China ’ s Hydropower Resources and Development

: Hydroenergy is developed due to its low-cost and near-zero pollution emission properties; therefore, the efficient management of hydroenergy is an important goal of sustainable development for any nation, especially for China, since it owns the most abundant water resources around the world. Developing hydroenergy is not only an effective response to the energy crisis but also a positive way to cope with climate change in China. Nevertheless, research on hydroenergy in China is still not comprehensive. This study reviews hydroenergy development in China by combining its geographical characteristics and hydroenergy reserves. The general condition of hydro-power development including large-and medium-scale hydropower stations and small hydro-power development is presented. This article illuminates the potential problems and existing challenges in China’s hydropower development and relevant exploitation suggestions are provided for hydropower development in the future.


Background
Hydropower plays a critical role in decarbonizing the power system and improving system flexibility.In 2021, hydropower generation decreased for the first time in two decades, mainly due to the persistent droughts in hydropower-rich countries, such as Brazil, the United States, Turkey, China, India, and Canada, leading to lower-than-usual hydro capacity utilization.Although the persistent droughts have led to the slow development of hydropower generation, the increasing capacity growth indicates that the future of hydropower can still be looked forward to, such as capacity additions in 2021 reaching 35 GW and China owning 66% of the capacity growth [1].
To date, the majority of the energy demand is provided by fossil fuels; however, since fossil fuels are finite resources and will eventually decrease, they have become very expensive or very environmentally damaging to retrieve.In recent years, renewable energy has been emerging as an alternative energy source to mitigate the disadvantages of fossil fuels.It will increase from 20% to 31%, despite the fact that integrating high levels of variable sources, such as wind and solar will be difficult in some markets.The greatest potential for renewable power will be precisely in these growing markets, where demand is highest and renewable integration is most cost-competitive; the shift to cleaner power generation may owe more to economics than climate policy [2].
Electricity power, as one of the most widely used energies, is essential for the dayto-day activities of human beings, since its use is important in improving people's standard of living.However, electricity generation can also damage the environment.Whether this damage occurs depends largely on how electricity is generated.For example, burning coal releases two times the amount of carbon dioxide-a major contributor to global warming-as burning an equivalent amount of natural gas.Anthropogenic carbon dioxide emissions primarily result from fossil fuel combustion and cement manufacturing.In combustion, different fossil fuels release different amounts of carbon dioxide for the same level of energy use: oil releases about 50% more carbon dioxide than natural gas, and coal releases about two times that amount.Nuclear energy does not generate carbon dioxide emissions, but it produces other dangerous waste products [3].
The principal scene of the IEA World Energy Outlook observes that world electricity demand will increase by more than two-thirds from 2011 to 2035.Thermal power generation, which is the main source of electricity in many countries despite its share, will decrease from 41% to 33% [4].With gas holding at 22%, fossil fuels will still account for more than half of all generations, which is bad news for the low-carbon transition.Hydropower attracts attention for its potential nature, along with low cost, near-zero pollution emissions, and the ability to quickly respond to peak lads, making it a valuable energy source [5].
The hydropower sector currently comprises 80% of global capacity for renewable energy generation and is considered a conduit between dependence on fossil-based energy sources and alternative energy futures.With the inherent technical advantages and its economic and environmental benefits, hydropower has made an important contribution to the world's energy structure.It has accounted for a high percentage of the electricity market and developed rapidly in both developing and developed countries [6].Over the next 5 years, the increase of 9% in hydropower capacity will be led by China, India, and Brazil.One-quarter of global growth is expected to come from only three megaprojects: Two in China and one in Ethiopia.However, apart from these three large projects, new capacity additions are forecasted to continue to decline.This is mainly due to a slowdown in the two largest markets, China and Brazil, where growth is challenged by the rising investment costs that result from the remaining limited economical sites.Moreover, extra expenditures are needed to address social and environmental impacts [1].Therefore, a review of Chinese hydropower development is essential to analyze hydropower development all over the world.
Hydroenergy takes an active role in electricity generation, and it meets about 20% of the electricity demand in China.Furthermore, due to its large reserves as well as its economic, environmental, and social benefits, hydroenergy will orient the development of renewable energy in future decades.China is working to expand its electricity generation capacity to sustain its rapid growth.Therefore, it is particularly urgent to develop hydropower for efficient application in industrial productions.Hydropower exploitation has been increasing in recent decades [7,8].Despite the fact that the amount of hydroenergy resources in China ranked first in the world, the installed hydropower capacity only takes up 26% of the technically exploitable hydropower resources [9].The Chinese government has adopted a series of policies for electrical power development, with the aim to accelerate hydropower development for saving fossil energy resources and decreasing air pollution [10].
China has abundant hydroenergy resources that account for 1/6, ranking first in the world for abundant rivers and a high drop.The results of the survey indicate that the total theoretical hydropower potential is about 6.94 × 10 8 KW and is mainly distributed in the southwest of China.There are many rivers in mainland China, among them, 3886 rivers with hydropower potential greater than 10 4 KW [11].It is foreseeable that the annual power generation is 6.0829 × 10 12 KWh if it is fully developed.To this end, the economically available hydroenergy capacity can be estimated at 5.416 × 10 5 KW [12].In China, water resource distribution is extremely uneven and does not match the regional economic development.The resource is mainly in the west, while market demand is higher in the east, which makes it difficult to exploit and utilize hydropower.Furthermore, the distribution of precipitation throughout China is very uneven, which makes river runoff vary greatly at different times.Therefore, reasonable planning for the construction of hydropower stations exerts a positive influence on hydropower exploitation.From 1989 to 2050, thirteen large hydropower bases (Figure 1) are arranged in China for the efficient development of hydropower cascade scrolling.Consequently, this generates over half the amount of hydropower resources in China.The nearly 60-year experience shows that Chinese hydropower construction has attained a quite high technical level, and China has the capability of independently designing and building various hydropower stations under different complex situations.

Contributions and Organizations
As so far, existing studies mainly focused on a viewpoint on hydropower development in China with minimal attention paid to a global view of China's exploitation.In this study, we review the hydropower reserves in ten main river systems and give detailed expressions about hydropower exploitation, hydropower station construction, the economic benefits, and the prospect of their development.This study reviews the hydropower development in recent years in China, the paper not only gives a detailed description of the hydrological characteristics of the ten major river systems but also reviews the hydropower development in all of these basins.
Compared with the existing literature, this study gives a detailed review of hydropower development according to ten main river systems, and the hydropower stations have been divided into large and medium-scale hydropower stations and small hydropower (SHP), and the benefits of them are listed in the study.After reviewing the situation of the hydropower development, we have summarized the problems and suggestions for hydropower in China.The study can provide a reference for researchers who devote themselves to further research on hydropower in China.
This paper is organized into four sections: Section 2 introduces hydropower reserves and their distribution in China.Section 3 is devoted to the analysis of hydropower resource development in China.Problems and suggestions for hydropower in China have been analyzed in Section 4. Finally, the paper concludes the discussion of this study.

Hydropower Reserves and Their Distribution in China
Due to the varied topography of China, the distribution of the river has a great influence on hydroenergy reserves that is useful for hydropower exploitation.According to the Aggregate of Confirmed China's Hydropower Resources issued by the National Development and Reform Commission in 2005, there are 3886 rivers with theoretical hydropower storage of more than 10 MW in the mainland of China and that makes China's hydropower ranks the world's first in theoretical storage [13].In this part, an overview of China's hydropower reserves and distribution is given.
China is located in the eastern part of Eurasia, the topography of China is high in the West and low in the East with a ladder-like distribution tilting to sea.On the one hand, the topography is beneficial to forming precipitation; on the other hand, it links the eastern and western from traffic to economy.Rivers flow from west to east with a high drop, which contributes to abundant hydropower.In the southwest region, due to rivers possessing both abundant runoffs and very steep slopes, they have rich hydropower potential.In addition, rivers following through the central south, the southeast and the northeast regions are quite rich in plentiful runoffs and considerable drops.By contrast, on the northwestern and eastern plains, due to either lack of river flow or lack of falls, the potential of hydropower is generally not attractive.However, the upper Yellow River has favorable hydropower potential due to its great falls [11].
Meanwhile, rivers roughly flow along the Greater Khingan Mountains, the Yin Mountains, the Helan Mountains, the Qilian Mountains, the Riyue Mountains, the eastern of the Hoh Xil Mountains, and the northwest of the Gangdise Mountains forming an inland river basin in northwest China and Tibet.The south part and the eastern part of the above line are assigned to the sea river system [14,15], therefore, the rivers are possessed of large discharge and large power.According to the distribution of the hydropower resources, the inland areas of China are divided into ten basins and each of them is described in Figure 2.

The Hydropower in the Yangtze River Basin (YRB)
The Yangtze River is the longest river that flows through over 20% of the land of China with a length of over 6300 km.The river origins from the Tangelo Mountains and the trunk streams and flows through 19 provinces with a natural fall of 5400 m.The Yangtze River trunk stream runs through three economical regions in China from west to east with 180,000 km 2 areas.The average annual rainfall of the Yangtze River is 1100 mm and there are more than 996 billion m 3 of water resources per year, which accounts for 36% of the amount of river runoff in China.The Yangtze River ranks third only second to the Amazon and the Congo River in the world.
The Yangtze River consists of 7000 branches.The Han River is the largest branch with a length of 1577 km, the Jialing River is second to them with a length of 1571 km, the Min River has the most abundant annual water resource at 87.7 billion m 3 , the Jialing River is ranked second in all of them with water resource of 70.4 billion m 3 .The information on the main streams in the Yangtze River is listed in Figure 3.According to the survey, the Yangtze River basin has a theory reserve of 268 million KW, the technology installed capacity of the Yangtze River basin is 2.81 × 10 8 KW, and the annual power generation of the Yangtze River basin is 1027 billion KWh, which accounts for about 53.4% of the total power generation in China [http://www.sasac.gov.cn/(accessed on).].There are about half of the thirteen large hydropower bases in China located in the Yangtze River basin, such as the Jinsha River hydropower base, the Yalong River hydropower base, the Dadu River hydropower base, the Wu River hydropower base, the Three Gorge hydropower base, the Xiangxi hydropower base.The installed capacity of the above six hydropower bases accounts for 60% of the total capacity in China.The hydropower resources of the Yangtze River basin are concentrated upstream of the river, the installed capacity accounts for 87% of the whole basin.The Jinsha River is famous in the world for its abundant hydropower.The total theoretical hydropower of the Jinsha River is about 1.21 × 10 8 KW and the available hydropower resources reach 0.9 × 10 8 KW.Four cascade hydropower stations with a total installed capacity of 4.65 × 10 7 KW are constructed in the lower reaches of the Jinsha River [16].The Minjiang River is the largest tributary in terms of the amount of water.It has abundant hydropower of more than 0.82 × 10 7 KW accounting for 1/5 of the Yangtze River.More than 129 cascade hydropower stations have been developed in the upper stream of the river with a total installed capacity of more than 3.0 × 10 6 KW [17].The Yalong River is one of the tributaries of the upper reaches of the Yangtze River and has abundant hydropower of theoretical reserves of 3.37 × 10 7 KW with 2.2 × 10 7 KW in the main streams and 1.14 × 10 7 KW in the tributaries, the available hydropower amount is 3.0 × 10 7 KW.There are 21 large and medium-sized cascade hydropower stations designed to make full use of the hydropower in the river.The Han River is the largest tributary of the Yangtze River, it owns the theoretical potential hydropower of 1.09 × 10 7 KW and the available amount is 6.14 × 10 6 KW.However, the developed hydropower resources only hold about 15% of the total.There are more than 900 hydropower stations built in the tributaries of the Han River.Other main streams of the Yangtze River with abundant hydropower include the Xiang River, the Yuan River, the Wu River, the Gan River, the Zi River, and so on.Remark 1. Figure 3 shows that the Han River and the Yalong River are the two longest tributaries in the Jialing River, but the total install capacity is lower than other tributaries.Otherwise, the Jialing River and the Jinsha River own the majority of theoretical potential hydropower in the Yangtze River, however, most of the capacity is located in the Xiang River.In this way, the theoretical potential of hydropower is largely related to the flow rate of rivers.Future exploitation should focus on the Jialing River and the Yalong River.

The Pearl River Basin and Its Hydropower
The Pearl River is the third largest river in China in terms of drainage area corresponding to the second largest in terms of stream flow, and it has abundant water resources.Moreover, it is the largest river flowing through the south of China.It consists of three major tributaries including the West River, the North River, and the East River (Table 1) [18].Three major tributaries have formed an alluvial delta in the mouth of rivers, which is known as the Pearl Delta.The Pearl River flows from the Yunnan-Guizhou plateau through six provinces with a length of 2214 km and a drainage area of 4.42 × 10 5 km 2 and pours into the South China Sea.The total annual runoff is just second to the Yangtze River in China.The hydropower reserves are up to 3.32 × 10 7 KW and mainly concentrate in upstream of the West River, the Qian River and the Hongshui River.Despite abundant hydropower, it is difficult to exploit its special territory.Otherwise, water resource distribution is inconsistent with economic development, which is a restriction to promote economic development.The West River is the largest tributary accounting for 77.8% of the total drainage area of the basin.It has potential hydropower resources of 2.29 × 10 7 KW that hold about 89% of the Pearl River basin.However, the exploited amount is just only 2.771 × 10 6 KW accounting for 12% of the total.The constructed hydropower stations have a total installed capacity of 1.58 × 10 8 KW with small regulating storage and scattered distributions so the hydropower does not attain effective use.The North River has hydropower reserves of 2.38 ×10 6 KW and technique exploitation amount of 2.34 × 10 6 KW.Several hydropower stations including the Baishiyao hydropower station, the Mengli hydropower station, the Mengzhouba hydropower station, and others with a total installed capacity of 1.44 × 10 6 KW are constructed in the basin.The East River is the smallest branch of the Pearl River.It contains hydropower reserves of 0.93 ×10 6 KW with 29% of the Pear River basin.Several hydropower stations such as the Dongjiang hydropower station, the Dongjiangmujing hydropower station are constructed with a total installed capacity of 7.06 × 10 5 KW. 1, the hydropower resources are concentrated in the West River, but the exploitation ratio is about 30% for the three main streams.Therefore, further research should develop the West River.

The Yellow River Basin and Its Hydropower
The Yellow River basin plays a critical role in China's economic and social development and ecological security [19].As the second largest river in China, the Yellow River originates from the Bayankala Mountain in the Qinghai Province and covers nine Provinces with an area of 7.95 × 10 7 km 2 .The Yellow River is 1900 km east-west and 1100 km south-north.The Yellow River basin is known for its hydropower resources in the upper reaches area.It has hydropower theoretical reserves of 4.33 × 10 7 KW and the available exploitation amount is 3.73 × 10 7 KW.However, the hydropower resource distribution is unbalanced and occurred years-lasted and provinces-extended droughts many times in history [20]).Therefore, a reasonable hydropower exploitation plan contributes to the ecoenvironmental change and benefits to efficient utilization of hydropower.
The Yellow River is composed of upper reaches, middle reaches, and lower reaches (Table 2).The upper reach covers 51.3% of the Yellow River and it has theoretical hydropower of 2.6 × 10 7 KW with a drop of 3496 km.The hydropower development of the upper stream of the Yellow River has rapid growth.However, the troubles of the lack of management and unbalanced development mode caused by rapid development are also increasing.The middle stream of the Yellow River has a length of 1206 km and an area of 3.44 × 10 6 km 2 .It covers the theoretical hydropower of 1.26 × 10 7 KW and available hydropower of 1.06 × 10 7 KW.The upper reach covers the downward of the Mengjin and it has a length of 786 km and an area of 2.3 × 10 4 km 2 .There is only 1/3 of the total hydropower development concentrated in the upper reaches and middle reaches of the Yellow River, and 2/3 of them focused on the lower reaches.There are 13 hydropower stations distributed along with the lower streams with a total installed capacity of 7.91 × 10 6 KW which has an important effect on the "West power to East" Project.

The Brahmaputra and Other Interior Rivers of the Tibet Basin
The Brahmaputra drains an area of around 530,000 km 2 and a length of 3848 km and crosses four different countries, the drainage area in China accounts for 50.5% of the total catchment area [21,22].It is the world's fourth largest in terms of the average discharge at the mouth, with a flow of about 20,000 m 3 •s −1 .Originating in the glaciated Kailas range of southern Tibet at 5300 m above mean sea level, the Brahmaputra traverses 1625 km in China and there are three branches of its origin including the northern branch, medium branch, and southern branch.The Brahmaputra has an enormous hydropower potential of the order of 7.92 × 10 7 KW just second to the Yangtze River.A famous "The Yarlung Zangbo Grand Canyon" has been formed, and it is the largest Gorge in the world owning abundant hydropower of 4.5 × 10 4 KW.However, it is difficult to exploit hydropower because of its complex terrains.Therefore, tunnels are worked on "The Yarlung Zangbo Grand Canyon" to have efficient utilization of the hydropower.
In addition, other inland rivers flowing through Tibet also have an amount of hydropower.Rivers between the Qiangtang area and intermountain of the southern Tibet are collected as the interior rivers of the Tibet basin.There are about 101 rivers that flow through above 1000 km 2 of drainage areas [15].The theoretical hydropower reserves of the Brahmaputra and other interior rivers of the Tibet basin are up to 7.98 × 10 7 KW [23].

The Southwest International Rivers Basin
The Southwest International Rivers include the Nu River-the Saerwen River, the Lancang River-the Mekong River, and other tributaries (Table 3).These rivers flow through several regions with complex terrains that make different climates from west to east and south to north.Hydropower resources are abundant in southwest China, the theoretical hydropower potential of the rivers is 2.57 × 10 8 KW accounting for 37.9% of China.However, the exploited hydropower only accounts for 34% of the total, corresponding to the annual power generation of over 5.06 × 10 11 KWh.
The Lancang River is of great importance not only to southwestern China but also to the rest of Southeast Asia.It is the mainstream of the Mekong River which is the largest international river in Southeast Asia [24].The geomorphology of the Lancang River varies from high mountains and deep valleys that run in a north-south direction to medium/low Mountains and wide valleys [25].The Lancang River has potential hydropower of 3.66 × 10 7 KW with 2.55 × 10 7 KW in the main streams and it has a natural drop of 5000 m.Thus, it has been regarded as one of the main areas of hydropower exploitation in China.However, 80% of the annual occurs during the rainy season which makes hydropower exploitation difficult [26].
The Nu River-Saerwen River has a length of 3240 km and a drainage area of 3.24 × 105 km 2 only a length of 2013 km, annual runoff of 7.1 × 10 7 m 3 and a drainage area of 1.37 × 10 5 km 2 in China.After entering Burma, it is named the Saerwen River.The Nu River lies between approximately 91 to 100° E and 23°5′ to 32°48′ N in China, it has a natural drop of 4840 m and which guarantees the abundant potential hydropower of 4.6 × 10 6 KW distributed in Tibet and Yunnan Province.The abundant water resources are distributed in the mainstream, middle reaches, and lower reaches of the Nu River.The Nu River has the greatest developing potential and the development capacity to be developed ranks second among all of the hydropower bases in China.

Remark 4. The upper reaches of the Nu Riv-er-Saerwen River and The Lancang Riv-er-Mengong
River are located in China, therefore, reasonable development of upstream hydropower Nu Riv-er-Saerwen River is not beneficial to China but also important to the countries downstream.In general, the complexity of the topography causes the hydropower development of the Southwest International Rivers basin challenging.Otherwise, the majority of hydropower reserves spread in other tributaries (as seen in Figure 4), which makes hydropower development difficult.

The Southeastern Rivers Basin
The southeastern Rivers basin is located in China's South-East coastal areas, such as Zhejiang Province, Fujian Province, Taiwan, Anhui Province, and Jiangxi Province with an area of 2.45 × 10 4 km 2 .Most of the rivers establish their own system and run into the sea alone.The Minjiang River and the Qiantang River are the two biggest rivers in the Southeastern River basin (Table 4).The Minjiang River has a length of 541 km in the instream and an area of 6.10 × 10 4 km 2 and flows from the Wuyi mountain.The three major tributaries of the upper reaches include the Sha River, the Futun River, and the Jian River, they join near Nanping forming the mainstream of the Minjiang River, additionally, lower reaches include regions down the Shuikou Dam.The Minjiang River basin has a warm climate and abundant rainfall all year and it owns the most abundant hydropower in eastern China, the theory of reserves is 6.26 × 10 6 KW and the total available installed capacity is 5.58 × 10 6 KW.The constructed hydropower stations have reached about 3.7 × 10 8 KW.Most cascade hydropower stations in the mainstream of the Minjiang River are constructed in 1993, especially after the year 2000 and the exploitation of cascade hydropower stations rate has reached 85%.However, cascade hydropower station construction has a complicated affection for the water environment that remains to be discussed [27].The Qiantang River basin is in the northern Zhejiang Province between 117.62° to 121.87° E and 28.17° and 30.48°N in China, which is the largest and longest (484 km) river system in Zhejiang Province.It has a length of 688 km and an area of 5.56 × 10 4 km 2 with abundant hydropower of 2.41 × 10 6 KW and a constructed installed capacity of 1.88 × 10 6 KW accounting for 16.1% of the total.
Remark 5.The hydropower of the Southeastern Rivers basin has been fully developed, and the further policy should focus on the environment protections.

The Northeastern Rivers Basin
The Northeastern Rivers covers three provinces including Liaoning, Jilin, and Heilongjiang with an area of 1.24 × 10 6 km 2 .There are 290 rivers with a drainage area of more than 1000 km 2 .The main streams include the Heilong River, Liao River, and the northeastern international rivers which have abundant hydropower of 1.53 × 10 7 KW and a total installed capacity of 2.4 × 10 6 KW (Table 5).The Northeastern River has abundant hydropower but uneven spatial and temporal distribution.The Heilong River flows through the north of northeastern China as the boundary of between China and Russia.It covers three countries including China, Russia, and Mongolia with a length of 4370 km ranking 11 all over the world, and drainage of 1.84 × 106 km 2 ranking 10 over the world.The basin in China accounts for 48% of the total area.There are over 201 rivers with a drainage area of more than 1000 km 2 , there are plenty of small streams.The Heilong River is rich in hydropower with water reserves of 5.2 × 10 6 KW [15].There are about 200 tributaries of the Heilong River and the Songhua River and the Wusuli River are the largest of them.The Songhua River is the largest mainstream of the Heilong River.It covers Jilin Province, Inner Mongolia autonomous region, and Heilongjiang Province.It has a length of 2308 km and a drainage area of 5.46 × 10 6 km 2 .With abundant water resources and a high drop in the midstream of the river, the Songhua River has water resources reserves are up to 7.08 × 10 6 KW, however, the exploited hydropower is less than 50% of the total.
The Liao River is the largest river in northeast China.The basin covers an area of 2.19 × 10 5 km 2 , and it is covered under the temperate and warm temperate belt, subjective to the monsoon climate [28].It has a natural drop of 1200 m with hydropower of 1.17 × 10 6 KW only about 10% of them being exploited.The Tumen River and the Yalu River are the borders between China and Korea and they have abundant hydropower of 2.89 million KW.The Tumen River has a drop of 1200 m and a length of 525 km while the Yalu River has abundant rainfall with a length of 795 km and hydropower of 2.13 million KW.

The Huai River Basin
The Huai River is the sixth largest river in China.It is located between the Yangtze River and the Yellow River of China covering five provinces of 27,000 km 2 [29].It is the boundary between northern and southern China.The hydropower of Huai River is mainly focused on the upper reaches and the midstream of the river.The theoretical potential hydropower resources are up to 1.45 × 10 6 KW and there are 30 rivers with theoretical potential hydropower resources of more than 10 MW.However, developed and developing hydropower just accounts for 48% of the total.There is no large hydropower station in the basin of the Huai River and it is suitable to build middle small-sized hydropower stations.The distinguishing feature of the hydropower development in the Huai River is its efficient utilization.

The Hai River and Luan River Basins
The Hai River and the Luan River play the dominant role in the northern regions of China.The Hai River covers Tianjin, Beijing, Hebei, Shanxi, Shandong, Henan, and Liaoning Provinces with an area of 3.18 × 10 5 km 2 and the basin has an important strategic location for it is located in the heart of China.The Luan River is one of the largest rivers in North China, it originates from the northern Hebei Province and flows southeast, and then pours into the Bohai Sea.The Luan River has a length of 885 km and covers an area of 4.46 × 10 4 km 2 .The Hai River and the Luan River basin have hydropower resources of 2.94 × 10 6 KW, there are 59 rivers with hydropower resources of more than 10 MW.However, it is not suitable to build large and medium-sized hydropower stations.Therefore, the hydropower stations in the Hai River and Luan River basins have an installed capacity of less than 5 × 10 4 KW.The Hai River and the Luan River basins always must face water shortages.The hydropower exploitation ratio is as high as 70%.

The Northwest Inland Rivers Basin
The Northwest Inland Rivers basin covers more than 2.6 × 10 6 km 2 accounting for 1/4 of China.Most rivers flowing through the basin originate from mountains, downstream to the basins and plans, and then disappear into the deserts and lakes.Thus, the tributaries are difficult to collect [15].The Northwest Inland Rivers have numerous tributaries but no larger ones, which makes the hydropower distribution scattered and difficult to build large and medium-sized hydropower stations.The hydropower stations in the Northwest Inland Rivers basin have a total installed capacity of less than 5 × 10 4 KW.The rivers have theoretical hydropower reserves of more than 3.70 × 10 7 KW and a total installed capacity of 4.45 × 10 6 KW.

Hydropower Resources Development in China
The first hydropower station Shilongba in China was built in 1912.During the past decades, the construction of hydropower stations in China is stumbled.The development of hydropower resources contributes to reducing greenhouse gas emissions and it is one of the main supports of the power industry in China.According to the data published by the National Bureau of Statistics [30], By the end of 2019, the total hydropower generation is 1339.00 billion KWh accounting for 17.38% of the total power.There are 13 large-scale hydropower bases as seen in Table 1 constructed or planned on large rivers, including the Jinsha River, the Dadu River, the Yalong River, the Wujiang River, the Hongshui River, the Lancang River, and the Yellow River, the total installed capacity as high as 2.76 × 108 KW [31].
At the end of 2021, the number of recorded hydropower stations is more than 46,785 in China, with a combined installed capacity of 2376.92 million KW, while the hydropower is 391 million KW.Among them, 20,866 are completed, with a combined installed capacity of 217 GW and 1324 are under construction, with a combined installed capacity of 110 GW [32], exceeding the cumulative values of the USA, Canada, and Brazil which ranks just after China in terms of hydropower installed capacity [33].With building numerous hydropower stations, the installed capacity of large and medium-scale hydropower stations accounts for nearly half of them.Nevertheless, with several huge hydropower stations put into operation, small hydropower stations are also developed rapidly in recent decades for no pollution and flexibility in operation.

Large and Medium-Scale Hydropower Stations Construction and Its Benefit in the Yangtze River Basin
During the past decades, China has built numerous large and medium-scale hydropower stations on great rivers, especially on thirteen large hydropower bases.The number of large and medium-scale hydropower stations especially cascade hydropower stations have sustained growth in recent years.The Three Gorge hydropower station plays a dominant role in all hydropower stations for its large installed capacity and power generation amount.It is the largest hydropower station in the world with a 2.25 × 10 7 KW installed capacity, which generates power with about 9.81 × 10 10 KWh every year accounting for 14% of the hydropower generation in China.The construction of the Three Gorges Project reduces coal consumption by over 30 million tons as well as SO2 by 0.5 million tons [34].However, with the increasing development of hydropower in China, the share of the three gorges project will continue to decline.The Xiluodu hydropower station is second to the Three Gorge hydropower station in the mainstream of the Yangtze River with a total installed capacity of 13.86 million KW, the annual power generation of 5.7 × 10 10 KWh, the primary function of the hydropower station is generating power as well as flood-control.
The Xiangjiaba hydropower station is developed downstream of Jinsha River hydropower base level 4 at the end of cascade hydropower stations.It ranks third in total installed capacity (6.4 million KW) in China among the constructed stations.The Gezhouba hydropower station is another large hydropower station located in the mainstream of the Yangtze River, which is the first constructed hydropower station in the Yangtze River basin with an installed capacity of 2.72 × 10 6 KW and annual power generation of about 1.57 × 10 10 KWh.The Baihetan hydropower station is developed as the second rung of cascade hydropower stations downstream of Jinsha River hydropower base level 4 with a total installed capacity of 1.60 × 10 8 KW and annual power generation of 6.24 × 10 10 KWh.The Wudongde hydropower station is the first rung of cascade hydropower stations downstream of Jinsha River hydropower base level 4 with a total installed capacity of 1.02 × 10 7 KW and an annual power generation of 3.89 × 10 10 KWh, it is beneficial to improve the profit of cascade in the upstream and promote the economic development of the local government.
In addition, numerous hydropower stations are also constructed in the tributaries to the Yangtze, such as the Ertan hydropower station, the Gongzui hydropower station, the Pubugou hydropower station, the Wujiangdu hydropower station, the Geheyan hydropower station, the Danjiangkou hydropower station, the Wuqiangxi hydropower station, and the Ankang hydropower station.Moreover, great deals of small hydropower stations are constructed to make full use of the hydroenergy of the Yangtze River.Table 6 gives a brief view of the main hydropower stations located in the Yangtze River basin.5, the Three Gorge hydropower station is the largest hydropower station with a total capacity ofpp more than other constructed stations, however, the annual power generation ratio is lower than other hydropower stations.Other stations with lower total installed capacity have a higher ratio of annual power generation.Since the annual power generation is collected by the end of 2021, and it varies from year to year, the recorded data reflects the average level.There are numerous hydropower stations are constructing for Pumped-storage or flood control.

The Main Large and Medium-Sized Hydropower Stations in the Pearl River Basin
Numerous hydropower stations are distributed in the Pearl River basin for better hydropower generation.The Longtan hydropower station is the largest in the Hongshui River in Guangxi Province, it has a total installed capacity of 6.3 × 10 6 KW and plays an important role in economic promotion, flood control, and ecological benefits.Guangdong Pumped Storage Power Station is second to the Longtan hydropower station located in the Pearl River basin with a total installed capacity of 2.4 × 10 6 KW and an annual power generation of 1.57 × 10 9 KWh.Other constructed hydropower stations located in the Pear River basin include the Yantan hydropower station, the Tianshengqiao Fist cascade Hydropower Station, the Tianshengqiao Second cascade Hydropower Station, the Dahua Hydropower Station, the Datengxia Hydropower Station, and the Baiseli Hydropower Station et.The main hydropower stations can be reviewed in Table 7.  6 indicates that despite the Longtan station having the largest installed capacity, the annual power generation is behind other medium-sized hydropower stations, such as the Yantan station, the two cascade hydropower stations.

The Main Large and Medium-Sized Hydropower Stations in the Yellow River Basin
The Longyangxia reservoir located in Gonghe County of Qinghai Province is the first large, cascaded hydropower station upstream of the Yellow River with a total installed capacity of 1.28 × 10 6 KW.It has been incorporated into the national grid transporting electricity to Xining, Lanzhou, Xia, the Qaidam Basin of Qinghai Province, and the Hexi Corridor in the west of Gansu Province.The Longyangxia reservoir plays a positive role in hydropower generation, flood control, and irrigation.The Laxiwa hydropower station is the largest hydropower station in the Yellow River basin, which is the second cascaded hydropower station upstream of the Yellow River, it has an installed capacity of 4.2 × 10 6 KW and an annual power generation of 1.02 × 10 10 KWh.The Lijiaxia hydropower station is the third level of a large hydropower station upstream of the Yellow River, it has a total installed capacity of 0.2 × 10 7 KW that is just second to Laxiwa and the annual power generation is 5.90 × 10 9 KWh.Along with the river, Gongboxia hydropower station is constructed in Qinghai Province as the fourth large-size cascaded hydropower station with a total installed capacity of 1.5 × 10 6 KW and annual power generation of 5.14 × 10 9 KWh.Liujiaxia is another large-size hydropower station that connects the power grid of Shaanxi Province, Gansu Province, and Qinghai and it is praised as "A Pearl of Yellow River".
In addition, large numbers of medium-sized reservoirs such as Yanguoxia, Bapanxia, Daxia, Wanjiazhai, Qingtongxia, Longkou, Tianqiao, Sanmenxia, and Xiaodilang are constructed in the Yellow River basin, and they are benefiting hydropower generation, irrigation and flood-control.Table 8 shows the detail of the main hydropower stations located in the Yellow River basin.7).Because the Yellow River is the most heavily sediment concentration river, it suffers from heavy sedimentation, diverting and floods over history, hydropower stations constructed in the Yellow River basin should Consider both water reservoirs and hydropower generation.

The Main Large and Medium-Sized Hydropower Stations in the Southwest International Rivers Basin
Zangmu, Jiexu and Jiacha are designed as three largest hydropower located on the Brahmaputra.Zangmu hydropower station is the first hydropower station developing on the Brahmaputra with the total installed capacity of 5.10 × 10 6 KW, and an annual power generation of 2.5 billion KWh and it is also the largest developing project of hydropower in Tibet.Jiexu hydropower station is the third cascade of level 5 power stations on the Brahmaputra, which is developed installed capacity of 0.51 × 10 6 KW and an annual power generation of 2.58 × 10 9 KWh.The Jiacha hydropower station is the last cascade of the level 5 hydropower stations with designed total installed capacity of 0.36 × 10 6 KW and the annual power generation of 1.60 × 10 9 KWh.
Large and medium-sized hydropower stations located in the Southwestern International Rivers mainly focus on the Lancang River and the Nu River with installed capacity more than2.5× 10 6 KW at the Lancang River and 1.81 × 10 6 KW at the Nu River.There are eight cascade developing hydropower stations located in Lancang River including Gumu, Wunonglong, Lidi, Tuoba, Huangdeng, Dahuaqiao, Miaowei hydropower stations and other large and medium-sized hydropower stations include Gongguoqiao, Manwan, Dachaoshan, Nuozhadu, Jinghong, Ganlanba and Mengsong hydropower stations that are constructing or constructed on the Lancang River.13 cascade hydropower stations are constructing on the Nu River including Songta, Bingzhong, Maji, Luma, Fugong, Bijiang, Biluo, Lushui, Liuku, Shitouzhai, Saige, Yansangshu and Guangpo hydropower stations.The detail information about these hydropower stations is shown in Table 9.

The Main Large and Medium-Sized Hydropower Stations in the Southeastern Rivers Basin
There are 118 large and medium-sized hydropower stations located in the Southeastern Rivers with total installed capacity over 1.42 × 10 7 KW and 59 of them located in Fujian Province, 22 of them located in Zhejiang Province and the rest of them located in Jiangxi Province.Gutianxi with four levels located in Fujian Province is the first cascade hydropower station constructed in China and it has a total installed capacity of 2.76 × 10 7 KW and the annual power generation over 0.8 × 10 9 KWh.Furthermore, there are also a lot of large and medium-sized hydropower stations concentrate the Northeastern Rivers and Northwestern Rivers.For the little amounts of hydropower, very few large and mediumsized hydropower stations are constructed in Hai and Luan Rivers basins and Huai River basins.The 50-year experience shows that China's hydropower construction has attained a quite high technical level, and China has the capability of independently designing and building various hydropower stations under different Complex situations [31].

"Small Hydropower" (SHP) Development in China
In China, hydropower plants with installed capacities less than 50 MW are currently referred to as small hydropower (SHP) plants [35].China possesses abundant SHP resources that are economically feasible for exploration.A check of state water resources shows that the exploitable part of China's rural SHP resources using current techniques is 1.28 × 10 8 kW, which ranks first in the world [36].The history development of China's SHP can be roughly divided into three states.From 1950 to 1979, small hydropower is developed for irrigation and lighting, and then after the publishing of the Reform and Openup Policy, it is mainly developed for economic development.Up to now, SHP is developed for environmental protection as well as for promoting economic development.With a huge energy demand, small hydropower (SHP) has undertaken rapid development in recent years in China.Especially in the recent three decades, the increasing environmental pressure has promoted its development rate and the significance of its management and operation.The number of SHP stations in China has exceeded 45,000 with a total installed capacity of more than 6.8 × 10 7 KW, ranked first in the world.SHP provides approximately 5% of China's gross electricity generation and is the major backbone for rural electrification [37].It is widely distributed in more than 1700 mountainous counties around the country and most of them distribute in west of China.However, the distribution of SHP is uneven geographically.
Figure 8 shows that SHP installed capacity of different regions in China.It indicates that the Southwest Cfhina and the South China are two main regions collecting small hydropower stations.Among them, the Southwest China, which includes Yunnan, Sichuan, Guizhou, Tibet, and Chongqing provinces, has the largest installed capacity accounting for nearly 35% of the total.The Southwestern China followed by the Central-south, Northwest, East, Northeast and the North with potential shares of 31.7%, 23%, 8%, 1.8%, and 1%, respectively.Obviously, SHP development in China is mainly concentrated in the west, especially in the southwest.

Comparisons with Other Power Sources
At the end of 2020, China has the total install capacity of 2.20 billion KW with the power generations of 7.62 × 10 12 KWh, the thermal power is still accounting for half of them.Because of the economic benefits and China's green energy strategy, renewable energy has become the core of energy transformation.Since renewable energy is characterized by high efficiency, clean, low carbon and environmental protection.The development of renewable energy is conducive to the sustainable development of the ecological environment and social economy.As seen in Table 10, renewable energy maintains good growth in recent years, hydropower is the mainstay of renewable energy.Figure 9 lists the average percentage of recent years, and hydropower accounts for 67.40% of renewable energy, which is far more than other renewable energy.Although China hydropower has already achieved economic, social and environmental benefits it still faces many challenges such as uneven regional distribution of hydropower resources, cost-effectiveness, imbalanced development, migration settlement, environmental problems, the immature industry system and hydropower project management risks [34,38].

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Uneven regional distribution of hydropower resources.The water resources of China are richly endowed by nature and rank top of the world.However, due to distinguishing features of topography and precipitation, the distribution of hydropower resources is very uneven in China.For instance, in the Southwest Region, many rivers possess both abundant runoffs and very steep slopes, thus, there exists very abundant hydropower potential (accounting for 70.0% of the total) while North China lacks hydropower resources (accounting for 1.8% of the total) due to either lack of river or lack of falls.

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Cost-effectiveness and imbalanced development.Hydropower development requires large initial capital outlays, and this is traditionally conducted at the government level.In terms of generation equipment, hydropower is about 40 percent higher than thermal power in unit cost.The initial investment is huge, and the funds needed are mostly from loans, therefore asset liability ratio of hydropower enterprise, especially new hydropower, is almost beyond 70%, which increase the reimbursement pressure for large hydropower enterprises [34].Otherwise, most of the hydropower resources are in the western regions, whereas most of the electricity consumption is required in the eastern and the southern coastal regions as well as some part of the central region.Hence, a national strategy for electricity transmission from western to eastern China has been formulated and that will cost a lot of money.

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Migration settlement.Due to a huge population, the migration settlement of reservoir regions in hydropower development is troublesome.For many years, construction leads to more than 22 million reservoir resettlements to distribute in 31 provinces and municipalities directly under the Central Government [31].Along with the growing numbers of hydropower reservoirs construction, the contradiction between increasing migration settlement population and backward infrastructure construction in resettlement areas will become more prominent.Hence, it will become increasingly difficult to implement a hydropower project if the moving residents are not helped to settle down appropriately.In addition, population health and cultural relics and historic sites protections caused by migration have also caused extensive concern more and more.• Ecologic environmental problems.Although hydropower is clean renewable energy with no carbon emissions, it still affects the ecological environment of the reservoir, and the exploitation causes a series of ecological disasters.As the scale of hydropower becomes larger and larger, the impact of uncertain runoff quantity on hydropower and power grids will become more significant.The impact of the extreme climate on the operation of hydropower stations will be even more.The drought in the southwestern region of the country in 2022 exhibits an important warning.As well as the development of hydropower projects changes the hydrology of the rivers and leads to water quality variation and geology state change.Thus, the rivers lost their self-protection ability and ecologic disasters such as earthquakes and mud-rock flow arise frequently.Moreover, the aquatic community that consists of fishes and plants will die because of reservoir inundation and the changes in living conditions, such as eutrophication [14].According to the report released by the Ministry of Environment Protection (MEP), the water quality of the mainstream Yangtze River and its tributaries is slightly poorer in 2008.MEP asserted that it is mainly due to the major flooding during the autumn season [38].

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The immature industry system.While rapid development has been demonstrated in hydropower, a lot of challenges remain to be solved.A low level of industry concentration is one of the most challenging things that make it difficult to manage hydropower, especially for SHP.Most SHP stations are mountainous and have a wide distribution.The transmission network in such areas is usually weak and not well-developed [39].Otherwise, the immature estimation system may cause excessive exploitation and utilization of hydropower resources and a few small hydropower projects don't take into consideration of downstream production, living and ecological water requirement that cause discontinuous flow [34].Furthermore, the overproduction of hydropower gives rise to the waste of power and increases the load instability of the grid in the grid-connection system.Indetermination of SHP property leads to market confusion in the SHP supply system regions.With little access to information and weak dispatching management, it is hard to control the output of SHP stations which consequently results in some large hydropower stations having no way but spillage due to network congestion [37].China's hydropower is at the stage of rapid development of industry, while large-scale construction is desired, few managements and research have been devoted.

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Hydropower project management risks.Effective management should be based on how to systematically balance the multiple objectives of sustainable development and address the balance among stakeholders.Thus, some risks that appear in the management should be taken into consideration.Hydropower project risk management involves a wide variety of factors, including estimations, contracts, clients, contractors, designers, unknown geological conditions, economic and political circumstances, technology issues, natural events, ecological and environmental impacts, affected local communities, management and control techniques, individual activities, organizational operations, and inter-organizational risk allocations [14] and there are a lot of studies have conducted these themes.

Suggestions
More research into hydropower exploitation.To resolve the current energy shortages, hydropower, due to its huge reserves as well as its economic, environmental, and social benefit, will orient the development of renewable energy during the 12th Five-Year Plan [34].Thus, more research should be put into hydropower exploitation, and not only that, but more attention should also be paid to balancing hydropower and ecological environment, hydropower management and stakeholders.Furthermore, the reasonable exploitation planning implemented in the hydropower project should fully consider the local hydrology and geology as well as the development of other energy sources.Otherwise, hydropower generation is deployed rationally to ensure the stable operation of power grids when put into the electricity grid.
Developing the cascade hydropower exploitation (CHE).Cascade hydropower station mainly refers to a group of hydropower stations distributed on different river sections from upstream to downstream in terms of river fall [40].Thus, CHE is generally focused on the great rivers.Considering that China has large quantities of great rivers, CHE is the most efficient use of hydropower in different river sections.Although CHE has developed gradually in China in recent years, total quantity management mainly includes the scientific management of the river fall of different sections, the multi-annual mean flow of typical hydrologic stations, the critical scale of CHE based on the minimum flow, as well as the number and installed capacity of cascade hydropower stations is the key strategic segment remained to be improved.CHE in China mainly focus on 13 large hydropower development bases and CHE in Southwest China such as the bases Jinsha River, the Dadu River, the Yalong River, Lancang River and the Brahmaputra River plays a strategically dominant role in all of them.
Developing SHP.SHP is a kind of favorable energy resource not only has the advantage of hydropower, but also it allows rural and mountainous areas to have access to electricity.Thus, SHP is of utmost importance to both the social and economic development of China nowadays [37].However, SHP had a disordered development in recent years although has increasing numbers.Therefore, sound SHP management especially the dispatching management system is necessary.SHP in Southwest China has a dominating role, the value in the Northwest and North is relatively low.Therefore, to ensure the sustainable development of hydropower adapt to economic development, it should be developed in Northwest and North China to reduce the cost of power transition.At the same time, a fair price mechanism and orderly management should be established for SHP in Southwest China as well as other regions in China.
Developing immigration policy.In the comprehensive development of hydropower, China needs to combine reservoir resettlement, ecological emigration, and anti-poverty immigration [33].Immigration resettlement is an important part of the hydropower project because it is related to social stability and the economic development in immigration regions.A good immigration policy should consider environmental capacity in immigration regions as well as sustainable development.Environment capacity in immigration regions is a reference to population size in a region that ensures a positive development of the ecological environment.But ecological virtuous cycle development as well as maintaining a good environmental quality .Hence, the environmental assessment of the immigration regions should be comprehensive and the immigrant policy should fully consider resource conditions, economic development, environment quality as well as the quality of immigrants.

Conclusions
As one type of renewable energy resource, hydropower development brings in several benefits such as relieving the energy crisis and delaying climate change.China has an abundant hydropower reserve that is conducive to hydropower development.However, most of the rivers have uneven runoff distribution within one year or between years and the hydropower does not match regional economic development.Therefore, China has a low level of hydropower exploitation and less than 30% of technical exploitable hydropower capacity has been built, thus, it has a broad development prospect.The scientific understanding of hydroelectricity including a scientific recognition of development states and a sound policy system is essential and it is bound to bring about vigorous development of hydroelectricity.This study gives a detailed review of the current situation of hydropower in China including the hydropower distribution and the status of hydropower exploitation.According to the review of hydropower resources in China and hydropower stations distribution, we have analyzed the influence of hydropower resource distribution on its development.Otherwise, the characteristics and the functions of hydropower stations have been discussed in each section.After then, emphasis should be laid on the relevant adverse impacts on society and the environment and the affected vulnerable groups should be given special attention during hydroelectric development.Lastly, the corresponding practical policies should be laid down and implemented in the hydropower projects.This study has provided a detailed review of the hydropower development in China and emphasized the challenges in hydropower development.Moreover, it can provide a reference for future research on hydropower development in China.

Figure 2 .
Figure 2.Ten hydropower resource basins in China.

Figure 3 .
Figure 3.The mainstreams of the Yangtze River.

Figure 4 .
Figure 4.The hydropower reserves of the Southwest International Rivers basin.

Figure 5 .
Figure 5.The main hydropower stations in the Yangtze River basin.

Figure 6 .
Figure 6.The main hydropower stations in the Pearl River basin.

Figure 7 .
Figure 7.The main hydropower stations in the Yellow River basin.

Figure 8 .
Figure 8. Distribution of the SHP in China.

Figure 9 .
Figure 9. Renewable energy development in recent years.

Table 1 .
Information on the main streams of the Pearl River.

Table 2 .
The information on the main streams of the Yellow River.

KW) Length (km) Area (×10 5 km 2 )
Remark 3. The hydropower development in the Yellow River basin should consider both hydropower resources and flood control.The Yellow River has experienced several breakings and river courses, and most of the hydropower projects have been constructed for flood control and hydropower development.

Table 3 .
The main steam of the Southwest Rivers.

Table 4 .
Main International River of Southeastern China.

Table 5 .
Main streams of the Northeastern River in China.

Table 6 .
The main large and medium-sized hydropower stations in the Yangtze River basin.
Remark 7. As seen in Figure

Table 7 .
The main large and medium-sized hydropower stations of the Pearl River basin.
Remark 8.There are numerous hydropower stations constructed in the Pearl River basin, and most of them concentrate on the Hongshui River. Figure

Table 8 .
The main large and medium-sized hydropower station of the Yellow River basin.

Table 9 .
The main hydropower stations of the Southwest International Rivers basin.

Table 10 .
Comparisons of power generation in China.