# Changes of Flow and Sediment Transport in the Lower Min River in Southeastern China under the Impacts of Climate Variability and Human Activities

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Data and Methods

#### 2.1. Introduction about the Min River Basin

^{2}. It originates from the Wuyi mountain range, flows from the west to the east, and pours into the East China Sea. The basin is mostly mountainous (Figure 1), and the average slope of the river is 5‰. The basin is dominated by the subtropical monsoon climate zone with a warm and humid climate. The average annual temperature is about 17–19 °C. The average annual precipitation is about 1600–1700 mm, declining gradually from upstream to downstream. The period during April to September is the flood season, and the runoff takes up about 75% of the total annual runoff. The maximum monthly runoff occurs in June when the main flood season in the river basin starts. From October to March of the following year, it is a low-flow season, and the average annual runoff reaches the minimum in January. The lower reaches of the Min River are often influenced by heavy typhoon rains from July to September, causing serious floods with high flood peaks and large water volume.

^{3}annually on average in the tributaries of Min River.

^{3}and an effective storage capacity of 700 million m

^{3}. The drainage area above the dam site is 52,438 km

^{2}, accounting for 86% of the total basin area. The reservoir is mainly used for hydroelectricity, also used for flood control and navigation.

#### 2.2. Data Used

- (1)
- Daily discharge data at Zhuqi station during 1950–2017. Zhuqi is the most important station along the lower reaches, located at about 45 km below the Shuikou Dam. It controls a drainage area of about 54,500 km
^{2}. The observation of water level and river discharge at Zhuqi started in 1950; - (2)
- Daily water level data at Xiapu, Zhuqi, Wenshanli, Liberation Bridge, and Baiyantan from 1950 to 2017 (except Xiapu, which started in 1993);
- (3)
- Daily suspended sediment concentration data at Zhuqi during 1952–2017;
- (4)
- Daily precipitation data at 19 precipitation gauging sites during 1950 (or 1951 or 1952) to 2017. The daily precipitation data are converted to the areal precipitation in Min River basin by the Thiesson polygon method.

#### 2.3. Methods

#### 2.3.1. Indicators of Flow Regime Changes

- The mean annual discharges (MAD)
- The annual coefficient of variation of daily discharges (CV)
- The annual minimum 7-day average discharge (Min7d)
- The annual maximum 1-day average discharge (Max1d)
- Julian date of each annual 1-day maximum (Dmax1d)
- Occurrence day of each annual 7-day minimum starting from August 1 (Dmin7d)
- The number of days with the discharge below the 15% percentile (N15p)
- The number of days with the discharge exceeding the 90% percentile (N90p)
- The mean annual concentration of suspended sediment (MSSC)
- The annual total load of suspended sediment (TSSL)

#### 2.3.2. Methods of Change Detection

- (1)
- Exploratory change analysis

- (2)
- Mann-Kendall trend test

- (3)
- Pettitt test for change-point detection

_{t,T}is equivalent to a Mann–Whitney statistic for testing that the two samples X

_{1}, …, X

_{t}and X

_{t}

_{+1},..., X

_{T}come from the same population, given by:

_{T}is approximately given by:

_{T}, provided that the statistic K

_{T}is significant at a given significance level (e.g., α = 0.05).

#### 2.3.3. Quantification of the Attribution of Sediment Discharge Change

_{S}can be expressed in an elasticity form as (Zhang et al., 2019) [19]:

_{j}is the average daily discharge in class j (j = 1, …, n), which is one of the n non-overlapping discharge classes with equal length of 1/n of the range of streamflow variation in log-space; p

_{j}

_{,1}and p

_{j}

_{,2}are the occurrence probability of discharge q

_{j}in two periods (i.e., period 1 and period 2) when the C~Q relationship changed; C

_{j}

_{,1}and C

_{j}

_{,2}are the sediment concentrations for discharge p

_{j}in period 1 and period 2, respectively; $\overline{{Q}_{S}}$ is the average daily SSL; $\overline{{Q}_{1}}$, $\overline{{Q}_{2}}$, and $\overline{Q}$ denote mean values of daily discharge in period 1, period 2, and the whole period, respectively; $\overline{{C}_{1}}$, $\overline{{C}_{2}}$, and $\overline{C}$ denote mean values of sediment concentration in period 1, period 2, and the whole period, respectively.

_{0}) and catchment property (m) is given by (Michael and Farquhar, 2011) [20]:

_{0}is estimated using the FAO-56 method [22]. By combing Equations (10) and (11), we can estimate m by solving the nonlinear equation.

_{S}change given as

_{0}, respectively, and ${\eta}_{m}=-{\eta}_{Q}\left[\frac{m}{Q}\frac{\partial E}{\partial m}\right]$, denoting the elasticity of SSL to streamflow caused by changes of catchment property (such as topography, soil type/depth, geologic substrate, land cover, etc.). Here, ${\eta}_{P}$ and ${\eta}_{{E}_{0}}$ represent the effects of climate variability on SSL change, while ${\eta}_{m}$ and ${\eta}_{C}$ represent mostly the effects of human activities.

## 3. Alteration of Streamflow and Sediment Process in the Lower Reaches of the Min River

#### 3.1. Changes of Streamflow Process

^{3}/s during the whole 68 years, 1685 m

^{3}/s during 1950–1992, and 1775 m

^{3}/s during 1993–2017. Figure 2a shows no obvious overall trend during the whole 68 years. In comparison, the annual coefficient of variation (CV) of daily discharges exhibits a significant downward trend, as shown in Figure 2b.

^{3}/s) (N15p), and the number of days with the discharge exceeding the 90% percentile (~3470 m

^{3}/s) (N90p) are displayed in Figure 3. By visual inspection, we find an obvious positive trend in Min7d and an obvious negative trend in Max1d, at the same time, both N15p and N90p decreased. While other sequences showed no obvious changing trend.

#### 3.2. Variation in Sediment Transport

^{3}during the period from 1952 to 1992 with a maximum of 0.261 kg/m

^{3}in 1962 and the minimum of 0.065 kg/m

^{3}in 1991, and the average suspended sediment transport capacity was 7.15 million tons every year during 1951 to 1992. After the impoundment of Shuikou Dam in 1993, the average suspended sediment concentration at Zhuqi was 0.038 kg/m

^{3}during 1993–2017, with a maximum of 0.136 kg/m

^{3}in 2010 and a minimum of less than 0.007 kg/m

^{3}in 2008, and the average annual suspended sediment transport decreased to 2.48 million tons during 1993 to 2017. In comparison with the average of 7.15 million tons every year during 1951 to 1992, about 4.67 million tons (about 65%) suspended sediment was deposited in all the reservoirs above Zhuqi every year. By comparing the average of 6.06 million tons of sediment transport during the more recent period from 1983 to 1992 with the 4.67 million tons of sediment after 1993 when Shuikou Reservoir was built, it is estimated that about 3.58 million tons of suspended sediment, which would deposit in the lower Min River channel without the effect of reservoirs, was deposited in the Shuikou reservoir every year.

^{3}/s, and the ratio of bedload to suspended load is about 4.3–12.6% with an average of 7.5%. As the average suspended sediment transport was 6.06 million tons at Zhuqi during 1983–1992 before the construction of Shuikou Reservoir, the average bedload sediment transport was about 0.455 (≈6.06 * 7.5%) million t. Xu et al. (2012) [23] estimated that the average annual bedload sediment below the Three Gorges Dam during 2003~2010 after the impoundment of the reservoir in 2003 decreased by 93% in comparison with that during 1991–2002. As the Shuikou dam controls 96.2% of the total drainage area above Zhuqi, it is estimated that after the completion of Shuikou reservoir, about 0.41 million tons (≈ 7.5% * 93% * 96.2% * 6.06 million t) of bedload sediment was deposited in the Shuikou reservoir every year due to the reservoir’s retention. Therefore, after the completion of Shuikou reservoir, an average of 3.99 million t of sediment (including 3.58 million t of suspended load and 0.41 million t of bedload) was deposited in the reservoir area every year compared with the 10 years before the completion. Assuming a sediment density of 1300 kg/m

^{3}, the Shuikou dam area loses about 3.07 million m

^{3}of its capacity per year due to sediment deposition, which means the reservoir loses about 1.2% of its initial capacity (2.6 billion m

^{3}) every 10 years.

^{2}) in the Min River Basin, started its cofferdam construction.

#### 3.3. Changes of Water Level and Riverbed along the Lower Min River below Shuikou Dam

^{3}/s, the average water level was 8.28, 8.3, 7.73, 6.93, 3.73, and 3.60 m in 1950, 1970, 1990, 2000, 2010, and 2017, respectively, that is, the average water level dropped 4.68 m in 68 years and such a drop mostly occurred in the last 30 years.

## 4. Contributing Factors of Sediment Reduction

#### 4.1. Precipitation

#### 4.2. Vegetation

#### 4.3. Dam Construction

^{3}as shown in Figure 1. Figure 16 shows the significant increase in the total reservoir capacity of the reservoirs in the Min River basin during 1954 to 2011.

#### 4.4. Sand Mining

#### 4.5. Relative Attribution of Sediment Discharge Changes

_{Q}and η

_{C}for the Min River are computed using Equations (7) and (8) for different number of discharge classes (n), and the results are plotted in Figure 17. For n approximately greater than 800, the variations of η

_{Q}and η

_{C}are generally stable, and we take η

_{Q}= 0.95 and η

_{C}= 1.09 at n = 1000 as the representative values of η

_{Q}and η

_{C}.

## 5. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Conflicts of Interest

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**Figure 1.**The elevation of the Min River basin and locations of major reservoirs and hydrologic gauging sites.

**Figure 2.**Mean annual discharge (

**a**) and annual coefficient of variation (CV) of daily discharge (

**b**) observed at Zhuqi during 1950–2017.

**Figure 3.**Changes of low flows and flood flows observed at Zhuqi during 1950 to 2017. (

**a**) The annual series of minimum 7-day average discharge (Min7d); (

**b**) minimum 7-day average flow occurrence time (Dmin7d); (

**c**) the number of days with the discharge below the 15% percentile (~510 m

^{3}/s) (N15p); (

**d**) maximum 1-day average discharge (Max1d); (

**e**) maximum 1-day average flow occurrence time (Dmax1d); (

**f**) the number of days with the discharge exceeding the 90% percentile (~3470 m

^{3}/s) (N90p).

**Figure 4.**(

**a**) Variation of areal precipitation above the drainage area above Zhuqi and (

**b**) average daily discharge observed at Zhuqi during three different periods.

**Figure 5.**(

**a**) Annual average suspended sediment concentration and (

**b**) annual suspended sediment load observed at Zhuqi during 1952–2017.

**Figure 6.**The double-mass curve of (

**a**) cumulative annual runoff versus cumulative annual suspended sediment load and (

**b**) cumulative runoff in June versus cumulative sediment load in June at Zhuqi.

**Figure 7.**The sediment rating curves between suspended sediment concentration (C) and streamflow discharge with 300 discharge classes during the period 1952–1992 (Period-I) and the period 1994–2018 (Period-II) observed at Zhuqi in the Min River.

**Figure 9.**Comparison of thalwegs of the lower Min River (start from the Shuikou Dam to the river mouth) in 2011 and 2015.

**Figure 13.**The correlation between annual areal precipitation and annual runoff at Zhuqi in the period before and after 1993.

**Figure 14.**Variation of the maximum 1-day precipitation and the total erosive precipitation of Yangkou and Zhuqi. (

**a**) Maximum 1-day precipitation of Yangkou, (

**b**) total erosive precipitation of Yangkou, (

**c**) maximum 1-day precipitation of Zhuqi, and (

**d**) total erosive precipitation of Zhuqi.

**Figure 15.**Changes of the annual areal average NDVI (

**a**) and the trend of monthly NDVI (

**b**) over the Min River basin during 1981 to 2015. (Note: the number +3, +2, +1, and 0 stand for very significant trend, significant trend, weak trend, and no trend, respectively, and the sign (+ or −) stands for negative or positive trend.).

**Figure 17.**Variation of elasticity indices η

_{Q}and η

_{C}for the Min River vary with increasing n.

τ | p-Value | Class | Trend Type |
---|---|---|---|

τ > 0 | p < 0.01 | +3 | Very significant increase |

0.01 ≤ p < 0.05 | +2 | Significant increase | |

0.05 < p ≤ 0.1 | +1 | Slight increase | |

τ = 0 | 0.1 < p | 0 | No trend |

τ < 0 | 0.05 < p ≤ 0.1 | −1 | Slight decrease |

0.01 ≤ p < 0.05 | −2 | Significant decrease | |

p < 0.01 | −3 | Very significant decrease |

Data Type | Index | Trend | Year of Change |
---|---|---|---|

Flow | MAD | 0 | – |

CV | − ** | 1984 ** | |

Min7d | + ** | 1981 ** | |

Dmin7d) | 0 | – | |

N15p | − ** | 1980 ** | |

Max1d | 0 | – | |

Dmax1d | 0 | – | |

N90p | − ** | 1984 ** | |

Sediment | MSSC | − ** | 1993 ** |

TSSL | − ** | 1993 ** |

**Table 3.**Changes of areal annual precipitation (P), potential evaporation (E

_{0}), and catchment property (m) in the drainage area above Zhuqi, and annual average streamflow discharge (Q), average suspended sediment yield (Q

_{S}), and suspended sediment concentration (C) observed at Zhuqi during period-1 (1951–1992) and period-2 (1993–2017).

P (mm/year) | E_{0} (mm/year) | Q (mm/year) | Q_{S} (t/km^{2}/year) | C (kg/m^{3}) | m (−) | ||||||
---|---|---|---|---|---|---|---|---|---|---|---|

P_{1} | ΔP | E_{0,1} | ΔE_{0} | Q_{1} | ΔQ | Q_{S}_{,1} | ΔQ_{S} | C_{1} | ΔC | m_{1} | Δm |

1653.7 | 50.6 | 1087.9 | −5.4 | 973.5 | +60.3 | 131.2 | −85.7 | 0.129 | −0.091 | 1.056 | −0.037 |

**Table 4.**Elasticity of suspended sediment load (Qs) and contribution to Qs reduction with respect to precipitation (P), potential evapotranspiration (E0), sediment concentration (C), and catchment property (m).

Elasticity of Q_{s} | Contribution to Q_{s} Reduction (%) | |||||||
---|---|---|---|---|---|---|---|---|

${\mathit{\eta}}_{\mathit{Q}}$ | ${\mathit{\eta}}_{\mathit{P}}$ | ${\mathit{\eta}}_{\mathit{E}\mathbf{0}}$ | ${\mathit{\eta}}_{\mathit{m}}$ | ${\mathit{\eta}}_{\mathit{C}}$ | $\mathit{\epsilon}$_{s,P} | $\mathit{\epsilon}$_{s,E}_{0} | $\mathit{\epsilon}$_{s,m} | $\mathit{\epsilon}$_{s,C} |

0.95 | 1.34 | −0.39 | −0.41 | 1.09 | −3.6 | −0.2 | −1.3 | 105.1 |

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**MDPI and ACS Style**

Wang, W.; Wang, T.; Cui, W.; Yao, Y.; Ma, F.; Chen, B.; Wu, J.
Changes of Flow and Sediment Transport in the Lower Min River in Southeastern China under the Impacts of Climate Variability and Human Activities. *Water* **2021**, *13*, 673.
https://doi.org/10.3390/w13050673

**AMA Style**

Wang W, Wang T, Cui W, Yao Y, Ma F, Chen B, Wu J.
Changes of Flow and Sediment Transport in the Lower Min River in Southeastern China under the Impacts of Climate Variability and Human Activities. *Water*. 2021; 13(5):673.
https://doi.org/10.3390/w13050673

**Chicago/Turabian Style**

Wang, Wen, Tianyue Wang, Wei Cui, Ying Yao, Fuming Ma, Benyue Chen, and Jing Wu.
2021. "Changes of Flow and Sediment Transport in the Lower Min River in Southeastern China under the Impacts of Climate Variability and Human Activities" *Water* 13, no. 5: 673.
https://doi.org/10.3390/w13050673