Analysis of runoff variation and driving factors in the Minjiang River Basin over the last 60 years

The distribution of water resources in time and space is widely affected by climate change and human activities (Ngongondo et al. 2011; Kale et al. 2016; Xu et al. 2018; Bombelli et al. 2021). Precipitation and temperature are vital climate variables proven to affect the hydrological environment (Nadal-Romero et al. 2008; Zhang et al. 2011; Li et al. 2016). Precipitation and evaporation are the main meteorological factors affecting watershed runoff (Cheng et al. 2011; Chang et al. 2016; Wu et al. 2020; Nazeer et al. 2022). Human activities (e.g., land use/cover variations, hydroelectric exploitation, afforestation, urbanization, water use, etc.) have obvious effects on the water cycle, resulting in great changes in the spatial-temporal distribution of water resources (Xu et al. 2008; Guo et al. 2018a, 2018b; Korkanç 2018; Lu et al. 2020). Many researchers have conducted studies on the impact of climate and human activities on runoff and employed various methods to distinguish the contributions of climate change and human activities to runoff changes (Cui et al. 2011; Maalim et al. 2013; Ejder et al. 2016; Xu et al. 2020). Trends in runoff, temperature, and precipitation in the Qom Rood Watershed in Iran during 1979–2016 were studied at monthly and annual time scales using the Mann-Kendall test, as well as the double-mass curve and a multiple regression model (Callow & Smettem 2009; Arslan et al. 2020; Yang et al. 2022). According to Yaghmaei et al. (2018), the results proved that natural driving forces showed no effect on runoff changes, and the observed decreasing tendency in runoff resulted from dam operations. Wu et al. (2020) used integrated methods of hydrological modelling and numerical analysis to show that runoff characteristics responded to climatic variations and hydroelectric exploitation in the Jinsha River Basin. Additionally, Chang et al. (2016) reported that climate changes and human activities accounted for ∼20 and ∼80%, respectively, of runoff changes. Hydroelectric exploitation has a major effect on the hydrological environment (Zhai et al. 2010; Tuset et al. 2016).

The Minjiang River is the fifth largest river in China. It is also the largest tributary of the Yangtze River and therefore generates important considerations and concerns regarding runoff. Located in the upper reaches of the Yangtze River, it plays an important role in the ecological security of the Yangtze River Basin. Additionally, the Minjiang River is one of the tributaries with the most fragile ecosystem, the most frequent human activities, and the largest engineering development intensity in the Yangtze River Basin. At present, the Minjiang River Basin (MRB) is facing prominent ecological and environmental problems, such as water reduction or even river cutoff, water pollution, biodiversity reduction, frequent geological disasters, and so on (Guo et al. 2018a, 2018b; Fan et al. 2021). Due to overdevelopment of water diversion along the Minjiang River (especially in the upper reaches), runoff of the Jinma River (a tributary of the Minjiang River) has been seriously reduced, and even river cutoff has occurred, potentially causing damage to the ecological environment of the MRB, as well as loss of fish and other resources (Wei et al. 2021). With the operation of the Zipingpu Hydropower Station on the Minjiang River and strict environmental management, both water reduction and river cutoff have been effectively alleviated (Liu et al. 2020).

The middle and lower reaches of the Minjiang River are the most economically and socially developed areas in Sichuan Province, especially in the Chengdu Plain where many large-scale irrigation areas are located for grain production (e.g., Dujiangyan and Tongjiyan). A large amount of water consumption in these areas has a strong impact on the distribution of water resources in the MRB. The water resources of the Minjiang River are not as rich as those of the Dadu River, however, for economic and social development, there is an extremely large water demand, resulting in a great disparity between water supply and water demand. The development rate of water resources in the MRB is high, and more reservoirs and dams are planned along the mainstream portions of the Minjiang River, the Dadu River, and other tributaries. According to comprehensive planning for the MRB, the runoff change of the entire basin will be regulated by hydropower and water diversion projects in the future and will have a great influence on the runoff of the Yangtze River. Since the implementation of the Yangtze River Protection Law, ecological and environmental protection in the Yangtze River Basin has attracted much more attention.

At present, many researchers have studied runoff changes in the Yangtze River (Zhang et al. 2006; Yan et al. 2011; Naveed et al. 2022). Some researchers have also studied runoff changes in the upper reaches of the MRB. Several studies have confirmed a downward trend in annual flow and sediment discharge for the Yangtze River over the past decades (Liu et al. 2007; Yan et al. 2011). Hou et al. (2018) demonstrated that the effects of human activities and climate change on runoff variation accounted for ∼77 and 23%, respectively, of annual runoff variation in the upstream portion of the MRB. Zhang et al. (2020) reported that the contributions of climate variation and land use/land cover change to runoff depth were at the ratio of 5:1 in the upper reaches of the MRB.

Some researchers have conducted studies on runoff changes at the Zhenjiangguan (ZJG) station in the upstream region of the Minjiang River, revealing the contributions of climate and forest vegetation to the runoff change (Nie et al. 2020). The results provided a good reference for our research. However, the runoff variation of the entire river basin has always been ignored. Economic and social development along the Minjiang River is highly developed, and the Dadu River is an important hydropower development base in China. Consequently, their impacts on the runoff change of the MRB should not be ignored. Decision makers need to know the flow variation of the entire MRB and the roles of variables affecting runoff. Taking the Minjiang River and its main tributary (the Dadu River) as the research objects, the temporal and spatial runoff changes in the most recent 60-year period were studied. The specific objectives of the study were to: (1) determine the change rule of the temporal and spatial water resources in the MRB; (2) determine the variation characteristics of inter-annual and annual runoff; (3) discuss the impact of artificial disturbance, precipitation, and vegetation on runoff; and (4) provide technical support for ecological flow guarantees and ecological restoration of the MRB and the Yangtze River Basin.

The Minjiang River, with a total length of 735 km and an average annual flow of 3022 m³/s, is one of the primary tributaries on the north bank of the Yangtze River. The main tributaries of the MRB are the Dadu River and the Qingyi River. The Dadu River, with a length of 1062 km and an average annual flow of 1670 m³/s, is the largest tributary of the Minjiang River. There is a high degree of development and utilization in the MRB, and the current utilization ratio of water resources is approximately 11.4%. Historically, the disordered development of diversion power stations has seriously damaged the ecological environment. So far, 10 hydropower stations have been built along the Minjiang River. Except for the Zipingpu Reservoir hydropower station with seasonal regulation performance, other stations are diversion hydropower stations. According to official statistics, excessive diversion development in the upper reaches of the Minjiang River before 2005 led to intermittent river cutoff over approximately 80 km. In addition, 15 hydropower stations have been built along the Dadu River. The control point for the Dadu River is the Pubugou hydropower station. Zipingpu and Pubugou are control reservoirs, and have a total storage capacity of 1.112 billion m³ and 5.39 billion m³, respectively, and were put into operation in 2006 and 2010, respectively. In addition, there is another control project under construction named the Shuangjiangkou Reservoir which is an annual regulation reservoir.

Monthly streamflow at six hydrological stations was collected for analysis. Those stations were Zhenjiangguan (ZJG), Dujiangyan (DJY), Gaochang (GC) located on the Minjiang River, and Dajin (DJ), Luding (LD), and Shawan (SW) located on the Dadu River. ZJG is the first hydrological station on the upper reaches of the Minjiang River. DJY is located downstream of the Zipingpu Hydropower Station, which is the representative station in the middle reaches of the Minjiang River. GC is located at the end of the lower reaches of the Minjiang River and is the gateway station of the MRB. DJ is the hydrological station on the upper reaches of the Dadu River, LD is the hydrological station on the middle reaches of the Dadu River, and SW is the gateway station of the Dadu River. The monthly runoff data (1957–2016) at ZJG, DJY, GC, DJ, LD, and SW were provided by the Yangtze River Water Commission and the Hydrological Bureau of Sichuan Province. Irrigation area, afforestation area, population, water consumption, number of reservoirs, and total storage capacity of reservoirs were obtained from the Sichuan Provincial Bureau of Statistics. Annual precipitation data were obtained from the Sichuan Meteorological Bureau. Normalized difference vegetation index (NDVI) was derived from remote sensing interpretation (MODIS MOD13).

(1) Analysis of runoff variation

Eight indexes, including irrigation area, afforestation area, population, water consumption, number of reservoirs, the total storage capacity of reservoirs, NDVI, and annual precipitation, were selected as potential driving factors to analyze their influence on average annual runoff and runoff in the non-flood season at hydrological stations. We used Canoco 4.5 and SPSS software to conduct canonical correspondence analysis and regression analysis to study the influence of each driving factor on runoff change in the basin.

In general, the runoff of the Minjiang River increased from upstream to downstream. The average annual runoff values at ZJG and DJY were 1.7 and 14.4 billion m³, respectively, and the discharges were 55 and 464 m³/s, respectively. Additionally, the average annual runoff values at DJ, LD, and SW were 16.5, 27.5, and 46.1 billion m³, respectively, and the discharges were 522, 885, and 1,481 m³/s, respectively. The Dadu River is the largest tributary of the Minjiang River and accounts for approximately 55% of the total runoff of the Minjiang River. After the Dadu River feeds into the Minjiang River, the runoff at GC increases greatly, producing an average annual runoff of 84 billion m³ and discharge of 2693 m³/s.

The inclination rates of runoff variation at ZJG and DJY were −0.1955 and −0.5373, respectively, indicating that the runoff at ZJG did not change significantly, while the runoff at DJY showed a slight reduction. The inclination rates at ZJG in the flood and non-flood seasons were consistent with the inclination rate of the average annual variation, whereas the inclination rate of DJY in the flood and non-flood seasons was significantly greater than the inclination rate of the average annual variation, showing a downward trend in the flood season and an upward trend in the non-flood season.

The inclination rate of the average annual variation at LD was −0.0139, indicating that it has changed insignificantly over the past 60 years. The inclination rate at SW was 0.4197, showing a slight downward trend. The inclination rates at LD in the non-flood and flood seasons were consistent with that of average annual variation, whereas the inclination rates at SW in the flood and non-flood seasons were significantly greater than that of the average annual variation, indicating that the discharge at SW changed significantly in flood and non-flood seasons, showing a downward trend in flood seasons and an upward trend in non-flood seasons.

After the Dadu River drains into the Minjiang River, the runoff at GC was observed to change with an inclination rate of −6.3652 over the past 60 years, showing an obvious downward trend. Additionally, GC was affected by the runoff change in the upper and middle reaches of the Minjiang and Dadu Rivers. The inclination rates of runoff variation in the flood and non-flood seasons were significantly greater than that of average annual variation. In recent years, runoff in the flood season has decreased significantly and increased significantly in the non-flood season.

The discharge at SW in the non-flood season showed a slight downward trend before the 1990s and thereafter was seen to increase slowly. A large change was observed around 2010 when discharge at SW increased significantly.

The runoff change at GC in the non-flood season was affected by the superposition of runoff changes at DJY and SW and was similar to that at DJY and SW before 2010. There was a large change around 2010 when runoff in the non-flood season showed a significant upward trend.

The numbers 1–56 represent the years from 1961 to 2016. That is, the number 1 represents 1961, the number 2 represents 1962, and so on, and the number 56 represents 2016; A_ICL: irrigation area; A_AFFO: afforestation area; N_POPU: population; T_WSPU: water consumption; N_RESE: number of reservoirs; NDVI: normalized difference vegetation index; T_PREC: annual precipitation; T_SC: total reservoir capacity

From the distribution diagram of runoff samples and driving factors (Figure 5), it can be seen that the right part of the diagram mainly reflects the influence of human activities and vegetation types on runoff, while the left part reflects the combined influence of precipitation and human activities on runoff. The samples on the right side of the diagram are KF_GC, KF_SW, and KF_DJY (i.e., runoff samples at GC, SW, and DJY in the non-flood season). Runoff values were positively correlated with total reservoir capacity and the vegetation situation in the basin. Runoff at SW was most significantly affected by the total storage capacity, followed by runoff at GC, and finally by runoff at DJY. The Pubugou Reservoir is upstream of SW and is a control project on the Dadu River. The Zipingpu Reservoir is upstream of DJY and is a control project on the Minjiang River. GC is located downstream of the Zipingpu and Pubugou Reservoirs. Runoff in the non-flood season was also significantly affected by vegetation. On the left side of Figure 5, there are runoff samples in the non-flood season at ZJG and DJ, as well as the annual runoff samples at all stations in the basin. Because ZJG and DJ are located upstream of regulatory reservoirs, the runoff changes at the two stations were mainly affected by both precipitation and human activities.

F_ZJG: average annual runoff at ZJG; F_DJY: average annual runoff at DJY; F_GC: average annual runoff at GC; F_DJ: average annual runoff at DJ; F_LD: average annual runoff at LD; F_SW: average annual runoff at SW; KF_ZJG: runoff at ZJG in non-flood season; KF_DJY: runoff at DJY in non-flood season; KF_GC: runoff at GC in non-flood season; KF_DJ: runoff at DJ in non-flood season; KF_LD: runoff at LD in non-flood season; KF_SW: runoff at SW in non-flood season.

ZJG is located upstream of the Zipingpu Reservoir, so the operation of the reservoir has no impact on runoff, and annual runoff variation at ZJG was generally consistent in the two periods. In contrast, DJY (located downstream of the reservoir) was affected by its operation, so the annual runoff processes during the two periods showed obvious differences: runoff increased significantly in the non-flood season and decreased in the flood season. The average annual runoff at DJY in the non-flood season was approximately 46.64% higher in 1961–2007 than in 2008–2016. In the flood season, the average annual runoff was approximately 8.23% lower in 1961–2007 than in 2008–2016.

The average annual runoff at SW was influenced by the operation of the Pubugou Reservoir. Compared with 1961–2010, the average annual runoff at SW in 2011–2016 increased by approximately 26.21% in the non-flood season and decreased by approximately 15.20% in the flood season.

The average annual runoff at GC was jointly influenced by the operation of the Zipingpu and Pubugou Reservoirs. Compared with 1961–2010, the average annual runoff at GC in 2011–2016 increased by approximately 36.47% in the non-flood season and decreased by approximately 11.77% in the flood season.

The Shuangjiangkou Hydropower Station located upstream of the Dadu River Basin is a control project of cascade development, with a dam height of 314 m, and serves as an annual regulation reservoir.

At present, the Shuangjiangkou Hydropower Station is still under construction and will have a future significant effect on runoff changes in the Dadu and Minjiang Rivers.

Due to irrigation in the middle and lower reaches of the Minjiang River and economic and social development, the increasing water demand of these areas has resulted in a decrease in annual runoff at DJY and GC over the last 60 years. The water supply in the MRB will continue to increase due to economic and social development. According to comprehensive planning for the MRB, by 2030 the water supply of the Minjiang River will increase from 633,985 to 810,161 × 10⁴ m³, and the water supply of the Dadu River will increase from 72,350 to 113,943 × 10⁴ m³ (Figure 9). With the construction of the Tianfu New District and the expansion of the DJY irrigated area, the disparity between the water supply and demand around the Chengdu Plain has become increasingly prominent. At present, water diversion projects are planned to alleviate the disparity, including a water diversion project from the Dadu River to the Minjiang River. By 2030, the temporal and spatial water distribution in the basin will be further affected by the utilization of water resources.

Continuous investigation and monitoring of the ecological water environment in the MRB and scientifical evaluation of the impact of water conservancy and hydropower project development on the ecosystem (especially the adverse impact on important sensitive targets and protected objects) are conductive to optimizing the dispatching mode of cascade stations and subsequent hydropower development schemes.

Runoff variation will have an adverse impact on the aquatic ecosystem of the basin. Based on the identification of the impact of hydropower cascade development, combined with the need for aquatic ecological protection, cascade joint dispatching should be studied and implemented to effectively reduce the impact of human activities.

The development of the MRB has produced a great demand for water. The Dadu River is rich in water resources, but the water demand is low at present. The further increase in water demand will have an adverse impact on the water resources in the MRB (Xu et al. 2019) by 2030. On the one hand, measures of prioritizing water-savings should be implemented in the MRB to improve the irrigation assurance rate and reduce industrial water consumption. On the other hand, consideration should be given to the potential diversion of water from the Dadu River which has low utilization of abundant water resources in order to optimize the allocation of water resources in the basin, build and improve the water network system in the Sichuan region, and rationally allocate and utilize water resources (Xu et al. 2021; Cheng et al. 2022).

The development and utilization rate of water resources in the MRB is high, and the ecological environment has seriously deteriorated. On the one hand, previous studies mostly focus on the upper reaches of the Minjiang River, but ignore changes of runoff in the whole basin. On the other hand, previous studies mostly focus on driving effects of single factors such as vegetation, precipitation or dams on runoff, without analyzing the contribution of each influencing factor to runoff change. In this study, runoff variations of the Minjiang River and its main tributary (the Dadu River), as well as the runoff distribution between flood and non-flood seasons over the past 60 years were analyzed. Furthermore, the impacts of reservoir operation, water consumption, precipitation, and vegetation on runoff were analyzed. Ultimately, some suggestions regarding ecological environment protection and ecological restoration were put forward. This study provides a new perspective for identifying the contribution of anthropogenic disturbance and climate change to runoff, and our results can improve the effectiveness of ecological restoration at a basin scale.

The results indicated that water resources in the MRB have gradually decreased over the past 60 years, especially in the Minjiang River. Before the 1980s, the runoff was mainly affected by precipitation and other climatic conditions. From the 1980s to 2000, both anthropogenic disturbances and climate change had significant effects on runoff, after 2000, the anthropogenic disturbance was the main factor affecting runoff. In general, the contributions of anthropogenic disturbance, vegetation, and climatic conditions to runoff change in the Minjiang River Basin were 76.24, 13.62, and 10.14% respectively. There was an obvious relationship between runoff distribution and water resource development and utilization. In the past two decades, runoff has been affected by the operation of hydropower projects and the development and utilization of water resources such as water supply and irrigation. Due to increasing water demand in the Chengdu Plain, the annual runoff of the Minjiang River has gradually decreased over the past 60 years. Therefore, it is suggested that (i) ecological environment monitoring should be strengthened, (ii) water conservation measures should be implemented, and (iii) investigations and implementation of water resource allocation projects in the MRB should be increased.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.