Dry-season soil water repellency affects Tahoe Basin infiltration rates

Lake Tahoe's declining clarity makes the identification of runoff and erosion sources and evaluation of control measures vitally important. We treated relatively undisturbed, native, forested sites of 10% to 15% slope with surfactant and used a rainfall simulator to investigate the effects of repellency. We compared infiltration measurements made by the simulator and a mini-disk infiltrometer (MDI). Runoff was produced by all plots with untreated water, but only two of 12 plots with surfactant. At volcanic soil sites, infiltration rates using surfactant exceeded those with water by only 20% when there was little litter cover, but with substantial litter the infiltration rates increased threefold. Similarly, at the granitic soil sites surfactant-enhanced infiltration rates were four times greater with scant litter, and eight times greater with substantial litter cover. Postsimulation soil moisture content and wetting depths were greater with the surfactant treatment. Excavations under surfactant treatments revealed that discontinuities in the soil's hydrophobic organic layer resulted in preferential infiltration zones in the mineral soils below.