Regional Hydroclimatic Effect And Its Trend Prediction In The Three Gorges Reservoir

As one of the largest hydropower projects in the world,the Three Gorges Project has comprehensive benefits such as flood control,power generation,shipping,and environmental protection,aquaculture and water supply,and plays an important role in the economic development and ecological conditions of the upper reaches of the Yangtze River(UYRB).Since the storage of water in 2003,the reservoir area has formed an artificial lake with a total area of 1084 km~2,which is more than 600 km long and 1-2 km wide.Under the influence of climate change and human activities,the natural state and surface hydrological regime of the UYRB have changed obviously,which brings new challenges to the comprehensive utilization and management of water resources,flood control and drought relief.Quantitative analysis of the effects of global climate change and regional climate effect of the Three Gorges Reservoir(TGR)on hydrological climate over the UYRB have an important scientific significance and application value in understanding the regional hydrological climate effect and action mechanism of large-scale water conservancy project,studying the occurrence law of future meteorological disasters,disaster early warning and efficient use of water resources.The main research objective of this thesis is to study the variation law of hydrological process over the UYRB under the comprehensive influence of global climate change and regional climate effect of the TGR and to reveal the influence mechanism of reservoir impoundment on land surface hydrological process and regional climate.Focusing on the above research objectives,the thesis mainly studies the four aspects of the TGR and the UYRB,including the sensitivity assessment of regional climate model parameter schemes,regional climate effects and future extreme climate,land-atmosphere coupling simulation system construction and application and runoff process prediction.The main conclusions and innovative results of the research are as follows:(1)Based on multi-objective function,the simulation performance of the regional climate model for the UYRB cwas evaluated,and the effects of different cumulus convective parameterization schemes and land surface process schemes on simulation performance of the UYRB were discussed.The 72 parameterization schemes have excellent simulation performance for the temperature of the UYRB,but poor simulation performance for precipitation.Precipitation is sensitive to cumulus convective parametric scheme,and the Kian-Fritsch scheme has the best comprehensive simulation performance on precipitation in the UYRB,while the Biosphere-Atmosphere Transfer Scheme has the best comprehensive simulation performance on temperature in the UYRB.In the simulation results of the Kain-Fritsch scheme,compared with the BATS land surface scheme,the CLM land surface scheme has higher soil moisture and sensible heat flux as well as less evapotranspiration and precipitation,which directly leads to higher surface temperature simulated by CLM scheme.Compared with the BATS scheme,warmer surface temperature and less evapotranspiration of the CLM scheme lead to weaker simulated water vapor transport capacity,resulting in less precipitation simulated in the CLM scheme.At the same time,the dry atmosphere of the CLM scheme also increases the net radiation flux reaching the land surface and changes the surface energy budget,which leads to a large difference in surface temperature between CLM and BATS.(2)Based on the regional climate model(Reg CM4),the regional climate effects of the TGR and its influence on extreme precipitation were analyzed,and the mechanism of the regional climate effect of the TGR was revealed.Under the lake scheme L1,the air temperature over the TGR area increased in all seasons except spring,and the annual average temperature increased by 0.12?.The annual average precipitation decreased by 0.28mm/day,with the largest decrease in spring and summer.The average annual evapotranspiration increases by 0.04mm/day,which increases in autumn and winter,but decreases in spring and summer.According to the difference of MSE,CAPE and CIN before and after reservoir construction,the change of precipitation of the TGR area is mainly caused by the inhibition of convection activities under the cooling of the water body,which leads to a significant reduction in convection precipitation.The change of air temperature is mainly due to a large amount of energy exchange between the reservoir and the surrounding land,which plays an important role in regulating the energy budget in the region during the year.The change of evaporation is mainly affected by the seasonal variation of temperature gradient and direction between water temperature at a depth of 0.05m and air temperature at 2m height in the CLM4.5 lake model,as well as by the temperature gradient between the shallow water temperature and the deep water temperature.Under the lake scheme L2,the influence of reservoir regional climate effect on weak precipitation events is stronger than that of heavy precipitation events.The intensity and frequency of weak precipitation events(below 50th)in the TGR decreased significantly,while the frequency of heavy precipitation events(above 90th)decreased slightly,but their intensity and contribution to annual precipitation increased slightly.The impact of reservoir regional climate effect on precipitation is mainly concentrated within 20 km,and there is no obvious influence on the interannual variation trend of each extreme precipitation index in 2021-2050.(3)Based on Regional Climate Model(Reg CM),Variable Infiltration Capacity(VIC)Model and correction model of dynamic downscaling climatic factors based on quantile mapping method,one-way land-atmosphere coupling simulation system for the upper reaches of the Yangtze River was established.Based on the Generalized Likelihood Uncertainty Estimation(GLUE),the sensitivity of the VIC hydrological model parameters is analyzed.The results show that the infiltration shape parameter(B)and the depth of the second soil layerare the sensitivity parameters in the model.The 95%confidence interval calculated by the GLUE method covers the measured runoff of 4 hydrologic stations during the verification period,which indicates that the VIC hydrological model has certain applicability to the simulation of runoff over the UYRB.The VIC model has good simulation ability for daily-scale and monthly-scale runoff processes and peak-times of the UYRB.The NSE in the calibration and verification periods is above 0.9,while the BIAS is within?10%.The simulation performance of the VIC model for wet year is better than that for dry year,and the annual runoff of wet year is underestimated,while the annual runoff of dry water is overestimated.Based on the quantile mapping method,correction model of dynamic downscaling climatic factors is constructed,and the correction performance of the method based on single distribution and mixed distribution is evaluated.According to the evaluation indexes such as root mean square error,mean absolute error and correlation coefficient,the correction effect of the mixed distribution quantile mapping method on precipitation is better than that of the single distribution method.(4)Based on the land-atmosphere coupling system,the future changes of climate and hydrological process over the upper reaches of the Yangtze river basin were simulated,and the effects of climate change and reservoir climate effect on runoff process and runoff components were quantitatively analyzed.According to the prediction results from the Reg CM4 dynamic downscaling,compared with the reference period of1971-2000,the eastern part of the UYRB showed a warm and dry trend during 2021-2050,while the western part of the UYRB showed a warm and wet trend.In 2021-2050,the total runoff of the UYRB decreased by 4.1%-5%,the snow runoff decreased by 36%-39%,and the extreme runoff decreased slightly.The direct cause of the significant decrease of runoff in the southeastern part of the UYRB is due to the decrease in precipitation and the increase in evaporation in the region.The influence of regional climate effect induced by the TGR on total runoff is comparable to that of global climate change and changes the small scale cycle of runoff.Under the RCP4.5 scenario,the regional climate effects of Lake Scheme L1 and L2increase the total annual runoff by 2.9%and 3.7%,respectively,and extreme runoff(Q95 and)increase slightly,indicating that the climate effect of the reservoir alleviated the adverse effects of climate change on runoff to some extent.The change of spatial pattern and structure of precipitation caused by regional climate effect from the TGR is the main factor causing the change of annual runoff in the basin.