A Mechanism for Storm Runoff Generation During Large Rainfall Events
McKinnon, R. J.
Dowd, John F.
Endale, Dinku M.
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Flowpaths of stormwater from upland areas have long been the subject of major debate. A series of subsurface gutter experiments, situated on the mid- slope of a Piedmont catchment, were conducted to investigate a potential mechanism for the rapid mobilization of storm runoff from the unsaturated zone. Gutters were 1.45 m long and installed approximately 10 cm below the ground surface. Direct surface runoff was excluded from entering the gutters. Nearly a year of natural rainfall monitoring data showed a close relationship between rainfall intensity and the resulting runoff in the subsurface gutters. The gutter response closely followed the onset of intense rainfall and likewise “switched off” with the cessation of storm events. This behavior is not indicative of a saturated subsurface flow mechanism. Stable isotope analysis of runoff samples demonstrated that stormflow was comprised primarily of “old water,” which is water that was in the soil before the initiation of rainfall. Thus, the traditional explanations, macropore flow and overland flow, could not have been the dominant processes because they produce mainly “new water”. The data suggest that runoff from large storm events occurs when high intensity rainfall generates pressure waves that rapidly travel through the soil and induce pre-event water. Some hydrologists refer to this as a kinematic process. Research on this process at the field level will lead to understanding of stormflow pathways and the associated potential for transport of pollutants at the landscape scale.