Stoch Environ Res Risk Assess (2009) 23:1103–1111 DOI 10.1007/s00477-008-0285-8 ORIGINAL PAPER Scaling properties of the runoff variations in the arid and semi-arid regions of China: a case study of the Yellow River basin Qiang Zhang Æ Chong-Yu Xu Æ Tao Yang Published online: 8 October 2008 Springer-Verlag 2008 Abstract We analyzed long daily runoff series at six hydrological stations located along the mainstem Yellow River basin by using power spectra analysis and multifractal detrended fluctuation analysis (MF-DFA) technique with aim to deeply understand the scaling properties of the hydrological series in the Yellow River basin. Research results indicate that: (1) the runoff fluctuations of the Yellow River basin exhibit self-affine fractal behavior and different memory properties at different time scales. Different crossover frequency (1/f) indicates that lower crossover frequency usually corresponds to larger basin area, and vice versa, showing the influences of river size on higher frequency of runoff variations. This may be due to considerable regulations of river channel on the runoff variations in river basin of larger basin size; (2) the runoff fluctuations in the Yellow River basin exhibit short-term memory properties at smaller time scales. Crossover analysis by MF-DFA indicates unchanged annual cycle within the runoff variations, implying dominant influences of climatic changes on changes of runoff amount at longer time scales, e.g. 1 year. Human activities, such as human withdrawal of freshwater and construction of water reservoirs, in different reaches of the Yellow River basin may be responsible for different scaling properties of runoff variations in the Yellow River basin. The results of this study will be helpful for hydrological modeling in different time scales and also for water resource management in the arid and semi-arid regions of China. Keywords River flow Scaling features Multifractal detrended fluctuation analysis (MF-DFA) Spectra analysis The Yellow River 1 Introduction Q. Zhang (&) State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Science, 210008 Nanjing, China e-mail: zhangqnj@gmail.com Q. Zhang Institute of Space and Earth Information Science, The Chinese University of Hong Kong, Shatin, Hong Kong, China C.-Y. Xu Department of Geosciences, University of Oslo, PO Box 1047, Blindern, 0316 Oslo, Norway T. Yang State Key Laboratory of Hydrology-Water Resources and Hydraulics Engineering, Hohai University, 210098 Nanjing, China The well-evidenced global warming has the potential to alter the hydrological cycle over the earth and will give rise to altered spatial–temporal distribution of water resources (USEPA 1995; Xu and Singh 2004; Zhou et al. 2006; Zhang et al. 2008a). Hydrologists have been endeavoring to develop and/or employed appropriate hydrological models and robust statistical techniques in the study of hydrological characteristics and possible mechanisms behind them in the watersheds (e.g. Gupta et al. 1994; Xu et al. 2005; Zhang et al. 2005). However, more and more scholars have found that the hydrological characteristics and hydrological responses to the climate variability depend heavily on the season and the geographic regions (e.g. Xu et al. 2006). It should be noted that the characteristics of the runoff fluctuations depend on the size of the watersheds, the topography, land-use 123 1104 patterns, hydrogeology and drainage network morphology, the transferability of hydrological models hinges on how well they can be extrapolated across spatial and temporal scales (Zhou et al. 2006). The pioneering work by Hurst (1951) leads to massive study on scaling properties of hydrological series and scaling behaviors have also been identified in hydrological series (e.g. Puente et al. 2001; Koutsoyiannis 2002; Bunde et al. 2006; Zhang et al. 2008c). Scaling properties can well reflect physical and dynamical mechanisms of hydrological phenomena and are also helpful for hydrological modeling on different time scales and for better understanding of other geophysical phenomena (Kestener and Arneodo 2008). Therefore, better understanding of the scaling features of the runoff fluctuations will be greatly important for prediction and extrapolation of the runoff series from a shorter time scale to a longer time scale. Thus, the study of the scaling properties of the hydrological series is one of the major issues in hydrological sciences (e.g. Sposito 1998). Research of scaling characteristics in runoff series can be dated back to 1950s when Hurst (1951) performed the R/S analysis on annual runoff series from various rivers (e.g. Nile River) and found that the annual runoff series exhibited long-term dependencies. Hurst’s finding is now accepted as the first example for self-affine fractal behavior in hydrological series (e.g. Feder 1988). Bunde et al. (2006) systematically studied the temporal correlations and multifractal properties of long river discharge records from 41 hydrological stations around the globe Stoch Environ Res Risk Assess (2009) 23:1103–1111 using detrended fluctuation analysis (DFA), wavelet analysis and multifractal DFA, suggesting that the daily runoff are long-term correlated and are characterized by a correlation function C(s) that decays as C(s)*s-c. The exponent c varies from river to river in a wide range between 0.1 and 0.9. Livina et al. (2003) studied the spectra properties of the magnitudes of daily river flux increments, the volatility, indicating that the volatility series exhibits strong seasonal periodicity and power–law correlations for time scales less than 1 year. The above researches demonstrated that the hydrological system is a complex dynamic system characterized by nonstationary input (precipitation) and output (evaporation, human withdrawal of freshwater and infiltration), which display self-similar and exhibit self-affine fractal behaviors over a certain range of time scales (Feder 1988; Mandelbrot and Wallis 1969). Better understanding of the scaling and multifractality features of the runoff fluctuations will be greatly useful in hydrological modeling (Rodriguez-Iturbe and Rinaldo 1997) and will be beneficial for extrapolation between scales of observations of hydrology of the watersheds (Zhu et al. 2006). This is the main motivation of this study. Located in the semi-humid, semi-arid and arid climatic zones, the Yellow River (Huanghe in Chinese) is the second largest river in China, being the important source for water supply in the North-western and Northern China (Fig. 1). However, it is also the region of shortage of water resource and this water resource shortage tends to be increasingly serious reflected by more frequent zero-flow Fig. 1 Location of the study region and the hydrological stations (the river reach between the headwater and Hekou is the upper Yellow River; Hekou and Taohuayu is the middle Yellow River and the river downstream to the Taohuayu is the lower Yellow River) 123 Stoch Environ Res Risk Assess (2009) 23:1103–1111 Fig. 2 Six streamflow series from six hydrologic gauging stations along the mainstem Yellow River. The x axis denotes time in days. Locations of these six hydrologic stations are shown in Fig. 1 1105 6000 Tangnaihai station 4000 2000 0 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 6000 Lanzhou station 4000 2000 Streamflow (m3/s) 0 0 2000 4000 6000 8000 10000 12000 5500 4000 14000 15000 Toudaoguai station 2000 0 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 Longmen station 10000 0 0 2000 4000 6000 8000 10000 12000 14000 15000 20000 Huayuankou station 10000 0 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 10000 Lijin station 5000 0 0 2000 events in the Yellow River basin during the last decades (Zhang et al. 2008b). Thus, integrated water resource management based on good knowledge of hydrological principles becomes urgent. Due to massive impacts of deteriorating water shortage on regional economic development such as agricultural production, more and more relevant studies have been conducted on runoff changes and possible underlying causes (e.g. Liu and Zheng 2004). However, so far, no report is available in terms of the study of scale invariance features of the runoff series in the Yellow River basin although researches about the scaling, non-linearity, multifractal behaviors of the runoff series of the rivers of the world can be found widely in the literatures (Schertzer and Lovejoy 1987; Gupta and Waymire 1990; Rodriguez-Iturbe and Rinaldo 1997; Pandey et al. 1998; Zhang et al. 2008c). Therefore some questions still remain unanswered: (1) Do the statistics of the river flow exhibit scaling, if so, what type of scaling? (2) Are the scaling behaviors, if any, similar to those of the river flow of other rivers of the world? If not, what are the differences? We approach these issues by analyzing runoff data collected from six hydrological stations, corresponding to different basin areas, along the mainstem Yellow River. Therefore, the objectives of the current paper are: (1) to analyze characteristics of the hydrological stochastic processes; and (2) to characterize the scaling properties of 4000 6000 8000 10000 12000 14000 16000 different time scales in the hydrological series of the Yellow River basin. 2 Data and methods 2.1 Data The long daily runoff records from six hydrological stations along the mainstem Yellow River basin were analyzed (Figs. 1, 2). The detailed information of the data can be referred to Table 1 and the locations of the hydrological stations can be referred to Fig. 1. The data series are of good quality with no missing data within the study time interval. Table 1 Detailed information of the hydrological gauging stations Station name Drainage area (km2) Series length Tangnaihai st. Lanzhou st. 121,972 222,551 1956.1.1–2005.12.31 1967.1.1–2005.12.31 Toudaoguai st. 367,898 1958.1.1–2005.12.31 Longmen st. 497,552 1965.1.1–2005.12.31 Huayuankou st. 730,036 1957.1.1–2005.12.31 Lijin st. 751,869 1964.1.1–2005.12.31 The location of the stations can be referred to Fig. 1 123 1106 2.2 Methods Stoch Environ Res Risk Assess (2009) 23:1103–1111 Step 4 2.2.1 Power spectra analysis We apply the spectra analysis techniques and calculate the power spectrum S(f) of the time series Ri as the function of frequency f (Bunde et al. 2006). If short- or long-term memory exists in the series, we have S(f) * f-b, where b is related to the correlation exponent c by b = 1-c. Generally, the power spectra of runoff will have different frequency regimes of power law like nature (Tessier et al. 1996). The regimes are usually separated by the crossover frequency fc. Most river runoff series present multiscaling/ multifractal features with respect to time (Pandey et al. 1998). The power spectrum density is the Fourier transform of the autocorrelation function C(r), b = 1 - c. The shortor long-term memory can be distinguished by the integral R1 timescale T ¼ 0 CðrÞdr being finite or infinitely large (Trenberth 1985). The 1/f, likes the power spectrum scaling, is an interesting feature for complex system. In such scaling, the power spectrum of a given time series is characterized by inverse power law (Pritchard 1992). 2.2.2 Multifractal detrended fluctuation analysis (MF-DFA) MF-DFA is a generalized standard DFA by identifying the scaling of the qth-order moments of the time series (Kantelhardt et al. 2002). The MF-DFA procedure involves of five steps (Movahed et al. 2006). The first three steps are the same as those in the conventional DFA algorithm. Assume that xk is a time series, k = 1,…, N. P Step 1 Determine the ‘profile’ Yi ¼ ik¼1 ½xk hxi; i ¼ 1; . . .; N; where hxi is the mean of xk Step 2 Divide the profile Y(i) into Ns = int(N/s) nonoverlapping segments of equal lengths s; int(N/s) denotes the integer part of N/s. Since the length N of the series is often not a multiple of the timescale s considered, a short part at the end of the profile may remain. To retain this part of the series, the same procedure is repeated starting from the opposite end. Thereby, 2Ns segments are obtained Step 3 Calculate the local trend for each of the 2Ns segments by a least squares fit of the series. Then P determine the variance F 2 ðs; mÞ ¼ 1s si¼1 fY½ðm 2 1Þs þ i ym ðiÞg ; for each segment m, m = 1,…, Ns, P and F 2 ðs; mÞ ¼ 1s si¼1 fY½N ðm Ns Þs þ i ym ðiÞg2 ; for m = Ns ? 1,…, 2Ns. Here, ym(i) is the fitting polynomial in segment m. Linear, quadratic, cubic or higher order polynomials can be used in the fitting procedure (DFA1, DFA2, DFA3,…, DFAm) 123 Step 5 Average over all segments to obtain the qthorder fluctuation function, defined as Fq ðsÞ ¼ P s 2 q=2 1=q g ; where q 6¼ 0, s C f2N1 s 2N m¼1 ½F ðs; mÞ m?2 Determine the scaling behavior Fq(s) sh(q) of the fluctuation functions by the log–log plot of Fq(s) versus s for each value of q For stationary time series, the exponent h(2) for small time scales is identical to the well-known Hurst exponent H. For non-stationary signal, the relation between the exponent h(2) for small scales and the Hurst exponent H is H = h(2) - 1 (Hu et al. 2001). It is well known that for uncorrelated series, the scaling exponent H equals to 0.5; 0.5 \ H \ 1 indicates long memory or persistence; 0 \ H \ 0.5 indicates short memory or anti-persistence. This method was successfully used in characterization of scaling properties of the runoff series of the Yangtze River basin (Zhang et al. 2008c). Hence we can use the value of h(2) to determine its correlations and scaling properties. 3 Results 3.1 Scaling properties revealed by power spectra Figure 3 demonstrates the power spectra of the daily runoff records of Yellow River basin in a log S(f)–log (f) plot. The spectra of the runoff series of these six stations are almost in the same shape with different frequency regimes of power–law-like nature. The difference is the crossover frequency 1/f: 15 days for Tangnaihai station and Lanzhou station; 18 days for Toudaoguai station; 25 days for Longmen station and Huayuankou station and 39 days for Lijin station. Therefore, inference can be obtained about the changes of the crossover frequency with different basin areas: shorter crossover frequency usually corresponds to larger basin area, and vice versa. The finding is in good agreement with that of the Danube River, being the second longest river in Europe (2,850 km long with drainage area of 817,000 km2): the crossover f shifts with basin area with frequencies corresponding to approximately 1/f = 60 days for Ceatal Izmail (807,000 km2) and Orsova (576,232 km2), 20 days for Nagymaros (183,534 km2) and 10 days for Ingolstadt (20,001 km2) and Dillingen (11,315 km2) (Dahlstedt and Jensen 2005). In the low frequency domain, the power–law has a b exponent closing to 0.8 for the Tangnaihai station, Lanzhou stations, Huayuankou station and Lijin station: b = 0.77 for Tangnaihai station; b = 0.82 for Lanzhou station; b = 0.69 for Huayuankou station and b = 0.7 for Lijin station (Fig. 3a), which corresponds to the logarithmic decaying correlation Stoch Environ Res Risk Assess (2009) 23:1103–1111 1107 Fig. 3 Power spectra of the daily streamflow records of Yellow River basin in log S(f)– log (f) plot. a Tangnaihai station; b Lanzhou station; c Toudaoguai station; d Longmen station; e Huayuankou station and f Lijin station. The dashed lines indicate spectra exponents b = 1 (1/f noise) and 2 (white noise). Solid lines show the local slope of the power spectra curves in the long time limit. This value range means that in the longer time scale, the runoff fluctuations in the upper and lower Yellow River basin are denoted as flicker (or 1/f) noise (acceptable for b from 0.7 to 1.3, e.g. Zhu et al. 2006). The crossover frequency is lower in the lower Yellow River than that in the upper Yellow River. In the low frequency domain, the power law has the b close to 1 indicating logarithmic decaying correlations in the longer time scale. In the time scales of about 30 days, the spectra are dominated by the inverse power law, showing the features of 1/f noise (Pritchard 1992). The power law has b exponent of 0.61 and 0.54 for Toudaoguai and Longmen station in the middle Yellow River basin. In the higher frequency domain, the b values fall inside the range of 1.7–2.55: b = 2.54 for Tangnaihai station; b = 1.7 for Lanzhou station; b = 2.42 for Toudaoguai station; b = 1.89 for Huayuankou station and b = 2.3 for Lijin station, which indicate long-term correlations but cease to follow a power–law. In the Mississippi River (Dahlstedt and Jensen 2005), similar phenomenon was identified: in the lower frequency, the power–law has an exponent close to 1 corresponding to logarithmic decaying correlation in the long time limit, and the power–law exponent close to 3 can be identified in the higher frequency (Jensen 1998). The power law for the Longmen station in the higher frequency domain has the b value of 1.2, close to 1 and in an intermediate range. This intermediate range spectrum can be interpreted as a Lorentzian (red-noise, with f-2), or as a reminiscent of 1/f noise (Zhu et al. 2006). Different scaling behaviors of the river flow are identified in the different parts of the Yellow River and in the different magnitude of frequency domains. 123 1108 Stoch Environ Res Risk Assess (2009) 23:1103–1111 exploring scaling properties of compartments of climatic changes and hydrological series (Pandey et al. 1998; Zhang et al. 2008c). In this paper, MF-DFA analysis was also performed on the daily runoff series of the six hydrological stations along the mainstem Yellow River. Figure 4 illustrates log–log plots of Fq(s) versus s of runoff series of the six hydrological gauging stations in the Yellow River basin. Based on log–log plots of Fq(s) versus s of runoff series, one crossover point can be detected for six curves of Fq(s) versus s. The timing for these crossover points is between 338 and 407 days. It is attributed to the 1-yearperiod. To determine statistical properties of the fluctuations of runoff series, we compute the three scaling exponents for smaller time scales. The h(2) values of the runoff series of these six stations are 1.45 ± 0.0151 (Tangnaihai station), 1.3628 ± 0.0118 (Lanzhou station), Generally, the runoff fluctuations in the upper and lower Yellow River basin demonstrate similar scaling properties and the runoff fluctuations in the middle Yellow River basin exhibit different scaling properties: in the longer time scale, the runoff demonstrates long-term persistence and in the shorter time scale, however the runoff exhibits a Lorentzian or a reminiscent of 1/f noise (Zhu et al. 2006). Different drainage areas, topography, land-use pattern, drainage network morphology and different intensity of human exploitation of water resources in the different river reaches can be responsible for these differences. 3.2 Scaling properties revealed by MF-DFA The temporal correlations are further studied by using MFDFA technique, and this method was widely used in 6 6 Lanzhou station Tangnaihai station 0.5 ± 0.0231 5 Crossover point 4 3 1 0 1 2 Crossover point 4 3 1.3628 ± 0.0118 1.45 ± 0.0151 2 0.6276 ± 0.0291 5 log10 F(s) log10 F(s) Fig. 4 Log–log plots of Fq(s) versus s of streamflow series of Tangnaihai station, Lanzhou station, Toudaoguai station, Longmen station, Huayuankou station and Lijin station of the Yellow River basin 2 3 1 4 0 1 2 log10 s 6 0.6665 ± 0.0185 5 4.5 Crossover point 4 log10 F(s) log10 F(s) Longmen station 0.6674 ± 0.0207 5 3 4 Crossover point 3.5 1.2933 ± 0.0172 3 2 1.1980 ± 0.0098 2.5 1 0 1 2 3 2 4 0 1 2 6 4 6 Huayuankou station Lijin station 0.6758 ± 0.0293 0.7268 ± 0.0206 5 log10 F(s) 5 Crossover point 4 3 2 3 log10 s log10 s log10 F(s) 4 5.5 Toudaoguai station 0 1 2 Crossover point 4 3 1.2261 ± 0.0062 log10 s 123 3 log10 s 3 4 2 1.3131 ± 0.0114 0 1 2 log10 s 3 4 Stoch Environ Res Risk Assess (2009) 23:1103–1111 1.2933 ± 0.0172 (Toudaoguai station), 1.1980 ± 0.0098 (Longmen station), 1.2261 ± 0.0062 (Huayuankou station) and 1.3131 ± 0.0114 (Lijin station), respectively. Based on the relation between the exponent h(2) for small scales and the Hurst exponent H, i.e. H = h(2) – 1, the h(2) values of these six hydrological stations are all larger than 0 but are smaller than 0.5, indicating short-term memory. Moreover, h(2) values of Tangnaihai, Lanzhou, and Lijin are all larger than 0.3, and the h(2) values of the rest stations are all smaller than 0.3. It implies that scaling properties of the runoff series in the middle Yellow River basin are different from those of the upper and lower Yellow River basin. This result is in good agreement with those by spectra analysis technique (Fig. 3). The crossovers indicate that the periods of the runoff series roughly equal to 1 year. Therefore, we can say that the runoff series are mainly controlled by precipitation variations. As for the shifts of frequency revealed by spectra analysis, for the shorter period, less than 1–2 months, periods of crossover tend to be longer from the upper to the lower Yellow River basin, it may be because of other influencing such as human withdrawal of fresh water, regulations of water reservoirs (Fig. 1 also illustrates the location of the reservoirs) other than climatic changes. The mechanisms for runoff fluctuations are different from one rive to another in the world. The Mary River in Australia is rather dry in summer and the Gaula River in Norway is frozen in winter (Bunde et al. 2006). Therefore no universal scaling behaviors exist in the runoff series of the rivers over the globe, which is different from the climate data with universal long-term persistence of observed temperature variations (e.g. Eichner et al. 2003). 4 Discussions and conclusions With the help of power spectra analysis and MF-DFA, we studied systematically the scaling properties of the runoff fluctuations within the Yellow River basin based on the long daily runoff data of six hydrological stations. Research results indicate that the runoff fluctuations exhibit self-affine fractal behavior and memory properties at different time scales. In the Fq(s) versus s plot, the crossover scales separate regimes with different scaling exponents, showing the multiple scaling exponents or multifractal properties in the hydrological series in the Yellow River basin. The crossover frequency (1/f) analysis results indicate that lower crossover frequency usually corresponds to larger basin area, and vice versa. Similar phenomenon was found within the Danube River, the second longest river in Europe. Besides, the b exponents in the lower frequency domain and higher frequency domain are different 1109 significantly. The b exponent in the lower frequency domain lies between 0.7 and 0.82 for the hydrological stations in the upper and lower Yellow River basin and is about 0.6 for the hydrological stations in the middle Yellow River basin. In the low frequency domain, the runoff fluctuation is in the logarithmic decaying correlation in the long time limit. The scaling properties of hydrological series of Longmen station are different from those of the rest stations of the Yellow River basin. This point can be further confirmed by MF-DFA analysis. Increasing crossover frequency (i.e. 1/f) may be due to accumulative regulation effect of water reservoirs and river channels from the upper to the lower Yellow River basin. Increasing intensity of human activities from the upper to the lower Yellow River basin may also be responsible for the increasing crossover frequency (i.e. 1/f). MF-DFA analysis results indicate that the runoff variations of the Yellow River basin are characterized by shortterm memory. h(2) values indicate different scaling properties of the hydrological processes in the different reaches of the Yellow River basin. Generally, similar scaling properties can be identified in the runoff fluctuations in the upper and lower Yellow River basin. And the runoff variations in the middle Yellow River basin demonstrate different scaling properties, particularly the hydrological processes of the Longmen station. The h(2) value of the runoff variations of the Longmen station is far less than 3, being distinctly different from those of the runoff series of the other hydrological stations, particularly of the stations in the upper and the lower Yellow River basin. This point is in agreement with the result obtained by spectra analysis technique. Different scaling properties are identified at the different time scales and in the different reaches of the Yellow River basin, indicating the different and uneven features of topography, drainage sizes, land-use patterns, hydrogeology and drainage network morphology, different intensity of human exploitation of water resources in the different river reaches of the Yellow River. The mechanisms for runoff fluctuations are different from one rive to another in the world. The Mary River in Australia is dry in summer and the Gaula River in Norway is frozen in winter (Bunde et al. 2006). Therefore no universal scaling behaviors exist in the runoff series of the rivers over the globe, which is different from the climate data with universal long-term persistence of observed temperature variations (e.g. Eichner et al. 2003). Similarly, the Yellow River is of large drainage area and is featured by complicated and extremely uneven spatial and temporal distribution of precipitation variability, evaporation change, and also different intensity of human influences on runoff fluctuations. In the Yellow River basin, intensifying human activities heavily influence the hydrological processes via increasing human withdrawal of 123 1110 freshwater, construction of water reservoir (see Fig. 1 for the locations of the water reservoirs in the Yellow River basin). The influence intensity and even the ways in which the above-mentioned factors impact the hydrological series of the Yellow River basin may decide the scaling properties of the hydrological series of the study region. In the headwater region of the Yellow River basin, more influences of the climatic changes are exerted on hydrological processes. In the middle and lower Yellow River basin, influences of human activities tend to be increasing. This is why the scaling properties of runoff variations of the two stations in the upper Yellow River basin are different from those in the middle and the Yellow River basin. Furthermore, booming agriculture in the middle Yellow River basin leads to more human withdrawal of freshwater than in the upper and the lower Yellow River basin, giving rise to different scaling properties of runoff series in the middle Yellow River when compared to those in the upper and the lower Yellow River basin. Our study also imply impacts of regulations of water reservoir on scaling properties of the runoff series, i.e. regulation of water reservoir seems to cause smaller h(2) value. This point can be well proved by the locations of the stations and water reservoirs and also the changes of h(2) values (Figs. 1, 4). Even so, crossover analysis indicates that annual cycles of the runoff series are kept unchanged but periods of tens of days are different in different reaches of the Yellow River basin. Thus, we can tentatively conclude that human activities, particularly human withdrawal of freshwater, water reservoir, can modify the shorter periods of runoff variations. However, long periods, e.g. 1 year, are mainly controlled by climatic variations. The result of this study will be of scientific and practical merits in regional hydrological modeling and basin-scale water resource management in the arid and semi-arid regions of China. Acknowledgments The research was financially supported by the innovative project from Nanjing Institute of Geography and Limnology, CAS (Grant No.: CXNIGLAS200814; 08SL141001), National Natural Science Foundation of China (Grant No.: 40701015), National Scientific and Technological Support Program (Grant No.: 2007BAC03A0604), and by the Outstanding Overseas Chinese Scholars Fund from CAS (The Chinese Academy of Sciences). Thanks should be given to Yellow River Conservancy Commission for providing hydrological data. 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