Interactive effects of damming, forest loss and climate change on the hydrology of the Baleh River, Malaysia

This paper combines empirical and modelling studies to assess historic and future changes in the hydrological regime and sediment yield of the Baleh River, Malaysia. In the most recent 50-year period, the river has experienced a reduction in flow magnitude, mainly as a result of changes occurring in the wet season. Simulations using the Soil and Water Assessment Tool (SWAT) suggested that annual rates of forest loss even less than those currently witnessed in other parts of Malaysia may result in increases in sediment yield of more than 300% in some sub-catchments of the Baleh over the next 50 years. However, the lower rainfall and higher temperatures expected under climate change were predicted to result in reductions in flow and increases in sediment yield of more than 100%. A new dam currently under construction was predicted to trap approximately 95% of sediment and affect flood frequency and magnitude up to 90 km downstream. Multivariate analyses of the historical flow regime show that several identifiable types of high flow event (defined by their shape) occur naturally in the river, with some of these having distinct temporal patterns of occurrence. Efforts are needed to develop an operational regime for the dam to mimic the most functionally important of these events, although designing this regime is complicated by changes in flow and sediment loads anticipated due to climate and landcover change. An adaptive approach to dam operation is therefore needed.