As a clean and renewable energy source, hydropower meets the growing energy needs of mankind while mitigating global warming trends [1
]. Hydropower continues to grow worldwide, especially in developing countries, such as China [2
]. However, large-scale hydropower development and construction has also brought controversies [3
], such as the effects on the water environment and the changes to the river hydrological, including runoff, flood peak flow, water temperature [8
]; effects on the ecological environment, such as migratory fish breeding [9
], environmental flow [10
], and ecological compensation [11
]; effects on the social environment, such as immigration issues and land occupation issues [14
]. In addition, the risk and the environmental impact of dam break are not negligible [15
]. The traditional water resource index “water intake” can no longer truly measure the water consumption of hydropower stations and their positive and negative impacts on the environment, ecology, and society. Therefore, how to objectively measure the comprehensive performance of cascade hydropower development is a difficult problem [17
The water footprint (WF) is a method to quantify the water consumption of hydropower stations and reservoirs and to assess the increasing pressure on local water resources considering the water surface evaporation of the reservoir (i.e., blue water footprint) [18
]. The water footprint concept is based on virtual water, proposed by Dutch scholar Hoekstra in 2002 and is an indicator of water use by consumers or producers, including direct and indirect water use [20
]. The water footprint consists of three parts: green water (soil water), blue water (surface water and groundwater), and grey water (polluted water).
The product water footprint (PWF) of hydropower plants characterizes the water consumption per unit of electricity production. There are three main calculation methods, which are the gross water consumption method, the net water consumption method, and the water balance method [21
], respectively, involving spatial scales such as the world, countries, and river basins. For example, Mekonnen et al. [23
] calculated the PWF of hydropower plants on a global scale using the total water consumption method, and the PWF ranges from 0.3 to 850 m3
and the average PWF is 68 m3
. The calculation accuracy is questionable because only 35 hydropower stations were selected for the whole world. Herath et al. [24
] used the water balance method to calculate the PWF of 17 hydropower stations in New Zealand, and the calculated results range from −2.80 to 19.80 m3
. However, WF is an indicator for quantifying the use of freshwater, and there should be no negative values. Therefore, the water balance method is not suitable for calculating the PWF.
Primary energy plays an important role in the primary stage of the energy supply chain, including raw coal, crude oil, natural gas, biomass, hydropower, nuclear energy, wind energy, and solar energy. These energy development and utilization processes have different levels of consumption of water resources. Gerbens-Leenes et al. [26
] estimated the biomass water footprint of four countries (the Netherlands, the United States, Brazil, and Zimbabwe) with an average of 72 m3
, which is the largest energy source in primary energy consumption. Gleick [27
] estimated raw coal, crude oil, natural gas, nuclear energy, wind energy, and solar water footprint. The wind power water footprint is negligible, and the value is 0 m3
. Monthly blue water scarcity in the basin is defined as the ratio of the total blue WF of the basin over the month to the available blue WF [28
]. Currently, there are various monthly water scarcity studies for specific river basins (Heihe River Basin [29
] and Yellow River Basin [19
] in China), national-level (China [18
] and Morocco [30
]), transboundary-level [31
], and global-level [28
]. However, those studies paid less attention to water consumption and accumulated blue water scarcity at different phases of the development and utilization of cascade hydropower stations in the basin.
Based on the water footprint method, this paper takes the Yalong River Basin as the research object, considers the four phases of hydropower development in the basin, calculates the PWF and the blue water scarcity, analyzes its influencing factors, and evaluates the cumulative environmental effects of hydropower stations (The relationship between the degree of hydropower development and utilization in the basin and the environment).
The traditional water resource index “water intake” can no longer truly measure the water consumption of cascade hydropower stations and their positive and negative impacts on the environment, ecology, and society. In this paper, we took the Yalong River Basin as the research object, considered the four phases of hydropower development in the basin, calculated the evaporated water footprint (EWF), product water footprint (PWF), and the blue water scarcity (BWS), analyzed its influencing factors, and evaluated the cumulative environmental effects of hydropower stations based on the water footprint method. Two main conclusions are summarized:
(a) The EWFs in established, ongoing, proposed, and planning phases of the 19 hydropower stations were 243, 123, 59, and 42 Mm3, respectively. The PWF of the 19 hydropower stations in the Yalong River Basin was 0.01 to 4.49 m3GJ−1, the average water footprint was 1.20 m3GJ−1, and the water use efficiency of hydropower development in the basin was greater than that of other basins and regions. The differences between the PWF of different hydropower stations were mainly related to the energy efficiency factor.
(b) The BWS belongs to low blue water scarcity (<100%) with or without considering the EWF of the four phases of the hydropower station hydropower stations in Yalong River Basin, which means that the cumulative environmental effects of development and utilization of cascade hydropower stations in the basin will not affect the local environmental flow requirements from the perspective of water footprint.
Our study has several limitations. First, cascade reservoirs with good reservoir regulation performance can reduce the risks of devastating floods in downstream regions and protect people’s lives and property. Unfortunately, these beneficial impacts were not reflected in this paper, as it is beyond the scope of our study. Second, the consumption of blue water of reservoirs in different months during the year was averaged in this paper. Actually, the consumption of blue water from reservoirs was different for different months over a year because the climate is seasonal. However, the effect was acceptable compared to the total blue water consumption. Third, we assumed that 80% environmental flow requirement equals 80% of the natural runoff in the Yalong River Basin. The environmental flow requirement was determined by factors such as natural geographical conditions and environmental conditions necessary for living and social and economic conditions. Therefore, different river basins will have different environmental flow requirements.