•Many Mekong basin ecosystem services depend on sediment.•Anthropogenic changes to basin sediment budget threaten basin livelihoods.•Highest cumulative threat is to lower Mekong floodplains and the ...Delta.•The Delta is a recently deposited landform (<7ka) vulnerable to subsidence/erosion.•The Delta's>17M population and thriving economy are existentially threatened.
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Two decades after the construction of the first major dam, the Mekong basin and its six riparian countries have seen rapid economic growth and development of the river system. Hydropower dams, aggregate mines, flood-control dykes, and groundwater-irrigated agriculture have all provided short-term economic benefits throughout the basin. However, it is becoming evident that anthropic changes are significantly affecting the natural functioning of the river and its floodplains. We now ask if these changes are risking major adverse impacts for the 70 million people living in the Mekong Basin. Many livelihoods in the basin depend on ecosystem services that will be strongly impacted by alterations of the sediment transport processes that drive river and delta morpho-dynamics, which underpin a sustainable future for the Mekong basin and Delta.
Drawing upon ongoing and recently published research, we provide an overview of key drivers of change (hydropower development, sand mining, dyking and water infrastructures, climate change, and accelerated subsidence from pumping) for the Mekong's sediment budget, and their likely individual and cumulative impacts on the river system. Our results quantify the degree to which the Mekong delta, which receives the impacts from the entire connected river basin, is increasingly vulnerable in the face of declining sediment loads, rising seas and subsiding land. Without concerted action, it is likely that nearly half of the Delta's land surface will be below sea level by 2100, with the remaining areas impacted by salinization and frequent flooding. The threat to the Delta can be understood only in the context of processes in the entire river basin. The Mekong River case can serve to raise awareness of how the connected functions of river systems in general depend on undisturbed sediment transport, thereby informing planning for other large river basins currently embarking on rapid economic development.
Untreated runoff was collected over multiple rain events from 19 impermeable urban surfaces, including nine roofs, six roads and four carparks, to quantify the differences in water quality due to ...surface type, age, condition and location. All 19 sites were exposed to the same climatic conditions. Samples were analysed for key urban pollutants of concern, namely total suspended solids and total and dissolved copper and zinc. Results showed uncoated zinc-based roofs produced zinc concentrations (up to 55 mg/L) several orders of magnitude higher than receiving environment water quality guidelines in New Zealand, of which the vast majority was in dissolved form. Even non-metallic roofs with zinc-based guttering produced zinc concentrations over ten times higher than the same roof material without zinc-based guttering. Older zinc-based roofs had approximately five times higher zinc concentrations, demonstrating a substantial age effect on the untreated runoff quality. Similarly, copper roofs produced more than an order of magnitude higher copper concentrations (up to 7.8 mg/L) above the next highest copper-producing surfaces: higher trafficked roads and carparks. Regardless of traffic volume or function, all roads and carparks produced high TSS concentrations. Dissolved metal concentrations were high across the dataset confirming that metal partitioning is an important consideration for effective pollutant control as different removal processes need to be used for dissolved versus particulate metals. This dataset provides an important benchmark of untreated runoff quality across different impermeable surface types within the same geographical area and clearly shows the influence of surface characteristics on water quality runoff regardless of the local differences in land use. These findings provide valuable guidance to stormwater managers in identifying priority surfaces and selection of appropriate treatment strategies for effective stormwater management for total suspended solids, zinc and copper.
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•Untreated runoff compared from 19 urban surfaces under the same climatic conditions.•Dataset compares TSS, zinc and copper across road, carpark and roof runoff.•Surface type drives runoff quality regardless of local differences in land use.•New, uncoated metal roofs leach more zinc than old, coated roofs.•Coated roofs leach zinc levels comparable to roads, but primarily in dissolved form.
•The dominant sources of uncertainty vary temporally and are scale dependent.•Model parameterization is the major source of uncertainty in the short term (2030s).•GCM is the major source of ...uncertainty to flow projections in long term (2060s).•RCP is the major source of uncertainty to sediment projections in long term (2060s).•The uncertainty in sediment load projections is larger than flow projections.
Reliable projections of discharge and sediment are essential for future water and sediment management plans under climate change, but these are subject to numerous uncertainties. This study assessed the uncertainty in flow and sediment projections using the Soil and Water Assessment Tool (SWAT) associated with three Global Climate Models (GCMs), three Representative Concentration Pathways (RCPs) and three model parameter (MP) sets for the 3S Rivers in the Mekong River Basin. The uncertainty was analyzed for the short term future (2021–2040 or 2030s) and long term future (2051–2070 or 2060s) time horizons. Results show that dominant sources of uncertainty in flow and sediment constituents vary spatially across the 3S basin. For peak flow, peak sediment, and wet seasonal flows projection, the greatest uncertainty sources also vary with time horizon. For 95% low flows and for seasonal and annual flow projections, GCM and MP were the major sources of uncertainty, whereas RCPs had less of an effect. The uncertainty due to RCPs is large for annual sediment load projections. While model parameterization is the major source of uncertainty in the short term (2030s), GCMs and RCPs are the major contributors to uncertainty in flow and sediment projections in the longer term (2060s). Overall, the uncertainty in sediment load projections is larger than the uncertainty in flow projections. In general, our results suggest the need to investigate the major contributing sources of uncertainty in large basins temporally and at different scales, as this can have major consequences for water and sediment management decisions. Further, since model parameterization uncertainty can play a significant role for flow and sediment projections, there is a need to incorporate hydrological model parameter uncertainty in climate change studies and efforts to reduce the parameter uncertainty as much as possible should be considered through a careful calibration and validation process.
Sediment and heavy metals in stormwater runoff are key pollutants of urban waterways, and their presence in stormwater is driven by climatic factors such as rainfall intensity. This study describes ...the total suspended solids (TSS) and heavy metal concentrations found in runoff from four different urban surfaces within a residential/institutional catchment, in a climate where rainfall is typically of low intensity (<5.1mm·h−1). The results were compared to untreated runoff quality from a compilation of international studies. The road runoff had the highest TSS concentrations, while copper and galvanized roof runoff had the highest copper and zinc concentrations, respectively. Pollutant concentrations were found to be significantly different between surfaces; quantification and prediction of pollutant contributions from urban surfaces should thus take account of the different surface materials, instead of being aggregated into more generalized categories such as land use. The TSS and heavy metal concentrations were found to be at the low to medium end of ranges observed internationally, except for total copper and zinc concentrations generated by dissolution of copper and galvanized roofing material respectively; these concentrations were at least as high as those reported internationally. TSS wash-off from the roofs was seen to be a source-limited process, where all available TSS is washed off during the rain event despite the low intensity rainfall, whereas both road TSS and heavy metals wash-off from roof and road surfaces appeared to all be transport-limited and therefore some carryover of pollutants occurs between rain events. A first flush effect was seen from most surfaces for TSS, but not for heavy metals. This study demonstrates that in low intensity rainfall climates, quantification of untreated runoff quality from key individual surface types in a catchment are needed to enable development of targeted and appropriately sized stormwater treatment systems.
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•TSS and metals runoff from 4 surfaces quantified under low intensity rainfall•Pollutant concentrations significantly different between roads and roof surfaces•Road runoff is key source of TSS, transport-limited process with first flush effect.•Metal roofs are key sources of zinc and copper, primarily in dissolved form.•Modelling of individual surfaces is needed for low intensity rainfall dynamics.
...RDR was supported by an MRC Partnership Grant for the PROGnosis RESearch Strategy (PROGRESS) group (grant reference number: ...IPD-MAs of randomized intervention and etiological studies, which ...are beyond the scope of this paper and are instead addressed in the accompanying paper 16, differ from IPD-MAs of multivariable prediction models, which are the focus of this paper.
The economic value of the Tonle Sap Lake Floodplain to Cambodia is arguably among the highest provided to a nation by a single ecosystem around the world. Nonetheless, the Mekong River Basin is ...changing rapidly due to accelerating water infrastructure development (hydropower, irrigation, flood control, and water supply) and climate change, bringing considerable modifications to the flood pulse of the Tonle Sap Lake in the foreseeable future. This paper presents research conducted to determine how the historical flooding regime, together with human action, influenced landscape patterns of habitats in the Tonle Sap Lake, and how these habitats might shift as a result of hydrological changes. Maps of water depth, annual flood duration, and flood frequency were created for recent historical hydrological conditions and for simulated future scenarios of water infrastructure development and climate change. Relationships were then established between the historical flood maps and land cover, and these were subsequently applied to assess potential changes to habitat cover in future decades. Five habitat groups were clearly distinguishable based on flood regime, physiognomic patterns, and human activity: (1) Open water, flooded for 12 months in an average hydrological year; (2) Gallery forest, with flood duration of 9 months annually; (3) Seasonally flooded habitats, flooded 5–8 months and dominated by shrublands and grasslands; (4) transitional habitats, flooded 1–5 months and dominated by abandoned agricultural fields, receding rice/floating rice, and lowland grasslands; and (5) Rainfed habitats, flooded up to 1 month and consisting mainly of wet season rice fields and village crops. It was found that water infrastructure development could increase the area of open water (+18 to +21%) and the area of rainfed habitats (+10 to +14%), while reducing the area covered with seasonally flooded habitats (−13 to −22%) and gallery forest (−75 to −83%). Habitat cover shifts as a result of climate change include a net increase of open water (2–21%), as well as a reduction of rainfed habitats by 2–5% and seasonally flooded habitats by 5–11%. Findings from this study will help guide on-going and future conservation and restoration efforts throughout this unique and critical ecosystem.
► We quantified spatial and temporal changes using geographical and hydrologic data. ► We found good correlation between flood duration and habitat cover. ► We quantified changes on habitat cover caused by alterations in the Mekong Basin. ► Conservation efforts must consider flooding seasonality and low agricultural use.
Addressing uncertainty in sediment predictions due to land use/land cover (LULC) change could better inform the selection of sediment management options for reservoir sustainability. We used the Nam ...Kong catchment of the Mekong River Basin in Southern Laos, with two hydropower dams in series, to understand the implications of LULC change uncertainty for catchment‐level and reservoir‐level sediment management options. The catchment‐level sediment management options of terracing, vegetative filter strips, and no tillage were evaluated applying the Soil and Water Assessment Tool (SWAT). The reservoir‐level sediment management option of flushing was assessed using the Sediment Simulation Screening Python Model (PySedSim). Costs of sediment management options were assessed via the economic value of the loss in hydropower production and the avoided cost of dredging. Our results suggest that LULC projections resulted in high variability in loss of reservoir capacity and cost of sediment management. Terracing was found to be the best catchment‐level management option at decreasing both the magnitude and variability in loss of reservoir storage for both dams, but it was also the most expensive option. Flushing was also effective in reducing sedimentation, but it was less economically beneficial compared to catchment‐level sediment management options. Combinations of catchment‐level and reservoir‐level management strategies, however, can be effective in reducing the magnitude and variability in loss of reservoir storage and associated costs in response to LULC change uncertainty.
Key Points
Costs of sediment management options are linked to the economic value of the loss in hydropower production and the avoided cost of dredging
The sediment management strategy best at increasing the life span of a reservoir may not be the most cost‐effective option to implement
Integration of suitable sediment management options can reduce the amount and variability in reservoir storage loss and associated cost
•A 3D model was used to quantify net sedimentation and aquatic productivity.•The model was run with scenarios of hydrological and habitat cover change.•Hydropower causes distinct seasonal changes and ...climate change increase uncertainty.•Annual net sedimentation is projected to decrease by 53% due to hydropower.•Aquatic net primary production is projected to decrease by 9–39%.
The Tonle Sap is the largest lake in Southeast Asia and its fishery supports the livelihood and nutrition of millions of people in Cambodia. However, the hydrological and ecological drivers of this ecosystem are changing as a result of hydropower development on the Mekong River and global climate change. The objective of this study was to quantify the impacts of the Mekong's future hydrological alterations on aquatic net primary production (NPP) of the Tonle Sap. A three-dimensional (3D) hydrodynamic model was used to evaluate eleven scenarios of hydropower development and climate change, with respect to water flows, suspended sediments, and floodplain habitat cover, which were identified as the key drivers of productivity change. We found that hydropower development would cause the most distinct changes in seasonality by reducing wet season water levels and increasing dry season water levels. Combined scenarios of hydropower and climate change revealed that areas of open water and rainfed/irrigated rice would expand by 35±3% and 16±5%, respectively, while optimal area for gallery forest would decrease by 40±27%. The estimated annual net sedimentation was projected to decrease by 56±3% from the 3.28±0.93 million tons baseline values. Annual average NPP in the open water and in the floodplain was 1.07±0.06 and 3.67±0.61 million tons C, respectively, and a reduction of 34±4% is expected. Our study concludes that Tonle Sap's drivers of ecological productivity – habitat cover, sedimentation, and NPP – will face a significant change, and a decline of its ecosystem's services should be expected if mitigation and adaptation strategies are not implemented.
The Tonle Sap is the most fertile and diverse freshwater ecosystem in Southeast Asia, receiving nurturing water flows from the Mekong and its immediate basin. In addition to rapid development in the ...Tonle Sap basin, climate change may threaten natural flow patterns that sustain its diversity. The impacts of climate change on river flows in 11 sub-basins contributing to the Tonle Sap Lake were assessed using the Soil and Water Assessment Tool (SWAT) model to quantify the potential magnitude of future hydrological alterations. Projected river flows from three General Circulation Models (GFDL-CM3, GISS-E2-R-CC and IPSL-CM5A-MR) for three time horizons (2030s, 2060s and 2090s) indicate a likely decrease in both the wet and dry season flows. The mean annual projected flow reductions range from 9 to 29%, 10 to 35% and 7 to 41% for the 2030s, 2060s and 2090s projections, respectively. Moreover, a decrease in extreme river flows (Q5 and Q95) was also found, which implies there could be a decline in flood magnitudes and an increase in drought occurrences throughout the basin. The results of this study provide insight for water resources planning and adaptation strategies for the river ecosystems during the dry season, when water flows are projected to decrease.
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