Author(s): Xin Yi Chong; Xin Yi Lum; Ramon J. Batalla; Damia Vericat; Fang Yenn Teo; Karen Suan Ping Lee; Christopher Gibbins

Linked Author(s): Fang Yenn Teo

Keywords: Dams; Functional flow; Sediment; Tropical rivers; Landcover change

Abstract: Tropical rivers face multiple interacting pressures, as a result of climate change, loss of forest cover, and increasing number of new dams. These pressures threaten the physical and ecological integrity of tropical rivers, as well as the livelihoods of local communities who rely on them directly for ecosystem services. This paper combines empirical and modelling studies to assess long-term changes in the hydrological regime of a large tropical river and assess the impacts of landcover change, climate change, and damming on water and sediment yields. We developed a new tool (HyFFlow) to characterise the river’s flow regime in geomorphically and ecologically meaningful ways. HyFFlow analyses indicated that the flow regime is characterised by distinct types of high flow event (defined by duration, magnitude and rates of rise and fall), with seasonal patterns to the timing of some but not all of these types. Despite some changes in rainfall and temperature, there have been no major changes in the flow regime over the last 50 years. However, flow magnitude has subtly reduced over time, with a shift to a ‘lower flow’ regime. Analyses of catchment landcover data indicate only very limited forest loss over the last 20 years (<2%). Simulations using the Soil and Water Assessment Tool (SWAT) suggest that greater rates of clearance than observed historically may not lead to major changes in runoff, but that flows could reduce significantly (20-50%) in coming decades if predictions for climate change hold true. However, sediment yield appears highly sensitive to land cover change, with annual sediment yield predicted to increase by 322% by 2050 if annual rates of forest loss increase to 2%. A new dam is predicted to trap more than 95% of the fine sediment carried by the river, reducing loads immediately downstream. While this will counteract some of the effects of landcover change on fine sediment loads in the short section of river downstream from the dam, model simulations suggest that sediment yield at the catchment outlet will change little compared to historic levels. Assessment using the SedInConnect model indicates that construction of a dam, access road, and the associated increase in illicit forest clearance as a result of newly accessible areas will alter the degree of connectivity between sediment source areas and the catchment outlet. The work illustrates the complex interactions between climate, landcover and damming, and the challenges of maintaining the integrity of tropical catchments in the face of multiple pressures. Together, the HyFFlow analyses and SWAT help provide recommendations for an operational regime for the dam and catchment management to help limit changes to the river’s flow regime and sediment loads.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221279

Year: 2022