| This paper made a study on hydro-logical and drought characteristics for rivers and the security of Urban water supply in Xi'an City,which is pivotal for planning and scheduling the water resources in the City.Since Xi'an is the link connecting the east and the west of China,it has vital significance on political and economic status in the country.As it is known to all that water is the basis of human existence,and it lays solid foundation for people's livelihood and urban production.Furthermore,with the development of Xi'an City,urban water demand and water supply project are changing.In accordance with the plan,after Hanjiang to Weihe River Project been completed,Shitouhe Reservoir mainly supplies water to Baoji City.Since then,Heihe River,Shibianyu River and Wangchuanhe River are the major local surface water resources for urban areas in Xi'an City.However,with the changing climate and environment,the runoff for each river may probably change accordingly.Besides,wetness-dryness compensation and drought characteristics of the rivers would also vary.How to effectively guarantee the safety of urban water supply under the premise of maintaining urban ecological environment is an urgent problem to be solved.Then,the study selected Heihe River,Shibianyu River and Wangchuanhe River as the targets.The three rivers are situating at the northern foothill of Qinling Mountains,and they matter a lot for supplying water to Xi'an City.By following Kernel distribution principle,marginal distribution for runoff has been set up in the study;encounter probability and recurrence interval of rich-poor runoff for the three rivers were studied by applying function theories of two-dimensional Copula and Vine Copula;drought characteristics were also researched by the use of runs theory and completely-nest Archimedes Copula function;the variation trends and mutations of the main hydro-logical variables for the three rivers were analyzed by adopting linear method,trend free prewhitening Mann-Kendall,Bootstrap test and partial trend method of clustering analysis;at last,in combination of the Hanjiang to Weihe River Project,this paper studied how to adjust the urban water resources and ensure the safety of urban water supply under the changing environment.(1)The characteristics of runoff distribution in Heihe River have changed greatly in recent years,the multi-year average runoff of Jinpen Reservoir at Heihe River,which is 537million m~3,130million m~3 less than the calculated value when constructing the Reservoir.The variation coefficient and skewness coefficient of the annual runoff of the station are obtained,the variation coefficient is 0.56 and the skewness coefficient is 1.4 times the variation coefficient.The values are quite different from those of used in the construction of Jinpan Reservoir,the variation coefficient is 0.37 and the skewness coefficient is 3 times the variation coefficient.At the same time,the distribution characteristics of Shibianyu hydrologic station and Lijiahe hydrologic station had some changes in recent years.(2)Runoff of 3 rivers:Heihe River,Shibianyu River and Wangchuanhe River in spring,summer and autumn are having higher synchronous wetness-dryness probabilities of any two rivers:it is 54.7%in spring,47.2%in summer,58.5%in autumn and 20%in winter.With regard to wetness-dryness probabilities,Shibianyu River and Wangchuanhe River has the limited regulation for Heihe River in spring,summer and autumn;while in winter,the two rivers could have certain regulation for supplying water to Heihe River.(3)From the point of water demand,the mean duration of drought in Heiyukou is about 3.79months,the mean intensity is about 47.26 million m~3,and the mean peak value is about 13.51million m~3.After the year of 1993,both intensity of drought duration and peak value of Heihe River had been intensifying obviously.When calculating drought duration,intensity and peak value by single threshold,for mild,medium and severe drought events that appear simultaneously,the calculation result of co-occurrence probability for Shibianyu River is relatively large,the results are 0.174,0.350,0.177,with short recurrence interval,the values are 3.113,1.549 and 3.060 years respectively;co-occurrence probability for Wangchuanhe River is the lowest with the longest recurrence interval.(4)In the seasonal scale,the runoffs of the three rivers show significant declining in spring,a slight decline or no change evidently in summer,and no obvious variation in autumn,while the variations of the runoffs at three stations in winter are quite different.When it comes to annual scale,there is a significant decrease in Heihe River,then seconded by Wangchuanhe River as for the decrease,and no significant change in Shibianyu River.From annual scale and flow interval,the Index of overall downward trend of annual runoff in Heiyukou from 1984 to 2011was-4.1692,-1.5485 in Shibianyu and-2.9511 in Lijiahe,compared with that from 1956 to1983.(5)In case that it has been changed for Shitouhe River to supply water to Baoji City,Hanjiang to Weihe River Project plans that the water volume regulated to Xi'an City is 832million m~3,it is approximately 45miiion m~3 residual water volume in 50%level year in 2030 after returning the occupied agricultural and ecological water.Nevertheless,water deficit in 75%level year is about 168 million m~3,the water shortage rate is 18.7%;and the deficit is 498 million m~3 and the water shortage rate is 18.7%in 95%level year;within which,urban water supply in 50%level year the residual water volume is 35 million m~3,water deficit in 75%level year is about11 million m~3,the water shortage rate is 0.6%;and the deficit is 194 million m~3 and the water shortage rate is 11%in 95%level year.Thus,it can be seen that,when it is dry year,the City can increase groundwater extraction,and decrease agricultural irrigation and ecological water at the same time;on the other hand,it shall increase the water distributed from Hanjiang to Weihe River Project to balance supply and demand. |
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