Runoff generation and sediment production on unpaved roads, footpaths and agricultural land surfaces in northern Thailand

Rainfall simulation was used to examine runoff generation and sediment tran sport on roads, paths and three types of agricultural fields in Pang Khum E xperimental Watershed (PKEW), in mountainous northern Thailand. Because int erception of subsurface flow by the road prism is rare in PKEW, work focuse d on Horton overland flow (HOF). Under dry antecedent soil moisture conditi ons, roads generated HOF in c. 1 min and have event runoff coefficients (RO Cs) of 80 per cent, during 45 min, c. 105 mm h(-1) simulations. Runoff gene ration on agricultural fields required greater rainfall depths to initiate HOF; these surfaces had total ROCs ranging from 0 to 20 per cent. Footpaths are capable of generating erosion-producing overland flow within agricultu ral surfaces where HOF generation is otherwise rare. Paths had saturated hy draulic conductivity (K-s) values 80-120 mm h(-1) lower than those of adjac ent agricultural surfaces. Sediment production on roads exceeded that of fo otpaths and agricultural lands by more than eight times (1.23 versus < 0.15 g J(-1)). Typically, high road runoff volumes (owing to low K-s, c. 15mm h (-1)) transported relatively high sediment loads. Initial road sediment con centrations exceeded 100 g l(-1), but decayed with time as loose surface ma terial was removed. Compared with the loose surface layer, the compacted, u nderlying road surface was resistant to detachment forces. Sediment concent ration values for the road simulations were slightly higher than data obtai ned from a 165 m road section during a comparable natural event. Initial si mulation concentration values were substantially higher, but were nearly eq uivalent to those of the natural event after 20 min simulation time. Higher sediment concentration in the simulations was related to differences in th e availability of loose surface material, which was more abundant during th e dry-season simulations than during the rainy season natural event. Sedime nt production on PKEW roads is sensitive to surface preparation processes a ffecting the supply of surface sediment, including vehicle detachment, main tenance activities, and mass wasting. The simulation data represent a found ation from which to begin parameterizing a physically based runoff/erosion model to study erosional impacts of roads in the study area. Copyright (C) 2000 John Wiley & Sons, Ltd.