Toward understanding the cumulative impacts of roads in upland agricultural watersheds of northern Thailand

This paper describes the interactions of various physical processes that allow unpaved roads to contribute disproportionately to basin-wide runoff and stream sediment in the 93.7 ha Pang Khum Experimental Watershed (PKEW) in northern Thailand. Many road sections in PKEW are constant sources of sediment entering the stream during most rain events because: (1) Horton overland flow is generated on the compacted surfaces after small depths of rainfall; (2) surface preparation processes, including vehicle detachment and maintenance activities, renew the supply of easily transportable surface sediment on inter- and intra-storm time scales; (3) erosion of the road surface is accelerated in locations where slopes are steep, overland flow distances are long, and/or vehicle usage is high; (4) surface runoff typically exits from the road directly into the stream. Owing to these collective processes, sediment delivery rate on PKEW roads is more than an order of magnitude higher than that on adjacent fields (approximate to120 Mg ha(-1) per year versus 9 Mg ha(-1) per year). Thus, unpaved roads appear to be on the same order of importance as agricultural lands in contributing sediment to the stream network, despite occupying a fraction of the total surface area in the basin (approximate to0.5% versus 12%). A more thorough assessment of linkages between all hillslope runoff/sediment sources and the stream network, however, is still needed to fully evaluate the relative impacts of roads versus those of agriculture practices in PKEW. (C) 2004 Elsevier B.V. All rights reserved.