Interflow Moving over Leaky Impeding Layers: How Far Can We Expect It to Go?
Wednesday, 17 December 2014
Interflow can occur in any slope where higher conductivity topsoils are underlain by a low conductivity impeding layer which could include B horizons, till layers, hardpans, C horizons and bedrock of various permeabilities. Impeding layers of essentially impermeable bedrock seem to be a rarity, as studies that have evaluated interflow on slopes underlain by apparently solid crystalline rock have still found leakage into the rock. Hewlett’s concrete soil trough studies thus comprise an endpoint of interflow boundary conditions. In many hillslope environments, downslope interflow necessarily includes a normal flow component into the leaky impeding layer. By making a simplifying assumption of zero pressure at the base of the impeding layer (for perched conditions), the downslope travel distance as interflow of a parcel of water can be estimated from the ratio of the hydraulic conductivities, the ratio of the downslope and normal hydraulic gradients, and the thickness of the saturated zone above the impeding layer. For many hillslopes, downslope travel distances imply that only the slope segments adjacent to the riparian valley can be expected to deliver interflow to the valley during a storm. Over most of the hillslope, interflow acts only to redistribute recharge downslope from the point of infiltration. Therefore, continuous perching of water moving as interflow from the ridge to the valley does not imply continuous connectivity. In terms of stormflow contributions, only the lower slopes within the range of the downslope travel distance may be connected to stream valleys.