The variation and attribution analysis of the runoff and sediment in the lower reach of the Yellow River during the past 60 years

The water and sediment regimes of the Yellow River are the basis of decision-making of major projects of the Yellow River. Based on the water and sediment data at the Huayuankou station, Gaocun station, Aishan station, Lijin station in the lower reach of the Yellow River, the Mann-Kendall test, the T-test for differences, wavelet analysis, slope change ratio method and the double cumulative curve method were applied to analyze the runoff and sediment regimes alteration. The results show that the water and sediment of the lower Yellow River have a signi ﬁ cant downward trend, and the annual sediment decreases signi ﬁ cantly compared with the annual runoff. The annual runoff and sediment of the four hydrological stations changed around the 1980 and 1990s, respectively. The water and sediment of hydrological stations have periodic variations on multiple time scales, but the variation scales are different. Precipitation, human activities and other factors lead to the decrease trend of water and sediment in the lower Yellow River, and their contribution rates to the change of water and sediment are also different. Precipitation contributed 0.15% – 8.71% and 0.06% – 22.32% to the reduction of runoff and sediment load at hydrological stations, while human activities contributed 91.29% – 99.85% and 77.68% – 102.21% to the reduction of runoff and sediment load, respectively. Human activity is the main factor of runoff and sediment reduction. Yellow


INTRODUCTION
Rivers are the indispensable and important resource for human survival and development. For most rivers, the impacts of climate change and human activities on water and sediment processes are particularly significant. The change of land use, large-scale drainage and overgrazing have reduced the water holding capacity of the basin, resulting in an increase in the frequency and scale of downstream floods in the wet season, and a serious decrease in river water volume in the dry season (Leopold ). The construction and storage of reservoirs obviously changes the hydrological process of rivers, which is the human activity that has the largest impact on rivers and the most far-reaching disturbance from rivers (Petts ) The Yellow River is the second longest river in China. The complex and intractable crux of the Yellow River lies in the nature of less water and more sand and the inharmonious relationship between water and sand (Wang ). With the superposition of natural factors and human activities, the amount of water and sediment in the basin shows an obvious trend of fluctuation, and the historical water and sediment conditions in the lower reaches of the Yellow River are completely different from those in modern times. In the new period, the relationship between water and sediment in the Yellow River has changed, and the sediment inflow in the lower reaches of the Yellow River has decreased sharply (Hu & Zhang ). The serious shrinkage of the lower reaches of the Yellow River, the reduction of flood discharge and sediment load capacity, etc., seriously threatens the flood control safety of the basin and seriously restricts the sustainable development of regional economy and society.
The evolution of water and sediment in the Yellow River basin has always been a concern of human beings, and it is also a focus and difficult problem in the Yellow River manage- How much did each factor contribute to the change? Therefore, the purpose of this study is (a) to statistically detect the trend, change-points and periodic relationship of the annual runoff and sediment discharge of the four main hydrological stations in the lower Yellow River; (b) to analyze of the contribution of climate change and human activities to runoff and sediment in the lower Reaches of the Yellow River basin; The study will provide scientific guidance for the management of the Yellow River basin.

STUDY AREA AND DATA
The Yellow River is known as the cradle of Chinese civilization and is a world-famous river with high sand content. The Yellow River Basin is located between 95 53 0 to 119 05 0 E longitude and 32 10 0 to 41 50 0 N latitude. It has a continental climate with a multi-year average annual precipitation of 446 mm. The basin covers an area of 752,443 km 2 , showing a trend of 'high in the west and low in the east', descending gradually from west to east. The western section of the Yellow River is located on the Qinghai-Tibet Plateau with an altitude of more than 4,000 m. The middle section is dominated by the Loess Plateau, with an altitude of 1,000 to 2,000 m. The eastern section is located in the North China Great Plain, with the main landforms being plains.
The total area of the Yellow River Basin is 795,000 square kilometers, the main stream is 5,464 kilometers in length, and the water drop is 4,480 meters. The main stream has a variable curvature and uneven distribution of tributaries. There are 76 first-level tributaries with a drainage area greater than 1,000 km 2 , among which 13 are the first-level tributaries with a drainage area greater than 10,000 km 2 or with a sediment inflow of more than 50 million tons.
According to the characteristics of the river, with Hekou Town and Taohuayu as the dividing point, the Yellow River is divided into three parts: the upper, middle and lower reaches. The section from Mengjin to Gaocun on the lower Yellow River is a typical wandering section. The section has the characteristics of wide and shallow water flow, unsteady mainstream swing, and drastic changes in river regime. During the operation of the Sanmenxia Reservoir for water storage and sediment interception, the amount of sand entering the downstream river channel was greatly reduced, resulting in strong erosion in the lower Yellow River. During this period, the lower reaches of the Yellow River scoured a total of 2.14 billion m 3 of sediment, of which the total scouring of the upper reaches of Gaocun (1.52 billion m 3 ) accounted for 71% of the total erosion of the lower channel. After the operation of the Xiaolangdi Reservoir, the amount of sediment entering the lower reaches of the Yellow River has been greatly reduced, and the riverbed in the lower reaches of the river has been scoured severely.
In order to analyze the temporal changes of runoff and sediment conditions, four major hydrological stations in the lower reaches of the Yellow River, Huayuankou station, Gaocun station, Aishan station, Lijin station, were selected as the case study locations (Figure 1). Hydrological data (annual runoff and sediment load) from the Yellow River Yearbook and China River Sediment Bulletin, as well as annual rainfall series from China Meteorological Data Network  were used in this study. Three missing rainfall data were supplemented by linear interpolation method.

Mann-Kendall trend test
The Mann-Kendall test is the rank-based nonparametric test for assessing the significance of a trend and has been widely used in hydrological trend detection studies. The Mann-Kendall test statistic (S) is calculated as follows: (2) where x i and x j are the date values of x in years i and j, and n indicates the length of the date values. When n > 40, the statistic S is approximately normally distributed with the mean, and the variance is given by the following: Based on S and Var, the standardized Mann-Kendall statistics Z is computed as follows: The standardized Mann-Kendall statistic Z follows the standard normal distribution with a mean of zero and variance of one. In the test for a trend, if jZj > Z α=2 , where Z is asymptotically normally distributed and Z α=2 is the critical value of the standard normal distribution with a probability α=2, the trend of the sequence will be significant. A positive value of Z denotes an increasing trend, and the opposite corresponds to a decreasing trend.
The ability of M-K test to detect trends can be influenced by autocorrelation in the data. Given this, the 'trend-free pre-

Mann-Kendall abrupt change test
The Mann-Kendall test was also applied to determine the abrupt changing year for time series. The statistic S k of the abrupt change test was given by: Calculate the parameter UF using the following The Mann-Kendall mutation test calculates the normalized variable UF of time series data and compares it with a critical variable at a certain confidence level α (taken as 0.01). When UF is greater than 0, it indicates an upward trend, and when it is less than 0, it indicates a downward trend; When UF exceeds a critical value, it indicates a significant upward or downward trend. At the same time, the same statistical calculation is performed on the inverse sequence of the original time series, so that UB ¼ ÀUF. If the two curves appear at the intersection point within the 99% confidence level, it indicates that the mutation occurred at this time point.

T-test for differences
Define the sequence mutation index of the sample length N as AI, and use the method of continuously moving the base year to calculate the time series of mutation index AI. S P is the joint sample variance. The statistic t follows the t distribution with degrees of freedom M 1 þ M 2 À 2. When a certain significant level α is given, such as t < t α , at the significant level of α, the mean values of M 1 and M 2 on both sides of the reference total there are obvious differences, that is, a sudden change occurs at the reference point. The calculation formula is given by: Among them: X 1P are the average and standard deviation of M 1 years before the baseline; X 2P and S 2 are the mean and standard deviation of the M 2 years after the baseline. M 1 and M 2 are the sample lengths of the two sequences before and after the benchmark.

Wavelet analysis
The complex Morlet wavelet was used to analyze the periodicity and variation tendency in runoff and sediment load at the four stations. The continuous wavelet transform (CWT) is defined as the sum over time of the real signal, f(t), multiplied by the scaled (stretched or compressed) shifted versions of the wavelet function, ψ(t) as follows: where the wavelet coefficients, W, are the result of the CWT of signal f(t). The function, ψ(t), can be real or complex, playing the role of a convolution-kernel. The scale or dilation parameter, a, scales a function by compressing or stretching it, whereas b is the translation of the wavelet function along the time axis.
In this study, the complex Morlet wavelet function is applied to distinguish temporal runoff and sediment load oscillations. Using the wavelet transform, wavelet coefficients and their variances are calculated. Wavelet power spectra and multiscale periodicity features are obtained with wavelet coefficients.

Slope change ratio
The change rate of cumulative slope is used to quantitatively assess the contribution of human activities and climate change to river runoff and sediment. The sum of all the influencing factors of the variable was defined as 1, and the influence degree on the variable was calculated according to the ratio of the cumulative slope of various influencing factors over time to the change rate of the cumulative slope of the variable. Assuming that the cumulative runoff changes over time and there is an inflection point in a given year, the slopes of the variables before and after the inflection point are Y Rb and Y Ra respectively, and the slopes of the cumulative precipitation before and after the inflection point are Y Pb an Y Pa respectively. The formula of K p , K R , C R , C H are given as:

Analysis of variation characteristics
Trend analysis of the runoff and sediment The annual runoff and sediment load of each hydrological station is shown in Figure 2. During this period, generally speaking, the annual runoff and sediment load show a downward trend. In

Abrupt changes analysis of the runoff and sediment
The   After entering the 20th century, with the increase of human activities, the capacity of sediment load further decreased.
The Mann-Kendall non-parametric test may have multiple mutation points or low mutation credibility in the test process, so the T-test of mean difference was used to verify the mutation results. In this paper, the critical value is t ¼ 2.704 and the significance level 0.01, and the statistical   19861996AS 1980, 1981, 19851997LJ 19791995 Table 5. According to the characteristics of hydrological abrupt transitions, the relationship curves of rainfall and runoff, rainfall and sediment load accumulation before and after abrupt transitions of the four stations were drawn as shown in Figure 6. The determination coefficient R of the fitting relation in each stage is above 0.95, and the fitting degree is good.

Natural factors
The contribution rate of precipitation and human activities to the variation of water and sediment volume of the four hydrological stations is calculated quantitatively by the cumulative slope change rate method. See Tables 6 and 7.
The influence of rainfall on the yield of water and sediment mainly lies in the size and amount of rainfall in the region.
When the rainfall is too much, it will even lead to the damage of water conservancy and water conservation projects with low standards and cause the change of water and sediment quantity. In recent years, the magnitude and frequency of heavy rain and heavy rain have decreased. Compared with the period of T b and T a , the contribution of precipitation to the reduction of runoff volume of the four hydrological stations is 3.89%, 5.52%, 8.71% and 0.15% respectively, and the contribution of precipitation to the reduction of sediment volume is À2.21%, À0.06%, 22.32% and À0.04% respectively. It can be concluded that the change of water and sediment quantity mainly depends on the influence of human activities.   Table 8.
To sum up, since the abrupt change of annual runoff in the lower Yellow River, the sediment load volume of all hydrological stations has decreased significantly and the sediment reduction volume has decreased along the river.
Due to the impact of human activities, the sediment load volume of each hydrological station decreased by about 381 million t, 269 million t, 224 million t and 67.2 million t    1960-19861987-20191960-20002001-2019GC 1960-19861987-20191960-19961997-2019AS 1960-19851986-20191960-19971998-2019LJ 1960-19791980-20191960-19951996-2019 respectively every year, which is closely At the end of 1970s, the key method to reduce sediment flow into the Yellow River was defined as strengthening     (Li ). Therefore, the irrigation project along the Yellow River is another main factor that causes the change of water and sediment.

CONCLUSION
Based on the hydrological data of four hydrological stations in the lower Reaches of the Yellow River, a variety of statistical methods were used to analyze the variation trend, abrupt year, period, influencing factors and their contribution rates to the variation of runoff and sediment volume. The major findings can be summarized as follows: (1) With the passage of time, the annual variation of runoff and sediment discharge of the Yellow River from 1960 to 2019 showed an obvious decreasing trend, and the decreasing trend of annual sediment discharge was more obvious than that of annual runoff. (2) There were abrupt changes in runoff and sediment load at all hydrological stations, among which the abrupt changes in runoff occurred in the 1980s and in the 1990s.
(3) There are multi-time scale periodic changes in the watersediment series of hydrological stations, and the cycle changes of runoff are the same, while the variation scales of sediment discharge are slightly different.
(4) Since the 1980s, the abrupt change of annual runoff and sediment is caused by the combined action of precipitation and human activities, and human activities are the main reason for the change of runoff and sediment in the lower reaches of the Yellow River.