Runoff observation uncertainty is one of the key unresolved issues in the field of hydrology. Current studies mainly focus on the uncertainty sources and their impacts, but the effects of observation uncertainty on changes of entire flow regime characteristics are still rare. This study collects daily runoff observation series from 1971 to 2020 at five hydrological stations in the water conservation zone of the Yellow River (Huangheyan, Tangnaihai, and Lanzhou stations in the Yellow River Source Region, Xianyang Station in the Weihe River, and Heishiguan Station in the Yiluo River). Changes in 16 metrics from five main flow regime characteristics (magnitude, frequency of events, variability, duration, and timing) are detected by the trend tests, and the effects of observation uncertainty on trends of flow regime metrics are evaluated by adopting a normal distribution error model and some metrics, i.e., the uncertainty width, significance change rate of slopes, coefficient of variation, and degree of deviation. Results showed that: (1) At all the stations, flow regimes showed significant increases in the low flow magnitude, and significant decreases in the high and average flow magnitude, variability and duration. At the Heishiguan Station, the magnitude, variability and duration metrics significantly decreased, while the frequency metrics significantly increased. At the Xianyang Station, the low flow magnitude and timing metrics significantly increased, while the high flow magnitude, frequency and variability metrics significantly decreased metrics. In the Yellow River Source Region, the low flow magnitude and high flow timing metrics significantly increased, while the low flow frequency, high flow magnitude and variability metrics significantly decreased. (2) Observation uncertainty considerably affected the trend changes of 28.75% of total flow regime metrics at all the five stations. Among these, the trends of 11.25% of total metrics changed from significance to insignificance, and those of 17.5% of total metrics changed from insignificance to significance. For the rest metrics, the trends remained the same, i.e., significant trends (18.75%) and insignificant trends (52.50%). (3) Observation uncertainty had the greatest impacts on frequency metrics, especially at the Xianyang Station, followed by the duration, variability and timing metrics. The magnitude metrics were impacted least.

Projecting changes in flow regime under climate change and reservoir operations is crucial for safeguarding water security, preserving biodiversity and restoring aquatic ecosystems of a basin. Leveraging bias-corrected climate model data, this study utilized the Soil and Water Assessment Tool (SWAT) with an improved reservoir module and Indicators of Hydrologic Alteration to project the impacts of climate change and reservoir operations on the flow regime of the Yangtze River basin in the 21st century. The results showed that: (1) The improved SWAT model outperformed the original version, enhancing the simulation accuracy of daily streamflow and hydrological indicators at representative stations. The NSE (Nash-Sutcliffe efficiency coefficient) and KGE (Kling-Gupta efficiency coefficient) of simulation and observation of daily streamflow increased by 0.01-0.26 and 0.01-0.08, respectively. (2) Climate change is projected to increase streamflow, in which the streamflow increases significantly from January to April (the maximum increase is 17%). Meanwhile, climate change increases the variability of pre-flood streamflow (the maximum increase is 18%), the frequency of extremely high pulse, the duration of extremely high pulse and the rise rate of flow. (3) Reservoir operations effectively replenish dry-season flows (the maximum increase is 67%), reduce the frequency of extremely high pulse (the maximum decrease is 16%), and shorten the duration of both extremely high pulse (by -10% to -4%) and extremely low pulse (by -51% to -38%). Reservoir operations improve the stability of flow. While upstream reservoir operations can partially counteract climate-induced streamflow changes, their regulatory capacity is constrained by operational rules and total storage capacity. Therefore, adjusting reservoir operation strategies in response to the impacts of climate change on streamflow becomes imperative.

Water provisioning in the Asian Water Tower (AWT) is crucial to meet the demand of economic development in basin countries, whereas both climate change and economic development are altering the balance of water supply and demand. Previous researches mostly focus on the problem of how the availability of upstream water resources affects downstream uses, but the benefits to economic development are yet addressed. On the basis of hydrological runoff data in the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP), we developed an index representing the degree of water stress mitigation by the AWT, and established a production-oriented model using Adaptive Regional Input-Output (ARIO), for the evaluation of economic benefit and its relationship with the degree of water stress mitigation. The results show that the water stress mitigation index of basin countries exhibits dependency on geographical location, and the Central Asian countries display as a core area with higher mitigation index and temporal variation. Meanwhile, the overall economic benefit of the AWT to mitigate water stress reaches as high as 13.06% of Gross Domestic Product (GDP) of all basin countries. The benefit exhibits a significant spatial heterogeneity, with the beneficiary countries mainly concentrated in Central and South Asia, exceeding up to 80% of GDP, such as Bhutan, Nepal, Tajikistan and Kyrgyzstan. Importantly, the benefit not only depends on the degree of water stress mitigation, but also on economic structure, notably demonstrated by Bangladesh. Furthermore, the marginal economic benefit of water stress mitigation by the AWT increases with the degree of mitigation. This implies that the reduced water supply will have a much greater impact on the countries highly benefited from the AWT, especially in the low developing countries. The disparity between countries that benefit the most and least, respectively, can reach up to 2.63 times. As mentioned above, the low developing countries receive the greatest economic benefits when there is an increasing water supply of the AWT. Therefore, the greater significance of AWT is illustrated by its role, not only in mitigating water stress, but also in advancing economic development in the basin countries. In this regard, the basin countries are suggested to take joint actions, so as to safeguard the AWT, and establish a long-term comprehensive water resources management that takes into account the balance of supply and demand. This study can provide evidences for promoting sustainable development of the economic circles around the AWT, especially in low developing countries.

As a conceptual framework for understanding contemporary sustainability challenges, the metacoupling framework emphasizes the dynamic interactions and multi-regional connections within human-environment systems. Utilizing this framework to investigate the interactions between water supply and demand within and between regions can provide theoretical support for the sustainable management of water resources. In this study, we applied the environmentally extended input-output method and the Wang, Wei, and Zhu (WWZ) value-added decomposition method to quantify the metacoupling intensity and connections in the sending-receiving and spillover systems of China's virtual water trade. We also assessed the impact of virtual water trade on various regions. The results indicate the following: (1) Internal consumption and trade dynamics (2012-2017): The average internal consumption of virtual water within a province accounted for 78.05% of the total virtual water volume. Provinces primarily engaged in long-distance virtual water trade, which was on average 4.96 times the volume of peripheral virtual water trade. The manufacturing and agricultural sectors were the largest consumers of virtual water. (2) Spillover systems and trends: Upstream and downstream virtual water spillovers associated with the spillover systems exhibited an increasing trend, averaging 46.28% of the total traded virtual water volume. Jiangsu and Jilin produced the highest upstream virtual water spillover, whereas Xinjiang and Heilongjiang produced the highest downstream virtual water spillover. (3) Sectoral drivers and spillovers (2017): In 2017, the manufacturing and construction industries were the primary drivers of the upstream virtual water spillover, whereas the manufacturing and agricultural sectors were the primary drivers of the downstream virtual water spillovers. Long-distance trade generated a greater amount of both upstream and downstream virtual water spillovers. (4) Contribution to SDG 6.4: Virtual water trade contributed to a 5.75% improvement in achieving China's SDG 6.4 target, with long-distance trade contributing more significantly than peripheral trade. The virtual water trade was the most beneficial in alleviating water stress in economically developed provinces with poor water resource endowments, but it had negative impacts on some economically underdeveloped provinces.