Pressure wave generation of runoff in a convergent zone
Thomas, Jason Butler
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Two subsurface gutter experiments designed to emulate the response in an ephemeral network were conducted simultaneously along a uniform hillslope and within an adjacent area of topographic convergence in a Piedmont catchment to investigate and compare pressure wave-generated runoff from the unsaturated zone. Rainfall and runoff data reveal a close relationship between rainfall intensity, soil moisture conditions, and runoff in the subsurface gutters. Data suggest that runoff occurs when rainfall causes fluctuations in soil pressure at the soil surface that generate pressure waves that propagate downward and “push” water into macropores and other preferential flow paths. Data also show that two different, but not mutually exclusive, stormflow mechanisms operate within the convergent zone depending on soil moisture conditions and rainfall intensity. Translatory flow occurred when rainfall generated kinematic pressure waves that traveled rapidly through the soil and released pre-event water to ephemeral flowpaths. Saturated or near saturated Darcian flow (i.e. throughflow) occurred as water moved laterally through the soil during and immediately following rain events. Stable isotope data support the findings that runoff in the subsurface gutters is dissimilar to rainfall.