| In recent years,the global climate is gradually warming,and abnormal climate events occur frequently,which have a great impact on the hydrological cycle.Compared with average hydrological events,extreme hydrology is more significant in water supply,flood control and drought resistance and ecological environment protection.Taking the upper reaches of the Heihe River Basin as the study area,the annual maximum runoff(used to characterize flood),annual minimum runoff(used to characterize dry water)and standardized runoff index(SRI)were defined to analyze the trend,frequency of extreme runoff in the historical period(1961-2000)and the future period(2031-2070),as well as the evolution characteristics of drought and flood.Among them,the runoff data in the historical period came from the measured data of hydrological stations,and the runoff data in the future period was predicted by the CSIRO-MK-360 model driven SWAT model under the scenario of RCP4.5.The main achievements are as follows:(1)The generalized extreme value model(GEV)was used to analyze the frequency of maximum monthly runoff and minimum monthly runoff in the study area from 1961 to 2000.The results showed that there were more than once in five years floods for 10 times and dry water for7 times in the historical period.20-year,50-year and 100-year floods are 192m~3/s,204m~3/s and 211m~3/s,respectively,and the dry water is 9.6 m~3/s,9.0 m~3/s and 8.6 m~3/s,respectively.The SRI calculation results show that the basin has a trend of changing from drought to flood in historical period,and gradually becomes wetter on the whole.In the 40 years,the total number of drought events is greater than that of flood events,but the number of extreme flood events is 3 times as much as that of extreme drought,indicating that the risk of extreme flood disaster events is higher.(2)To predict the future runoff in the study area,a SWAT model was constructed based on the collected data of the study area(including DEM data,soil data,land use data,meteorological data and hydrological data)in the upper reaches of the basin.The period from 1990 to 2005 was taken as the model rate period,and the period from 2006 to 2014 was taken as the model validation period.The results show that the Nash efficiency coefficient(NSE)and determination coefficient(R~2)are 0.91 and 0.92,respectively,and the two indexes are 0.80 and 0.88 in the validation period,indicating that the SWAT model has good applicability in the study area.Furthermore,the CSIRO-MK-360 model 2031-2070 is used to drive the validated SWAT model to forecast the future runoff in the study area.The results show that the average runoff in the next 40 years is 51m~3/s,which is8%higher than that in the historical period.(3)The GEV model was used to fit the future extreme runoff series from 2031 to 2070.The results showed that the frequency of annual maximum and minimum runoff in the return period of more than 5 years in the future was less than that in the base period,with 9 and 3 times,respectively.The floods that occurred once in 20 years,50 years and 100 years all increased compared with the historical period.An increase of 3.9%,6.6%and 8.4%,respectively.The increase of dry water was more significant.SRI value shows that the continuous drought last year in the future in the study area is not too long,the change trend of performance as normal to drought,floods,overall and more moist,but drought event dramatically increased,the proportion of 4%by the original increased to 7%now,this shows that in the future,become more abundant water resources in the study area,a certain drought relief,However,the risks of major and extremely large floods and droughts should not be underestimated and should be addressed actively. |
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