The transboundary Mekong River is facing two ongoing changes that are expected to significantly impact its hydrology and the characteristics of its exceptional flood pulse. The rapid economic development of the riparian countries has led to massive plans for hydropower construction, and projected climate change is expected to alter the monsoon patterns and increase temperature in the basin. The aim of this study is to assess the cumulative impact of these factors on the hydrology of the Mekong within next 20–30 yr. We downscaled the output of five general circulation models (GCMs) that were found to perform well in the Mekong region. For the simulation of reservoir operation, we used an optimisation approach to estimate the operation of multiple reservoirs, including both existing and planned hydropower reservoirs. For the hydrological assessment, we used a distributed hydrological model, VMod, with a grid resolution of 5 km × 5 km. In terms of climate change's impact on hydrology, we found a high variation in the discharge results depending on which of the GCMs is used as input. The simulated change in discharge at Kratie (Cambodia) between the baseline (1982–1992) and projected time period (2032–2042) ranges from −11% to +15% for the wet season and −10% to +13% for the dry season. Our analysis also shows that the changes in discharge due to planned reservoir operations are clearly larger than those simulated due to climate change: 25–160% higher dry season flows and 5–24% lower flood peaks in Kratie. The projected cumulative impacts follow rather closely the reservoir operation impacts, with an envelope around them induced by the different GCMs. Our results thus indicate that within the coming 20–30 yr, the operation of planned hydropower reservoirs is likely to have a larger impact on the Mekong hydrograph than the impacts of climate change, particularly during the dry season. On the other hand, climate change will increase the uncertainty of the estimated reservoir operation impacts: our results indicate that even the direction of the flow-related changes induced by climate change is partly unclear. Consequently, both dam planners and dam operators should pay closer attention to the cumulative impacts of climate change and reservoir operation on aquatic ecosystems, including the multibillion-dollar Mekong fisheries

The world's rivers deliver 19 billion tonnes of sediment to the coastal zone annually 1 , 17 with a significant fraction being sequestered in large deltas, home to over 500 million 18 people. Most (>70%) large deltas are under threat from a combination of rising sea 19 levels, ground surface subsidence and anthropogenic sediment trapping 2,3 , and a 20 sustainable supply of fluvial sediment is therefore critical in preventing deltas being 21 'drowned' by rising relative sea levels 2,3,4 . Here, we combine suspended sediment 22 load data from the Mekong River with hydrological model simulations to isolate the 23 role of tropical cyclones (TCs) in transmitting suspended sediment to one of the 24 world's great deltas. We demonstrate that spatial variations in the Mekong's 25 suspended sediment load are correlated (r = 0.765, p < 0.1) with observed variations 26 in TC climatology, and that a significant portion (32%) of the suspended sediment 27 load reaching the delta is delivered by runoff generated by TC-associated rainfall. 28Furthermore, we estimate that the suspended load to the delta has declined by 52.6 ± 29 explaining past 5,6,7 , and anticipating future 8,9 , declines in suspended sediment loads 34 reaching the world's major deltas. However, our study shows that changes in TC 35 climatology affect trends in fluvial suspended sediment loads and thus are also key to 36 fully assessing the risk posed to vulnerable coastal systems. 37 Mt over recent years (1981-2005The world's largest rivers contribute a disproportionately large fraction (Extended 38Data Table 1) of the terrestrial sediment flux, which has both created, and is critical in 39 sustaining, their great deltas. Moreover, river borne sediments are a key vector for carbon 40 and nutrients, thereby playing a vital role in global biogeochemical cycles 10,11 . However, a 41 significant majority (>70%) of large deltas are now recognized as being under severe 42 threat from rising relative sea levels 2,3 , in part due to reported anthropogenically-driven 43 reductions in sediment loads 5,6,7 . Many large rivers are located in tropical regions 44 (Extended Data Figure 1) that exhibit highly seasonal flow regimes affected by tropical 45 cyclones (TCs). The potential destructive or constructive impacts of tropical cyclones that 46 directly strike deltas are well established 12,13 . However, when they strike further upstream 47TCs deliver much higher than normal levels of rainfall, effectively triggering landslides 48 and mobilizing sediments into the river network, thereby generating very high 49 instantaneous sediment loads 14,15,16 . Such high sediment loads could compensate for the 50 potential destructive effects of TCs striking deltas proper but, notwithstanding some prior 51 studies in smaller drainage basins 17,18 , the role of TCs in driving sediment delivery to the 52 lowlands and coast remains unclear. As noted, this is particularly the case for large rivers 53 that carry much of the terrestrial sediment flux because these rivers are, in t...

No abstract

Abstract. Climate change poses critical threats to water-related safety and sustainability in the Mekong River basin. Hydrological impact signals from earlier Coupled Model Intercomparison Project phase 3 (CMIP3)-based assessments, however, are highly uncertain and largely ignore hydrological extremes. This paper provides one of the first hydrological impact assessments using the CMIP5 climate projections. Furthermore, we model and analyse changes in river flow regimes and hydrological extremes (i.e. high-flow and low-flow conditions). In general, the Mekong's hydrological cycle intensifies under future climate change. The scenario's ensemble mean shows increases in both seasonal and annual river discharges (annual change between +5 and +16 %, depending on location). Despite the overall increasing trend, the individual scenarios show differences in the magnitude of discharge changes and, to a lesser extent, contrasting directional changes. The scenario's ensemble, however, shows reduced uncertainties in climate projection and hydrological impacts compared to earlier CMIP3-based assessments. We further found that extremely high-flow events increase in both magnitude and frequency. Extremely low flows, on the other hand, are projected to occur less often under climate change. Higher low flows can help reducing dry season water shortage and controlling salinization in the downstream Mekong Delta. However, higher and more frequent peak discharges will exacerbate flood risks in the basin. Climate-change-induced hydrological changes will have important implications for safety, economic development, and ecosystem dynamics and thus require special attention in climate change adaptation and water management.

The transboundary Mekong River is facing two on-going changes that are estimated to significantly impact its hydrology and the characteristics of its exceptional flood pulse. The rapid economic development of the riparian countries has led to massive plans for hydropower construction, and the projected climate change is expected to alter the monsoon patterns and increase temperature in the basin. The aim of this study is to assess the cumulative impact of these factors on the hydrology of the Mekong within next 20–30 yr. We downscaled output of five General Circulation Models (GCMs) that were found to perform well in the Mekong region. For the simulation of reservoir operation, we used an optimisation approach to estimate the operation of multiple reservoirs, including both existing and planned hydropower reservoirs. For hydrological assessment, we used a distributed hydrological model, VMod, with a grid resolution of 5 km &times; 5 km. In terms of climate change's impact to hydrology, we found a high variation in the discharge results depending on which of the GCMs is used as input. The simulated change in discharge at Kratie (Cambodia) between the baseline (1982–1992) and projected time period (2032–2042) ranges from &minus;11% to +15% for the wet season and &minus;10% to +13% for the dry season. Our analysis also shows that the changes in discharge due to planned reservoir operations are clearly larger than those simulated due to climate change: 25–160% higher dry season flows and 5–24% lower flood peaks in Kratie. The projected cumulative impacts follow rather closely the reservoir operation impacts, with an envelope around them induced by the different GCMs. Our results thus indicate that within the coming 20–30 yr, the operation of planned hydropower reservoirs is likely to have a larger impact on the Mekong hydrograph than the impacts of climate change, particularly during the dry season. On the other hand, climate change will increase the uncertainty of the estimated hydropower impacts. Consequently, both dam planners and dam operators should pay better attention to the cumulative impacts of climate change and reservoir operation to the aquatic ecosystems, including the multibillion-dollar Mekong fisheries