Damping of Periodic Variations in the Unsaturated Zone: Effects On Preferential Flow Initiation.

Observations in the unsaturated zone demonstrate that periodic fluctuations of water supply or extraction in the near-surface have decreasing amplitude with depth of penetration, eventually becoming quasi-steady. This “damping” process is observed in other systems where physical processes are driven by gradients in potential, such as heat flow. The damping of pressure and flux signals is important for many problems in the hydrologic sciences including preferential flow initiation. We systematically examine the depth at which signals are damped to a quasi-steady value, using analytical solutions for diffuse unsaturated flow, for a variety of hydrologic signal periodicities, soil-hydraulic properties, and soil-water conditions. Complexities such as heterogeneity in soil-hydraulic properties are examined using numerical solution of Richards Equation. Preliminary results suggest that shorter periodicities and lower soil-water diffusivities are the primary factors promoting damping of hydrologic signals (i.e. reducing the depth of penetration of a coherent periodic variation). Lower soil-water diffusivity can result from drier soil-water conditions or a porous media with low hydraulic conductivity. Two case studies are considered to evaluate the effects of the damping process on the potential for preferential flow initiation in thick unsaturated zones: (i) Rainier Mesa and Shoshone Mountain, Nevada and (ii) Idaho National Laboratory (INL), Idaho. Vitric tuffs at Rainier Mesa and Shoshone Mountain may be dominated by diffuse unsaturated flow, which could dampen episodic percolation variations and prevent unsaturated fracture flow below those lithologies. At INL, we investigate the potential for sediments interlayered with fractured basalt to dampen recharge fluctuations and possibly prevent preferential flow initiation at depth.