Watershed Water Policy based on a Glacier Water Service Perspective in the Typical Glacier Basins, Western China

Taking the Yangtze River Source Basin (YRSB) and Shule River Basin (SRB) as two typical cases, the sustainability of the water resources in these two basins was evaluated using the Level of Water Stress (LWS) from Sustainable Development Goal (SDG) 6.4.2, and the regulating eﬀect of the glacier runoﬀ on the LWS was quantiﬁed. From 2000 to 2030, the level of socioeconomic development in the YRSB is low, and the total water consumption is only about 0.18 × 10 8 m 3 whereas the SRB has a relatively high level of socioeconomic development and total water consumption is about 10 × 10 8 m 3 , i.e., 50 times higher than that in the YRSB. For the above reasons, the SRB’s LWS is much higher than the YRSB’s, resulting in a very low sustainability of water resources. As natural assets, glaciers ﬂow downstream in runoﬀ mode, so compensation at the watershed scale should be considered. In the basin, the optimal allocation of water resources is needed. At the inter-basin scale, the compensation mechanism of glacier water resources needs to be improved


Introduction
Water shortage is a challenging environmental problem in arid regions around the world in the 21st century (World Economic Forum, 2015) (Doungmanee 2016).Since 1960, the amount of globally available fresh water per capita has decreased by 55%.The global demand for fresh water is expected to increase by 50% by 2030.By 2050, 2.3 billion people are expected to live in areas with severe water scarcity, and agriculture will account for 70% of all the water withdrawal globally.The total economic cost of the global water crisis is estimated to be $500 billion per year (WWAP/UN-WATER, 2018).In particular, in arid regions, global climate change and urban economic development have exacerbated the problems caused by the limited water supply and water shortages (Ercin andHoekstra 2014, Vorosmarty et al. 2000) ( IPCC, 2013).The Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (AR6, 2021) stated that without significant reductions in greenhouse gas emissions, global surface temperatures are projected to rise by at least 2.1 °C by 2100 (Veal 2021).Under future climate change scenarios, the glacier retreat in global arid regions will intensify further.Glaciers, acting as a "water tower" or solid reservoir, provide a copious amount of freshwater to both the natural environment and humans, playing a crucial role in the sustainable utilization of water resources in arid regional oases across the world (Immerzeel et al. 2020).The global population inhabiting arid regions, which is highly reliant on glacier water resources, amounts to nearly 2 billion.The expeditious shrinkage of glaciers has had far-reaching effects on both socioeconomic and natural ecosystems in areas downstream of glaciers (Ding and Zhang 2015, 2018, Ding et al. 2020), encompassing food production, industrial and agricultural water requirements, ecosystem services, and economic activities, all of which extensively depend on glacier runoff (Chen et al. 2023, Milner et al. 2017).Due to fragile natural conditions, very limited water resources, and serious human disturbances, the risk of desertification in arid regions will continue to be an important negative factor affecting local socioeconomic activities (Allen et al. 2010).On the contrary, in the river source region of the alpine region, the glacier scale is larger, the glacier runoff changes are more stable, but the social water demand is less, and the water stress degree is lower.In view of the differences between the glaciers and their runoff and the social water demand in the lower reaches of the arid region and the alpine river source region, the watershed water resources policy needs to be treated differently (Wang et al. 2021).
The United Nations Food and Agriculture Organization (UN-FAO) and the Water Resources Organization (UN-Water) have used SDG 6.4.2 indicators (Level of the Water Stress, LWS) and their accounting methods to produce water stress progress reports in five pilot countries (Jordan, the Netherlands, Peru, Senegal, and Uganda) (FAO and UN-Water, 2018).However, the current LWS method and its assessment progress are mainly concentrated on arid areas that have no glaciers, and the assessment cases are limited to the national scale (Gleick 2014, Vanham et al. 2018) (Hannah and Max, 2020) and lack the scale of Global Glacier-covered Arid Areas (GGAAs).It is necessary to adapt the LWS assessment method for use at the GGAA scale (Wang et al. 2021).
The Shule River Basin (SRB) and Yangtze River Source Basin (YRSB) in western China are representative inland river basins in arid regions and the upstream areas of these great rivers contain glacier runoff.The natural and social environments in these two basins are significantly different, and their glacier water resource services have also exhibit significantly different characteristics, and thus, these two basins may require the implementation of completely different sustainable development strategies in the future.Therefore, it is necessary to conduct a comparative study of the glacier water resource services in these two basins.Based on research on the impact and adaptation of cryosphere changes, Chinese scientists were the first to propose the definition of cryospheric services: the benefits that the cryosphere provides to human society, including the resources, products, and benefits obtained directly or indirectly from the cryosphere system (Xiao et al. 2015).Therefore, glacier water resource services can be defined as the benefits that glacier water resources provide to human society, including resources, products, and benefits.Previous research related to glacier water resource services has mainly concentrated on the functional regionalization of glacier water resource services (Lin et al. 2019, Liu et al. 2018) and valuation of glacier services (Hong-Feng et al. 2009, Sun et al. 2021a, Zhang et al. 2019b), and the corresponding research results in other fields are very limited.Research on cryospheric services has produced a series of results.Although these studies still have many problems related to scientific understanding, research methods, and uncertainties (Qin et al. 2020), they still have great significance to the study of glacier water resource services in terms of theoretical guidance and methodological references.
The study improves the existing method of calculating the LWS, and the improved method considers the impact of glacier runoff.Based on the calculated Water Withdrawal Quantities (WWQs) of the socioeconomic systems in the YRSB and SRB from 2000 to 2030, as well as the LWS values of these two basins, the sustainability of the water resources in the two basins were evaluated, and corresponding strategies for sustainable development in these two basins were identified.The results of the study provide a reference for the assessment of water resource sustainability in glacial basins worldwide.

study area
The Yangtze River Source Basin (YRSB) (90°43'-97°13'E; 32°30'-35°35'N) is located in the upstream area of the Yangzi River on the Qinghai-Tibetan Plateau (QTP).The YRSB defined as the region above Zhimenda in the Yangtze River Basin.In terms of the administrative division, the YRSB is located in the Yushu Tibetan Autonomous Prefecture and the Haixi Mongolian-Tibetan Prefecture in Qinghai Province.The area of the YRSB is about 14.03×10 4 km 2 , and it contains many extremely high mountains (>3500 m).The YRSB has a semi-humid and semi-arid climate and is located in the plateau sub-frigid zone.The main vegetation types in the YRSB are grasslands and alpine meadows.There are 627 glaciers in the YRSB, with a glacier coverage area of 1010.39 km 2 , total glacier reserves of about 450 km 3 , an equivalent water volume of 5000×10 8 m 3 , and a glacier melting volume of about 9.89×10 8 m 3 (Fig. 1) (Gen andGeocryology 2001, Liu et al. 2015).In the YRSB, the population is very small, and human activities are very limited.About 272000 permanent residents lived in this region in 2017, and it had an urbanization rate of 30.40% and an annual Gross Domestic Product (GDP) of 4.25 billion CNY (Huang et al. 2019) (Table 1).The ratio of the three industries (primary, secondary, and tertiary) was 35.01:28.39:17.13.Due to the loose Family Planning Policy in the region, the population growth rate in the YRSB has been very high, the average annual increase was greater than 2.70% from 2000 to 2017.The primary industry is the largest and most important type of industry in the YRSB, but its proportion has been shrinking continuously, while the proportions of the secondary industry and tertiary industry have been increasing rapidly.
Figure 1 Location of the study area.In the figure, (a) is for the glacier region of western China, (b, c) are for the Shule River Basin and Yangtze River Basin The Shule River originates in the Qilian Mountains, northeastern QTP.The area of the Shule River Basin (SRB) (92°11'˜98deg30'E; 38deg00'˜42deg48'N) is about 5.28x10 4 km 2 .The upstream area of the SRB mainly contains mountain valleys at the western end of the Qilian Mountains, while its midstream and downstream areas are in the western section of the Hexi Corridor, and the terrain in these regions is flat.The SRB is the second-largest inland river basin in the Hexi Corridor, situated in northwestern China.The runoff of the mountain river is influenced by the snow cover, frozen ground, and glaciers.The average temperature per year ranges from 6.9 to 8.8degC, while the precipitation averages between 47 to 63mm.Additionally, the mean pan evaporation averages from 2897 to 3042mm per year (Wang et al. 2021).Therefore, the SRB is subject to a standard continental drought climate.There are 510 glaciers in the SRB, with a glacier coverage area of 409 km 2 , total glacier reserves of about 134 km 3 , an equivalent water volume of 1500x10 8 m 3 , and a glacier melting volume of about 1.65x10 8 m 3 (Fig. 1) (Gen andGeocryology 2001, Liu et al. 2015).The SRB in northwestern China is representative of global glacier-covered arid areas (Wang et al. 2021).In the global arid areas, "If there is water, there is an oasis.Without water, the oases cannot survive and become desertified."Water is the most critical controlling factor for eco-systems in arid inland watersheds (Cheng et al. 2014).Numerous research endeavors concerning agriculture, industry, ecology, and hydroelectric water usage have been implemented in the SRB, and the outcomes of these investigations hold paramount significance for the region's sustainable socioeconomic advancement.As of 2017, the region presented a permanent population of approximately 315,200 individuals, an urbanization rate of 38.50%, and an annual GDP of 20 billion yuan (Table 1).The proportion of the three industries was 9.27:52.43:38.48.Given the substantial migration of the population, the urbanization rate within the basin has transitioned from its prior regression to a gradual increase throughout the last two decades.
The YRSB has a single industry centering around animal husbandry, while the SRB has a variety of industries.Although irrigation agriculture has been developed in the SRB, there is also a high level of industrial development.The level of socioeconomic development in the YRSB is lower than that in the SRB.From 2000 to 2018, the population in the YRSB increased by 2.7%, which was much higher than that in the SRB (0.6%), but the population density was still much lower than that in the SRB (less than 1/3 that in the SRB).The area of sown crops in the YRSB was small, accounting for about 0.04% of the total basin area, while that in the SRB was large, accounting for 0.6-1.4% of the total basin area.The differences in the socioeconomic indicators in the two basins directly lead to obvious differences in the water consumption of their socioeconomic systems, which is also the main reason for the huge difference in the water stress in these two basins.

Water resources
Freshwater resources are what we usually call water resources that are directly utilized on land.They are composed of the water in rivers and lakes, mountain snow, glaciers, and groundwater.In arid and semi-arid areas, precipitation provides little recharge to the groundwater, and therefore, the sustainable exploitation of groundwater is limited.Because there is continuous mutual transformation between surface water resources and groundwater resources, there is very little non-duplicative quantity between groundwater resources and surface water resources in the YRSB and SRB (http://slt.gansu.gov.cn/xxgk/gknb/,http://slt.qinghai.gov.cn/),so the freshwater resources mainly consist of surface water in this study.
The water resource data used in this study included runoff data and water consumption data for the YRSB and SRB.Among them, the runoff simulation data were the output results of the Variable Infiltration Capacity-Chinese Academy of Sciences (VIC-CAS) model of Zhao et al. (2019).The input parameters and model data included the following: meteorological data from 52 national meteorological stations on and around the QTP, soil data, digital elevation model data, monthly river runoff data collected at the Changma and Zhimenda hydrological stations (Fig. 1b, c) between 1971 and 2013, 1 km resolution global surface coverage data products, glacier distribution data from the Second Chinese Glacier Inventory, and the downscaling scheme of the data outputs from four Coupled Model Inter-comparison Project five (CMIP5) Global Climate Models (GCMs) under the Representative Concentration Pathway 2.6 (Supplementary Table 1).The runoff simulation data have been scientifically verified in many studies (Wang et al. 2021, Zhang et al. 2019a, Zhao et al. 2019b).The interannual variation trend of the total runoff from 2014 to 2030 remained consistent under both RCP 2.6 and 4.5 scenarios, with minimal differences observed in the variation range.Therefore, in this study, we solely employed the RCP 2.6 scenario to simulate the runoff component of surface-water resources, given the aforementioned findings.
The Environmental Flow Requirement (EFR) is important to maintaining the sustainability of a basin's natural ecosystem and human development.Considering the available data, in this study the hydrology method was used, namely, the minimum monthly average flow method (Q90), to calculate the EFR (Wang et al. 2021).Based on the actual runoff data for 2000-2010, the average annual EFRs in the YRSB and SRB were calculated to be 19.11×10 8 m 3 and 4.20×10 8 m 3 , accounting for 12.58% and 40.60% of their average annual surface runoff, respectively.Due to the reduction in glacial runoff, the basins' ecological base flows have exhibited downward trends.During 2020-2030, their EFRs will reach 16.80×10 8 m 3 and 3.84×10 8 m 3 , respectively.

Water withdrawal quantity
The WWQ involves many indicators of the water resource system.The WWQ is composed of the Primary Industry Water Demand (PWD), the Secondary Industry Water Demand (SWD), the Tertiary Industry Water Demand (TWD), the urban and rural Residents' domestic Water Demand (RWD), and the Artificial Ecosystem Water Demand (AEWD).In this study, the WWQ between 2000 and 2017 was obtained from the 2000-2018 Water Resources Bulletin of Gansu and Qinghai (http://slt.gansu.gov.cn/xxgk/gknb/,http://slt.qinghai.gov.cn/).
To calculate the carrying capacities of the water resources in the YRSB and SRB under the current trends of socioeconomic development and water utilization between 2018 and 2030, we assumed that the main socioeconomic indicators and water demand would continue to follow the current trends over the next 13 years, during which time the rates of change of the main and unit indicators of socioeconomic development and its water demand in the future were set (Supplementary Table 2).Restricted by the availability of data, we used the socioeconomic indicators (e.g., population, GDP, and farmland area) multiplied by the indicators of the average water demand (i.e., the water demand by each resident for life, for irrigation per hectare of farmland, and for every 10,000 CNY of industrial GDP generated).

Level of water stress
In this study, we utilized the LWS metric to gauge the degree of exploitation of the regional water resources and the l extent of the carrying capacity and overload of said resources.The formula employed to calculate the LWS is as follows: LWS = D S , (2) where S represents the freshwater resources, and D stands for the freshwater withdrawal.The LWS can be classified into the following categories: low stress (<10%); low to medium stress (10-20%); medium to high stress (20-40%); high stress (40-80%); and extremely high stress (>80%).In this study, the freshwater withdrawal mainly refers to the WWQ , and the available freshwater resources should also be deducted the EFR value from the freshwater resources for maintaining the stability of the natural ecosystem.In addition, based on the setting of the available freshwater resources described in Section 3.1, the equation becomes LWS glacier = Dp R−R glacier −E , (4) where D P stands for the WWQ , Rrepresents the total runoff in the upstream area, E stands for the EFR , LWS glacier is the LWS when the total runoff does not include glacier runoff, andR glacier denotes the glacier runoff.

Compensation theory of glacier water resources
Natural resources are scarce.The premise of value accounting is that it can bring economic benefits and be measured reliably (El Serafy 2015)(United Nations, 2012).Glaciers are an important component of natural resources.As an important freshwater reservoir, glacier water resources undoubtedly provide economic benefits to humans, including a static frozen ice body and dynamic glacial meltwater.Static glacial solid ice produces considerable economic benefits including tourism and other aspects (Wang et al. 2020, Yuan andWang 2018), while dynamic glacial meltwater also produces huge economic benefits including agricultural irrigation, supplying fresh water, water conservancy, and power generation (Wang 1996, Wang et al. 2021).Therefore, glacial water resources are an important component of water resource assets and can be included in natural resource assets (Sun et al. 2021b).In contrast, some cryospheric elements do not meet the strict definition of natural resource assets, such as snow water resources (El Serafy 2015).Some cryospheric elements are rarely directly used by humans, and even cause disasters, such as ice avalanche, avalanches, and hail (Wang and Xiao 2019).Basic data for some cryospheric elements is lacking, such as data for frozen soil water resources (Zhao et al. 2019a).
Glacier water resources have spatial mobility.With the flow of glacier water resources, their services also produce spatial mobility.Among them, the supply area and the beneficiary area of glacier water resources and services may have the following four types of spatial relationships. 1) The supply area and the beneficiary area are in the same region.2) The beneficiary area is adjacent the supply area, and in the flow area which glacial water resources and services from provisioning area can potentially be delivered.3) The beneficiary area is distributed the flow area, but not adjacent the supply area.4) The beneficiary area is distributed outside the flow zone (Costanza 2008, Fisher et al. 2009, Serna-Chavez et al. 2014) (Fig. 2).
In addition to the glacial water resources and services that each river source region of the QTP receives from itself, most of the glacial water resources and services in the river source region of the QTP flow into southern Asia's transboundary countries and the lower reaches of China, while most of the glacial water resources and services in the arid inland river basin in the northern QTP flow from the upper reaches to the lower reaches (Figs. 1 and 3).It can be said that the loss of glacial water resources and services in the source areas of each basin and the upstream areas benefits other regions for free (or at low cost).Due to the special natural environment and the limiting government policies such as environmental protection and glaciers being seldom affected by upstream local human activities, the loss of glacial water resources and services mainly comes from the over use of glacier melt water resources caused by downstream human activities, environmental damage of the glacier melt water, the decline of glacial runoff services, the loss of glacier retreat caused by global warming and the resulting decline in solid ice services, which mainly occurs in the cross-border and across river regions (Sun et al. 2021b).
Figure 2 Possible spatial relationships between the service supply area and the beneficiary area of glacial water resources and resources.The blue circle marked with the letter P is the supply area of glacial water resources services, whereas the yellow circle marked with the letter B is the beneficiary area of glacial water resources and services.The gray circle marked with the letter F is the flow area within which glacial water resources and services from provisioning area can potentially be delivered.The benefiting area outside the provisioning area but within the flow area is marked as b f , while the benefiting area outside both the provisioning area and the flow area is marked as b n .Finally, the benefiting area that overlaps with the provisioning area is marked as b p .(repainted by Costanza 2008, Fisher et al. 2009, Serna-Chavez et al. 2014)

Total runoff and glacial runoff changes
Overall, the runoff in the upstream area of the YRSB is 10 times that in the SRB.Under the context of global warming, increasing precipitation leads to an upward trend in the total upstream surface runoff, while the glacier runoff changes little during 2000-2030.From 2000 to 2010, the average annual amounts of runoff in the YRSB and SRB were about 134.02×10 8 m 3 and 10.48×10 8 m 3 , respectively, including average annual amounts of glacial runoff of about 5.54×10 8 m 3 and 2.64×10 8 m 3 , respectively.From 2000 to 2010, glacier runoff accounted for 25.2% of the total annual runoff in the SRB, while it only accounted for 4.1% in the YRSB (Fig. 3).
Under the RCP 2.6 scenario, surface runoff in the SRB will maintain an increasing trend from 2010 to 2030.The surface runoff in the YRSB will maintain an increasing trend during 2010-2025; but after 2025, the surface runoff will decrease slightly.The variation trends of the available freshwater resources in these two basins are different due to the difference in the runoff variations.Forecasts for 2020-2030 have indicated that compared with 2000-2010, the average runoff in the YRSB and SRB will increase by 27.03% and 25.10%, respectively, whereas the average glacial runoff will decrease by 3.70% and 11.74%, respectively (Fig. 3).

Changes in the water withdrawal quantity
The WWQs in the YRSB and SRB exhibit significant differences from 2000 to 2030 due to the difference in their socioeconomic conditions.In the YRSB, because the population density and the level of the socioeconomic development are lower and the water use efficiency is further improved, the WWQ exhibits a decreasing trend from 2000 to 2030.In the SRB, because the population density is larger, the level of socioeconomic development is higher, and the basin is affected by theComprehensive Plan for Reasonable Utilization of Water Resources and Ecological Protection in Dunhuang (2011-2020)(http://www.gov.cn/gzdt/2011-06/22/content1890180.htm)(amongthem, the quota for water resource utilization is an important part), the WWQ exhibited a previous increase, followed by a decrease, and finally an increase during 2000-2030.From 2000 to 2030, the average annual WWQs in the YRSB and SRB are about 0.18×10 8 m 3 and 8.91×10 8 m 3 , respectively.The average annual WWQ in the YRSB is only 1/50 that in the SRB.In particular, the primary industry is the largest water use sector in both basins, and the other categories of water demand accounted for only a small proportion.Thus, the trend of the PWD is consistent with that of the WWQ (Fig. 4).
From 2000 to 2018, the WWQ in the YRSB decreased from 0.18×10 8 m 3 in 2000 to 0.16×10 8 m 3 in 2018, whereas in the SRB, it increased from 5×10 8 m 3 in 2000 to 8.39×10 8 m 3 in 2018.Under the current trends of the climate and economic changes, during 2019-2030, the WWQ in the YRSB will gradually decrease, while in the SRB, it will continue to increase (Fig. 4).By 2030, the WWQ of the two watersheds will reach 0.18×10 8 m 3 and 12.01×10 8 m 3 respectively.In the YRSB, the PWD significantly decreases from 2003 to 2030 mainly because of water use efficiency improvements and enhanced ecological protection.In contrast, the PWD in the SRB continuously increases from about 5.27×10 8 m 3 in 2000 to 10.99×10 8 m 3 in 2030 due to the continuous expansion of the farmland area and the increased water demand for irrigation (Ma et al. 2019).

Changes in the level of water stress
From 2000 to 2030, the LWS in the YRSB is very low (<0.003), which indicates that the quantity of available freshwater resources is much higher than the WWQ of the socioeconomic system.In contrast, during this period, the LWS in the SRB is always at a high level, the LWS projected to maintain a remarkably high level of 0.68-1.66throughout 2000-2030 in the SRB.The annual average LWS in the SRB is anticipated to rise from 0.93 during 2000-2017 to 1.74 during 2018-2030.The LWS is even greater than 1.0 in some years, that is, the total water consumption is close to or greater than the available water resources, which indicates that the water resources are close to or in a state of overload.The LWS in the YRSB exhibits a fluctuating decreasing trend from 2014 to 2030, while the SRB exhibits a upward trend because the lack of expansion of the cultivated land and the implementation of agricultural water quota planning in the SRB.In the YRSB, the LWS only increases by an average of 0.00007 per year if we subtract the glacial runoff from the available freshwater resources during 2000-2030.If the proportion of glacial runoff in the total runoff continues to decrease from 4.00% in 2000-2017 to 3.20% in 2018-2030, the inhibitory effect of the glacial runoff on theLWS will decrease slightly, which also shows that glacial runoff has very little influence on the LWS in the YRSB (Fig. 5).The vast majority of glacial water resources in the YRSB flow to downstream areas, the contribution of the glacial water resources to the water stress in a basin is small, but it makes a large contribution to the water resources across basins.In the SRB, if the glacial runoff is deducted from the available freshwater resources, theLWS in the SRB rises by an average of 0.71 annum (Fig. 5).This reflects the regulating service of glaciers on the balance of regional water resource supply and demand.A decrease in the proportion of glacial runoff in total runoff from 23.86% in 2000-2017 to 18.47% in 2018-2030 significantly diminishes the inhibitory effect of glacier runoff on LWS, exacerbating the future water crisis.Regardless of glacier runoff, SRB will struggle to achieve the water supply and demand balance target of SDG 6.4.2 by 2030.

Glacier water compensation
Based on the above analysis, glaciers are crucial to the sustainable development of the water resources in these two basins.The spatial flow of the glacier water resources and the related services are the basis of water resource compensation, and the principle of compensation is that the user and beneficiary/owner pay.The value of the glacier water resources and their total service supply is the maximum amount of compensation, and the theoretical amount is based on the actual use of the glacier water resources.In the source region of the YRSB, the glacier water resources flow to the middle and lower reaches of Yangtze River, mainly produce benefits in there, while the upper reaches receive little benefit.Therefore, the compensation of the glacier water resources in the source region of the YRSB should follow the outside-in compensation mode and the national vertical compensation mode, that is, the inter-basin compensation mode from outside the source region to the source region.
The beneficiaries of the glacial water resources in the SRB include the upper, middle, and lower reaches.Among them, the middle and lower reaches are the main beneficiary areas.The glacial water resources and their services are mostly consumed within the basins.According to the principle of user pays and beneficiary pays, the compensation of glacier water resources in the SRB should mainly follow the horizontal compensation from the middle-lower reaches of the basin to the upper reaches.For the above two types of basins, in addition to clear compensation subjects, for the other beneficiary areas that are difficult to define, the compensation amount of the glacier water resources should be borne by the state (Xu et al. 2019).Generally, the inter-basin compensation of the glacial water resources in the source region of the YRSB should be adopted to ensure the sustainable utilization of the basin's water resources and to improve the ability to achieve synergistic ecological environmental protection and social and economic development.Internal regulation and compensation are mainly used to improve water resource sustainability and to promote sustainable development of the social and economic system in the SRB.

Discussion
By comparing the two basins, we identified several characteristics.As the most important ecological barrier area (Sun et al. 2012) and restricted development area in China (Fan 2015), the level of socioeconomic development in the YRSB was very low, its industrial structure was simple, and its proportion of animal husbandry was very high.The level of socioeconomic development in the SRB was higher than that in the YRSB, its industry was varied, and its irrigated agriculture was developed.The WWQ in the YRSB was very small, and it was mainly composed of the water demand for animal husbandry and the RWD.The WWQ in the SRB was very large, and the water demand for irrigated agriculture accounted for the vast majority of the WWQ.The LWS in the YRSB was very low, and the glacier runoff only reduced the LWS in the YRSB slightly.However, in the SRB, the LWS was very high, and the glacial runoff reduced the LWS significantly.
Based on these results, as typical representatives of inland basins in northwestern China's arid region and the upstream areas of many large rivers in China, the glacier water services in these two basins were significantly different.The upstream areas of the large rivers in China represented by the YRSB are important supply areas for the ecosystem services in China, their level of socioeconomic development is low, and their WWQs are limited.Affected by governmental policies, restrictions, and other factors, these basins are unlikely to contain large-scale economic development activities in the future, so their WWQs will be maintained at a very low level.Correspondingly, the sustainability of the water resources in these basins will always be at a very high level, and they are the output areas of the glacier water resources and their services.The inland basins in northwestern China's arid region represented by the SRB have a low level of socioeconomic development, which results in their WWQs being at high level.Therefore, the sustainability of the water resources in these basins will remain at very low levels, and most of their glacier water resources and their services are mainly consumed within the same basin.
The inland basins in northwestern China's arid region represented by the SRB face the problem of high LWS values, which are mainly caused by irrational water withdrawal, such as inefficient and excessive withdrawal of the basins' water resources.Therefore, for such basins, internal control measures should immediately be adopted in order to control the PWD, especially to control the water withdrawal for farmland irrigation, to guide the agricultural population transferrin a transition to non-agricultural industries, resulting in a lower water demand and more water efficient industries.This plan would improve the water resource carrying capacities of these basins and support sustainable development of the socioeconomic systems in these basins (Wang and Wei 2019).The upstream areas of the great rivers in China represented by the YRSB are the supply regions of glacier water resources and their services.Most of their glacier water resources and services flow to their midstream and downstream areas and support the socioeconomic development of their midstream and downstream areas.According to Chinese law, the property rights of natural resources belong to the country, are supervised and used by local governments or relevant departments, and local residents have the right to use them (Diao, 2011).Therefore, these basins have experienced the phenomenon of the loss of the right to use the glacier water resources due to the spatial flow of the glacier water resources and their services, as well as the phenomenon of deviation between the benefits and costs (benefits: the benefits provided by the glacier water in these basins to the entire society; costs: the social cost required to maintain the stability of the quantity and quality of the glacier water resources in these basins).A cross-basin compensation system should be implemented in these basins to make up as much as possible for the loss of glacier water resources and their related cost deviations and to improve the ability to achieve synergistic ecological environmental protection and socioeconomic development in the QTP water tower areas such as the upstream areas of great rivers.
Previous studies have shown that under the background of global warming, the cryospheric elements in the alpine mountainous regions of western China have undergone significant changes.In particular, in recent decades, mountain glaciers have rapidly melted, which has led to significant changes in the river runoff in the mountainous basins of western China (Ding and Zhang 2015, 2018, Ding et al. 2020) (Chen et al., 2019).These changes were specifically manifested as follows.In the basins in the western part of the Heihe River Basin, the runoff increased, and the drier the basin, the greater the proportion of glacier runoff.In the basins in which the number and areas of small glaciers are large, the glacier runoff and its proportion have continued to decrease.In the basins in which the number and areas of small glaciers are large, the glacier runoff and its proportion initially increased and then decreased.In some basins, the peak of the glacier runoff has or is about to occur (Ding and Zhang 2015, 2018, Ding et al. 2020) (Chen et al., 2019).After 2030, in some river basins of western China, especially the arid regions in northwestern China, their surface water resources will continue to decrease, their glacier water resource services will continue to decrease or even disappear completely, and the problem of the increasing LWS will gradually occur.It is urgent that various measures be implemented to reduce the various negative impacts brought about by the reduction of glacier water resources and the weakening of their services in order to improve the adaptability of the residents in the western China to climate change and its impacts.
The upstream areas of the great rivers on the QTP are the debt region for glacial resources, and this debt benefits the midstream and downstream areas.Therefore, it is necessary to establish a national or downstream-led water resource compensation mechanism.Based on the changes in the flow of the glacier water resources, the river basin water resources on the QTP should be managed comprehensively, and the source area should be appropriately compensated for the loss of glacial water resources.

Conclusion
The proportion of glacial runoff in the YRSB is very small and will continue to decrease in the future.The glacial runoff in the SRB accounts for a large proportion, and it may increase in the next few years.The level of social and economic development in the YRSB is very low, the industry is singular, and the proportion of animal husbandry is very high.The SRB has diversified industries and relatively developed irrigation agriculture.The water consumption of the social and economic systems in the YRSB is very small and is mainly used for animal husbandry and residential life.The water consumption of the social and economic system in the SRB is very large, and the water consumption for farmland irrigation accounts for the majority.
The water stress in the YRSB is very low, and the glacial water resources have a very weak alleviating effect on the water stress, while the water stress in the SRB is very high, and the glacial water resources have a large alleviating effect on the water stress.
Most of the glacier water resources and services in the SRB are consumed within the basin, and its sustainable development strategy should focus on internal control.In the future, the improvement of the irrigation efficiency and efficient water-saving measures for use in agriculture will be the key measures needed to reduce the LWS and ensure sustainable development in the SRB.The vast majority of glacial water resources and their services in the YRSB flow to the downstream areas, and a cross-basin water compensation system should be implemented to enhance the coordination of the ecological environmental protection and the social and economic development.
Overall, the YRSB is rich in freshwater resources, while these resources are very limited in the SRB.The glacial water resources and their services differ significantly in the source regions of major rivers and their inland river basins in arid regions.The YRSB and other major rivers have high water resource sustainability and are the supply and export regions of glacier water resources and their services.The sustainability of the water resources in the SRB and even in other inland river basins in arid areas is very low, and the glacial water resources and their services are mainly supplied from within the basin.

Figure 3
Figure 3 Runoff in the YRSB (left) and SRB (right) from 2000 to 2030

Figure 4
Figure 4 Changes in the WWQ in the YRSB (left) and SRB (right) from 2000 to 2030

Figure 5
Figure 5 Changes in the LWS in the YRSB (left) and SRB (right) from 2000 to 2030

Nomenclature
SDGs: Sustainable Development Goals GGAAs: Global Glacier-covered Arid Areas SRB: Shule River Basin YRSB: Upstream region of the Yangtze River Basin LWS: Level of Water Stress UN-FAO: United Nations Food and Agriculture Organization EFR: Environmental Flow Requirement VIC-CAS: Variable Infiltration Capacity-Chinese Academy of Sciences IPCC: Intergovernmental Panel on Climate Change CMIP: Coupled Model Inter-comparison Project GCM: Global Climate Model RCP: Representative Concentration Pathway WWQ: Water Withdrawal Quantity PWD: Primary industry Water Demand SWD: Secondary industry Water Demand TWD: Tertiary industry Water Demand RWD: Residents' domestic Water Demand AEWD: Artificial Ecosystem Water Demand

Table 1
Comparison of natural and social attributes in the two basins