Horton overland flow contribution to runoff on unpaved mountain roads: A case study in northern Thailand

Two indirect methods are used to detect evidence of intercepted subsurface flow (ISSF) by the road prism in the Pang Khum Experimental Watershed (PKEW ) in northern Thailand. During the 12-month study period we failed to. obse rve a soil moisture change that corresponds with ISSF being generated by. t he water table rising above the road surface. In support of the soil moistu re data, delta O-18 signatures of rain water, road runoff, and stream water (a proxy for soil water) suggest Horton overland flow (HOF) generated on t he road surface, not ISSF, is the dominant source of observed road runoff d uring typical rainfall events in the study area. This finding, which is con trary to the ISSF-dominant road runoff regime found typically on unpaved mo untain roads in the US Pacific NW, suggests that the use of a HOF-based mod el to simulate runoff and sediment transport on unpaved roads in PKEW provi des not only lower bound estimates of these processes, but realistic approx imations for typical events. Copyright (C) 2001 John Wiley & Sons, Ltd.