Study region The Greater Mekong Region, comprising Myanmar, Thailand, Laos, Cambodia, and Vietnam, covering an area of nearly 2 million km2. Study focus Groundwater resources have become increasingly crucial to support the region's rapid development. Advancements in hydrogeological knowledge have led to a better understanding of groundwater availability and water quality at both national and sub-national levels. However, very little attention has been given to integrating groundwater knowledge at the regional scale, leading to major data gaps between national studies and global hydrogeological knowledge. This study aims to address these major data and knowledge gaps by conducting a comprehensive hydrogeological review of 179 documents focused on the Greater Mekong Region in Southeast Asia. New hydrogeological insights for the region The review provides the first synthesised overview of the hydrogeology of the Greater Mekong Region. The results are new regional hydrogeological maps of aquifer distribution and groundwater threats from arsenic pollution, high salinity, and over-abstraction. The maps serve as the basis for a unified transboundary framework and an overview of the most significant groundwater challenges. The review also shows that, beyond transboundary aquifers, there are similarities among major aquifer systems and significant differences in the management and development of water resources across borders. Hence, regional cooperation and exchange of lessons learnt in similar hydrogeological settings would be highly beneficial for sustainable groundwater development across the region.

Mangrove forests can help to mitigate climate change by storing a significant amount of carbon (C) in soils. Planted mangrove forests have been established to combat anthropogenic threats posed by climate change. However, the efficiency of planted forests in terms of soil organic carbon (SOC) storage and dynamics relative to that of natural forests is unclear. We assessed SOC and nutrient storage, SOC sources and drivers in a natural and a planted forest in southern Thailand. Although the planted forest stored more C and nutrients than the natural forest, the early-stage planted forest was not a strong sink relative to mudflat. Both forests were predominated by allochthonous organic C and nitrogen limited, with total nitrogen being a major driver of SOC in both cases. SOC showed a significant decline along land-to-sea and depth gradients as a result of soil texture, nutrient availability, and pH in the natural forest.

The Lower Mekong Basin has had extensive hydropower dam development, which changes its hydrologic conditions and threatens the exceptional aquatic biodiversity. This study quantifies the degree of hydrologic change between pre-impact (1965–1968) and post-impact (2018–2021) peak hydropower development in two major tributaries of the Lower Mekong Basin—the Sekong River, with the fewest dams, and the Sesan River, with the most dams. Both rivers have historically supported migratory fishes. We used daily pre- and post-impact data and the Indicators of Hydrologic Alteration framework to evaluate streamflow changes from dam development. We found significant changes in low- and high-magnitude flows in the pre- and post-impact periods of dam development. For the Sekong River, minimum flow had large fluctuations, with increases of 290% to 412% compared to the pre-impact period, while the Sesan River’s minimum flow ranged from 120% to 160% more than pre-impact. Dry season flows increased by 200 ± 63% on average in the Sekong River, which was caused by releases from upstream dams. Meanwhile, the Sesan River’s dry season flows increased by 100 ± 55% on average. This study indicates that seasonal flow changes and extreme flow events occurred more frequently in the two basins following dam construction, which may threaten the ecosystem’s function.

Mangrove forests serve as significant carbon sinks and play a crucial role in mitigating climate change. Currently, the response of mangroves to intensified climate change and human activities, and the factors that influence the magnitude of carbon storage in their sediments remain uncertain. To address these questions, two sediment cores were collected from the mangrove reserve in Pearl Bay, Guangxi, China. The activity of 210Pb in the sediment, grain size, bulk elemental composition, stable carbon isotopes, lignin, and different sediment organic matter (OM) fractions were investigated to determine the local mangrove’s response to climate change and human activities, as well as the factors influencing its carbon storage. The results showed mangrove forests with lower tidal ranges, slower sedimentation rates, and where OM predominantly originated locally tend to have larger carbon stocks. The mangrove OM (MOM) decreased progressively from the bottom to the top of the cores, indicating that the mangroves in Pearl Bay have possibly undergone degradation, which was further substantiated by the decrease in lignin content. Based on these results, the entire cores were divided into two stages: stable stage 1 (1963–2001) and degradation stage 2 (2001–2020). The cause of the mangrove degradation is likely due to the impact of human activities; however, these impacts are anticipated to gradually lessen in the future due to mangrove protection policies. Our results indicate that lignin can track and predict mangrove growth trends and provide guidance for the sustainable management of mangrove ecosystems.

Direct and indirect photolysis are important abiotic processes in aquatic environments through which plastics can be transformed physically and chemically. Transport of biodegradable plastics in water is influenced by vertical mixing and turbulent flow, which make biodegradable plastics remain susceptible to sunlight and photolysis despite their high density. In general, biodegradable plastics are composed of ester containing polymers (e.g., poly(butylene succinate), polyhydroxyalkanoate, and polylactic acid), whereas non-biodegradable plastics are composed of long chains of saturated aliphatic hydrocarbons in their backbones (e.g., polyethylene, polypropylene, and polystyrene). Based on the reviewed knowledge and discussion, we may hypothesize that 1) direct photolysis is more pronounced for non-biodegradation than for biodegradable plastics, 2) smaller plastics such as micro/nano-plastics are more prone to photodegradation and photo-transformation by direct and indirect photolysis, 3) the production rate of reactive oxygen species (ROS) on the surface of biodegradable plastics is higher than that of non-biodegradable plastics, 4) the photodegradation of biodegradable plastics may be promoted by ROS produced from biodegradable plastics themselves, and 5) the subsequent reactions of ROS are more active on biodegradable plastics than non-biodegradable plastics. Moreover, micro/nanoplastics derived from biodegradable plastics serve as more effective carriers of organic pollutants than those from non-biodegradable plastics and thus biodegradable plastics may not necessarily be more ecofriendly than non-biodegradable plastics. However, biodegradable plastics have been largely unexplored from the viewpoint of direct or indirect photolysis. Roles of reactive oxygen species originating from biodegradable plastics should be further explored for comprehensively understanding the photodegradation of biodegradable plastics.

Hydropower, although an attractive renewable energy source, can alter the flux of water, sediments, and biota, producing detrimental impacts in downstream regions. The Mekong River illustrates the impacts of large dams and the limitations of conventional dam regulating strategies. Even under the most optimistic sluicing scenario, sediment load at the Mekong Delta could only recover to 62.3 ± 8.2 million tonnes (1 million tonnes = 10 9 kilograms), short of the (100 to 160)–million tonne historical level. Furthermore, unless retrofit to reroute sediments, the dams are doomed to continue trapping sediment for at least 170 years and thus starve downstream reaches of sediment, contributing to the impending disappearance of the Mekong Delta. Therefore, we explicitly challenge the widespread use of large dead storages—the portion of the reservoirs that cannot be emptied—in dam designs. Smaller dead storages can ease sediment starvation in downstream regions, thereby buffering against sinking deltas or relative sea level rises.

People who live in Cambodia's flood-prone areas are at a heightened risk of experiencing a flood disaster due to climate change and urbanization. Annual flood disasters have an impact on the Stung Sen River Basin, the largest tributary river of Tonle Sap Lake. Through interviews with pertinent stakeholders, we investigate the current gaps in flood control risk management at the Stung Sen Catchment, the national to local levels. At a common level, the Kampong Kor, Kampong Svay, and Meanchey were selected for the interview and field survey. The findings highlight that a top-down approach is used to alert locals about potential flood danger. The results emphasized the difficulties in addressing cross-border flood warning issues, the effects of incorrect urbanization development and climate change on vulnerable populations, and the inadequate readiness for the safety of children in flood-prone locations. Consequently, we suggest the integrated flood risk management framework as a very effective way to lessen the impact of flood risk for both present and future flood risk harm.

A modified piano key weir with a rounded nose and a parapet wall (MPKW) can improve the discharge capacity significantly compared to a standard piano key weir. However, the optimum of the inlet/outlet width ratio (Wi/Wo) on the discharge efficiency of MPKW is still not investigated numerically. The present work utilized the numerical modeling to investigate and analyze the effects of the inlet/outlet key width ratios on the hydraulic characteristics and discharge capacity of the MPKW. To validate the numerical model with the experimental data, the results indicate that the average relative error is 2.96%, which confirms that the numerical model is fairly well to predictthe specifications of flow over on the MPKW. Numerical simulation results indicated that the discharge capacity of the MPKW can be improved up to 8.5% by optimizing the Wi/Wo ratio ranging from 1.53 to 1.67 even if the other parameters of the MPKW keep unchanged. A big Wi/Wo ratio generally leads to an increase in discharge capacity at low heads and a little effect on the discharge efficiency at high heads. The discharge efficiency of the inlet and outlet crests increases up to 9.6% for high heads, while discharge efficiency of the lateral crest decreases up to 23.5% compared with the reference model. The findings of the study revealed that the intrinsic influencing mechanism of the Wi/Wo ratio on the discharge performance of MPKWs.

Plastic pollution in rivers is of increased global concern. Rivers act both as pathways for land-based plastic waste into the ocean, and as plastic reservoirs for long-term retention. Reliable observations are key to designing, optimizing and evaluating strategies to prevent and reduce plastic pollution. Several measurement methods have been developed to quantify macroplastic (>0.5 cm) storage and transport in rivers, including visual counting from bridges, net sampling, and images-based techniques. Method harmonization is crucial to make sure data collected using different techniques remains consistent. In turn, this would allow for comparative analysis of plastic pollution within and between rivers. In this paper, we present a harmonization approach to estimate floating plastic item and mass transport from data collected using different methods. The approach allows estimating the same values based on different measurement methods and data collection protocols. We applied our approach to the Mekong-Tonlé Sap-Bassac river system around the city of Phnom Penh, Cambodia. We estimated the floating plastic item and mass transport in the wet and dry season by combining data from net sampling and visual counting. During the wet season, plastic transport in the Mekong increased with a factor of up to 170 (item transport) and 294 (mass transport) compared to the dry season. The river plastic mass balance around Phnom Penh changed considerably, which was mainly due to the flow reversal of Tonlé Sap river between the wet and dry season. Downstream of Phnom Penh, the total plastic transport was consistently higher than upstream, although less in the wet season (1.5–1.7 times) compared to the dry season (3.8–5.9 times), emphasizing the city’s role as entry point of plastic pollution into the Mekong. The largest sources of uncertainty are assumed to be caused by key differences between methods, including the size ranges, extrapolation from observation point to full river width, and the contribution of submerged plastic to the total transport. Future work should focus on including data from other methods than net sampling and visual counting, and reducing the uncertainties related to combining data from different methods. Our results show that river plastic transport dynamics are highly variable over time and space, especially around confluences, bifurcations and urban areas. With our paper we aim to contribute to further harmonization of river plastic monitoring.

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.

Study Region: Mekong River Basin Study focus: Climate change has become a global environmental and socio-economical issue potentially affecting river hydrology and downstream flood characteristics. Climate change and upstream dam construction are the two main driving factors in altering the hydrology in the Mekong River Basin. Cambodian floodplain is located in the vulnerable flood zone to be affected by climate change and upstream dam development. This study investigated the potential impacts of future climate change and dam construction on flood hazards and agricultural damages in the Cambodian floodplain of the Mekong River. Climate outputs from MRI-AGCM3.2S under different sea surface temperature (SST) scenarios and a large ensemble climate dataset (d4PDF) under 4 K increasing scenario were used as input to a fully distributed rainfall-runoff-inundation (RRI) model for future projection of flood in the Lower Mekong Basin. New hydrological insights for the region: The results indicated that the changes in extreme flood events (25-year flood) showed an increase of agricultural flood damages under climate change impact by 18-28% under different SST scenarios. These increases would be only 5-13% under integrated impacts from future climate change and dam construction. Under the 4 K increasing scenario, the agricultural damages for extreme flood events of 10-year, 50-year, and 100-year return periods would increase by 32%, 38%, and 39% for climate change impact alone and 17%, 29%, and 31% for combined climate change and dam development scenario, respectively. The climate change impacts would have more influential on increasing the magnitude of extreme floods than the reduction caused by dam reservoir constructions in the future projection in the Cambodian floodplain of the Mekong River Basin. The impact of dam constructions tends to have greater impact on reducing smaller extreme events; however, their impacts are relatively smaller for larger extreme flood events (i.e., 100-year flood).

Identification of the sources of the spatio-temporal information of flooding is important for flood control and understanding the water dynamic. Flood disasters are generally caused by two main sources: fluvial and pluvial flooding. However, there is a gap in information and challenge of such particular information in the Mekong River Basin (MRB) known as the largest river basin in Southeast Asia. This paper aims to analyze the spatio-temporal hydrograph separations of flooding and to determine the fluvial and pluvial sources of inundation water in the Lower Mekong Basin (LMB) by using a distributed rainfall–runoff–inundation (RRI) model and time–space accounting scheme (T–SAS) method. This study focuses on the two major flood events in 2000 and 2011, whose characteristics were different. The one in 2000 was long in terms of flood duration and it was the historically largest annual total flood water with twice the peak times in July and September. The 2011 flood had the highest flooded water during the peak time; however, its annual total flood water was less than the one in 2000. The results of spatio-temporal sources of flooding indicated that during the flow peak time in 2000 and 2011, the flow at Kratie was mainly contributed by 30-day (67%) and 100-day (98%) precipitation from the upstream. The drainage area of the MRB in China, northern Lao PDR, southern Lao PDR and eastern Thailand, and Cambodia and Vietnam contributed to peak flood at Kratie by 13%, 27%, 33%, and 27% for the 2000 flood and 12%, 33%, 38%, and 16% for the 2011 flood. The source of inundated water in the LMB was derived from upstream flow (fluvial source) of 35–36% and local rainfall (pluvial source) of 64–65%. Even though flood events in 2000 and 2011 had different characteristics, the sources of flood inundation in the LMB for both events were majorly from the local rainfall rather than the upstream flow. The large annual flood volume and long during of flooding in 2000 caused severe total economic damages up to 517 million US$ in the LMB countries (Cambodia, Lao PDR, Thailand, and Vietnam), while the high peak inundated water in 2011 with shorter flood duration caused damages of 493 million US$.

The Tonle Sap Lake (TSL) Basins of the Lower Mekong are one of the world’s most productive ecosystems and have recently been disturbed by climate change. The SWAT (Soil & Water Assessment Tool) hydrological model is utilized to investigate the effect of future climate scenarios. This study focused on two climate scenarios (RCP2.6 and RCP8.5) with three GCMs (GFDL-CM3, GISS-E2-R-CC, and IPSL-CM5A-MR) and their impact on the hydrological process and extremes in the Sen River Basin, the largest tributary of the TSL basin. The annual precipitation, surface runoff, lateral flow, groundwater flow, and total water yield are projected to decrease in both the near-future (2020–2040) and mid-future period (2050–2070), while actual evapotranspiration is projected to increase by 3.3% and 5.3%. Monthly precipitation is projected to increase by 11.2% during the rainy season and decrease by 7.5% during the dry season. Two climate models (GISS and IPSL model) lead to decreases in 1-day, 3-day, 7-day, 30-day, and 90-day maximum flows and minimum flows flow. Thus, the prediction results depend on the climate model used.