Importance of rural roads as source areas for runoff in mountainous areas of northern Thailand

Abstract

Unpaved road surfaces have extremely low infiltration rates compared with other watershed land surfaces and, therefore, are significant source areas for erosion-producing Horton overland flow. The hydrologic role of roads is an important issue in mountainous areas of the tropics where erosion control efforts are predominately focused on deforestation and agricultural practices. We report on an investigation of soil physical properties that control excess rainfall (rainfall intensity in excess of infiltration capacity) on rural roads and surrounding lands in a mountainous watershed in northern Thailand. The results of our disk permeameter measurements indicate that saturated hydraulic conductivity on unpaved roads is about one order of magnitude lower than on any other land-surface type. Median saturated hydraulic conductivities were not exceeded by measured rainfall intensity on any land use except road surfaces and roadside margins. By simulating excess rainfall, we found that in contrast with other areas of the watershed, the road surface tends to generate excess rainfall early in a rain event, and on nearly all of its area. Despite the relatively small areal extent of road-related surfaces (<0.5% of basin area), they contribute a large portion of basin-wide total excess rainfall during frequently occurring, small rainfall events. However, during larger events, agricultural, secondary vegetation, and forested areas assume greater importance because of their larger areal extent. ©1997 Elsevier Science B.V.