HYDROLOGICAL PROCESSES Hydrol. Process. 15, 337–358 (2001) Erosion prediction on unpaved mountain roads in northern Thailand: validation of dynamic erodibility modelling using KINEROS2 Alan D. Ziegler,* Thomas W. Giambelluca and Ross A. Sutherland University of Hawaii at Manoa, Department of Geography, 55B 445, 2424 Maile Way, Honolulu, Hawaii 96822, USA Abstract: The event- and physics-based KINEROS2 runoff/erosion model for predicting overland flow generation and sediment production was applied to unpaved mountain roads. Field rainfall simulations conducted in northern Thailand provided independent data for model calibration and validation. Validation shows that KINEROS2 can be parameterized to simulate total discharge, sediment transport and sediment concentration on small-scale road plots, for a range of slopes, during simulated rainfall events. The KINEROS2 model, however, did not accurately predict time-dependent changes in sediment output and concentration. In particular, early flush peaks and the temporal decay in sediment output were not predicted, owing to the inability of KINEROS2 to model removal of a surface sediment layer of finite depth. After 15–20 min, sediment transport declines as the supply of loose superficial material becomes depleted. Modelled erosion response was improved by allowing road erodibility to vary during an event. Changing the model values of erosion detachment parameters in response to changes in surface sediment availability improved model accuracy of predicted sediment transport by 30–40%. A predictive relationship between road erodibility ‘states’ and road surface sediment depth is presented. This relationship allows implementation of the dynamic erodibility (DE) method to events where pre-storm sediment depth can be estimated (e.g., from traffic usage variables). Copyright  2001 John Wiley & Sons, Ltd. KEY WORDS road erosion modelling; model validation; runoff generation; erodibility INTRODUCTION Roads and road-building disrupt watershed hydrological and geomorphological systems and contribute to adverse cumulative watershed effects (Reid, 1993; Montgomery, 1994). In some instances, road impacts may be greater than those of other recognized disruptive activities. Megahan and Ketcheson (1996) state that the primary sediment source from logging activities in western USA is forest access roads, rather than other timber management activities (e.g. Megahan and Kidd, 1972). In a field study near Melbourne, Australia Grayson et al. (1993) determined that timber harvesting activities did not greatly affect stream physical and chemical water quality, but improperly placed or maintained roads contributed substantial sediment quantities. In mountainous northern Thailand, we have demonstrated that unpaved rural roads can disrupt hydrological and erosional processes disproportionately to their areal extent, compared with agriculture-related lands (Ziegler and Giambelluca, 1997a,b). Despite evidence that road-related impacts often outweigh those of other activities, conservation efforts historically have focused on agricultural and timber removal activities. In an attempt to better understand road impacts, geomorphologists and hydrologists have been approaching the goal of modelling road-related physical processes realistically (e.g. Simons et al., 1977, 1978; Ward and Seiger, 1983; Flerchinger and Watts, 1987; Luce and Cundy, 1994; Elliot et al., 1995; Anderson and MacDonald, 1998; * Correspondence to: Dr Alan Ziegler, University of Hawaii at Manoa, Department of Geography, 55B 445, 2424 Maile Way, Honolulu, Hawaii 96822, USA. E-mail: adz@hawaii.edu Received 28 October 1999 Copyright  2001 John Wiley & Sons, Ltd. Accepted 21 March 2000