Study On The Identification Method Of Hydrological Variation Types In River Streamflow Series In Yulin City In Northern Shaanxi Province

Recent years,the “natural-artificial” dual factors driving the hydrological cycle are constantly changing.This leads to the transformation mechanism and occurrence conditions of water change in different degrees,different scales and different levels,which is called hydrological variance.Hydrological variation poses a major threat to regional water security,which has attracted the attention of the academic circle in recent years,and its statistical law and attribution have become one of the hot research objects in the international field.Runoff is the important existence forms of surficial water resources,the time series of which is the important observation variable and has recorded and expressed surficial runoff process changed over time,and its statistical characteristics and change form reflect and contain the evolution information of the hydrologic system.The impact of hydrological variation to runoff series is embodied in many aspects,such as the mean,median,variance,regression coefficient and other statistical parameters.or the variance characteristic such as the distribution probability,correlation structure and cycle period.Yulin region is located in the junction of Inner Mongolia and Shaanxi province,in northern loess plateau,arid and rainless for a long time.Its ecological environment is fragile.Because the local people and government has carried out of soil and water conservation and desertification control activities,combined with the energy chemical industry base construction since the 90 s and the urbanization process,the hydrological cycle by nature change and human activity of interference,complex hydrological variations occur in river runoff process,and various types of variation are interlaced and superimposed.It is difficult to recognize its variation law only through a single method.To this,this article studied the runoff variation types in Yulin region from three aspects,which were the driving factors and their evolution,building runoff variation type identification method system,and variation types of diagnosis and analysis,in order to realize the law of the evolution of the hydrologic system and its driver.(1)The driving factors and their evolution.The variation of runoff is due to the change of hydrological cycle in the basin,so it is not an isolated event.Along with the variation of runoff sequence,the correlation factor sequence must show abnormal fluctuation characteristics and statistical characteristics.In view of this,it is necessary to inspect and analyze the driving factors of runoff sequence variation and its evolutionary process before identifying the variation types of runoff sequence.Through inspecting meteorological conditions and underlying surface conditions in Yulin area,it is found that except for the runoff sequence variation,the meteorological elements and underlying surface condition are observed unusual,which affects the hydrologic cycle.Rainfall sequences of most sites shows a downward trend,and reference crop transpiration and evaporation rises steadily year by year since the mid-90 s.In addition,through preliminary analysis of horizontal and vertical in runoff sequence,it is found the runoff in Yulin region is great differences in spatial distribution and presents the characteristics of south to north much less.The variation coefficients of runoff series at different hydrologic station slant whole big,which shows the sequence fluctuation amplitude is large,and the annual distribution of runoff is uneven.On the other hand,the rainfall runoff relationship of hydrological cycle in the natural state is stable and good.Compared to the rainfall runoff relationship in Yulin area,which has been seriously deteriorated,so it can be seen that the river runoff has been mutated by human activities.(2)The system of identification of runoff sequence variation type.There are many methods of runoff mutation identification,but the advantages and disadvantages of various methods are different.Therefore,by sorting out the advantages and disadvantages of the existing methods and choosing the methods suitable for the statistical characteristics of runoff sequence,the system of runoff sequence variation identification is constructed.Firstly stripping sequence trend item with the EMD decomposition,secondly adopting the method of STARS to fluctuation item to test mean variance,thirdly testing its variance variations after mean stabilization according to the testing results of mean variance using ICSS method.According to the calculation results,the variation type of runoff series at Zhaoshiyao station is complicated,the trend variation,mean variation,variance variations exist at the same time and intertwine with each other,belonging to the composite variation types,which also illustrates the rationality and scientificity of the method system of runoff sequence variation identification in this paper.The above system is used to identify the variation types of other runoff series at 21 hydrological stations in Yulin region.The results shows that except for the Shenmu and Caoping station,the runoff series at the rest station occur to different degree of variation,and mostly complex type.(3)Diagnosis and analysis of variance types.Because there are many kinds of interwoven complex variance types in Yulin region,the sequence of complex variation types is decomposed into single mutation type sequences,namely stationary sequence,only containing trend item sequence,mean variation sequence and variance mutation sequence.Then the correlation coefficient and regression coefficient between the single mutation type sequences and the 8 sequences in each station control basin,which are precipitation,evaporation(the evapotranspiration of reference crops),annual maximization of NDVI,interceptive water in silting dam,reservoir impoundment,agricultural irrigation water,urbanization rate and population density,are used to ascertain the specific physical cause that leads to the variation of the corresponding parameters.According to the analysis of correlation coefficient and regression coefficient,it shows that in the Wuding river basin with high vegetation coverage,transpiration of vegetation is the main cause of runoff decline,and precipitation is the main cause of the descending trend in Tuwei river basin,Kuye river basin and Huangfuchuan basin,but the variation of the mean and variance of the sequence is mainly caused by human activities,such as the interceptive water volume in silting dam,the agricultural irrigation water and the urbanization.