Dynamics of the Urban Water Footprint on the Tibetan Plateau: A Case Study of Xining, China

Determining the changes in the urban water footprint (WF) of the Tibetan Plateau is important for sustainable development within this region and in downstream regions. Taking Xining, the largest city on the Tibetan Plateau, as an example, this study quantified the changes in the WF of this region in the 2005–2018 period. We found that Xining’s total WF increased by 22.6%, from 8.9 billion to 10.9 billion m3 in this period. The increase in Xining’s gray WF (WFgray) resulting from the intensification of urban point-source pollution was the primary cause of the increase in its total WF. Xining’s WFgray from point-source pollution increased by 75.3%, from 3.1 billion to 5.4 billion m3. In addition, Xining’s WF far surpassed the amount of available water resources (WA) in this region. It is possible to prevent Xining’s WF from exceeding its WA only by simultaneously controlling point- and nonpoint-source pollution in the future. Thus, it is recommended that great importance be attached to the rapid increase in the WFgray of the Tibetan Plateau resulting from rapid urbanization and that effective measures be implemented to control point- and nonpoint-source pollution, so as to safeguard sustainable development within the Tibetan Plateau and in downstream regions.


Introduction
As the "Asian Water Tower" [1], the Tibetan Plateau spawns rivers that are well-known both domestically and internationally, such as the Yellow River, the Yangtze River, and the Lancang River. It is a region with concentrated lakes, glaciers, multiyear snow cover, and permafrost, and is strategically important for producing, storing, and transporting water resources in China and even Asia [2]. The Tibetan Plateau abounds with water resources, and its water-use dynamics are vitally important for its own sustainable development and that in downstream regions [3]. With rapid socioeconomic development, the Tibetan Plateau has been undergoing rapid urbanization, which has significantly affected its water consumption and water-use structure [4]. Urban water use has become an important factor that affects regional water resources [5]. Accurately assessing the state and dynamics of urban water use on the Tibetan Plateau and, on this basis, reasonably planning the allocation of water resources are crucial for facilitating sustainable development within the Tibetan Plateau and in its downstream regions [6].
In recent years, researchers in China and elsewhere have conducted a series of studies on the entire Tibetan Plateau and its urban areas to assess their state of water use. Some researchers analyzed and assessed the spatiotemporal distribution and dynamic variation in water resources within the entire Tibetan Plateau through field observations, statistical analyses, and remote sensing monitoring. However, these studies often failed to sufficiently account for the effects of urbanization and human activity on water use [7,8]. Some researchers quantitatively assessed the state of water use in urban areas of the Tibetan Plateau. However, these studies often took into consideration the dynamics of the use of water resources for direct consumption (e.g., water used for domestic and industrial purposes) but failed to take into account the needs of such areas as environments and sewage purification for water resources [9][10][11] or considered only the impact of urban water contamination on the utilization of water resources [12]. Currently, an integrated assessment of urban water use on the Tibetan Plateau remains lacking.
The water footprint (WF) offers an efficacious means to understand urban water use. The WF refers to the amount of water resources required by all the products and services consumed by a certain population (an individual, a region, a country, or the world) within a certain time [13]. Different from the conventional "water withdraw" index, the WF includes not only blue water but also green and gray water. The blue WF (WF blue ) refers to the total amount of surface water and groundwater used during the manufacture of products and the provision of services. The green WF (WF green ) refers to the amount of rainwater consumed by evapotranspiration during the manufacture of products (mainly crops) [13]. The gray WF (WF gray ) refers to the amount of freshwater required to dilute the pollutant to meet the water-quality standards [13]. Thus, the WF can be used to comprehensively understand urban water use on the Tibetan Plateau, but the relevant research is still scarce.
The objective of this study was to take Xining, the largest city on Tibetan Plateau (Figure 1), as an example to understand the dynamics of WF under urbanization in this region. To achieve this, Xining's total WF, WF green , WF blue , and WF gray were first quantified year-by-year for the 2005-2018 period in a production way. Then, the difference between the WF and the amount of available water resources W A was determined, and the sustainability of water use in the region was assessed. Finally, the primary causes of the changes in Xining's WF and their impact on midstream and downstream regions were analyzed. The results of this study can provide a basis for thoroughly understanding the dynamics of urban water use in Xining and even the Tibetan Plateau and, thereby, offer support to facilitate sustainable water utilization in this region.

Study Area
Situated on the northeastern Tibetan Plateau and in the middle and lower reaches of the Huangshui River Basin, Xining (100 • 54 -101 • 56 E, 36 • 13-37 • 23 N) is a typical dendritic, semi-open valley city [14] that is surrounded by mountains and encompasses a total area of 7679 ha (Figure 1). With a continental plateau semiarid climate, Xining has an annual average temperature of 7.6 • C, a maximum temperature of 34.6 • C, a minimum temperature of −18.9 • C, annual average precipitation of 380 mm, and annual average evaporation of 1364 mm [15]. Xining has four districts (Chengdong, Chengxi, Chengzhong, and Chengbei), three counties (Huangzhong, Huangyuan, and Datong), a national economic and technological development zone, Chengnan New District, a biological science and technology industrial park, and Haihu New District. Xining is the only city on the Tibetan Plateau with a population exceeding one million people. By the end of 2019, Xining's permanent resident population had reached 2.39 million people.
Xining has a well-developed, dense water system that comprises the mainstream of the Huangshui River and its 56 tributaries, with a drainage area of more than 50 ha, including the Beichuan, Nanchuan, Shatangchuan, Yaoshui, Xinachuan, and Dongxia rivers. In 2015, Xining's water resources amounted to 1.21 billion m 3 , of which 1.19 billion m 3 (98.35%) was surface water. In addition, in 2015, Xining consumed 567.76 million m 3 of water [16]. In 2015, Xining discharged 99.81 million tons of urban wastewater and sewage [17]. In recent years, with the development of the Lanzhou-Xining urban agglomeration, Xining's urbanization process has accelerated. As a result, there has been a further increase in its urban water withdrawal and wastewater discharge.

Data Sources
Total water withdrawal on irrigation, livestock, industrial, and domestic sectors for Xining for the 2005-2018 period, used in this study to calculate WF blue , originated from the 2005-2018 Water Resources Bulletins of Qinghai Province [16].
Meteorological, land-use/cover, and river basin division data for Xining, used in this study to calculate WF green , originated from the China Meteorological Data Service Center (http://data.cam.cn, accessed on 7 November 2020), the European Space Agency (ESA) GlobCover Dataset [18], and the Third Water Resource Survey Data produced by the Hydrological and Water Resources Bureau of Qinghai Province, respectively. Based on Yang et al.'s study [19], a threshold of 20,000 (i.e., minimum sub-river basin area ≥20,000 m 2 ) was selected to divide the river basin into several sub-river basins.
Total wastewater discharge data and data for ammonia nitrogen and chemical oxygen demand (COD) discharged in wastewater for Xining, used in this study to calculate WF resulting from point-source pollution (WF gray,p ), originated from the 2005-2018 Statistical Yearbooks of Xining and Qinghai Province [17,20]. The Class III water standard stipulated in the Standard Limits for the Basic Items of the Environmental Quality Standards for Surface Water (GB 3838-2002) was adopted (i.e., ammonia nitrogen ≤ 1.0 mg/L, COD ≤ 20 mg/L) [21]. The natural background pollutant concentration in the absence of human impact was set to 0 mg/L [22].
Digital elevation model data, used in this study to calculate Xining's WF resulting from nonpoint-source pollution (WF gray,np ), originated from the National Geomatics Center of China (http://ngcc.sbsm.gov.cn/, accessed on 7 November 2020).
Data for Xining's amount of precipitation P, used in this study to calculate Xining's W A and water deficit W D , originated from the 2005-2018 Water Resources Bulletins of Qinghai Province [16].

Methods
Based on Liu et al.'s study [23], Xining's WF is given by where WF is the total water footprint, WF blue is the blue water footprint, WF green is the green water footprint, and WF gray is the gray water footprint ( Figure 2).

Calculation of WF blue
The WF blue measures the amount of surface water and groundwater used for production and service purposes within a region and within a certain period of time [24]. The WF blue is calculated as follows: where WU i is the amount of type i (e.g., irrigation, livestock, and domestic) water withdrawal, which was directly obtained from the Water Resources Bulletins of Qinghai Province.

Calculation of WF green
The WF green mainly refers to the total amount of rainwater consumed by evapotranspiration in cropland [25]. The WF green is calculated as follows [13]: where ET cropland,i is the evapotranspiration from cropland in the i th pixel. Cropland distribution used in this study originated from the ESA GlobCover Dataset [18]. The evapotranspiration (ET) was estimated using the Integrated Valuation of Ecosystem Services and Tradeoffs (InVEST) model [26].

Calculation of WF gray
The WF gray refers to the amount of freshwater required to dilute a certain pollutant load to meet a certain water-quality standard [13]. WF gray is calculated as follows: where WF gray,p is the water footprint resulting from point-source pollution, and WF gray,np is the water footprint resulting from nonpoint-source pollution. WF gray,p was calculated as follows [27,28]: where WF grey,p,(COD) is the water required to meet the standard of COD, WF grey,p,(NH 3 −N) is the water required to meet the standard of ammonia nitrogen. L i is the emission amount of pollutant i (ammonia nitrogen or COD), and C max,i is the maximum allowable concentration of ammonia nitrogen or COD in a water body, C nat is the natural background concentration of ammonia nitrogen or COD in a water body. V is the wastewater discharge volume in Xining. In this study, the values of C max for ammonia nitrogen and COD were 1 and 20 mg/L, respectively; the value of C nat was 0 mg/L [22]. WF gray,np was calculated as follows: where L is the amount of nitrogen, which was calculated based on the InVEST model, and relevant parameters were determined based on Sharps et al.'s study [29]. C max is the maximum allowable concentration of nitrogen in water body and C nat is the natural background concentration of nitrogen in water body.

Calculation of W A and W D
The W A refers to the amount of water that can be used by humans without causing substantial harm to ecosystems. The W A was calculated by deducting the regional ecological water demand W E from the regional precipitation as follows [30,31]: where P is the amount of precipitation, and W E is the ecological water demand (i.e., the amount of water resources required by plants to grow in the region). W E is calculated as follows: where ET ecology,i is the evapotranspiration from ecological lands (e.g., forestlands, grasslands, and wetlands) in the i th pixel, calculated based on the InVEST model [26]. W D , a concept analogous to the ecological deficit, is a measure of water resource stress. The W D is the difference between the W A and the WF, as shown below: where W D is the water deficit. When the WF of a region is greater than its W A (i.e., WF > W A ), there is a W D . In this case, the W A of the region is unable to meet the water demand. Conversely, when WF < W A , there is a water surplus. In this case, the W A of the region is able to meet the water demand.

Xining's WF in 2018
In 2018, Xining's WF exceeded the regional W A . Specifically, Xining had a WF of 10.9 billion m 3 and a W A of 3.5 billion m 3 . This translates to a W D of 7.4 billion m 3 (Table 1). Of the three types of WF, the WF gray was the largest contributor to the total WF. Xining had a WF gray of 9.7 billion m 3 , accounting for 89.4% of its total WF (Figure 3a). Its WF blue and WF green were 587 million and 563 million m 3 , respectively, accounting for 5.4% and 5.2% of its total WF, respectively. With respect to Xining's WF blue , water used for cropland irrigation was the largest contributor (30.5%) (Figure 3b), followed by water used for forestry, fishery, and animal husbandry (23.5%). Water used for industrial, domestic, and urban public purposes accounted for 15.5%, 14.8%, and 11.8% of Xining's WF blue , respectively. Water used for ecological and environmental purposes contributed the least (3.9%) to Xining's WF blue . With respect to Xining's WF gray , Xining's WF gray,p accounted for a relatively high proportion (55.7%) of its WF gray , whereas Xining's WF gray,np accounted for 44.3% of its WF gray (Figure 3c).   Figure 5 and Table 1 Figure 6b and Table 3 show the trends of Xining's WF gray . Xining's WF gray,p increased year-on-year. Specifically, it increased by 75.3%, from 3.1 billion to 5.4 billion m 3 . By contrast, Xining's WF gray,np decreased year-on-year. Specifically, it decreased by 3.4%, from 4.5 billion to 4.3 billion m 3 .

Comprehensive Assessment of Xining's WF
Known as "China's Water Tower", the Tibetan Plateau is the source of such major rivers as the Yangtze River and the Yellow River, and it is an important ecological barrier that ensures water safety in China and even the world. Many researchers in China and elsewhere have assessed the state of water use in Qinghai Province and Xining Municipality on the Tibetan Plateau based on the WF. However, the relevant research considered the changes in the WF primarily from the perspective of domestic use and was unable to satisfactorily reflect the respective trends of various WF types (e.g., WF blue , WF green , and WF gray ) [32,33]. Some researchers calculated the WF gray and used it to reflect the changes in water pollution in Qinghai Province. However, the relevant research focused on assessing the changes in the WF gray from the perspective of agricultural nonpoint-source pollution failed to simultaneously account for urban point-source pollution and agricultural nonpoint-source pollution and often underestimated the water deficits in this region [12,34]. For example, Sun et al. concluded that water scarcity occurred in 1.0% and 10.4% of the total areas of Qinghai Province and Tibet Autonomous Region [9], respectively. Zhang et al. noted that the Tibetan Plateau abounds with groundwater resources [35], showing an, overall, increasing trend.
In this study, the water use in Xining in the 2005-2018 period was comprehensively assessed based on the WF. By calculating Xining's WF blue , WF green , and WF gray and differentiating point-and nonpoint-source pollution when calculating its WF gray , the trend of Xining's WF was sufficiently determined. It was found that Xining was facing a severe water deficit. The results of this study are consistent with those of the Third Water Resource Survey of Qinghai Province and can provide a reliable scientific basis for regional water resource management.

Primary Causes of the Changes in Xining's WF
The increase in Xining's WF gray was the primary cause of the increase in its total WF (Figure 7), and the increase in its WF gray,p was the predominant cause of the increase in its WF gray (Figure 8). Xining's W Fblue and WF gray,np decreased year-on-year. These decreases, however, were unable to cancel out the increase in Xining's WF gray,p . Figure 8 shows the rates of contribution of the changes in various types of water use to the changes in Xining's total WF. As demonstrated in Figure 8, the rates of contribution of the changes in Xining's WF gray,p , the amount of water used for forestry, fishery, and animal husbandry, the amount of water used for urban public purposes, and the amount of water used for public environments are positive, whereas the rates of contribution of the changes in the WF gray,np , the amount of water used for industrial purposes, and the amount of water used for cropland irrigation are negative. Figure 9 shows the correlations between Xining's WF gray,p and socioeconomic factors. As demonstrated in Figure 9, Xining's WF gray,p is positively correlated with its urban population (R 2 = 0.95, p < 0.01), its urbanization rate (R 2 = 0.97, p < 0.01), the GDP (in 100 million RMB) of its secondary industries (R 2 = 0.84, p < 0.01), and the GDP (in 100 million RMB) of its tertiary industries (R 2 = 0.93, p < 0.01). Thus, urbanization played an important role in the increase in Xining's WF gray,p .

Impact of the Rapid Increase in Xining's WF on Sustainability within and outside the Region
The increase in Xining's WF gray has disrupted the balance between the supply of and demand for water resources for diluting the pollutant. This has significantly affected downstream water quality and posed a threat to ecological safety in downstream rivers and water supply safety in downstream cities ( Figure 10). The Huangshui River is an important water source for Xining. The region through which the Huangshui River passes is relatively densely populated and economically developed, and it is the political, economic, transportation, and cultural center of Qinghai Province. However, this region has a shortage of water resources and a low water-use efficiency [36], with an amount of water resources per person of 670 m 3 (approximately 30% of the national average), an amount of water resources per mu of cropland of 537 m 3 (approximately 37% of the national average), and an amount of water consumption per person of 278 m 3 (approximately 60% of the national average). Lanzhou plays a central role in the economic zones in the upper reaches of the Yellow River and the construction of the Lanzhou-Xining urban agglomerations, and it is also an important transportation hub and a commercial and financial center in northwestern China and a major inland port city that links the West Europe Land Bridge. The Huangshui River enters Lanzhou via Haishiwan in the Honggu District and is a primary tributary of the upper Yellow River [37]. In recent years, the Huangshui River has been relatively heavily polluted, which has attracted the attention of relevant authorities [38]. Pollution in the Huangshui River significantly affects water supply safety in downstream cities, including Lanzhou. This urgently necessitates the simultaneous control of point-and nonpoint-source pollution in Xining, with a view to achieving the goal of regional sustainable development. According to the United Nations Sustainable Development Goal 6-clean water and sanitation, there is an urgent need to effectively address water scarcity and water pollution across the globe before 2030. However, if the trend seen in the 2005-2018 period continues, Xining's WF will reach 12.7 billion m 3 in 2030, and its W D will increase to 9.3 billion m 3 , 3.1 times the regional W A (Figure 11). If measures are implemented to completely eradicate nonpoint-source pollution by 2030, Xining's WF will reach 8.6 billion m 3 , still 2.8 times higher than its W A . If point-source pollution is completely eliminated, Xining's WF will reach 5.0 billion m 3 , and its W D will reach 2.0 billion m 3 . Xining's WF will be less than its W A only when nonpoint-and point-source pollution are controlled simultaneously. To prevent Xining's total WF from exceeding its W A , its WF gray in 2030 needs to be reduced by 7.4 billion m 3 , 76.3% of its current WF gray .

Future Perspectives
In this study, the dynamics of water use in Xining were comprehensively assessed based on the WF. In addition, the driving forces of changes in WF and their effects on sustainability were also discussed. However, when calculating Xining's WF gray , only the most key pollutants were taken into consideration. Other toxic substances (e.g., heavy metals) in trace amounts may cause more harm to the human body and require more water to dilute. Limited by data availability, the environmental flow requirements were not considered. Thus, Xining's WF gray and water deficit may have been underestimated in this study. In addition, the model used to estimate Xining's WF gray,np is relatively simple and requires further improvement. Nevertheless, the results of this study basically agree with those obtained by the Third Water Resource Survey of Qinghai Province. In the future, process-based models (e.g., the Soil and Water Assessment Tool) and urban metabolism models will be employed to more accurately assess the historical process and future trend of the region's WF. In addition, we only calculated the water footprint and revealed the water deficit in this region, and the potential solutions as well as their feasibility and cost need further analyses.

Conclusions
Xining's WF displayed an increasing trend in the 2005-2018 period. Xining's total WF increased by 22.6% in the 2005-2018 period, from 8.9 billion to 10.9 billion m 3 . Its W A remained stable. Its W D increased by 23.7% in the 2005-2018 period, from 6.0 billion to 7.4 billion m 3 .
Of the changes in Xining's various types of WF, the changes in its WF gray contributed significantly to the changes in its total WF. The increase in Xining's WF gray resulting from intensifying urban point-source pollution was the primary cause of the increase in its total WF. Xining's WF gray,p increased by 75.3% in the 2005-2018 period, from 3.1 billion to 5.4 billion m 3 . While Xining's WF blue and WF gray,np decreased year-on-year, these decreases were unable to cancel out the increase in Xining's WF gray . A correlation analysis between Xining's WF gray,p and socioeconomic development showed that urban socioeconomic development was the predominant cause of the increase in Xining's WF gray,p .
Xining's WF has far exceeded the regional W A . This has caused the water quality in the middle and lower reaches of the Huangshui River, where Xining is located, to be significantly lower than the national standard and has even affected water supply safety in downstream cities, including Lanzhou. In the future, it is possible to prevent Xining's WF from exceeding its W A only by simultaneously controlling point-and nonpoint-source pollution. Thus, it is recommended that great attention be paid to the rapid increase in the WF gray of the Tibetan Plateau resulting from rapid urbanization and that the government establish a WF management system and implement effective measures to control pointand nonpoint-source pollution, so as to safeguard sustainable development within the region and in downstream regions.