The roles of roads and agricultural land use in altering hydrological processes in Nam Mae Rim watershed, northern Thailand

Abstract

The distributed hydrology soil vegetation model (DHSVM) is applied in the 107 km2 Nam Mae Rim watershed (NMRW) in northern Thailand. Simulations using land cover scenarios for 1989 and 2002, extreme deforestation, and forest, each run with and without roads, show that roads have very small effects on the mean water fluxes, but significantly increase peak flows for all land cover scenarios. The magnitude of the road effect on peak flow depends on the land cover context in which the roads are placed. Roads have the smallest effect on peaks within the extreme deforestation scenario, and affect mainly the smallest peaks in the two homogeneous land cover scenarios (forest and extreme deforestation). Roads have the largest effect on peaks within the heterogeneous landscapes of the two historical land cover scenarios (1989 and 2002). This result indicates that, by conveying surface flow quickly to the stream, roads within a fragmented landscape are especially important in converting greater amounts of overland flow into higher peak flows. Without roads, the patchy land cover pattern buffers the impacts of the scattered overland flow source areas and limits increases in peak flows. The combined effects of land cover and roads are examined in comparison with forest. The 1989 and 2002 land cover patterns result in relatively small changes in the mean annual water balance, but with some significant changes in the seasonal distribution of fluxes. Streamflow is lower in the late wet season and higher in the dry season for the 1989 and 2002 scenarios compared with forest. Conversion from forest to any of the other land cover scenarios results in higher dry season evapotranspiration (ET) and lower wet season ET. The extreme deforestation scenario results in much lower wet season ET and much higher annual streamflow, but little change in dry season streamflow, in comparison with forest. Copyright © 2008 John Wiley & Sons, Ltd.