A clear understanding of the changes of water resources under the background of environmental changes is of great significance for scientific management and utilization of water resources in China. This study systematically analyzed the spatial-temporal variations of surface water resources in China since 2000. Water vulnerability in current (2010s) and its trends from 2000 to late-2010s in different regions of China were also summarized. In addition, the correspondingly adaptive measures to counter regional risks to water resources were proposed. We concluded that the runoff of major rivers had been decreasing in eastern China and increasing in western China during 2000-2018. In the arid area of Northwest China, the alpine runoff has shown an overall upward trend since the late-1990s/early-2000s, with a 10%-25% increase caused by the increase of glacial meltwater and precipitation. While the runoff of each hydrological station in the 2000s-2010s was 34.7% lower than that in the 1950s-2010s on average. The increases in precipitation and glacial meltwater with global warming caused a rapid expansion of lakes in the Qinghai-Tibet Plateau and Xinjiang, thus leading to an increase in total area and water quantity of lakes in China from 1995 to 2015. The mean contribution rates of climate change and human activity to runoff change in river basins of China were 53.5% and 46.5%, respectively, during the period of 2000-2010s. The driving factor of runoff change in many river basins has gradually changed from climate change (1950s-2000) to human activity (2000-2018). During 2000-2018, the contributions of human activities to runoff change were 50%-80% in major rivers of eastern China. The vulnerability in most areas of Northwest China and North China is generally high, with the vulnerability index greater than 0.6. Comparatively, in Northeast, East, South, and Central China, it is lower or not vulnerable. In Southwest China, the vulnerability varies greatly with Yunnan and Sichuan relatively low while Chongqing and Guizhou relatively high. The precipitation increase, the application of water-saving technology, the establishment of flood control and drought relief engineering facilities, and the introduction of relevant policies and measures have helped to gradually reduce the vulnerability of water resources in most areas of North and Northwest China (except Xinjiang) from 2000 to 2010s. Water vulnerability has been increasing in southern China, caused by climate change and the development of industry and agriculture, which increases water resource exposure since 2000. Based on the typical risk factors and vulnerability characteristics of water resources in different regions, this study proposed some targeted adaptive measures correspondingly so as to scientifically deal with the problems of surface water resources in China.

The source region of the Yellow River (SRYR) in the northeastern Tibetan Plateau is critical for supplying water resources to downstream areas. However, streamflow in the SRYR declined despite a slight increase in precip-itation during the past few decades. The SRYR experienced significant frozen ground degradation with climate warming, but how frozen ground degradation influences runoff remains unclear. This study investigated the changes of the precipitation-runoff relationship using the double-mass curve method and examined the impact of long-term spatiotemporal changes in frozen ground on the water balance components using the geomorphology -based eco-hydrological model (GBEHM). The results showed that the precipitation-runoff relationship changed significantly since 1989 in the SRYR from 1960 to 2019. In the same period, the areal mean value of the maximum thickness of seasonally frozen ground (MTSFG) decreased by 0.10 m/10a and the areal mean active layer thickness (ALT) of permafrost increased by 0.06 m/10a. Besides, 21.0 % of the entire SRYR has degraded from permafrost to seasonally frozen ground (SFG). Runoff decreased mainly in the region with elevation below 4200 m, where the evapotranspiration increase exceeded the precipitation increase. Frozen ground degradation significantly altered the hydrological processes, which is reflected by the increased subsurface runoff and the decreased surface runoff. The total water storage increased by 2.9 mm/a in the permafrost region due to the increase in active layer thickness and by 5.7 mm/a in the degradation region where permafrost completely thawed during 1960-2019. The runoff seasonality was also altered, being indicated as an increase in winter runoff. These findings help provide a better understanding of the runoff change under climate warming in permafrost-affected regions and provide insights into future water resources management in the Yellow River basin under the climate warming.

The impact of climate change on glaciers and the hydrological processes in the easternmost end of the eastern Tianshan Mountains has yet to be understood. This study investigated the glacier change (area, surface elevation and volume change) and analyzed the variation of the observed runoff series over the past four decades in the Yushugou Basin, Eastern Tianshan Mountains. The hydrological processes were also simulated through the HBV-light model to quantify the impact of climate change on the glacier and runoff. The results showed that the glacier area has decreased by 13% and the total volume has decreased by 0.018 km(3) over the past four decades. A significant increasing trend (p < 0.01) was detected for the annual runoff and monthly runoff (May to September; p < 0.01). The simulation results revealed that the Yushugou River was highly recharged by glacial runoff and a negative tendency was found for the glacier mass balance on the basin scale over the past 38 years. As a region with an extremely dry climate and the lowest precipitation in the Tianshan Mountains, the observation and simulation of glaciers is critical to the security assessment of local water resources.

Glacier shrinkage and permafrost degradation have significantly altered the hydrological processes in cryospheric regions through releasing water and absorbing more energy from the ground. Considering the upper Shule River Basin (USRB) as a typical cryospheric-dominated watershed on the Tibetan Plateau, an extended Budyko framework considering glacier shrinkage and permafrost degradation was constructed to investigate their contributions to runoff change. Runoff exhibited a significant increasing trend during 1970-2015, with a tipping point appearing around 1998. Thus, 1970-1998 and 1999-2015 were identified as the baseline and changing periods, respectively. During the two periods, runoff was the most sensitive to landscape alteration, followed by precipitation. The increase in precipitation contributed 93.1 % to the increase in runoff, while its effect was partially offset by the negative contribution of potential evapotranspiration (-3.9 %). Glacier shrinkage and permafrost degradation accounted for a 8.0 % and 24.8 % increase in runoff, respectively. Part of these increases were offset by changes in other landscape factors (-22.0 %). Our study elucidates the impacts of glacier shrinkage and permafrost degradation on hydrological processes in cryospheric basins.

The hydrological regulation function of glaciers in different watersheds is different. This study took the Yanggong River Basin (YRB) and Urumqi River Basin (URB) as two typical cases, to explore the runoff change differences and their responses to climate factors from 1979 to 2017. In the past 39 years, the YRB's annual runoff showed an insignificant trend of increasing first and then decreasing, while the URB's increasing trend was significant. From the 1980s to the 2010s, the YRB's monthly runoff extremum occurred earlier than before, and the peak value decreased. The time of monthly runoff extremum in the URB has not changed, but the peak value is increasing. Both basins experienced an increase in annual temperatures from 1979 to 2017, with the rate of warming being more pronounced in the URB. The precipitation in the YRB had no significant trend from 1979 to 2017, and the significant increasing trend in annual precipitation extended in the URB from 1998 to 2010. The YRB's runoff change was mainly influenced by the flood season precipitation, while the URB's runoff increase was due to the temperature rise causing faster glacier and snow melt, and the summer precipitation change. In continental glacier basins with many glaciers, the regulation function of glaciers on total runoff is more significant.

The source region of the Yangtze River (SRYR), located on the eastern Tibetan Plateau, is an essential part of the Asian Water Tower and plays an important role in the downstream water resources. Significant changes in frozen ground caused by increases in air temperature have been widely reported in the past several decades, which has greatly affected regional runoff. This study evaluated the spatiotemporal variations in frozen ground and hydrological components by utilizing a geomorphology-based eco-hydrological model (GBEHM) and investigated the reasons for runoff changes based on the Budyko framework. The results showed that the area with an elevation range of 4700-4800 m located in the permafrost region was the main source area of runoff generation from 1981 to 2015. Compared with the permafrost region, the seasonally frozen ground (SFG) region had a larger ratio of annual evapotranspiration to annual precipitation, although the aridity indices in the two regions were very similar. From 1981 to 2015, the mean value of the maximum frozen depth of SFG (MFDSFG) decreased by 12.3 cm/10 a and the mean value of the active layer thickness (ALT) of permafrost increased by 4.2 cm/10 a. The annual runoff in the SFG region decreased, while that in the permafrost region increased. Runoff change was more sensitive to precipitation change in the higher altitude regions that were mainly covered by permafrost than in the lower altitude regions that were mainly covered by the SFG, while the evapotranspiration change in the transition zone was more sensitive to climate change. An abrupt change in the annual runoff time series was detected in 1989, 2004, and 2004 in the SFG region, the permafrost region and the entire SRYR, respectively, and the annual runoff change from period 1 (1981 to change point) to period 2 (change point + 1 to 2015) were - 25.7 mm, 33.8 mm and 25.8 mm respectively. Frozen ground degradation contributed changes of -15.0 mm, - 8.8 mm and -11.6 mm to the annual runoff in the SFG region, the permafrost region and the entire SRYR, respectively. This result implied that frozen ground degradation had a negative impact on regional runoff in the SRYR. These findings deepen our understanding of frozen ground and its hydrological changes and are helpful for water resource management in the SRYR.

On the Tibetan Plateau, climate change, particularly increases in air temperature, significantly affects cryospheric and hydrological processes. Based on 5 typical future climate scenarios from the Coupled Model Intercomparison Project (CMIP5) under emission scenario RCP4.5 and a distributed ecohydrological model (GBEHM), this study analyzes the potential characteristics of future climate change (from 2011 to 2060) and the associated effects on the cryospheric and hydrological processes in the upper Heihe River Basin, a typical cold mountain region located on the northeastern Tibetan Plateau. The precipitation, air temperature, and frozen ground elasticities of runoff/evapotranspiration are then estimated based on the simulation results. The typical future climate scenarios suggest that air temperature will increase at an average rate of 0.34 degrees C/10a in the future and that precipitation will increase slightly by 6 mm/10a under the RCP 4.5 emission scenario. Based on the GBEHM-simulated results, due to the increase in air temperature, glaciers would be reduced to less than 100 million m(3) by 2060, the permafrost area would shrink by 23%, the maximum frozen depth of seasonally frozen ground would decrease by 5.4 cm/10a and the active layer depth of the frozen ground would increase by 6.1 cm/ 10a. Additionally, runoff would decrease by approximately 5 mm/10a, and evapotranspiration would increase by approximately 9 mm/10a. The estimated elasticities indicate that annual runoff would decrease at an average rate of 24 mm/degrees C and evapotranspiration would increase at an average rate of 21 mm/degrees C with rising air temperature in the future. The impacts of increased air temperature on hydrological processes are mainly due to changes in frozen ground. The thickening of the active layer of the frozen ground increases the soil storage capacity, leading to decreased runoff and increased evapotranspiration. When the active layer depth increases by 1 cm, annual runoff decreases by approximately 1.3 mm, and annual evapotranspiration increases by approximately 0.9 mm. In addition, the shift from permafrost to seasonal frozen ground increases groundwater infiltration, which decreases surface runoff. Compared to that over the past 50 years, the effect of increased air temperature on the frozen ground in the upper Heihe River Basin will be greater in the future, which would result in a faster reduction in runoff in the future considering the effects of global warming.