Abrupt behaviours of streamflow and sediment load variations of the Yangtze River basin, China

HYDROLOGICAL PROCESSES Hydrol. Process. 7, 5 () Published online 9 March in Wiley Online Library (wileyonlinelibrary.com) DOI:./hyp.97 Abrupt behaviours of streamflow and sediment load variations of the Yangtze River basin, China Qiang Zhang,,, * Vijay P. Singh, Chong-Yu Xu 5 and Xiaohong Chen,, Department of Water Resources and Environment, Sun Yat-sen University, Guangdong, China Key Laboratory of Water Cycle and Water Security in Southern China of Guangdong High Education Institute, Sun Yat-sen University, Guangdong, China School of Geography and Planning, and Guangdong Key Laboratory for Urbanization and Geo-simulation, Sun Yat-sen University, Guangzhou, China Department of Biological and Agricultural Engineering, and Department of Civil and Environmental Engineering, Texas A & M University, College Station, TX, USA 5 Department of Geosciences and Hydrology, University of Oslo, Blindern, Oslo, Norway Abstract: Monthly sediment load and streamflow series spanning 96 from four hydrological stations situation in the main stem of the Yangtze River, China, are analysed using scanning t-test and the simple two-phase linear regression scheme. Results indicate significant changes in the sediment load and streamflow from the upper reach to the lower reach of the Yangtze River. Relatively consistent positive coherency relations can be detected between streamflow and sediment load in the upper reach and negative coherency in the middle and lower reaches. Interestingly, negative coherency is found mainly for larger time scales. Changes in sediment load are the result mainly of human influence; specifically, the construction of water reservoirs may be the major cause of negative coherency. Accentuating the human influence from the upper to the lower reach results in inconsistent correlations between sediment load and streamflow. Decreasing sediment load being observed in recent years has the potential to alter the topographical properties of the river channel and the consequent development and recession of the Yangtze Delta. Results of this study are of practical significance for river channel management and evaluation of the influence of human activities on the hydrological regimes of large rivers. Copyright John Wiley & Sons, Ltd. KEY WORDS abrupt behaviours; scanning t-test; the simple two-phase linear regression scheme; hydrological regimes; the Yangtze River basin Received October ; Accepted 7 February INTRODUCTION The hydrological regimes of a river represent an integrated basin response to climatic inputs, with precipitation being dominant (Zhang et al., ; Zhang et al., ). Statistical properties of streamflow variations are critical for the evaluation of regional availability and variability of water resources (e.g. George, 7; Brabets and Walvoord, 9; Chen et al., ; Zhang et al., a). Human activities heavily interfere with hydrological processes, particularly sediment load. These activities, especially the construction of dams and water reservoirs, are primarily responsible for the reduction in terrestrial sediments to coastal areas (James et al., 6). Walling and Fang () pointed out that reservoir construction was probably the most important influence on land ocean sediment fluxes. That is why a goal of the International Geosphere Biosphere Program and its core project, Land Ocean Interaction in the Coastal Zone, has been to survey the terrestrial sediment supply to coasts and analyse perturbations in this flux (Syvitski, ). Sediment load and streamflow changes of large rivers in China have been investigated a great deal. Analyzing *Correspondence to: Qiang Zhang, Department of Water Resources and Environment, Sun Yat-sen University, Guangzhou 575, China. E-mail: zhangqnj@gmail.com sediment load datasets from 95 to 5 from four gauging stations in the main stem of Yangtze River, Wang et al. (7a,b) found distinct stepwise decreases in sediment load, which they attributed to both natural and anthropogenic impacts. Using the 95 time series of annual sediment supply and coastal bathymetric data, Yang et al. (6) illustrated a significant decreasing trend in riverine sediment supply since the late 96s and attributed this decreasing sediment to dam construction. They indicated that the subsequent result of decreasing sediment load may be responsible for the recession of deltaic coast, which poses a great challenge to coastal management. Zhang et al. (6) found that water reservoirs exerted more influence on sediment transport than water discharge, and this influence was more significant in the tributaries than in the main stem of the Yangtze River. Analyzing annual water discharge and sediment load series (from the 95s to ) from nine stations in the main channel and main tributaries of the Zhujiang (Pearl River), Zhang et al. (c) demonstrated a significant decreasing sediment load at some stations in the main tributaries, and since the s, more stations have witnessed significantly decreasing sediment loads. It should be noted that the influence of human activities or natural factors, such as precipitation change, is subjected to different time scales, for example, climate change usually influences the Copyright John Wiley & Sons, Ltd.

HYDROLOGICAL PROCESSES AT DIFFERENT TIME SCALES IN THE YANGTZE RIVER 5 hydrological processes at longer time scales as compared with that of human activities, such as the construction of water reservoirs. The previous studies seem to have mostly ignored this important consideration, which is the major reason for this study (Chen et al., ; Lu et al., ). The Yangtze River (Changjiang, Figure ), the longest river in China and the third longest river in the world, plays a vital role in the economic development and ecological environmental conservation within China. Numerous water reservoirs have been built in the Yangtze River basin, and most of these reservoirs are located in the tributaries. Xu (5) stated that up to the end of s, there were 9 water reservoirs in the upper Yangtze River basin with a total storage capacity of.5 m. The construction of the Three Gorges Dam started on December, 99, and ended in 9 with a total storage capacity of 9. 9 m. The construction of Gezhouba Dam started in May, and its operation started in December 9, with a total storage capacity of.5 9 m. The building of water reservoirs has greatly altered the hydrological processes in the Yangtze River basin (Zhang et al., 6). It should be noted here that, when compared with the Three-Gorge Dam, the Gezhouba Dam is much smaller in terms of volume capacity (.5 9 m of Gezhouba Dam vs 9. 9 m for Three Gorges Dam). It has only a limited influence on hydrological processes except sediment transport several years afterwards. However, it may have considerable impacts on ecological environment such as the migratory fishes from the middle and lower reaches of the Yangtze River to the upper reaches. Using monthly data from four stations in the Yangtze River, the objectives of this study therefore were as follows: () to investigate abrupt changes in sediment load and streamflow at different time scales, () to investigate the causes of the abrupt changes, and () discuss implications of these changes. DATA Monthly sediment load (kg/s) and monthly streamflow (m /s) from January 96 to December (Table I) from four major hydrological stations in the Yangtze River were obtained collected from the Changjiang (Yangtze) Water Resource Commission, which firmly controls the quality of data before release. Locations of the stations are shown in Figure. There were no missing data in the streamflow series, but missing data were found in the sediment load series in 979 at the Yichang station and in, 966, 96, 976 and 97 at the Datong station. No missing data can be found in the sediment load and streamflow series at other stations considered in this study. The existence of missing data results in a decrease in the sample size available for analysis. To make full use of the data without loss of its statistical properties, missing values were filled in with multi-annual mean sediment load of the respective individual months. 9 E 96 E E E E E E E 6 N Nanjing N TGD GZB Yichang Wuhan Shanghai N Chongqing N Jinshajiang R. Yayanjiang R. Minjiang R. Danjiangkou water reservoir Hanjiang R. Dongting Lake CHINA Huanghe R. Yangtze R. basin Jialingjiang R. Wujiang R. Poyang Lake Taihu Lake Hydrological station Pingshan st. Datong st. Yichang st. Gezhouba Dam Hankou st. (GZB) Three Gorges Dam (TGD) Figure. Location of the Yangtze River basin and the hydrological stations Copyright John Wiley & Sons, Ltd. Hydrol. Process. 7, 5 ()

6 Q. ZHANG ET AL. Table I. Sediment load and streamflow data at four hydrological stations of the Yangtze River basin Station name METHODOLOGY Zhang et al. (9) revised a method developed by Lund and Reeves () for detecting abrupt hydrological changes, and the revised method was applied in this study. The original method (Solow, 97; Easterling and Peterson, ; Lund and Reeves, ) can be written as X t ¼ m þ a t þ e t () m þ a t þ e t where X t is the dependant variable representing a hydrometeorological series; m and m are the mean values of the two sub-series, respectively, divided by c, the assumed change point; a and a are the regressive coefficients of the two sub-series, respectively; t and t are the time interval of the two sub-series, respectively, being defined as: t c and c < t n, respectively, where c is the possible change point to be tested; n is the total length of the two sub-series; and e t is mean zero independent random error with a constant variance. Relations between streamflow and sediment load were analysed by using a coherency technique, which entails two steps: scanning t-test and coherency analysis based on the scanning t-test. In the scanning t-test (Jiang et al., ), statistic t(n, j) isdefined as the difference of the subsample averages between every two adjoining subseries of equal subseries size (n) expressed as = tn; ð jþ ¼ x j x j n = s j þ s j () where x j ¼ n X j xi ðþ; x j ¼ n i¼j n jþn s j ¼ n Drainage area (km ) i¼j jþn X i¼j X ; xi ðþ x j Streamflow series xi ðþ; s j ¼ n Sediment load series Pingshan 5 99 96 96 Yichang 5 5 96 96 Hankou 6 96 96 Datong 75 96 96 X j i¼j n ; xi ðþ x j in which n is the subsample size varying as n=,,..., < N/ or may be selected at suitable intervals; and j=n+, n+,...,n n+ is the reference time point. It should be noted that hydrological series are usually autocorrelated. Thus, the Table-Look-Up Test (von Storch and Zwiers, 999) was adopted to modify the significance criterion of statistic t(n, j) based on lag- autocorrelation coefficients of the pooled subsample and the subsample size n. Criterion t.5 for the correction of the dependence was employed to determine significant changes in time scales longer than years. For shorter subsample sizes, the critical values are overly restrictive. Because the significance level varies with n and j, the test statistic was normalized, to make values comparable, as t r ðn; jþ ¼ tn; ð jþ=t :5 () when t r (n,j) >., the abrupt change is significant at the 95% confidence level. t r (n,j) <. denotes a significant decrease, and t r (n,j) >. a significant increase. Finally, the coherency of abrupt changes between two series u and v was defined as t rc ðn; jþ ¼sign½t ru ðn; jþt rv ðn; jþšfjt ru ðn; jþt rv ðn; jþjg = () When statistic t rc (n, j) >. with both jt ru (n, j) j; jt rv (n, j)j>., the two series have abrupt changes in the same direction, whereas if t rc (n, j) <., the two series have abrupt changes in opposite directions (Jiang et al., ). The coherency of abrupt changes between monthly streamflow series and monthly sediment load series can be regarded as an indication of the interaction between these two series on decadal and basin scales. RESULTS AND DISCUSSIONS Changes in sediment load and streamflow at Pingshan station Pingshan station is located upstream to the Gezhouba Dam and Three Gorges Dam (Figure ). Streamflow has been increasing at a time scale of > months. The decrease and increase can be found to be intermittent at different time scales (Figure ). Regions dominated by significant change points also are found to distribute sporadically within the time scales versus time space. Comparatively, patterns of distribution of change points of sediment load are in approximate agreement with those of streamflow. Increasing tendency is found mainly at longer time scales, and the abrupt changes in sediment load at shorter time scales, such as < months, are relatively complicated. For sediment load and streamflow changes, the long-term tendency is dominated by the increasing trend, and these results are good line with those by Zhang et al. (6). Earlier study suggested that the river suspended sediments are mainly from the upper Yangtze River Catchment (Pan, 999); the sediments from Jinshajiang River alone accounts for about 9.% of that in Yichang station (Xu, 5). Over-exploitation in the upper Yangtze Catchment led to an increasing trend of sediment load in the Pingshan station (Pan, 999). Figure shows abrupt changes in streamflow (upper panel of Figure ) and sediment load (lower panel of Figure ). Different changing properties of streamflow can be identified during different time intervals. Specifically, before, streamflow was characterized to be increasing. Decreasing streamflow can be observed during and the early s. The s and s Copyright John Wiley & Sons, Ltd. Hydrol. Process. 7, 5 ()

7 streamﬂow (Figure ) also indicates positive relations between sediment load and streamﬂow by positive coherency at various time scales. Pingshan: streamflow Sediment load and streamﬂow changes at Yichang station 95 95 Pingshan: sediment load Standardized streamflow (m/s) Figure. Change points on different time scales of streamﬂow and sediment load variations of the Pingshan station. Dashed lines show decreasing trend after the change point, and solid lines indicate increasing trend after the change point. Thick solid and dashed lines denote signiﬁcant change points. The meanings of the line styles are the same for subsequent ﬁgures. The x-axis denotes time scales with unit of months; ﬁrst, we should identify the change points in the plot, that is, the regions circled by thick dashed or solid contours, and then read the time scales from x-axis and then the time when the change point occurred from the yaxis. In so doing, the time scales and the time when the change point occurred can be read from the plot. The same procedure will be followed in the subsequent ﬁgures 95 Figure. Coherency between sediment load and streamﬂow series at the Pingshan station 96 6 Standardized sediment load (kg/s) The upper panel of Figure 5 shows abrupt changes in streamﬂow at the Yichang station. Figure 6 displays abrupt behaviours of streamﬂow and sediment load using linear regressive technique. Increasing tendency can be found at time scales of > months. Changing characteristics of streamﬂow at time scales of < months are complicated. Signiﬁcant change points are identiﬁed in about, the mid-s and the early s. Comparison between the upper panel of Figure 5 and the upper panel of Figure indicates similarity in streamﬂow changes at the Yichang and Pingshan stations. The differences, if any, between streamﬂow changes at the Yichang and the Pingshan stations should be due to the streamﬂow from the tributaries between the Pingshan and Yichang: streamflow 96 Figure. Linear trends of time intervals separated by change points for the streamﬂow and sediment load series at the Pingshan station are featured by increasing streamﬂow. Decreasing streamﬂow is found after. Abrupt changes in sediment load shown in the lower panel of Figure are similar to those of streamﬂow changes, implying that the transport of sediment load is subjected mainly to the hydrodynamic characteristics of the river channel (Lu et al., ). Coherency analysis of sediment load and Copyright John Wiley & Sons, Ltd. HYDROLOGICAL PROCESSES AT DIFFERENT TIME SCALES IN THE YANGTZE RIVER 95 95 Yichang: sediment load Figure 5. Change points on different time scales of streamﬂow and sediment load variations of the Yichang station. Dashed lines show decreasing trend after the change point, and solid lines indicate increasing trend after the change point. Thick solid and dashed lines denote signiﬁcant change points. The meanings of the line styles are the same for subsequent ﬁgures Hydrol. Process. 7, 5 ()

Q. ZHANG ET AL. 96 95 5 6 result of the trapping effect of the Three Gorges Dam. Streamﬂow was not consistently decreasing or increasing before the early s and has exhibited moderate variations since the s. Changes in sediment load were different from those of the streamﬂow, being dominated by a general decreasing trend. A sudden decrease was observed after the early s and ~. Results of coherency analysis (Figure 7) indicated negative relations between streamﬂow and sediment load after the early s on a time scale of > months, showing tremendous impacts of the trapping effect of the Gezhouba Dam on the sediment transport. No negative relations were found during other time intervals on different time scales, particularly after the late s, which should be caused by concordant changes in the sediment load and streamﬂow, that is, decreasing tendency, although the magnitudes of changes were different. Sediment load and streamﬂow changes at Hankou station the Yichang stations, such as the Wujiang River (Figure ). Besides, the storage capacity of the river channel between these two stations should be one of the factors causing differences in streamﬂow changes at the Pingshan and Yichang stations. Changes in sediment load are subjected to different patterns of changes when compared with those at the Pingshan station. Several time intervals were identiﬁed to be dominated by different changing properties of sediment load. Speciﬁcally, signiﬁcant change points were detected in the early s, the early s and the late s. The construction of the Gezhouba Dam started in, and its operation started in the early s with a total storage of.5 9 m, which has been exerting a tremendous inﬂuence on sediment transport (Chen and Huang, 99). Furthermore, up to the end of the s, there were water reservoirs constructed in the Jinshajiang River basin with a total storage of. 9 m (Xu, 5). These water reservoirs on the mainstream and tributaries of the Yangtze River have been trapping large amounts of sediment and have given rise to signiﬁcant abrupt changes in the sediment load in the early s and the mid-s (Yang et al., 6). The effects of water reservoirs, that is, the Gezhouba and the Three Gorges Dam water reservoirs, are reﬂected by abrupt changes in streamﬂow. Since the start of the construction of the Three Gorges Dam in 99 and its completion in late with a total storage capacity of.9 m, the period of 6 was decided as the post-tgd (Three Gorges Dam) by Chen et al. (). Changes in annual sediment load indicate a decrease in the load since (Chen et al., ). The late s were identiﬁed as the signiﬁcant change point, and the sediment load has obviously been decreasing since this change point, which should be the Copyright John Wiley & Sons, Ltd. Figure 6. Linear trends of time intervals separated by change points for the streamﬂow and sediment load series at the Yichang station Figure shows abrupt changes in sediment load and streamﬂow at the Hankou station (see Figure for its 95 Figure 7. Coherency between sediment load and streamﬂow series at the Yichang station 96 95 5 Hankou: streamflow Standardized sediment load (kg/s) Standardized streamflow (m/s) 95 95 Hankou: sediment load Figure. Change points on different time scales of streamﬂow and sediment load variations of the Hankou station. Dashed lines show decreasing trend after the change point, and solid lines indicate increasing trend after the change point. Thick solid and dashed lines denote signiﬁcant change points. The meanings of the line styles are the same for subsequent ﬁgures Hydrol. Process. 7, 5 ()

9 HYDROLOGICAL PROCESSES AT DIFFERENT TIME SCALES IN THE YANGTZE RIVER Sediment load and streamﬂow changes at Datong station Changes in streamﬂow were evidently similar to those at the Yichang and Hankou stations in terms of abrupt changes (Figure ). No large tributaries and no massive streamﬂow input were available in the lower Yangtze River basin, which should contribute to the similarity of streamﬂow changes at the Yichang, Hankou and Datong 96 95 5 Figure 9. Linear trends of time intervals separated by change points for the streamﬂow and sediment load series at the Hankou station Copyright John Wiley & Sons, Ltd. 95 Datong: streamflow 96 95 5 Figure. Coherency between sediment load and streamﬂow series at the Hankou station Coherency analysis (Figure ) shows that on a longer time scale, such as > months, streamﬂow has a negative relation with sediment load since the mids. This should be the result of the construction of Gezhouba Dam, which decreased the sediment load in the lower river reach and caused inconsistent relations between sediment load and streamﬂow on longer time scales. On shorter time scales of < months, however, positive coherency was still observed. Therefore, sediment transport load also is heavily inﬂuenced by hydraulics on shorter time scales. Standardized sediment load (kg/s) Standardized streamflow (m/s) location). In general, similar changes can be found in streamﬂow when compared with those at the Yichang station in terms of the time when the change points occur. Changes in streamﬂow at the Pingshan station are different from those at the Yichang and Hankou stations. These results clearly show different inﬂuencing factors for streamﬂow changes at the foregoing three hydrological stations, which should be attributed to the uneven spatial distribution of precipitation changes (Zhang et al., a). These results also indicate that streamﬂow in the Yangtze River is inﬂuenced by climate change, such as precipitation, but not by anthropogenic factors, such as the construction of water reservoirs. Differences in patterns of streamﬂow changes at the Hankou station, when compared with those at the Yichang station, also should be caused by streamﬂow from the Hanjiang River and hydraulic regulation of the river channels between the Yichang and Hankou stations. It can be observed from the lower panel of Figure that the sediment load is dominated by a decreasing tendency except for a short time interval, that is, 95 to early s, which is characterized by increased sediment load. Figure 9 intuitively clariﬁes the changes in sediment load and streamﬂow. Streamﬂow follows a relatively complicated changing pattern, decreasing after the early s, which should be caused by the decreasing precipitation in the upper Yangtze River basin (Zhang et al., a). Changes in sediment load are dominated by a decrease, and this decrease is abruptly evident after, which should be attributed to the construction of the Three Gorges Dam because of the abrupt change stemming from the construction of the Three Gorges Dam, as reported in earlier studies (e.g. Zhang et al., b). 95 95 loadload Datong:sediment sediment Datong: Figure. Change points on different time scales of streamﬂow and sediment load variations of the Datong station. Dashed lines show decreasing trend after the change point, and solid lines indicate increasing trend after the change point. Thick solid and dashed lines denote signiﬁcant change points. The meanings of the line styles are the same for subsequent ﬁgures Hydrol. Process. 7, 5 ()

5 Q. ZHANG ET AL. stations. In addition, this result further confirms the conclusion that changes in streamflow are mainly the result of precipitation changes. Sediment load has generally been decreasing particularly since (Figures and ). The mid-lower basin is a kind of a sediment sink (Chen, 996). Increasing evident increase of the sediment load at the middle and the lower Yangtze River basin when compared with the upper Yangtze River basin is partly because of an important sink that significantly decreases the sediment discharge from the Yangtze River into the sea (Chen et al., 5). It is estimated that the annual sand extraction amounted to about million tons in the early s and increased to about million tons in the late s. In this case, sediment load changes are the integrated results of climate changes and human activities. However, the decrease in sediment load at the Datong station in recent decades has been moderate when compared with that at the Yichang and Hankou stations, which should be caused by the erosion of riverbed. The scouring process caused by the decreased sediment load from the upper and the middle Standardized sediment load (kg/s) Standardized streamflow (m /s) 96 95 5 6 96 95 5 Figure. Linear trends of time intervals separated by change points for the streamflow and sediment load series at the Datong station 95 Figure. Coherency between sediment load and streamflow series at the Datong station Yangtze River caused a moderate decrease, although the decreasing tendency was still evident. The time interval after witnessed a significant decreasing sediment load. The decreasing sediment since can be attributed to the trapping effect of the Three Gorges Dam. Streamflow at the Hankou station had been generally increasing, but it has been decreasing since. In the lower Yangtze River basin, precipitation has been increasing, and it is particularly true for precipitation maxima and precipitation intensity (Zhang et al., a, b). Spatial patterns of precipitation regimes over the Yangtze River basin result in spatial distribution of changes in streamflow. CONCLUSIONS Analysis of monthly streamflow and sediment load data from four hydrological stations along the mainstem of the Yangtze River shows abrupt changes. The following conclusions are drawn from this analysis:. Climate change and human activities induce changes in streamflow and sediment load at different time scales. Generally, the changes of hydrological regime influenced by climate factors, on a long-term basis, was comparable to the changes by anthropogenic factors. However, hydrological changes caused by human activities, such as the construction of water reservoirs, can reflect the anthropogenic influence on a longer time scale. Trapping by water reservoirs can decrease sediment load, and the decrease in sediment load can occur for a considerably long time. In this sense, the influence of water reservoirs on the hydrological regime, particularly the sediment load and subsequent effects, can be far reaching. Besides, the abrupt behaviours of sediment load and streamflow are presented in time versus temporal scales showing abrupt changes on different time scales, this is the novel points when compared with research on similar topics within the Yangtze River basin.. Coherency analysis shows that sediment transport in the upper Yangtze River heavily depends on the river hydraulics, as reflected by the positive coherency between sediment load and streamflow at the Pingshan station. Changes in sediment load in the middle and lower Yangtze River reaches are impacted by the trapping effects of the water reservoirs in the middle Yangtze River basin, for example, the Gezhouba Dam and the Three Gorges Dam. Massive trapping effects of water reservoirs of the Gezhouba Dam and the Three Gorges Dam directly cause a significant decrease in sediment load in the middle and the lower Yangtze River basin. The scouring of the riverbed in the lower Yangtze River leads to moderate changes in sediment load. The Three Gorges Dam still causes a considerable decrease in sediment load, resulting in a decrease in sediment load since about. However, changes in streamflow seem mainly to be the result of changes in Copyright John Wiley & Sons, Ltd. Hydrol. Process. 7, 5 ()

HYDROLOGICAL PROCESSES AT DIFFERENT TIME SCALES IN THE YANGTZE RIVER 5 precipitation. Comparison between the results of precipitation changes across the Yangtze River basin (Zhang et al., a) and abrupt changes in streamflow corroborates the tremendous impact of precipitation changes on the hydrological processes. Coherency analysis investigates abrupt changes of streamflow versus sediment load relations on different time scales, and this is another novelty of this study when compared with standing research.. Streamflow and sediment load are influenced by climate change and human activities, and their influence varies with time scales. Besides, influences of water reservoirs on sediment load changes also are affected by balance-effects of the storage of water reservoirs (e.g. Want et al., 7b). Therefore, retention effects of water reservoirs on sediment load are relatively complicated because the reservoirs act as sinks of sediment load. However, the storage capacity is decreasing with the operation of the water reservoirs. New reservoirs are completed; meanwhile, the old reservoirs will no longer have storage for sediment deposition. In this case, the retention capacity of water reservoirs for sediments is decreasing. Therefore, it is difficult to exactly estimate the total annual deposition in so many reservoirs within the Yangtze River drainage basin (Yang et al., ). An understanding of changes in the hydrological regime at different time scales and possible underlying causes is of scientific and practical significance. ACKNOWLEDGEMENTS This work is financially supported by Program for New Century Excellent Talents in University (the Fundamental Research Funds for the Central Universities), the National Natural Science Foundation of China (Grant No.: 7; 595), the Project from Guangdong Science and Technology Department (Grant No.: B5; B5), and by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. CUHK5). Cordial gratitude should be extended to the Changjiang the Changjiang (Yangtze) Water Resource Commission for providing the data analysed in this study. Our cordial gratitude also should be owed to the editor-in-chief, Prof. Dr Malcolm G. Anderson, and three anonymous reviewers for their professional and pertinent suggestions and comments, which are greatly helpful for further improvements of the quality of this manuscript. REFERENCES Brabets PT, Walvoord AM. 9. 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