A mechanism for storm runoff generation during large rainfall events

Authors: MCKINON, R - UNIVERSITY OF GEORGIA; DOWD, J - UNIVERSITY OF GEORGIA; Endale, Dinku

Submitted to: Georgia Water Resources Conference
Publication Type: Proceedings
Publication Acceptance Date: 2/28/2007
Publication Date: 3/27/2007
Citation: Mckinon, R.J., Dowd, J.F., Endale, D.M. 2007. A mechanism for storm runoff generation during large rainfall events. Georgia Water Resources Conference 2007, March 27-29, 2007, University of Georgia, Athens, Georgia.

Interpretive Summary: It is very important to quantitatively understand the movement of water through soil in order to limit pollutant delivery from agricultural practices to surface and subsurface waters. Research has shown a rapid mobilization of shallow subsurface water from hillslopes into storm pathways following large rainfall events. The mechanism that causes this response continues to be a matter of debate among hydrologists. Researchers from the University of Georgia in Athens, GA, and the USDA-ARS J. Phil Campbell Sr. Natural Resource Conservation Center near Watkinsville, GA, monitored for about a year shallow subsurface water release from a typical Southern Piedmont hillslope following rainfall events. They found, based on isotope composition of the released water and that of the rainfall, that the water released from the shallow subsurface resembled more of that residing in the soil (old water) than the rainfall (new water). This is contrary to commonly held view that such releases occur primarily due to saturation of the soil leading to release of ‘new water’. If that was so, the released water would have had similar isotopic composition as the rainfall. Rather, it appears that the release was triggered due to pressure waves initiated at the soil surface as a result of disturbance of the surface soil by rain storms, which subsequently traveled through the soil facilitating the release of the old water residing in the soil. Some hydrologists refer to this as a kinematic process. Research on this process will lead to better understanding of storm flow pathways and the associated potential for transport of pollutants. Such information will valuably be used by all government agencies and non government organizations involved in natural resources research, education, outreach, and management, particularly in the Southern Piedmont, a 41 million acre region in the southeastern United States.

Technical Abstract: Flow paths of storm water 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 storm flow 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 produce 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 storm flow pathways and the associated potential for transport of pollutants at the landscape scale.