The Mekong River Basin (MRB) in Southeast Asia is among the world’s ten largest rivers, both in terms of its discharge and sediment load. The spatial and temporal resolution to accurately determine the sediment load/yield from tributaries and sub-basin that enters the Mekong mainstream still lacks from the large-scale model. In this study, the SWAT model was applied to the MRB to assess long-term basin hydrology and to quantify the sediment load and spatial sediment yield in the MRB. The model was calibrated and validated (1985–2016) at a monthly time step. The overall proportions of streamflow in the Mekong River were 34% from surface runoff, 21% from lateral flow, 45% from groundwater contribution. The average annual sediments yield presented 1295 t/km²/year in the upper part of the basin, 218 t/km²/year in the middle, 78 t/km²/year in the intensive agricultural area and 138 t/km²/year in the highland area in the lower part. The annual average sediment yield for the Mekong River was 310 t/km²/year from upper 80% of the total MRB before entering the delta. The derived sediment yield and a spatial soil erosion map can explicitly illustrate the identification and prioritization of the critical soil erosion-prone areas of the MR sub-basins.

Microplastics are widespread in coastal and marine environments, and sediments serve as a sink for microplastics. In this study, four sediment cores were collected from the Kuala Gula Mangrove area. The abundance of microplastics in mangrove sediments ranged from 25–130 items/kg dry weight. The highest abundance of microplastic was observed at the KG04 site near the mouth of the river. The number of microplastics by sampling site was significantly different (p < 0.01), with station KG04 having the highest fiber content. The dominant color of microplastics was blue, and the main shape was fiber. Rayon, cotton, Polyethylene terephthalate (PET), and azlon were observed from FT-IR, indicating that the potential sources of microplastic and fiber could come from household laundry wastewater. Microplastics tended to accumulate in deeper depths at KG01 (p < 0.01), whereas other stations showed no significant difference (p > 0.05). However, this study provides evidence that mangroves can be a trap for microplastics and can be used as baseline data for future studies.

Climate change alters hydrological cycles and streamflow regimes at the local, regional and global levels. In this study, we aimed to assess the change in water balance change and hydrological extremes in the Prek Thnot River Basin of the Lower Mekong in Cambodia through a hydrological model (SWAT) under the two climate change scenarios (RCP2.6 and RCP8.5) following three different GCMs. An ensemble of 3 GCMs included GFDL-CM3, GISS-E2-R-CC and IPSL-CM5A-MR models and was applied to a well-calibrated SWAT model through climate change factors. Annual precipitation under RCP2.6 likely decreases by 0.1–0.5% for the near future (2021–2040) and mid-future (2051–2070) and decreases by 0.2–1.3% under RCP8.5. The decrease in precipitation will lead to reductions in water yield by 1–4% (RCP2.6) and 2–5% (RCP8.5). However, peak flow is expected to increase, while the low flow was projected to decrease (1–2% for RCP2.6 and 8–9% for RCP8.5). The study further found that high flow events will increase in both magnitude and frequency. The finding highlights water resources management issues in the Prek Thnot River Basin, including the frequency of future flood events.

Cambodia is located in one of the most severe flood-vulnerable zones in mainland Southeast Asia. Flooding is the country’s most recurrent and impactful hazard among other natural hazards. This hazard alone, observed in many river basins, has been inflicting huge damages on livelihoods, social infrastructure, and the country’s economy. This study aims to review the current status of flood hazards, impacts, driving factors, management capacity, and future research directions on floods in Cambodia. The findings of this study suggested that there is still a lack of flood-related studies on flood hazard mapping, risk and damage assessment, and future flood analysis in Cambodia. The existing related studies mainly focused on the Tonle Sap Basin and its tributaries, the Lower Mekong Basin, the whole Mekong River Basin, and some of the tributaries of the Mekong River in Cambodia. The fundamental driving factors of the current flooding in Cambodia are impacts of climate change, land-use change, water infrastructure development, and weather extremes. The applications of mathematical and statistical tests and indices, conceptual and physically-based modeling, artificial intelligence and machine learning, and remote sensing are recommended to focus on future research directions on flood in Cambodia in the areas of land-use change, existing and planned operation of water infrastructure, flood hazard and damage assessment, and flood forecasting. The outcomes from these studies and applications would improve the understanding of flood hazard characteristics, reinforce flood management, and achieve flood damage reduction.

Drought is a natural hazard that stresses ecosystems, agricultural production, food security, and local economies. Given ongoing hydropower dam development in the Sesan and Srepok Basins, the two most dammed tributaries in the Lower Mekong Basin, characterizing baseline drought events and understanding how dams modify downstream flow is needed to manage water resources and mitigate drought effects. We used the Soil & Water Assessment Tool (SWAT) to estimate streamflow data from 2001 to 2019. For both rivers, we found that runoff varied, but contributed about 75% of streamflow, followed by shallow and deep groundwater, which contributed up to 25%. We used the Standardized Runoff Index to characterize drought and detected frequent, severe, and prolonged drought events in the two basins. Severe and prolonged droughts in the 2009–2011 and 2015–2016 periods corresponded to the occurrence of Typhoon Ketsana and the El Niño-Southern Oscillation. Streamflow alteration can be caused by climatic conditions and anthropogenic activities such as hydropower dam development and operations (e.g., the timing and magnitude of water releases). Results from this study can be used as a baseline to gauge potential future droughts and design appropriate drought management plans to preserve ecosystems and food security in the Lower Mekong Basin and its tributaries.