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Environmental flows are defined as the flow required into a stream to maintain the river’s ecosystem. The notion of Environmental Flow Allocation (EFA) ensures that a sufficient amount of water is delivered to the stream to maintain ecological integrity. The objective of this study is to examine environmental flows and determine the best acceptable strategy for providing flows into the river in the Lower Tapi Basin. To achieve this objective, daily discharge data from three sites, Ukai (period 1975–2020), Motinaroli (period 1990–2021), and Ghala (period 1995–2005) were collected and analyzed using the Tennant, Tessman, variable monthly flow (VMF), and Smakhtin methodologies. A comparative analysis was carried out on all three sites using the four methodologies. The Tessman and VMF approaches have a strong connection with the computed environmental flow requirements (EFR), according to the results. The calculated EFR was found to be in the range of 30–35% of mean annual flows (MAF). The maximum EFR found at station Ghala is about 54.5% of MAF according to the Tessman method. Such research will help to prevent future degradation of the river by supplying flow in accordance with the EFR, and it will also be used by stakeholders and policymakers to allocate water to preserve the ecosystem.

The planetary boundary (PB) concept, introduced in 2009, aimed to define the environmental limits within which humanity can safely operate. This approach has proved influential in global sustainability policy development. Steffen et al. provide an updated and extended analysis of the PB framework. Of the original nine proposed boundaries, they identify three (including climate change) that might push the Earth system into a new state if crossed and that also have a pervasive influence on the remaining boundaries. They also develop the PB framework so that it can be applied usefully in a regional context. Science , this issue 10.1126/science.1259855 Developments in the planetary boundaries concept provide a framework to support global sustainability. The planetary boundaries framework defines a safe operating space for humanity based on the intrinsic biophysical processes that regulate the stability of the Earth system. Here, we revise and update the planetary boundary framework, with a focus on the underpinning biophysical science, based on targeted input from expert research communities and on more general scientific advances over the past 5 years. Several of the boundaries now have a two-tier approach, reflecting the importance of cross-scale interactions and the regional-level heterogeneity of the processes that underpin the boundaries. Two core boundaries—climate change and biosphere integrity—have been identified, each of which has the potential on its own to drive the Earth system into a new state should they be substantially and persistently transgressed.

Water scarcity has become a major constraint to socio‐economic development and a threat to livelihood in increasing parts of the world. Since the late 1980s, water scarcity research has attracted much political and public attention. We here review a variety of indicators that have been developed to capture different characteristics of water scarcity. Population, water availability, and water use are the key elements of these indicators. Most of the progress made in the last few decades has been on the quantification of water availability and use by applying spatially explicit models. However, challenges remain on appropriate incorporation of green water (soil moisture), water quality, environmental flow requirements, globalization, and virtual water trade in water scarcity assessment. Meanwhile, inter‐ and intra‐annual variability of water availability and use also calls for assessing the temporal dimension of water scarcity. It requires concerted efforts of hydrologists, economists, social scientists, and environmental scientists to develop integrated approaches to capture the multi‐faceted nature of water scarcity. Key Points We provide a comprehensive review of water scarcity indicators and reflect on their relevance in a rapidly changing world There is a need to incorporate green water, water quality, and environmental flow requirements in water scarcity assessment Integrated approaches are required to capture the multi‐faceted nature of water scarcity

With a growing population and a changing climate, competition for water resources in the water-energy-food (WEF) nexus is expected to increase. In this study, competing water demands between food production, freshwater ecosystems and utilities (energy, industries and households) are quantified. The potential trade-offs and related impacts are elaborated for different SSP scenarios with the integrated assessment model IMAGE, which includes the global vegetation and hydrology model Lund-Potsdam-Jena managed Land (LPJmL). Results for the 2045–2054 period are evaluated at the global scale and for a selection of 14 hotspot basins and coastal zones. On the global scale, we estimate that an additional 1.7 billion people could potentially face severe water shortage for electricity, industries and households if food production and environmental flows would be prioritized. Zooming in on the hotspots, this translates to up to 70% of the local population. Results furthermore show that up to 33% of river length in the hotspots risks not meeting environmental targets when prioritizing other water demands in the nexus. For local food production, up to 41% might be lost due to competing water demands. The potential trade-offs quantified in this study highlight the competition for resources in the WEF nexus, for which impacts are most notably felt at local scales. This emphasizes the need to simultaneously consider different dimensions of the nexus when developing scenarios that aim to achieve multiple sustainability targets.

Although the global agricultural system will need to provide more food for a growing and wealthier population in decades to come, increasing demands for water and potential impacts of climate change pose threats to food systems. We review the primary threats to agricultural water availability, and model the potential effects of increases in municipal and industrial (M&I) water demands, environmental flow requirements (EFRs) and changing water supplies given climate change. Our models show that, together, these factors cause an 18 per cent reduction in the availability of worldwide water for agriculture by 2050. Meeting EFRs, which can necessitate more than 50 per cent of the mean annual run-off in a basin depending on its hydrograph, presents the single biggest threat to agricultural water availability. Next are increases in M&I demands, which are projected to increase upwards of 200 per cent by 2050 in developing countries with rapidly increasing populations and incomes. Climate change will affect the spatial and temporal distribution of run-off, and thus affect availability from the supply side. The combined effect of these factors can be dramatic in particular hotspots, which include northern Africa, India, China, parts of Europe, the western US and eastern Australia, among others.

Increased water demand and overexploitation of limited freshwater resources lead to water scarcity, economic downturn, and conflicts over water in many places around the world. A sensible policy measure to bridle humanity's water footprint, then, is to set local and time‐specific water footprint caps, to ensure that water appropriation for human uses remains within ecological boundaries. This study estimates—for all river basins in the world—monthly blue water flows that can be allocated to human uses, while explicitly earmarking water for nature. Addressing some implications of temporal variability, we quantify trade‐offs between potentially violating environmental flow requirements versus underutilizing available flow—a trade‐off that is particularly pronounced in basins with a high seasonal and interannual variability. We discuss several limitations and challenges that need to be overcome if setting water footprint caps is to become a practically applicable policy instrument, including the need (for policy makers) to reach agreement on which specific capping procedure to follow. We conclude by relating local and time‐specific water footprint caps to the planetary boundary for freshwater use. Plain Language Summary Around the world, people are using too much water from rivers, lakes, and streams. As a result, rivers run dry, lake levels drop, and fish and other animals and plants that depend on freshwater are being harmed. We therefore need to strike a better balance between how much water we use for ourselves and how much water we leave in rivers and lakes for nature. In this study, we calculate this ceiling to water use by people, for every watershed in the world. We call this ceiling a water footprint cap. We first looked at how much water is available in total and subtracted what is needed for nature. What remains can be sustainably used by people, for example, to irrigate crop fields, to use at home, or to use in factories. While it makes perfect sense to not use more water than there is available, it is not so straightforward for policy makers to just make a rule that says: “Let's not use more water than this cap.” The reason is that the amount of water that is available changes from one year to the other and also from month to month. This means, for instance, that authorities cannot give out water use permits to farmers based on average water availability levels. After all, if the year or month turns out to be dry, there will not be enough water to meet the permit. We therefore discuss some steps policy makers should think of when planning to use a water footprint cap. Key Points Overexploitation of freshwater resources shows humanity's current inability to live within the means of the planet in terms of water use We propose setting water footprint caps per river basin to ensure human water appropriation remains within ecological boundaries We relate local and time‐specific water footprint caps to the planetary boundary for freshwater use

Limiting mean global warming to well below 2 °C will probably require substantial negative emissions (NEs) within the 21st century. To achieve these, bioenergy plantations with subsequent carbon capture and storage (BECCS) may have to be implemented at a large scale. Irrigation of these plantations might be necessary to increase the yield, which is likely to put further pressure on already stressed freshwater systems. Conversely, the potential of bioenergy plantations (BPs) dedicated to achieving NEs through CO2 assimilation may be limited in regions with low freshwater availability. This paper provides a first-order quantification of the biophysical potentials of BECCS as a negative emission technology contribution to reaching the 1.5 °C warming target, as constrained by associated water availabilities and requirements. Using a global biosphere model, we analyze the availability of freshwater for irrigation of BPs designed to meet the projected NEs to fulfill the 1.5 °C target, spatially explicitly on areas not reserved for ecosystem conservation or agriculture. We take account of the simultaneous water demands for agriculture, industries, and households and also account for environmental flow requirements (EFRs) needed to safeguard aquatic ecosystems. Furthermore, we assess to what extent different forms of improved water management on the suggested BPs and on cropland may help to reduce the freshwater abstractions. Results indicate that global water withdrawals for irrigation of BPs range between ∼400 and ∼3000 km3 yr−1, depending on the scenario and the conversion efficiency of the carbon capture and storage process. Consideration of EFRs reduces the NE potential significantly, but can partly be compensated for by improved on-field water management.

Environmental water requirements (EWRs) are set for South Africa's estuaries to ensure that they are maintained in a state that is both achievable and commensurate with their level of conservation and economic importance. However, these EWRs are typically determined on the basis of models and scenario analyses that require extrapolation beyond existing data and experience, especially if climate change is considered. In the case of the Berg Estuary, South Africa, available data on changes in freshwater flow and water quality span a period of at least five decades (1970s-present) during which significant reduction in lfows has been observed. Monitoring data also cover an extreme 3-year drought, from 2015−2017, which provided a unique opportunity to study the efects of severe freshwater starvation (zero-flow for an extended period) on this large, permanently open system. Our analyses show that mean annual runof (MAR) under present-day conditions has been reduced to around 50% of that under reference (natural) conditions and that reduction in runof during the low-flow season (summer) has been more severe (80-86% reduction) than for the high-flow season (39-42% reduction). The salinity gradient now extends much further upstream than under reference conditions. Hypersaline conditions along with a reverse salinity gradient were recorded in the estuary for the first time ever during the drought of 2015/17. Levels of dissolved inorganic nitrogen (NO x) reaching the estuary from the catchment have increased dramatically (6-7 fold) over the past five decades, dissolved reactive phosphate (PO4) slightly less so (2-3 fold), but ammonia (NH4) hardly at all. Increases in nutrient input from the catchment in the high-flow season are also much more dramatic than in the low-flow season. The estuary is no longer compliant with gazetted EWRs and requires urgent interventions to restore the quantity and quality of freshwater it receives.

Reducing the quantity of water in recent years has increased the competition between development projects and the environment. Wetlands are increasingly under pressure due to human activities. The most serious threats to wetlands are excessive agriculture and the diversion of water for irrigation. In recent years, due to water shortage and drought, wetland dryness in Iran has caused many problems, including the dust crisis. Therefore, planning at the basin scale is necessary to achieve sustainable development, which emphasizes the employment of mathematical models. In this study, using a reliability-based simulation–optimization approach, development planning in the Karkheh basin with the following two objectives is investigated: (1) total area under cultivation of agricultural development sectors and (2) supply reliability of the environmental flow requirement. The Water Evaluation and Planning (WEAP) model is used for the simulation of water resources and the multi-objective particle swarm optimization (MOPSO) algorithm is employed for optimization. The results show that in addition to significantly improving the supply reliability of the wetland requirement (from 55 to 79%), the design of agricultural development projects has been optimized. The reliability-based model has prevented unsustainable developments in the basin. Also, the average supply reliability of agricultural demands has increased from 51% (in previous studies) to 72%.

The habitat suitability index and environmental flow requirements were assessed for ten species of macroinvertebrates in a 2 km length section of the urban Botič creek (average flow 0.4 m 3  s −1 ) in Prague. Botič creek has been affected by two combined sewer overflows (CSO). Spring, summer and fall seasonal environmental flow requirements were identified using the Physical HABitat SIMulation System (PHABSIM) approach for the whole macroinvertebrate community: Spring – optimal flow 0.32–0.38 m 3  s −1 , minimal flow 0.20–0.21 m 3  s −1 and maximal acceptable flow 0.91–0.93 m 3  s −1 ; Summer - optimal flow 0.42–0.45 m 3  s −1 , minimal flow 0.19–0.21 m 3  s −1 and maximal acceptable flow 0.95–1.00 m 3  s −1 ; Fall - optimal flow 0.38–0.48 m 3  s −1 , minimal flow 0.22–0.23 m 3  s −1 and maximal acceptable flow 0.95–0.98 m 3  s −1 . The seasonal variability of environmental flow for all three categories is approximately 10%. Environmental flow requirements of the studied species and their life stages vary with depth, velocity and bottom substratum. Due to inflow from the CSOs, the optimal and maximal acceptable flow are not maintained and the maximal flow is exceeded by more than twice its value. Although the Instream Flow Incremental Methodology (IFIM) was primarily designed for large impounded rivers, the study proved its applicability in small streams affected by urbanization and urban drainage.