Severe decline in extent and seasonality of the Mekong plume after 2000

• Published in: Journal of hydrology (Amsterdam) 2024-11, Vol.643, p.132026, Article 132026
• Main Authors: Feng, Yikang, Park, Edward, Wang, Jingyu, Feng, Lian, Tran, Dung Duc
• Format: Article
• Language: English
• Subjects: algorithms; biodiversity; chronic exposure; coasts; decline; dry season; estuaries; governance; hydrologic data; Landsat; reflectance; river deltas; sea level; seasonal variation; sediment deposition; sediment yield; sediments; stream channels; suspended sediment; wavelet; wet season
• ISSN: 0022-1694
• DOI: 10.1016/j.jhydrol.2024.132026

•The Mekong plume declined by 83 % (wet season) and 63 % (dry) post-2000.•Significant decrease in seasonality of sediment discharge after 2014.•Increasing trends of alongshore drift and dry season sediment discharge after 2008.•Tropical cyclones correlate with sediment discharge in wet season (r = 0.60)•Annual exposure of mangroves to plume fell by ∼ 50 % post-2000. The Mekong plume, which sustains the geomorphology and rich biodiversity along the coastal zone of the Vietnamese Mekong Delta (VMD), has been under intensifying threats from dams, riverbed mining and sea level rise. However, our understanding of how much the intensifying stressors have altered the long-term spatiotemporal dynamics of the plume remains limited. In this paper, we investigate the spatiotemporal dynamics of the Mekong plume over 1988–2022 based on remote sensing, in-situ hydrological data, and continuous wavelet transform. Specifically, we analyze (1) the variability of the long-term average seasonal plume extent, (2) the interannual trends of the seasonal suspended sediment discharge and alongshore drift, and (3) changes in the long-term exposure of coastal mangroves to suspended sediment. Field suspended sediment concentration (SSC) data from five major gauging stations representing the upstream, midstream, and downstream sections of the VMD between 2010–2021 were used to calibrate 101 Landsat surface reflectance observations to estimate SSC. The top-performing algorithm with band combination (NIR-SWIR2) * (G/R) was selected for SSC retrieval (R2 = 0.72, SE = 40.6 mg L−1). Our results reveal a 63 % decline in plume extent in the dry season (Dec–May) and an 83 % decline in the wet season (Jun–Nov) between 1988–1999 and 2000–2022. The seasonal variation in suspended sediment discharge (SSD) remained relatively constant until 2014 but significantly weakened thereafter. Despite the decline in the plume, the surface SSC at the alongshore drift began to rise in 2008, marking a reversal in the trend after two decades of descent. The upswing in the surface SSC is associated with rising SSD in the dry season, which we attribute mainly to the release of water from dams upstream of the VMD. However, if the post-2014 decline of wet season SSD persists in the long term, it would deplete sediment deposition around the estuary and starve the alongshore drift. Along the coastline of the Mekong Delta, the annual exposure to suspended sediments declined by 37–54 % between 1988–1999 and 2000–2022,