| DAMS not only directly change the hydrological longitudinal connectivity of rivers,but also change the hydrological conditions of rivers to a great extent.Studying the impact of cascade DAMS on the hydrological regime of basins can improve the understanding of the hydrological effects of dam impounding and its future risks,and also provide scientific support for the management of water resources and ecological protection of basins.Although the hydrological effects of dam impoundment have been discussed,few studies have been conducted on the hydrological effects under the multi-stage dam combination model and the adaptation of river hydrological conditions to climate change under the influence of such combination model.Research objective: Using the Qingjiang River Basin with cascade dam sequence as a typical research area,to reveal the response characteristics of downstream hydrological conditions under the multi-stage dam combination model and the adaptability characteristics under the climate change scenario.Research methods:(1)The hydrological simulation model was used to simulate the hydrological responses of cascade dam impoundment under different climatic scenarios.(2)Selecting suitable typical concentration scenarios,using Tyson polygon method as the evaluation basis to select the best applicable BNU model,combined with the constructed SWAT model,to simulate the change characteristics of future temperature,precipitation and runoff in the basin,so as to study the seasonal changes of hydrological elements in the basin.(3)The IHA ecohydrological index system was used to study the impact of the dam on the hydrological regime of the basin.Due to the lack of hydrological data after the completion of Geheyan Dam,MODSIM method was used to simulate the data and analyze the response of the dam operation to the basin.(4)The IHA method was used to estimate the degree of hydrological change,and to reflect the change of specific hydrological index.The main findings are as follows.(1)Through the parameter calibration of the model,the model verification results show that the SWAT model can be used as a watershed hydrological model to simulate the hydrological process downstream of the dam.In the future climate change scenario,the change of the downstream hydrological status of the cascade dam in both wet and dry seasons,the two-dam and three-dam modes will change the downstream runoff and sediment discharge more than the single-dam mode.Moreover,the impact on the low-water season is greater than that on the wet season,and the impact on the runoff is greater than that on the sediment discharge.Thus,the difference of hydrological conditions in the period of abundance and drought is intensified.(2)The analysis based on the measured hydrological data shows that,even under the single dam operation mode,the sediment content in most months downstream of the dam is almost reduced by an order of magnitude,and the sediment transport rhythm of the river is changed to a large extent,that is,the duration of the month with high sediment content is significantly reduced,while that of the month with low sediment content is significantly increased.The monthly maximum sediment content decreased from 1.192mg/L to 0.088mg/L by54.2%.The decrease of sediment content was greater in dry months than in wet months,and the largest decrease occurred in April in dry months,which was about 99.7% less than that before impoundment.In terms of the influence on downstream discharge,there are differences among different hydrological years.For wet years,the downstream discharge of the dam increases from November to February in dry months,while the downstream discharge decreases significantly in April to May in relatively wet months.The downstream discharge changes little in June and July in wet seasons,and the influence on dry years has obvious characteristics of the reverse phase.In other words,the downstream discharge of the dam increases significantly in dry season,while the downstream discharge decreases significantly in wet season compared with the near-natural state,which reflects the emphasis of the dam on water resource allocation.(3)In terms of the impact on the downstream ecohydrology of the dam,8 of the 12 median ecohydrological indicators increased or decreased significantly under the single-dam mode,and the other 4 median indicators had little change,while 12 and 11 indicators changed significantly under the two-dam and threedam mode,respectively,with a change range of more than 25%.Among the 13 extreme ecohydrological indicators,12 of them increased or decreased in the single dam model,in which the annual 3-day minimum and annual 7-day minimum decreased by 75%,and the 13 extreme indicators changed significantly in the two-dam model and the three-dam model.Among the five discharge pulse eco-hydrological indicators,all of them increase or decrease to varying degrees under the single-dam mode,and the duration of low-pulse increases by 73%.These indicators also change to a large extent under the two-dam mode and the three-dam mode,but the trend of change is diverse,which reflects the complexity of the multi-dam mode on the downstream hydrological regulation.Research Conclusion: The watershed hydrological model can help reveal the response characteristics of downstream hydrological conditions under climate change,and the eco-hydrological index system with richer hydrological information can reflect the regulation effect of cascade DAMS on downstream water environment in more detail,as well as the complexity of such regulation and change effect. |
