Research On Water Supply-Energy Production-Environment Nexus Of The Lancang-Mekong River Basin

As the largest international river in Asia,the Lancang-Mekong River is rich in species.Its flow regime is critical to the ecological environment of the basin and the livelihoods of riparian people.At the same time,with a large elevation difference,the river basin is rich in hydropower resources.However,massive hydropower development in this basin is changing the flow regime(e.g.,runoff,nutrients)of the Lancang-Mekong River,which may affect the agricultural production and ecological security of riparian countries to some extent.With the successive completion of the Xiaowan and Nuozhadu hydropower plants on the mainstream of the Lancang River,substantial attentions have been paid to the ecological and environmental issues and water resources disputes relevant to the Lancang cascade hydropower dams in China.With the implementation of “the One Belt One Road Strategy”,China’s cooperation with countries in the lower Mekong River is getting closer and closer.It’s urgent to reveal the disparate interactions among water,energy and ecosystem sectors inherent in hydropower development of the basin.This work can help to clarify the comprehensive transboundary effects of the Lancang cascade reservoirs on the lower Mekong River,providing scientific support for crossborder game and international river cooperation negotiations relevant to the Mekong River.Taking the Lancang-Mekong River Basin as the study area,this study focuses on the ecological and environmental issues and water resources disputes related to the hydropower development of the Lancang River.The main objectives of this study are to develop the simulationoptimization model of the water supply-energy production-environment coupling system,to explore the energy production-environment nexus of the Langcang River Basin,to unravel the transboundary water supply-energy production-environment nexus of the Langcang-Mekong River Basin,and to illustrate the water supply-energy production-environment nexus under changing environmnets.The major study findings include the following.(1)A simulation-optimization modeling method for the water supply-energy productionenvironment coupling system was proposed to manage the conflict of low model development cost and high problem pertinence in complex system modeling.The system boundaries of the water supply,energy production,and environment subsystems were first determined according to the management needs of the Lancang-Mekong River Basin.Second,using mature models and methods in related fields,simulation models of each subsystem were established based on the available data of the river basin.Third,the input-output relationships between these models were systematically analysed to propose the coupling framework of multiple simulation models,with the coupling procedure of multiple models and the coupling model solving process described.Finally,the basic ideas of results analysis and rationality verification of the coupling model were illustrated.The model built in this paper incorporates the total phosphorus cycle into the water supply-energy production-environment coupling system,breaking through the limitations of previous studies which have difficulties in revealing the linkages between nutrients and water supply and power generation variables in runoff regulation due to the lack of description of the nutrient transport process in water bodies.Beyond this,the above modeling method and analysis ideas provide references for nexus analysis in similar watersheds.(2)Coupling the multiobjective reservoir operation optimization with the mass balance model of total phosphorus(TP),a multiobjective operation optimization model was established,which aims to maximize the total hydropower production(THP),firm power(FP),and total phosphorous export(TPE)of the Lancang River.With the ε-NSGAII algorithm applied to solve the above model,impacts of the Lancang cascade reservoirs on riverine TP transport were investigated through a series of numerical experiments.The major results include the following.First,although the Lancang cascade reservoirs impact the riverine TP transport to some extent,the TP flux of the lower Mekong River is not greatly affected by the TP interception of the Lancang cascade reservoirs,since the Lancang River’s contribution to the TP loads of the Lancang-Mekong River Basin is relatively small(about 5.8%).Second,the potential to enhance TPE is limited(with the change magnitude less than 10%)under traditional reservoir operations using total hydropower production and firm power as the objectives.Third,with a normal hydropower reservoir operation rule adopted for the Lancang cascade reservoirs,the TPE increases when the maximum allowed water storages during the wet season of Xiaowan and Nuozhadu reservoirs are reduced,but it decreases as the maximum allowed water storages during the dry season are declined.What’s more,the TPE is more sensitive to the constraint change in the wet season than in the dry season.Fourth,the TPE increases significantly(increment approaches approximately 22%)if the objective of maximizing TPE is added to the multiobjective operation optimization model.Finally,TPE yields a tradeoff against THP and FP,with the conflict of TPE and FP stronger than that of TPE and THP.(3)The TP mass balance model and downstream water supply model are coupled into the cascade reservoir operation optimization,forming a simulation-optimization model of a water supply-energy production-environment coupling system that maximizes the total hydropower production(THP),firm power(FP),the runoff satisfaction degree(RSD),total phosphorous export(TPE)and the satisfactory degree of the agricultural water supply(SDAWS).After that,simulation-optimization analyses were performed to reveal the water supply-energy production-environment nexus of the Lancang-Mekong River Basin.The major study findings are as follows.First,cascade reservoir operations in the Lancang River are beneficial for downstream agricultural water supply relative to natural conditions,which can increase the downstream SDAWS from 714.4% under natural conditions to 73.4-85.4%.However,an ambitious improvement in the downstream water supply situation would require a substantial shift in the time schedule of hydropower generation,which could lead to economic losses on the Chinese side and a reduction in the reliability of power grids during dry years.Second,the tradeoffs between SDAWS and RSD and between SDAWS and TPE are induced by different temporal distributions of water demand of ecosystem conservation objectives and downstream agricultural water supply,rather than the hydropower production of the Lancang cascade reservoirs in China.Third,the relationship between RSD and TPE is synergistic at first,followed by a weak tradeoff.Fourth,the agricultural water supply in the downstream countries results in more losses in ecosystem conservation objectives than hydropower production in China.(4)Based on the multiobjective simulation-optimization model with objectives of maximizing THP,FP,RSD and SDAWS as well as the performances of the SWAT model for the Lancang-Mekong River Basin under climate change scenarios,the evolution trajectory of the water supply-energy production-environment nexus under changing enrironments(e.g.,climate change,hydropower development)was explored through scenario analysis.The major study findings are as follows.First,driven by increasing precipitations and temperature,annual average streamflows of the Lancang-Mekong River Basin are likely to increase under future climate,with large increaments in the middle and far future but a small increase in the near future.Second,under climate change scenarios,the THP of the Lancang cascade reservoirs is expected to increase significantly,with a small increment in both FP and SDAWS but slight declines of the upper and lower limits of RSD.Meanwhile,the tradeoffs between water supply and energy production,between energy production and environment,and between water supply and environment are estimated to be weakened.Third,compared to the development plan with two reservoirs and five hydropower plants,THP,FP and SDAWS under the alternative development plan with two reservoirs and ten hydropower plants will increase significantly,but the lower limit of RSD in this case will decline slightly.What’s more,the tradeoffs between water supply and energy production and between energy production and environment are expected to be weakened,whereas the relationship between SDAWS and RSD will remain unchanged.Based on above works,this study clarifies the comprehensive transboundary effects of hydropower development in the Lancang River on the TP flux and available water for agriculture of the lower Mekong River and identifies the constraints for coordinated development of the water supply-energy production-environment coupling system in the Lancang-Mekong River Basin.The key findings include the following.First,there exist tradeoffs between hydropower production objectives and TP export,and the riverine TP transport is affected by the Lancang cascade reservoirs to some extent.But the TP flux of the lower Mekong River is not greatly affected by the TP interception of the Lancang cascade reservoirs,as the Lancang River’s contribution to the TP loads of the Lancang-Mekong River Basin is relatively small(about 5.8%).Second,cascade reservoir operations in the Lancang River are beneficial for downstream agricultural water supply,which can increase the downstream SDAWS from 714.4% under natural conditions to 73.4-85.4%.However,an ambitious improvement in the downstream water supply situation would require a substantial shift in the time schedule of hydropower generation,which could lead to economic losses on the Chinese side and a reduction in the reliability of power grids during dry years.Third,the agricultural water supply objective yields a tradeoff against the two ecosystem conservation objectives(RSD and TPE),and reservoir operations for downstream agricultural water supply have more constraining effects on ecosystem conservation than China’s reservoir operations for hydropower production.Some foreign media claim that the retention of nutrients and streamflows by the Lancang cascade reservoirs is not conducive to the maintenance of nutrient fluxes in the lower Mekong River and the utilization of water resources in downstream countries.However,the conclusions of this study suggest that the above viewpoints have no scientific basis.Beyond this,relevant conclusions of this study also provide references for China’s normalized water resources cooperation with downstream countries and negotiation of compensation mechanisms during the emergency water supplement period.