Hydrological consequences of landscape fragmentation in mountainous northern Vietnam: Evidence of accelerated overland flow generation

Abstract

Measurements of saturated hydraulic conductivity (Ks) and indices of Horton overland flow (HOF) generation are used to assess the influence of landscape fragmentation on near-surface hydrologic response in two upland watersheds in northern Vietnam. The fragmented landscape, which results from timber extraction and swidden agriculture, is a mosaic of surfaces having distinct infiltration characteristics. In general, human activity has reduced infiltration and altered near-surface flow paths on all disturbed land covers. Compacted roads, paths, and dwelling sites, for example, have the propensity to generate HOF for small rainfall depths. Although these surfaces occupy a small fraction of a basin land area (estimated at <1%), they contribute disproportionately to overland flow response during typical rainfall events. Recently abandoned fields have the lowest Ks of all non-consolidated, post-cultivation surfaces tested. Beginning 1-2 years following abandonment, diminished Ks recovers over time with the succession to more advanced types of secondary regrowth. If a grassland emerges on the abandoned site, rather than a bamboo-dominated cover, Ks recovers more rapidly. The decrease in Ks with depth below disturbed surfaces is more acute than that found at undisturbed sites. This enhanced anisotropy in near-surface Ks increases the likelihood of the development of a lateral subsurface flow component during large storms of the monsoon rain season. Subsequently, the likelihood of return flow generation is increased. Because the recovery time of subsurface Ks is greater than that for the surface Ks, the impact human activity has on hydrologic response in the fragmented basin may linger long after the surface vegetation has evolved to a mature forested association. © 2003 Elsevier B.V. All rights reserved.