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[Display omitted] • GEE-based water color inversion for rapid water quality monitoring. • The FUI local change index was constructed to quantify the water color change. • The annual average FUI value of large reservoirs in China was less than 11. • Construction of the reservoir reduced the FUI value by an average of 5 levels. • The FUI of reservoir water is positively correlated with its TLI and turbidity. Water color is an essential indicator of water quality, and research methods of water color based on remote sensing have become a trend in large-scale water quality monitoring. Large reservoirs are important for regional development. However, large-scale research into the quality of the water is lacking. Accordingly, to study the spatiotemporal water quality conditions of large reservoirs in China, this study conducted an inversion of water color over the past 23 years. Using Landsat series images, the Dynamic Surface Water Extent (DSWE) algorithm, and the Forel-Ule index (FUI), the water surface of 746 large reservoirs in China was sampled and the water color was inverted using the Google Earth Engine (GEE) platform. The FUI local change index was established to analyze changes over time. The results showed that the annual average FUI value of large reservoirs in China was less than 11, showing non-eutrophic and green water bodies. From the water color analysis of 91 large Type I reservoirs, it was found that the water quality of the reservoirs in the Northeast region was poor (FUI greater than 12), with many aquatic plants and algae or highly turbid and greenish-brown waters, mainly for flood control and irrigation. In southern China, there were primarily mesotrophic and oligotrophic reservoirs (FUI not greater than 8) with green or greenish-blue water color, which were mainly used for water supply and power generation. The construction of the southern reservoirs reduced the FUI values by an average of five levels, apparently changing the water color. The FUI local change index revealed that the FUI values of large reservoirs in China showed an overall decreasing trend from 1999 to 2007, stabilized after 2007, and then fluctuated after 2013. This study also found that the water color index of large reservoirs in China is positively correlated with the trophic status and turbidity of water bodies to some extent. Therefore, this study can compensate for the lack of water color remote sensing in large-scale reservoir water quality studies and support large-scale reservoir monitoring and management. [ABSTRACT FROM AUTHOR]

The Grand Ethiopian Renaissance Dam (GERD) filling and operation is a highly sensitive issue for Egypt and Sudan. A recently accepted manuscript by Heggy et al (2021 Environ. Res. Lett. 16 074022) assessed the water deficit for Egypt based on different scenarios for the first filling of GERD lake and estimated 31 billion cubic meters per year under a 3 year filling scenario. We would like to present grossly mischaracterized assumptions, inaccurate data, and controversial conclusions found in this accepted manuscript through this rebuttal. Although the accepted manuscript does not include any new analysis of the River Nile Hydrology, the results of previous substantive studies were misinterpreted or ignored. Moreover, we have serious concerns about the basic hydrological assumptions that are the basis for the economic impacts and the potential loss of the Egyptian agricultural lands. The main methodological flaws of concerns are (a) how the deficit is calculated, losses from GERD, especially the evaporation losses that contradicts several previous studies (e.g. Wheeler et al 2016 Water Int. 41 611–34; Eldardiry and Hossain 2020 J. Hydrol. 125708; Wheeler et al 2020 Nat. Commun. 11 1–9); (b) neglecting the normal role of High Aswan Dam (HAD) reservoir and directly linking the deficit of the water budget to an immediate loss of agricultural lands with all other associated exaggerated economic impacts estimates; (c) including highly exaggerated seepage losses from the GERD lake; (d) neglecting the updated situation of Aswan High Dam reservoir levels and the GERD’s infrastructure itself, and (e) quantifying the impacts of potential changes of water level on HAD reservoir on the Nubian aquifer. We herein present a direct fact-checking approach including the studies cited in the accepted manuscript. We believe that this critical comment paper can serve as a basis for defending scientific integrity and contributes to cooperation and peace in the region.

Sediment deposition is one of the key issues during the processes of reservoir construction and operation. On the basis of the observed sedimentation patterns in the dam area of Three Gorges Reservoir (TGR), a schematic diagram of fine-grained particles sedimentation in the dam area was proposed. Critical slope was used to judge the incipient motion of the fine-grained deposits. It was assumed that the mass transport processes of fine-grained deposits were dominated by the shallow flow equations. A numerical method of gravity-driven motion of fine-grained deposits was developed and combining with a 3-D flow and sediment transport model. Then, the sedimentation patterns of fine-grained particles in the dam area of the TGR were simulated by the developed method. The simulated results agreed well with the measured data. Sediment deposits in the dam area are mostly found at the bottom of the main channel, which have reshaped the river bed with a flat surface that is nearly horizontal. In contrast, very little fine sediment deposits can be found on the bank slopes of the channel. The developed method can be applied to improve our understanding of fine sediment transport mechanisms in large reservoirs. [ABSTRACT FROM AUTHOR]

• Drought propagation was re-examined by using three types of the propagation time. • An attribution framework was performed to quantify impacts of different factors. • Contribution rates are misjudged when ignoring climate change effects. • Climate change and large reservoirs mitigated drought while human activities exaggerated all drought characteristics. Enhancing the comprehension of hydrological drought evolution and drought propagation mechanisms is crucial for effective drought risk management in a changing environment. Previous studies have either inadequately considered the effects of climate change or conflated the impacts of large reservoirs and human activities. This study presents a comprehensive framework that integrates multiple comparison methods to quantify the effects of climate change, large reservoirs, and other human activities. The framework comprises three steps: (1) preparation of four types of streamflow based on critical time, large reservoirs inflow and outflow, and hydrological model; (2) estimation of hydrological drought characteristics, including duration, severity, peak intensity, instantaneous development speed, and instantaneous recovery speed and drought propagation characteristics, such as correlation, transition rate, and three types of the propagation time (PT) based on the start point (PT S), peak point (PT P), and end point (PT E); (3) quantification of the influences of various factors on the characteristic variables. The upper Yellow River Basin, a cold basin with large cascade reservoirs, was selected as the study area to utilize the proposed framework. The results revealed that climate change and large reservoirs reduced the duration and severity of drought events with relative contribution rates ranging from 146.6% to 340.2% and 381% to 707.7%, respectively, while those were increased by human activities with relative contribution rates from 428.0% to 1147.9%. Large reservoirs played the most crucial role in the development and recovery stages, with relative contribution rates of 75% and 85%, respectively. Climate change strengthened the correlation between the two drought indices, while human activities and large reservoirs weakened the correlation. The changes in three PT indicated that climate change shortened drought development and recovery period, while those were prolonged by human activities. Many hydrological droughts occurred before meteorological droughts due to large reservoir regulations. The changes in transition rates are −30.3%, 48.0%, and −28% under the influences of climate change, human activities, and large reservoirs, respectively. The framework and results of this study provide valuable insights into the mechanisms of hydrological drought and drought propagation and serve as a scientific basis for drought mitigation in changing environments. [ABSTRACT FROM AUTHOR]

Urban water demand is rapidly growing in India due to high growth in urban population and rapid industrialization. Meeting this demand is a big challenge for the urban planners in India. Incidentally, the large urban areas are experiencing faster growth in population, and most of them are in arid and semi arid regions, which are naturally water-scarce. As a result, water supplies from local water resources including aquifers are falling far short of the high and concentrated demands in most urban areas. Under such situations, these large cities have to rely on distant large reservoirs. The analysis of 302 urban centers shows that cities with larger population size have much higher level of dependence on surface water sources. Also, greater the share of surface water in the city water supplies, higher was the level of per capita water supply. Multiple regression models are estimated for Class I cities and Class II towns in India. The results show that Population Elasticity of Water Supply (PEWS) change with time and space—for Class I cities it was 1.127 in 1988, whereas that with respect to 1999 population is 1.289. It also shows that Class I cities have better water supply (PEWS is 1.127 in 1988 and 1.289 in 1999) than Class II towns (PEWS is 0.396 in 1988 and 0.675 in 1999). Given the structure and pattern of urban population growth, economic conditions and water demands, large reservoirs will have a much bigger role in meeting urban water supply needs. [ABSTRACT FROM AUTHOR]