Uncertainty and variability in sediment loads in the largest tributary of the Mekong Basin using the Soil and Water Assessment Tool

Abstract:

The Sesan, Srepok and Sekong (3S) rivers are the farthest downstream tributaries to the Mekong, and contribute nearly one quarter of the river’s annual water discharge. Very limited sediment monitoring has been carried out in the region, but aggressive plans for hydropower development in the 3S are likely to affect sediment loads and compromise their crucial role in the geomorphology, ecology, and agriculture of the Mekong floodplain. Thus, the main purpose of this study is to estimate the uncertainty and variability in the sediment contribution from the 3S. We used a multiyear dataset of water discharge and suspended solids to estimate monthly sediment loads at four key locations around the 3S. Surface runoff, erosion, and sediment transport were computed using the Soil and Water Assessment Tool (SWAT) in an area covering 101,400 km². The model was calibrated and subject to a global sensitivity analysis. Overall, the model was able to adequately characterize interannual and monthly patterns in suspended sediment loads. We estimated that 61% percent of mean monthly sediment loads from the 3S are discharged in the early rainy season months of July and August. Subbasins in the upper Sekong were shown to be particularly prone to erosion. The Sekong and Sesan were found to contribute similar sediment loads (4.5 ± 2.0 and 4.8 ± 4.2 Mt yr^-1, respectively), which are considerably larger than contribution from the Srepok (1.2 ± 0.5 Mt yr^-1). The model revealed the important the role of sediment deposition in the lower sections of the 3S, as 46-78% of the annual sediment eroded is not discharged through the 3S outlet. Annual sediment load from the 3S into the Mekong River varied from 2.2 to 18.5 Mt yr^-1. This study would facilitate the assessment of uncertainty and variability of ongoing monitoring and modeling studies in this important tributary where sediment transport is rapidly changing as a result of deforestation, hydropower development, and the changing climate.