Spatio-temporal Analysis of Precipitation data in the Poyang Lake Basin, China

. The monthly precipitation data from 14 stations were used to research the precipitation variation in Poyang lake basin during 1957–2012. The Mann–Kendall method was applied to analysis precipitation trends. There are two main periods of 1964~1975 and 1986~1999 with increasing annual precipitations for this basin. There is a highest coefficient of variation of the annual precipitation in the north mountain area of the basin. There are significant increasing trends in annual precipitation for Lushan station and Zhangshu station which located in the north. Six stations in the north had the significant upward trend in summer and eleven stations had the significant upward trend in winter. It can be determined that the Poyang lake basin has become wetter in winter over the period 1957-2012.


Introduction
Extreme hydrological disaster events such as droughts and floods are very common for the Poyang Lake Basin. It was recorded that droughts occurred mainly in 1963,1978, 1986,2003, 2006, 2007 and 2011, especially more severe during the last decade and floods occurred in 1954,1955,1967,1982,1992, 1995, 1996 and 1998 in this basin. In recent years, frequent occurrences of droughts hit this basin, and the long-term drought influences the water crisis of this basin. Precipitation is an important variable which influences greatly the occurence of extreme hydrological events. Therefore, fully understanding precipitation characteristics wich is the basis of drought and flood analysis is very important.
There have been many studies on precipitation trends (Liu et al., 2008;Tabari et al., 2012). Long-term or short-term precipitation trends (Gemmer et al., 2011;Wu et al.,2017) are often detected by using methods such as the Mann Kendall test and Sen's slope method. Moreover, some papers (Zhang et al., 2015) have been published on precipitation and drought analysis on this basin. However, few studies have focused on both long-term and short-term precipitation analysis for this basin. Thus, it is necessary to have a comprehensive precipitation analysis and variability for the Poyang Lake Basin. In this paper, we aim to look at the spatiotemporal change of the precipitation series. The purpose of this research is to: (1) gain the basis precipitation characteristics at different time scale; and (2) gain the trend and variability in precipitation and investigate the extreme precipitation events over the period of 1957~2012.

Study Area and data
The Poyang Lake Basin (Fig. 1) extends from 115°49′E to 116°46′E Longitude and 28°24′N to 29°46′N latitude with a total drainage area of 16.22×104 km 2 , about 97 % of Jiangxi province (Wang et al. 2012). The climate for this basin is a humid subtropical climate, with 1,680 mm annual mean precipitation during 1960-2007 (Liu et al. 2013). The average annual rainfall is 1570 mm in 1951-1984, and it is 1655 mm in 1991-2005.There is an obviously wet and a dry season for annual precipitation. About 54% of annual precipitation is in the wet season, but only 16% is in the dry season.
The precipitation data (monthly) from 14 meteorological stations in the Poyang lake basin (Table.1) are collected over the period from 1957 to 2012. These monthly groundbased meteorological observation data are from the National Climate Center (NCC) (http://cdc.cma.gov.cn.). All the observation sites are illustrated in Figure 1.
Where xi or xj means precipitation value at time i or j. For n 10, the statistic S can be considered as a normal distribution, and its statistic Z is calculated as: When |Z|>Z/2, the trend is rejected at the significance level  of 0.05. If |Z|>1.96, this means precipitation trends are significant. When Z >1.96, there is a significant rising trend, while when Z<-1.96, there is a significant downward trend. When |Z|<1.96, statistically insignificant trends are detected for precipitation.

Characteristics of average annual precipitation for all stations
The average annual precipitation is 1424~1909 mm for 14 meteorological stations during 1957-2012 (Table 1).The coefficient of variation value (CV) of the average annual precipitation were found highest at Lushan station (which located in the north mountain area of the basin), and the lowest CV with the value of 0.17 was at Yichun station (in the west of the basin). Fig. 2 shows the annual precipitation series for 14 meteorological stations.
As Fig. 2 shows that, great variations for the annual precipitation are found during1957~2012. The maximum precipitation with the value of 2997 mm was found in 1975 at the Lushan station and the minimum precipitation with the value of 766 mm was found in 1971 at Shuichuan station. Stations with annual precipitation over 2500mm are Guixi station (in 1975,1998,2010 and 2012, respectively), Lushan station (in 1975,1990, 1998, 1999,2005 Table 2 summarized some statistical characteristics of monthly precipitation series at 14 meteorological stations. The value of average monthly precipitation is 119~159 mm. The station located in the north (Lushan station) had the maximum value of mean monthly precipitation and three stations located in the south (Ganxian, Ji'an and Shuichuan station) had the minimum value. The maximum monthly precipitation for 14 stations is 509~1100 mm. The station in the north (Lushan station) had the maximum monthly precipitation and the Ganxian station which located in the south had the lowest. The maximum CV of the monthly precipitation was found at Boyang station and Guangchang at the rate of 87%, while the minimum CV with the value of 73% was detected at Shuichuan station and Yichun station located in the west. Table 3 shows the statistical data of average monthly precipitation for each station over the period 1957-2012. It is evident that the station in the north (Lushan station) had the highest average monthly precipitation from July to November, and the minimum average monthly precipitation is found in February. The station in the north (Guixi station) had also the highest maximum monthly precipitation in January, February, April and June, respectively. On the contrary, the station in the south (Shuichuan station) had the lowest average monthly precipitation in January, from March to May, December, respectively. Fig 3 shows the temporal distribution of the mean precipitation for each month (averaged by the Thiessen Polygon approach) of this basin. A lot of precipitation is found in April~June and less precipitation in October~January, which is concentrated in spring and summer.  'an  61  91  158  217  228  234  118  122  81  70  67  46  Jingdezhen 72  109  178  240  251  302  195  136  71  71  64  51  Lushan  56  80  143  214  257  291  231  258  149 101  82  47  Nanchang  69  99  164  220  248  296  125  116  68  55  66  47  Nancheng  67  100  187  238  253  306  134  123  84  64  66  52  Shuichuan  54  84  133  167  195  223  121  153  123 86  60  39  Xiushui  62  82  147  212  231  277  165  119  79  69  72  44  Yichun  69  97  168  217  231  239  146  127  81  79  78  54  Yushan  79  110  198  248  272  335  151  107  89  64  75  60  Zhangshu  72  101  174  225  258  286  134  113  75  59  76  51  Max  79  111  198  256  283  349  231  258  149 101  82  60  Min  54  80  133  167  195  202  111  107  62  55  53  39 The maximum of monthly precipitation is italic and in bold; the minimum of monthly precipitation is in bold.

Trend analysis of seasonal and annual precipitation for all stations
For four seasons and the whole year, precipitation trend tests are shown in Fig. 5 over the period 1951-2012, where the significance level  is 0.05. If |Z|>1.96, this means the trend of the precipitation series is significant. When Z>1.96, it is a significant increase trend (significant wet); when Z<-1.96, it is a significant decrease trend (significant dry). The value of Z below the 95% confidence level is defined as wet tendency or dry tendency. Also, Fig. 4 shows the spatial distribution of the results of trend tests (Z) for four seasons and a whole year.
There is an upward tendency in winter precipitation and a downward tendency in spring precipitation for most of stations. As shown in Fig. 4, six stations in the north had the significant upward trend in summer and eleven stations had the significant upward trend in winter. No statistically significant trends were found both in spring and autumn. On the annual scale, there are significant upward trends for two stations (Lushan station and Zhangshu station) which located in the north.

Conclusions
In this study, the changes of precipitation both in short term and long-term time scale for the Poyang lake basin between 1957 and 2012 were analyzed. The following conclusions may be made: (1) The annual precipitation decreased from north to south basically for the basin. The stations with annual precipitation often over 2500 mm are Guixi station, Lushan station, Jingdezhen station (in the north). There are two main periods of 1964~1975 and 1986~1999 with increasing annual precipitations for most of the basin. A maximum value of Cv for annual precipitation is found in the north mountain area of the basin, while the inter-annual precipitation changed slightly in the west area of the basin.
(2) The average monthly precipitation also decreases from north to south. There is a highest coefficient of variation for monthly precipitation in the north of the basin, while the lowest value in the west of the basin. Monthly precipitation concentrates in April to June and less distributes in October to January.
(3) On the seasonal scale, the precipitation concentrated in spring and summer. Meanwhile, precipitation shows an upward tendency in winter and a downward tendency in spring. In summer, six stations in the north had the significant upward trend; and in winter, eleven stations had the significant upward trend. On the annual scale, significant upward trends in precipitation are found at Lushan station and Zhangshu station which located in the north.