flowpath connecting them with the land surface includes some segment of purely matrix flow) to conduct fast macropore flow even when the surrounding matrix is unsaturated.

· Preferential flow to resume, even at considerable depths, after an interruption of the supply of water that generates the preferential flow.

· Macropore flow to be sustained for long times or considerable vertical distances, without requiring saturation of either the macropore or the matrix.

Processes like these may be important in many instances where preferential flow has been evidenced over longer ranges than expected.

Reexamination of recently published data from experiments with tuffs and sandstones permits approximate estimation of thresholds in matric pressure and flux density that might cause subsurface initiation of film flow. A theoretical generalization of these thresholds from rock to soil macropores is possible by adjusting for variations in matrix permeability, porosity, wall roughness and hydrophobicity, and for dynamics of macropore structure caused by shrink-swell processes and other effects.