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Northeast China is an important base for grain production, dominated by rain-fed agriculture that relies on green water. However, in the context of global climate change, rising regional temperatures, changing precipitation patterns, and increasing drought frequency pose threats and challenges to agricultural green water security. This study provides a detailed assessment of the spatiotemporal characteristics and development trends of green water security risks in the Northeast region under the base period (2001–2020) and the future (2031–2090) climate change scenarios (SSP245 and SSP585) using the green water scarcity (GWS) index based on raster-scale crop spatial distribution data, Delta downscaling bias-corrected ERA5 data, and CMIP6 multimodal data. During the base period, the green water risk-free zone for dry crops is mainly distributed in the center and east of the Northeast region (72.4% of the total area), the low-risk zone is primarily located in the center (14.0%), and the medium-risk (8.3%) and high-risk (5.3%) zones are mostly in the west. Under SSP245 and SSP585 future climate change scenarios, the green water security risk shows an overall expansion from the west to the center and east, with the low-risk zone increasing to 21.6% and 23.8%, the medium-risk zone increasing to 16.0% and 17.9%, and the high-risk zone increasing to 6.9% and 6.8%, respectively. Considering dry crops with GWS greater than 0.1 as in need of irrigation, the irrigated area increases from 27.6% (base period) to 44.5% (SSP245) and 48.6% (SSP585), with corresponding increases in irrigation water requirement (IWR) of 4.64 and 5.92 billion m 3 , respectively, which further exacerbates conflicts between supply and demand of agricultural water resources. In response to agricultural green water security risks, coping strategies such as evapotranspiration (ET)-based water resource management for dry crops and deficit irrigation are proposed. The results of this study can provide scientific basis and decision support for the development of Northeast irrigated agriculture and the construction planning of the national water network.

Water security assessments often rely on outputs from hydrological models that are applicable only in gauged regions where there are river discharge data to constrain the models. Therefore, there is an urgent need to explore new methods for assessing water security in ungauged regions. This study proposes the use of the water balance and water footprint concepts and satellite observations to assess water security in Anglophone Cameroon, which is an example of a typically ungauged region. Specifically, the study assesses demand-driven water scarcity in terms of blue and green water scarcities and population-driven water scarcity quantified using the Falkenmark index across all districts in Anglophone Cameroon. The study also performs a spatiotemporal trend analysis of precipitation and temperature in the study area using the Mann–Kendall test. Precipitation trend analysis returns varying strengths and magnitudes for different districts unlike temperature which demonstrates an upward trend in all districts. The water security assessment shows that blue water scarcity is substantially low across most districts, whereas population-driven water scarcity is observed in densely populated districts (<1,700 m3/capita/year). The results from this study suggest that the proposed method may be used to assess water security in ungauged regions irrespective of climate or population size.

The rise in global freshwater consumption, inefficient water use and population growth are the main drivers of water scarcity and food insecurity. This study analyzed the green, blue and economic water productivity (GWP, BWP and EWP, respectively) of the rainfed (teff, maize and sorghum) and irrigated (sugarcane) crops along with green water scarcity (GWS) in the Upper Awash Basin using water footprints (WFs) approaches. Teff has the lowest average GWP (∼0.3 kg/m3) and the highest average WF (4205 m3/ton) between 2000 and 2010 with an average EWP of ∼0.3 USD/m3 which is higher than those of other rainfed cereal crops (maize and sorghum). The highest GWS of maize was recorded in Metehara (269 mm/growing period) and the lowest in Debrezeit (70 mm/growing period) with an intermediate value of 90 and 117 mm in Wonji and Melkassa, respectively. All the rainfed crops have lower EWP (less than 0.3 USD/m3) compared with the irrigated sugarcane in the basin. This study demonstrates that increasing the value per unit of green/blue water through increasing yield per unit of supply and switching from high to low WFs crops has significant implications for addressing the water scarcity problem by setting a WP benchmark in the basin.

Water scarcity is the major concern that impacts the global economy and the livelihood of mankind. Climate change, rapid population growth, freshwater pollution, and depletion are among the factors that aggravate the situation. Although not yet exhaustively exploited, reclamation and reuse of wastewater are considered as potential mechanisms to mitigate the challenge. In relation to reclamation, conventional wastewater treatment plants are designed to remove organic matter, total solids, and nutrients but fail to remove the emerging micropollutants. A decentralized wastewater treatment system is another potential and emerging approach for sustainable water reuse at the point of the wastewater generation. However, its application is not exclusively independent of the centralized system; rather the integration of the two systems is recommendable to depend on the local situations. To remove micropollutants, integrating advanced wastewater technologies should be considered as well as advanced analytical instruments for proper monitoring. Although the reuse of reclaimed water in crop irrigation is a well-established practice, it lacks uniformity across the globe. Furthermore, if not properly monitored, the reuse of reclaimed water also has adverse effects on the soil properties and public health. Therefore, the aim of this work is to review the impacts of global freshwater scarcity, water resources management and monitoring practice, state-of-the-art (waste)water treatment technologies and experience of reusing reclaimed water, particularly in agricultural irrigation.

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

As water is an essential component of the planetary life support system, water deficiency constitutes an insecurity that has to be overcome in the process of socio-economic development. The paper analyses the origin and appearance of blue as well as green water scarcity on different scales and with particular focus on risks to food production and water supply for municipalities and industry. It analyses water scarcity originating from both climatic phenomena and water partitioning disturbances on different scales: crop field, country level and the global circulation system. The implications by 2050 of water scarcity in terms of potential country-level water deficits for food self-reliance are analysed, and the compensating dependence on trade in virtual water for almost half the world population is noted. Planetary-scale conditions for sustainability of the global water circulation system are discussed in terms of a recently proposed Planetary Freshwater Boundary, and the consumptive water use reserve left to be shared between water requirements for global food production, fuelwood production and carbon sequestration is discussed. Finally, the importance of a paradigm shift in the further conceptual development of water security is stressed, so that adequate attention is paid to water's fundamental role in both natural and socio-economic systems.

Green water—rainfall over land that eventually flows back to the atmosphere as evapotranspiration—is the main source of water to produce food, feed, fiber, timber, and bioenergy. To understand how freshwater scarcity constrains production of these goods, we need to consider limits to the green water footprint (WFg), the green water flow allocated to human society. However, research traditionally focuses on scarcity of blue water—groundwater and surface water. Here we expand the debate on water scarcity by considering green water scarcity (WSg). At 5 × 5 arc-minute spatial resolution, we quantify WFg and the maximum sustainable level to this footprint (WFg,m), while accounting for green water requirements to support biodiversity. We then estimate WSg per country as the ratio of the national aggregate WFg to the national aggregate WFg,m. We find that globally WFg amounts to 56% of WFg,m, and overshoots it in several places, for example in countries in Europe, Central America, the Middle East, and South Asia. The sustainably available green water flows in these countries are mostly or fully allocated to human activities (predominately agriculture and forestry), occasionally at the cost of green water flows earmarked for nature. By ignoring limits to the growing human WFg, we risk further loss of ecosystem values that depend on the remaining untouched green water flows. We emphasize that green water is a critical and limited resource that should explicitly be part of any assessment of water scarcity, food security, or bioenergy potential.

Water scarcity mitigation in regional agricultural systems contributes to water use efficiency improvement. Blue (WSIblue), green (WSIgreen) and grey (WSIgrey) water scarcity indices were proposed to describe various water stresses in detail and further determine the type of regional water scarcity. WSIblue and WSIgreen reveal resource-based water scarcities, and WSIgrey characterizes environment-based water shortages. Provincial water scarcity indices of China from 2000–2014 were calculated and analyzed in the current paper. The results indicated that the national WSI, WSIgrey, WSIblue and WSIgreen values are 0.84, 0.16, 0.39 and 0.89, respectively. China is facing a high water stress, manifested as a resource-based water shortage. Northwest and Northeast China experience a severe water quantity scarcity with high WSIblue and WSIgreen values, and the central and eastern regions exhibit a high WSIgrey value. Eastern China faces both serious resource-based and environmental water shortages. The constructed blue, green and grey water scarcity indices compensate for the inability of the existing index to determine the type of water shortage and indicate the reason for water scarcity. They also provide a targeted guiding significance for the formulation of effective measures to improve agricultural water resource management and alleviate regional water scarcity.

Saudi Arabia (SA) faces a water shortage, and it further challenges sustainable agriculture, industrial development and the well-being of people. SA uses more than 80% of its water resources for agricultural purposes. Groundwater extractions account for most of this demand, which is not sustainable. Hence, this study aims to analyze water management practices used in SA to propose viable and workable solutions to achieve sustainable management of scarce water resources. This study is based on a critical evaluation of information available on the water sector in SA. About 89% of the water demand in the Kingdom is non-sustainably met through over-pumping from groundwater resources and 9.3% by energy-intensive desalination. SA invested in dams and developed rainwater harvesting to enhance surface water availability and increase the recharge capacity of renewable aquifers. As there is a huge demand–supply gap, water demand management tools are the viable solutions leading to sustainability compared to supply enhancement that is capital intensive. A national agricultural policy, together with a water policy, can make agricultural systems more input efficient with higher productivity. Region-specific sustainable water resources management plans need to be implemented to match the demand–supply gap. Conjunctive water uses utilizing and prioritizing different water sources viz. harvested rainwater, treated wastewater, desalinized water, and groundwater, is vital in sustainable water resources management. In addition, climate change has exerted pressure on the available water resources and water uses as well as users, leading to adaptation for measures that are more sustainable in terms of water management. The most pressing problem SA faces in water resources management is the depletion and degradation of surface and subsurface water sources. SA has to implement many technological and legislative changes in addition to service management, conservation measures, paying a reasonable and justifiable price for water, and strengthening state agencies that will make water resources management in SA sustainable.

Water scarcity is exacerbating in many regions across North America because of climate change compounded by population growth, overexploitation of freshwater resources, and lack of proper management. Considering the regional urgency, there is an immense need to find more equitable solutions for water management. Therefore, this paper aims to examine the climatic factors impacting the hydrological regime in North America, including changes in surface runoff, groundwater storage, and the forested watershed, based on current trends and future projection scenarios. Moreover, this paper critically overviews need-based solutions for effective water management at a regional scale. The study shows that many areas of North America are exposed to extreme events such as prolonged droughts, devastating floods, wildfires, and altering precipitation patterns. Consequently, these changes are triggering wide-ranging impacts on water resources, leading to water supply deficiencies and influencing water flows and quality in the Southwestern United States, prairie provinces in Canada, and Mexico. The projection of warming around the region experiences spatial and seasonal variations because of the diversity in climatic conditions. Overall, in North America, winter is expected to warm more than other seasons causing earlier runoff and a decline in snowmelt. Given the physical and economic constraints that limit the development of new utilities, emphasis should be placed on strengthening nature-based solutions such as green infrastructure. Thus, the findings suggest an integrated water resource management approach is required, along with climate-induced innovative technologies, to secure the water resources while preparing for the forecasted challenges of tomorrow.