Alterational correlation analysis between the water level and the influencing factors of Hukou Station at Poyang Lake

Poyang Lake plays a restricts and interacts role with Yangtze River, the key node of water resources connection between them is Hukou Station. The water level of Hukou Station reflects the inflow, precipitation, evaporation of Poyang Lake Basin and the backflow of the main stream of Yangtze River. Influenced by the climate change and human activities, the water level of Hukou Station appears of the inconsistency characeteristics obviously, which induced abnormally low water level, ecological degradation of beach wetlands, and dramatic reduction of water storage capacity in Poyang Lake. In order to analyze the alterational correlation be between the water level and the influencing factors of hukou station, and finding out the highest correlation factor, the Hydrological Alteration Diagnosis System was used to diagnose the alteration of water level of Hukou Station and the influencing factors. The results show that, the water level of Hukou station experienced a moderate alteration in a leap-down manner in 2003, and the mean value of the water level decreased 0.8m after the hydrological alteration; the influencing factors experienced different hydrological alteration with different form, tha main alteration form is jump alteration, and there is no trend alteration happened in all the series; the alterational correlation between the water level of Hukou Station and Jiujiang Station is the highest, which means the mainstream flow of Yangtze River plays the most important role at the hydrological alteration of the water level of Hukou Station. The alterational correlation has a certain indicative role in analyzing the inconsistency evolution characteristics of Poyang Lake.


Introduction
Poyang Lake located in the mid-downstream of Yangtze River, which still remains its natural connection with it. Poyang Lake plays a interacts and restricts role with Yangtze River [1], and the water resources relationship between them has an impact on many aspects of drought risk management, flood disaster prevention, water ecological environment protection and water resources utilization, etc [2]. As the key node of water resources connection between Poyang Lake and Yangtze River, the water level of Hukou station is influenced by the hydrological factors such as runoff and evaporation in the Basin of Poyang Lake and the backflow of the main stream of Yangtze River. The change of water level at Hukou station reflects the evolution characteristics of water resources and the relationship between the lake and the river. In recent years, due to the impact of climate change and human activities, the spatial and temporal distribution regulations of water resources in Poyang Lake has changed, that induced a series of hydrological alteration problems such as abnormally low water level, ecological degradation of beach wetlands, dramatic reduction of water storage capacity in Poyang Lake, the occurrence of the dry season forward significantly, the duration of dry season significantly increased, etc [3]. Affected by the change above, the water level of Hukou station appears of the inconsistency characeteristics obviously. Finding out the influencing factors with the highest degree of correlation with the alteration of water level at the Hukou station, has a certain indicative role in analyzing the inconsistency evolution characteristics of Poyang Lake.
About the inconsistency characteristics of Poyang Lake, the study mainly focused on the the influence of the change in the hydrological regime at the main stream of the Yangtze River to the hydrological regime of Poyang Lake. Based on the data analysis from 2004 to 2008, Guo Hua et al [4] concluded that the large amount of water storage in the Three Gorges Reservoir in October, resulting in the decrease of the discharge of the Yangtze River, which significantly weakens the influence of the Yangtze River on the water level of Poyang Lake. The coupled hydrodynamic model of rivers and lakes in the middle steam of the Yangtze River was used by Wang Yingchun et al [5], and the results shown that the water level of Duchang Station in Poyang Lake will be reduced by 0.09-1.11 m due to the impoundment of the Three Gorges Reservoir. Fang Chunming et al [6] predicted that the dry season of Poyang Lake would be advanced by about one month under the combined action of impoundment and river erosion with the operation of the Three Gorges Reservoir. The study of sediment deposition changes with the influence of the operation of the Three Gorges Project in Poyang Lake and main stream of the Yangtze River was carried out by Hu [7] and Zhu [8], the results shown that the operation of the Three Gorges Project reduced the speed and quantity of sediment deposition in Poyang Lake, and caused the intensification of river erosion. Xu [9] considered that the water level of the Three Gorges Reservoir at the end of flood season (October-November) would decrease by 0.4-1.6 m, resulting in a corresponding decrease of 0.3-1.2 m in the water level of Poyang area.
In summary, many researchers take the operation of the Three Gorges Reservoir as the key factor of the inconsistent change of the hydrological elements of Poyang Lake, but there is few analysis of other hydrological elements that influencing the water level of Poyang Lake; at the same time, there is few analysis of alterational correlation among the influencing factors. In view of the problems above, the Poyang Lake's water level was taken as the research object, the Hydrological Alteration Diagnosis System was used to detect the hydrological alteration of the inflow runoff, precipitation, evaporation and the main stream runoff of the Yangtze River, and then based on the key factor analysis of alteration in the hydrological series to analyze the alterational correlation with the influencing factors and water level of Hukou station at Poyang Lake.

Study area
Poyang Lake is the largest surface water lake in China, which is located at the middle stream of the Yangtze River and the north of Jiangxi Province, the basin area is 162,200 km 2 . It adopts the incoming water major from five rivers: Raohe, Xiushui, Ganjiang, Fuhe, Xi'anjian and other small tributaries, such as Zhangtian and Boyang rivers. The water from Poyang Lake flow into the Yangtze River at Hukou Station, as figure 1 shows below.  Figure 1. A schematic diagram of the study area.

Database
The precipitation data comes from China Meteorological Data Network, which includes 14 stations of Ganzhou, Suichuan, Jinggangshan, Guangchang, Ji'an, Nanchang and Yichun, etc. The monthly and annual surface precipitation in the Poyang Lake Basin are counted by Tyson polygon method.
Using daily temperature, wind speed, sunshine and relative humidity, combined with the Penman-Monteith method recommended by FAO, potential evapotranspiration data of 14 stations were calculated. Monthly and annual potential evaporation of Poyang Lake Basin were counted by Tyson polygon method. The time duration of Ganzhou and other 14 stations is shown in table 1. Considering the flow from the upper stream of Yangtze River will affect the outflow of Poyang Lake, Jiujiang Station, which is nearest to Hukou, is used as the representative station of the Yangtze River mainstream. The monthly and annual average flow data of Jiujiang Station are obtained by compiling daily flow data. The time duration of Poyang Lake and Jiujiang Station of Yangtze River is shown in table 2.

Hydrological alteration diagnose system
The hydrological series mainly contains stochastic and definite composition, and the definite composition contains jump, tendency and circle elements. If the physical causes have not change with time, the definite composition will have no influence to the hydrological series, and in the time scale, the hydrological series is consistent and the hydrological data fluctuates in random around the mean value, which means no alteration happened in statistical regulations. Otherwise, the statistical regulations of hydrological series are inconsistent, which means the physical causes and parameters of hydrological series changed in time scale. So the hydrological alteration means the distribution style and parameters of hydrological series are changed obviously in the statistical scale [10].
The methods, such as Sliding T Test, Sequential Cluster etc. used to detect the jump alteration, the methods of Kendall Rank Correlation Test, Spearman Rank Correlation Test etc. used to detect the trend alteration. Based on the utilization of methods above, XIE [11] put forward the Hydrological Alteration Diagnosis System (HADS) at 2010. In this sysytem, the trend and jump alteration could be diagnosed comprehensively. The HADS contains 3 sections, Preliminary Diagnose (PD), Detailed Diagnose (DD) and Comprehensive Diagnose (CD).
In the section of PD, HADS uses the Hurst Coefficient Analysis, Sliding Average Analysis and Hydrograph Analysis to detect the hydrological alteration exist or not, and considering the correlation within Hurst coefficient and Fractional Brownian Motion parameter to decide the degree of hydrological alteration.
In the section of DD, the Spearman Rank Correlation, Linear Correlation Coefficient methods etc. are used to diagnose the trend alteration, the Rank Sum Test, Brown-Forsythe methods etc. are used to diagnose the jump alteration. If the jump alteration or trend alteration was diagnosed in the hydrological series, the section of CD will be activated.
In the section of CD, comprehensive jump and trend diagnose would be achieved. Based on coefficient of efficiency, the fitting degree of trend or jump proportion in the hydrological series will be estimated, and the better coefficient of efficiency will be taken as the hydrological alteration result. By actual conditions analysis and investigation, the hydrological alteration style conclusion could be confirmed at last.

Hukou station
With the first confidence degree α=0.05 and the second confidence degree β=0.01, the hydrological alteration diagnosis results by HADS were shown in table 3.
The result shows that, there is a moderate decrease jump alteration happened in the water level  Notes: the "+", "-", "0","↓"," ↑" means significant, not significant, could not be tested, decrease and increase. Same below.

Influence factors time series
Under the same degree of confidence, the yearly series from January to December and Annual average series of water level of Hukou Station, precipitation, evaporation and inflow of Poyang Lake Basin, flow of Jiujiang Station were diagnosed by HADS. The concise alteration diagnose results were shown in table 4. The result above shows the monthly and annual water level of Hukou station and the influencing factors experienced different alteration with different form, the main alteration form is jump alteration, and there is no trend alteration happened in all the series.

Alterational correlation analysis
Based on the hydrological alteration diagnose results of water level of Hukou Station, the alterational correlation with precipitation, evaporation and inflow of Poyang Lake Basin, flow of Jiujiang Station could be calculated as follows: Step 1: Alteration form correlation. As the alteration form could be divided into none alteration, jump alteration and trend alteration, the alteration form correlation could be defined as the alteration with the same form. For the alteration form of jump, it contains two elements, the alteration year and the alteration tendency.
Step 2: Weight of alteration correlation. If the different time series have the same alteration form, the assignment of alteration correlation weight is 1.0. If the alteration form is jump alteration, but only the alteration year or the alteration tendency has the same alteration form, the assignment of alteration correlation weight is 0.5.
As the steps above, the alterational correlation between the water level of Hukou Station and its influencing factors were calculated, the results were shown in table 5. Table 5. Alterational correlation between the water level of Hukou Station and its influencing factors. From the results in table 5, the alterational correlation between the water level of Hukou Station and Jiujiang Station is the highest, which means the mainstream flow of Yangtze River plays the most important role at the hydrological alteration of the water level of Hukou Station.

Conclusions
In this paper, the water level series in Hukou Station and its influecing factors, including precipitation, evaporation, inflow of Poyang Lake Basin, and the mainstream flow of Yangtze River were diagonsed by the HADS. Based on the alteration forms, the alterational correlation between water level of Hukou Station and its influecing factors was calculated. By the study results above, the following conclusions are drawn:

Discussion
There are still existing some problems need to be studied in the next step.

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The alterational correlation between the water level of Hukou Station and its influencing factors only show the qualitative relationship, but the quantitative relationship should be studied next step.

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The weight of alteration correlation should be reflected with the characteristics of different alteration form, and the value of weight should be studied further more.