Impacts Of Reservoir Regulation On The Characteristics Of Floods Over Mekong River Basin Under Climate Change

The water resource in Mekong River Basin(MRB)is facing two main challenges: climate change and dam constructions.The basic assumption of stationarity in hydrology thus no longer exists in MRB.The characteristics of future floods in MRB under the scenario of possible climate change and ongoing reservoir regulations is an important issue needed to be addressed,for the sustainable development in this transboundary and rapidly developing basin.A large scale distributed Geomorphology-Based Hydrological Model is successfully built over MRB(GBHM-MK),using remote sensing(RS)rainfall products as driving data.Compared with observed discharge,we assessed the feasibility of simulating hydrological processes using RS rainfall data over MRB.A detailed comparison was conducted between simulation driven by RS rainfall data and that driven by in situ data,and it shows that the discharge produced when RS rainfall was used to drive the GBHM-MK is much closer to the observations compared to the discharge generated when unevenly distributed gauge data were used to drive the model,meanwhile,the calibrated parameters of RS-driven simulation were more stable and reliable than those of gauge-driven simulation.Based on daily discharge observations,a non-linear filtering effect of reservoir regulation on flood frequency curve was found.A dimensionless index Reservoir Impact Index(RII)was defined to quantify the reservoir regulation effects.It is found that comparing the post-dam period with the pre-dam period,the mean annual maximum flood(MAF)generally decreases with increasing RII and stabilizes once RII exceeds a threshold value,and the Flood Frequency Curve(FFC)becomes steeper for smaller RII,but it becomes flatter after RII exceeds a threshold value.A process-based understanding is obtained using three reservoir operation models(SLM,SOP1 and SOP2)with various levels of complexity.All models captured the non-linear relationships of the MAF and coefficient of variations(CV)of flood with RII,suggesting that the basic flood control function of reservoirs is key to the non-linear relationships.The relative roles of reservoir storage capacity,operation objectives,available storage prior to a flood event,and reservoir inflow pattern were systematically investigated.Reservoirs and dams information on Mekong mainstream were collected,and SOP2 was coupled into the river routing procedure of GBHM-MK.Downscaling outputs from five General Circulation Models(GCMs)provided by ISI-MIP were used to drive the coupled GBHM-MK-SOP2 to project flood trends over MRB in 2010-2099.Through the analysis of projected precipitation,it is found that annual rainfall rate will increase over the basin,rainy days will decrease in the future,rainfall intensity will increase,while extreme rainfall events will be more intensified,especially for rainfall with higher probability of exceedance.Flood risk in MRB will increase with impacts of climate change: flood magnitude and inter-annual variation will increase,and flood frequency will increase at the same time.It is also found that upstream basin is more vulnerable to climate change.Taking reservoir regulations into consideration,the results demonstrate that reservoir regulation can efficiently reduce flood risk,particularly for upstream basin.From the temporal perspective,reservoir regulation can significantly alleviate flood risk brought by climate change in near future(2010-2039)and middle future(2040-2059);on the other hand,the impact of climate change will increase over time which results in the increase of flood risk in far further(2070-2099).From the spatial perspective,reservoir regulation can efficiently reduce flood risk in upstream basin due to large reservoirs mainly locate in upstream area;while for downstream area,reservoir reguations have limited impacts on flood,climate change will dominate flood pattern and make flood risk increase continuously.This paper demonstrates that climate change will have vital impacts on flood regime(magnitude,inter-annual variation and frequency)over MRB.With current and planned reservoirs,the magnitude and frequency of future flood could be reduced.However,further anthropogenic precaution measures are still needed to mitigate the adverse consequences brought by climate change in downstream basin.