The Impact of Land Use Change on Water Yield: The Case Study of Three Selected Urbanised and Newly Urbanised Catchments in Peninsular Malaysia

Economic development and increasing population has brought significant changes in land cover and placed stress on water quantity within tropical river catchments, including those of Malaysia. There is a need to investigate the impact of land use changes on water yield in moderate-sized (c. 1,000 km(2)) catchments, such as the Langat and Linggi, since these provide the water resources that cater for the rapid urbanization and industrialization that characterizes Malaysia. (On the contrary, most previous studies have been dealing with very small catchments i.e. <25 km(2), and frequently <1 km(2)). Findings from our study provides information for local river managers and development planners and will assist them in minimizing the negative impacts of development on water resources, while promoting sensible planning within river basins especially in the newly developed catchments such as the Bernam. An analysis of land-cover in the Langat, Linggi and Bernam basins indicates that there has been a significant change from forest (primary and secondary selva) to agriculture, especially tree crops (rubber, oil palm), ranging from 7 to 15% in the three water catchments, and an increase in the urban area that ranges from 183 to 394% during the period 1984-2002. Despite this, the runoff coefficient shows no significant increase during 42 years of development. The coefficient lies between 22 and 48%. The outcome is not straightforward and counter intuitive when comparison is made with results from other experimental catchments in the tropics. This study suggests that up-scaling the findings of small catchment studies of forest removal is far from simple, especially in the wet tropics, where the impact of tree crops on water relations may be insufficiently distinguished from primary or secondary forests.