Seasonal and interannual variations of flow discharge from Pearl River into sea

Wei ZHANG*, Shou-sheng MU, Yan-jing ZHANG, Kai-min CHEN

1. State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing 210098, P. R. China

2. College of Harbour, Coastal and Offshore Engineering, Hohai University, Nanjing 210098, P. R. China

3. China Institute of Water Resources and Hydropower Research, Beijing 100038, P. R. China

Seasonal and interannual variations of flow discharge from Pearl River into sea

Wei ZHANG*1,2, Shou-sheng MU1,2, Yan-jing ZHANG1,3, Kai-min CHEN1,2

1. State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing 210098, P. R. China

2. College of Harbour, Coastal and Offshore Engineering, Hohai University, Nanjing 210098, P. R. China

3. China Institute of Water Resources and Hydropower Research, Beijing 100038, P. R. China

Flow discharge from the river basin into the sea has severe impacts on the immediate vicinity of river channels, estuaries, and coastal areas. This paper analyzes the features and temporal trends of flow discharge at Pearl River’s three main gauge stations: the Wuzhou, Shijiao, and Boluo gauge stations on the West River, North River, and East River, respectively. The results show no significant trend in annual mean discharge into the sea at the three gauge stations. Changes of monthly mean discharge at the Boluo Gauge Station are evident, and a majority of monthly discharge in the dry season displays significant increasing trends. Furthermore, changes of the extreme discharge are quite evident, with a significant decreasing trend in the annual maximum discharge and a significant increasing trend in the minimum one. The significantly decreasing ratio of the flood discharge to annual discharge at the Boluo Gauge Station indicates that the flow discharge from the East River has increased in the dry season and decreased in the flood season since the construction of dams and reservoirs. At the other two gauge stations, the Wuzhou and Shijiao gauge stations, the seasonal discharge generally does not change perceptibly. Human impacts, especially those pertaining to reservoir and dam construction, appear to be responsible for the seasonal variation of flow discharge. The results indicate that the construction and operation of dams and reservoirs in the East River have a greater influence on flow discharge, which can well explain why the seasonal variation of flow discharge from the East River is more evident.

Pearl River; flow discharge; climate change; human impact; dam and reservoir

1 Introduction

River basins take the role of the major links between oceans and continents within the global geochemical cycle. They are major pathways for delivering terrestrial materials to the oceans (Walling and Fang 2003; Meybeck and V?r?smarty 2005). As the vector of these materials, flow discharge from the drainage basin into the sea attracts significant concern(Zhang et al. 2001; Chen et al. 2001; Yang et al. 2005; Wang et al. 2006). Both the amount and timing of freshwater inflow into the sea are important to ocean circulation, salinity, and sediment (Ye et al. 2003), and are a key research topic in the International Geosphere-Biosphere Programme and its core project, the Land-Ocean Interactions in the Coastal Zone (Syvitski 2003; Syvitski et al. 2005). However, flow discharge into the sea and other hydrological variables of global river basins change substantially because of the combined effects of climate change and human activities. As reported by the Intergovernmental Panel on Climate Change (Obasi and Dowdeswell 1998), climate change is likely to increase the runoff in higher latitude regions because of increased precipitation and snowmelt. Meanwhile, human activities, especially the construction of reservoirs and dams over the past 50 years, have resulted in water shortages (Wang et al. 2006). Variations in the patterns of flow discharge will lead to profound physical, ecological, and geographical impacts on the lower reaches and river estuaries because of the corresponding variation in the total dissolved solid flux. Therefore, advanced hydrologic models have been developed in recent years (Chau et al. 2005; Lin et al. 2006; Wang et al. 2009; Wu et al. 2009).

The patterns of flow discharge have changed dramatically in China’s river basins due to rapid socioeconomic growth and environmental degradation. The famous Yangtze River, for example, has exhibited a tendency toward two extremes: its floods are becoming increasingly severe, while, in sharp contrast, its flow discharge into the sea is getting smaller as more droughts occur (Chen et al. 2001). Research indicates that, although climate change should be responsible for discharge variations in the upper reach of the river, human impacts on flow discharge from the lower basin are mostly attributed to water transfer to both neighboring drainage basins and tributaries by a large number of electric pumping stations and sluices (Chen et al. 2001). The Yellow River, another sizeable river system in northern China, ceased to flow into the sea in 1972. Its discharge has steadily decreased since the 1950s, and the number of no-flow days in its lower reaches has increased rapidly, peaking at 226 days in 1997 (Xu 2004). The results of Wang et al. (2006) showed that the global El Ni?o-Southern Oscillation (ENSO) phenomena, which directly affected the regional annual precipitation in the Yellow River Basin, led to an approximately 51% decrease in river discharge, and that human impacts on river discharge (49%) seemed to be as great as natural influences, accelerating water losses in the hydrological cycle.

With the combined effects of climate change and human activities, the flow discharge from the Pearl River has also been changing. Changes in precipitation have important implications for hydrology and water resources, and are responses to climate change (McCarthy et al. 2001). However, the latest research on the Pearl River Basin (Zhang et al. 2009) indicates that although a significant decreasing trend in the number of rainy days can be identified, no significant trend in the annual precipitation and total summer or winter precipitation over the late five decades has been detected. Zhang et al. (2008) indicated thatlong-term changes of annual mean discharge from the Pearl River were not significant, and were mainly controlled by precipitation variations. More than 9 000 dams have been built in the Pearl River Basin since the 1950s for different purposes. The influence of intensive human activities on the seasonal variability of flow discharge is usually more obvious due to dam regulation. To date, however, no publication has clearly described the seasonal features and changes of flow discharge from the Pearl River into the South China Sea. Therefore, the aim of this study was (1) to detect the seasonal variabilities of flow discharge from three major tributaries of the Pearl River, and (2) to discuss the possible causes for changes in flow discharge and to study the influence of dam construction on these seasonal variabilities in detail.

2 Basin description, data sets, and method of analysis

The Pearl River (97°39′E to 117°18′E, 3°41′N to29°15′N) is China’s second largest river in terms of multi-annual mean discharge, with a value of 1.06 × 104m3/s (PRWRC 1991), which is less than that of the Yangtze River (2.85 × 104m3/s), but is considerably higher than that of the Yellow River (0.12 × 104m3/s). It plays a key role in fresh water supply to large cities in the Pearl River Delta region. The Pearl River is a compound river system including the West River, North River, and East River, as well as a number of small rivers (Fig. 1). The West River, the largest branch, originates in the Maxiong Mountain of Yunnan Province in southwest China and flows southeastward through Guizhou, Guangxi, and Guangdong provinces. Its total length is 2 214 km, and it has a drainage area of 0.35 × 106km2, accounting for 77.8% of the total drainage area of the Pearl River Basin. The North River is the second largest tributary, with a length of 468 km and a drainage area of 0.46 × 105km2. The East River, originating in Jiangxi Province and winding mainly through Guangdong Province, is approximately 562 km in length, with a drainage area of 0.27 × 105km2, which accounts for 5.96% of the total drainage area of the Pearl River Basin. The West River, North River, and East River flow into the Pearl River Delta and flow through eight large outlets into the South China Sea. The Pearl River Basin covers a region with a subtropical to tropical monsoon climate (Zhang et al. 2008). The annual mean temperature across the basin is 14 to 22 , and ℃

the annual mean precipitation ranges from 1 200 to 2 200 mm. The main precipitation occurs from April to September. This study analyzed the monthly and annual discharge data at three main gauge stations from the 1950s to 2006: the Wuzhou Gauge Station on the West River, the Shijiao Gauge Station on the North River, and the Boluo Gauge Station on the East River. These three gauge stations are located at the tidal limit, and the relationship between the water levels and their flow discharge downstream is influenced by tides and tidal flows. Therefore, flow discharge at these three gauge stations represents discharge from the Pearl River Basin into the sea. The locations, drainage areas, and series length are displayed in Fig. 1 and Table 1. These data were collected from the hydrological yearbooks of the People’s Republic of China. The data homogeneity and reliability were firmly controlled by the authorities before they were released. The series from the 1950s to 2006 employed in this study has good consistency.

Fig. 1Sketch of Pearl River Basin

Table 1Detailed information on hydrological records of Wuzhou, Shijiao and Boluo gauge stations in Pearl River Basin

The methods utilized in this study are briefly summarized below. First, certain characteristics of flow discharge, such as the distribution patterns of multi-monthly mean discharge and its standard deviation, and the distribution patterns of monthly mean discharge in the extremely wet and dry years at the three main gauge stations, were detected to understand the fundamental properties of flow discharge from the Pearl River Basin. Second, with the time series of annual and monthly mean discharge data, the trends in discharge were estimated utilizing the Mann-Kendall (MK) test (Mann 1945; Kendall 1975). One of advantages of the test is that it is distribution-free: it does not assume any special form for the data’s distribution function, and it prevents missing data (Yue et al. 2002). Thus, it is highly recommended for general use by the World Meteorological Organization (Mitchell et al. 1966). Prior to the application of the MK test, the time series of river discharge was pre-whitened using the methodology of Yue et al. (2002) to eliminate the effect of serial correlation in time series data with significant autocorrelation. Linear regression analysis was likewise applied to the annual mean discharge records in order to determine the changes in the annual mean discharge as a function of time in year scale.

3 Results and discussion

3.1 Features and changes of flow discharge into sea

The multi-annual mean discharge at the Wuzhou Gauge Station is 6 417 m3/s, accounting for 75.5% of the annual total discharge from the Pearl River into the South China Sea. Themonthly mean discharge at the Wuzhou Gauge Station displays large annual and seasonal fluctuations (Fig. 2), ranging from 24 200 m3/s in August 1994 to 1 110 m3/s in January 1963, extremely wet and dry years, respectively. The seasonal cycle of monthly mean discharge at the Wuzhou Gauge Station exhibits a high discharge of 4 600 to 14 463 m3/s from May to October and a relatively low discharge of 1 846 to 4 371 m3/s from November to April, with the maximum discharge usually in June or July and the minimum in January or February (Fig. 3). The interannual variation of monthly mean discharge at the Wuzhou Gauge Station is generally small in the dry season, with a standard deviation between 639 and 1 871 m3/s, and large in the wet season, with a standard deviation of 1 916 to 5 841 m3/s.

Fig. 2Fluctuation of monthly mean discharge in extremely dry and wet years at Wuzhou, Shijiao, and Boluo gauge stations

Fig. 3Multi-monthly mean discharge and its standard deviation at Wuzhou, Shijiao, and Boluo gauge stations

The discharge records at the Shijiao Gauge Station represent the discharge from the North River into the sea, accounting for approximately 15.7% of the total discharge from the basin. Fig. 4 shows that the maximum and minimum annual mean discharge at the Shijiao Gauge Station occurred in 1973 and 1963, respectively, with the monthly mean discharge ranging from 5 820 to 270 m3/s (Fig. 2). The seasonal cycle of the monthly mean discharge at the Shijiao Gauge Station shows a high discharge of 1 088 to 3 097 m3/s from April to September, and a low discharge of 415 to 1 020 m3/s from October to March (Fig. 3). Theinterannual variation of monthly mean discharge at the Shijiao Gauge Station is similar to that at the Wuzhou Gauge Station, with a relatively high standard deviation between 656 and 1 401 m3/s in the wet season and a low standard deviation between 232 and 853 m3/s in the dry season (Fig. 3).

Fig. 4Annual trends in discharge at Wuzhou, Shijiao, and Boluo gauge stations according to linear regression analysis

The multi-annual mean discharge at the Boluo Gauge Station is 751 m3/s. Although it accounts for a mere 8.8% of the total basin discharge, its features and variations attract equal attention because the Boluo Gauge Station is the main freshwater source of the metropolitan areas. The East River-Shenzhen water supply project, for example, is designed as a solution to the water demand in Hong Kong. Fig. 4 indicates that the minimum annual mean discharge at the Boluo Gauge Station occurred in 1963, just as that at the Wuzhou and Shijiao gauge stations. Fig. 3 demonstrates that the peak monthly mean discharge in the East River in June was roughly eight times greater than the lowest monthly mean discharge in January. It also shows that the interannual variation of monthly mean discharge at the Boluo Gauge Station is small in the dry season as well, with a standard deviation between 128 and 367 m3/s, and relatively large in the wet season, with a standard deviation of 362 to 910 m3/s.

The trend analysis of annual and monthly mean discharges at the Wuzhou Gauge Station is shown in Table 2 and Fig. 4. The results of the MK test show a negative trend in the annual mean discharge at the Wuzhou Gauge Station, suggesting a reduction in the annual mean discharge during the study period. The reduction, however, is insignificant, with an absolute value of the trend test statistic index z of 0.83, less than 1.96 at the confidence level of 95%. The results of linear regression analysis (Fig. 4), with the annual mean discharge as the dependent variable and time as the independent variable, indicate that changes in the annual mean discharge are insignificant. The slope of the linear regression line is ?6.15, and the significance level p is 0.550, which is more than 0.05. Table 2 shows that changes in the monthly mean discharge at the Wuzhou Gauge Station are characterized by negative trends from April to May and August to December, and positive trends from January to March and June to July. However, these changes are not significant. This indicates that almost all monthlymean discharge exhibits no significant change, except for September, which shows a strong downward trend.

Table 2Results of MK test in monthly, seasonal, and annual mean discharge into sea from Pearl River Basin

The MK test results reveal that no significant trend exists in annual and monthly mean discharge from the North River into the sea (Table 2). The trend test statistic index z is 0.81 (Table 2) and the significance level p is 0.481 (Fig. 4) by linear regression analysis at the Shijiao Gauging Station, indicating no obvious variation trend in the annual mean discharge. The monthly mean discharge in a majority of months, except that in May and June, exhibits a positive trend at the Shijiao Gauging Station, which may indicate a shift in the annual cycle of the hydrological regime. However, no month shows significant changes in discharge at the confidence level of 95% (Table 2).

The results of the MK test show that the value of z is 0.30 (Table 2), which is less than 1.96 at the confidence level of 95%. The results of the linear regression analysis show that the value of p is 0.893 (Fig. 4), which is more than 0.05. Both results demonstrate that no significant change exists in the annual mean discharge at the Boluo Gauge Station. However, changes in monthly mean discharge are relatively visible, in contrast to those at the Wuzhou and Shijiao gauge stations. It is clear that the monthly mean discharge at the Boluo Gauge Station displays significant changes for nearly half a year (Table 2), with a confidence level greater than 95%. It should be noted that all monthly mean discharge in the dry season exhibits an increasing trend, and variations are significant from December to March, leading to a significant upward trend over the whole dry season. Changes in monthly mean discharge during the wet season are statistically less significant compared with those in the dry season, and exhibit a strong downward trend only in June. Although the overall trend in monthly mean discharge in the wet season is insignificant, there is a general decreasing trend.

3.2 Impacts of climate change and human activities

The effect of human activities on water resources is one of the most severe crises faced by humanity. Among the various human activities in the river basin, the construction of dams and reservoirs is the most direct way of manipulating river water resources. Dams and reservoirs are globally essential to river fragmentation, creating significant impacts on water regulation and sediment retention in reservoirs (Nilsson et al. 2005; Syvitski et al. 2005; Wanget al. 2006). In the Pearl River Basin, it is reported that over 9 000 dams and reservoirs have been constructed since the 1950s, with a total storage capacity of 65 km3, approximately 23% of the multi-annual mean discharge of the Pearl River (Dai et al. 2008). Over 400 reservoirs are median-sized and large, with a storage capacity of over 107m3.

Dam and reservoir regulation may alter the seasonal discharge cycle. Although a number of median-sized and large dams and reservoirs are scattered widely across the West and North rivers, the previously described results of annual and monthly variations in flow discharge into the sea from the West River and the North River indicate that the influences of dams and reservoirs on flow discharge are relatively small. However, dam and reservoir construction in the East River weakens the seasonal variability of flow discharge from the river, indicating that the East River is a regulated river. This is the reason why flow discharge from the East River exhibits a significant increasing trend in the dry season. In reality, these variations induced by dams and reservoirs are fairly obvious in the extreme hydrological regime. Fig. 5 shows that the annual maximum discharge displays a significant downward trend ( p= 0.033 ＜ 0.05), while the annual minimum discharge exhibits a strong upward trend ( p＜ 0.001). This is because the dams and reservoirs drastically reduce the peak discharge by retaining discharge during the wet season and releasing it during the dry season to meet the water consumption demand. Meanwhile, the measured discharge in the wet season accounted for nearly 78% of the annual discharge in the 1950s, decreasing to 68% after 1990. Fig. 6(a) shows a significant decreasing trend ( p= 0.027 ＜ 0.05) in the ratio of the flood discharge to annual discharge at the Boluo Gauge Station over the past 50 years. All of this evidence indeed indicates that the discharge from the East River increases during the dry season and decreases during the wet season, as it is influenced by dams and reservoirs.

Fig. 5Annual maximum and minimum discharge at Boluo Gauge Station

The monthly mean discharge at the Boluo Gauge Station reveals that the annual distribution pattern of discharge has dramatically changed with the removal of seasonal peak discharge (Fig. 7(a)). The decreases in monthly mean discharge from 1969 to 1972, 1977 to 1981, 1990 to 1992, 1998 to 2000, and 2002 to 2004 were probably caused by the single or joint regulation of dams and reservoirs in the East River. Comparatively speaking, the ratios of the flood discharge to annual discharge and the monthly distribution patterns of discharge at the Wuzhou and Shijiao gauge stations (Fig. 6 and Fig. 7) do not show obvious changes.

Fig. 6Ratio of flood discharge to annual discharge at three gauge stations over past 50 years

Fig. 7Monthly mean discharge at three gauge stations over past 50 years

4 Conclusions

Based on systematic analysis of long-term annual and monthly mean discharge records at three main gauge stations in the Pearl River Basin, this study shows that the annual meandischarge into the sea does not exhibit significant changes in any of the three major sub-basins. However, an interesting phenomenon of inconsistency of the variation of monthly mean discharge with annual mean discharge at the Boluo Gauge Station, located in the East River, has been discovered. Strong upward trends in monthly mean discharges have been detected during most of the low-flow months at the Boluo Gauge Station, resulting in a significant increasing trend in the total discharge during the dry season. Although the decreasing trend in monthly mean discharge at the Boluo Gauge Station is insignificant during the wet season, except that the monthly mean discharge in June displays a significant downward trend, both the annual maximum discharge and the ratio of the flood discharge to annual discharge exhibit a significant decreasing trend. This indicates that the discharge from the East River has increased in the dry season and decreased in the wet season due to the construction of dams and reservoirs. At the other two gauge stations, Wuzhou and Shijiao gauge stations, the monthly mean discharge generally do not exhibit significant changes.

Human activities, mainly dam and reservoir construction, may be responsible for the seasonal variation of flow discharge. The dams and reservoirs in the Pearl River Basin alter the natural seasonal distribution of flow discharge to satisfy water consumption demand and to prevent flooding. The monthly variations in flow discharge in the West and North rivers are not obvious compared with those in the East River, indicating that the latter is a more regulated river. Although the impact of dam and reservoir construction has been discussed in this paper, the extent and magnitude of the influence of these kinds of human activities have not been quantitatively analyzed. Therefore, reconstruction of time series of flow discharge should be done in a future study for a better understanding of the degree of influence of human activities.

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(Edited by Yan LEI)

This work was supported by the National Natural Science Foundation of China (Grants No. 41006046 and 51061130545), the Special Fund for Public Welfare Industry of Ministry of Water Resources of China (Grant No. 201301072), the New Teachers’ Fund for Doctor Stations of the Ministry of Education of China (Grant No. 20100094120008), and the Special Fund of State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering of Hohai University (Grants No. 2009586712 and 2009585812).

*Corresponding author (e-mail: zhangweihhu@126.com)

Received Jul. 14, 2011; accepted Nov. 1, 2011