**4. Discussion**

#### *4.1. Impacts of Policies to the Virtual Water Transmission*

The evolution of the virtual water network embodied in China's power system was impacted by different policies implemented over periods. The decomposition model identified the change of power generation mix was a major factor impacting the evolution of VWT from 2005 to 2010 (Figure 9). The high proportion of thermal power generation has induced serious air pollution and made negative effects on climate change. The construction of hydropower plants was promoted to meet the power demand of developed regions. During the first period, the power generation from hydropower had increased from 397 TWh to 722 TWh, with a factor of 1.8. However, the water coefficient of hydropower is higher than that of thermal power, which drives more virtual water delivered from southwestern areas to load hubs concentrated in northeastern areas.

The power transmission and power demand are important factors that increased the virtual water transfers in the second period (2010–2014). They attributed to the increase of investment in power transmission lines. The maturity of UHV technologies exceeded the construction of the UHV-power transmission line. In the period, UHV electricity transmission in China had developed rapidly in relation to the long-distance transmission of alternating current and direct current electricity. In 2010, the 1000-kV Nanyang-Jingmen UHV AC project and the 800 kV UHV DC Yunnan-Guangdong project were put into operation. Besides, the 800 kV UHV DC transmission line from Xiangjiaba to Shanghai commenced operation. The UHV transmission project from Jinping to Southern Jiangsu was put into operation in 2012. The expand of transmission capacity made the power grid more feasible and released

the hydropower potential of southeastern provinces, including Yunnan, Sichuan, Hubei, and Guizhou. The change of power transmission dominated the increase of VWT at national level, but that was the main driver of the decrease of VWT in terms of the individual province (e.g., Hubei). Even the overall transmission capacity increased, the amount of power transmission from Hubei significantly decreased in this period. It is noted that the power transmission directly from Sichuan and Guizhou can satisfy the power demand in Guangdong and Yangtze River Delta, which was one reason for the decrease of power export in Hubei. The contributions from the changes in power generation and demand structure were little when compared with the aforementioned factors. It can be referred that power generation and demand structure change had no direct relationship with power transmission change.

#### *4.2. Advice for the Development of China's Power System*

Centralized spatial distribution of power generation and investment on the UHV project has led to the diverging between power generation and consumption. Driven by the soaring electricity consumption in load hubs, the magnitudes of electricity transmission will continuously increase in the future. For load hubs, importing electricity from western provinces could mitigate their water stress and air pollution, especially the already polluted Jing-Jin-Jiarea [42]. In the view of power generation provinces, exporting electricity can release their power generation potential and develop the local economy. However, the expanding of electricity transmission increasingly reallocated water resources in both the generation side and consumption side, which may aggravate the water scarcity in China because of the mismatch between water resources and energy resources.

We explored the evolution of VW transfers embodied in provincial electricity transmission and identified the related driving factors. According to the results, a large amount of VW was transferred from western to the eastern provinces, which exacerbated the water stress in western provinces. The water consumption of power plants competed with the water consumption of urbanization and agriculture. Many VW exporters su ffering serious hydrological challenges [43]. For instance, Inner Mongolia exported VW to Jing-Ji-Jin area for decades, but Inner Mongolia faced high water stress pressure and the challenge of groundwater depletion. Exporting VW from Inner Mongolia exacerbated the conflicts between di fferent sectors. Some northeastern exporters (i.e., Shanxi, Shaanxi, Ningxia, Xinjiang, and Gansu) facing the same conditions. The power-related VW in southwestern areas was mainly delivered to the Yangtze River Delta and Guangdong. In comparison with northwestern provinces dominated by thermal power, hydropower played a vital role in the southwestern provinces. The transfer of VW in hydroelectricity did not pose water stress to the local ecosystem because of rich abundant water resources in southwestern provinces. The transmission pattern of southeastern provinces is thus more positive than that of northeastern provinces.

Considering the competition for the demands of water resources among energy, urban consumption, and agriculture sectors [44], we sugges<sup>t</sup> that policymakers should integrate across the water-electricity nexus at regional and national levels. To reduce the water pressure in China, the governmen<sup>t</sup> issued the stringent regulation in 2012, which constrained the national freshwater withdrawals into a definite magnitude (670 billion Mm3). Although water-saving measures have been implemented on a single plant or sector, synergetic managemen<sup>t</sup> taking water-electricity nexus into account has not ye<sup>t</sup> been implemented by the government.

Instead of the water scarcity, other environmental impacts (e.g., carbon emission, air pollution, etc.) induced by power systems has raised attention worldwide as well [45]. How to deal with those problems with new technologies (renewable energies connecting technology, artificial intelligence technology, etc.) is associated with the development of power grids. Improving the share of renewable energies is a highlighted way to reduce pollution and emission.
