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Increasing crop yields to ensure an adequate food supply under water and land scarcity is dependent on intensive agricultural inputs (such as fertilizers, pesticides, agri-films, or energy) which consume water resources and generate water pollution. However, the burden shifting of water quantity and quality stress from producers to importer and consumers through agricultural input production, trade, and consumption have been largely overlooked. Here, taking maize in China as the study case, we mapped step-by-step indirect water footprint (IWF) of maize production, virtual water (VW) flows related to inputs driven by maize consumption, and the resulting burden shifting of water quantity and quality. Bottom-up WF accounting approach was applied. The maize consumption was intercepted into two stages: the crop production stage (CPS) from raw materials to the farm, and the crop trade stage (CTS) from farm to fork. Results show that the national average blue and grey IWF of maize production was 3.91 and 26.86 m 3 /t, respectively. In the CPS, the input-related VW flowed from the west and east coast to the north. In the CTS, the VW flows from the north to the south. Blue and grey VW flows in CTS caused by secondary flows of VW in CPS accounted for 48% and 18% of the total flows, respectively. In total VW flows along the maize supply chain, 63% of blue VW and 71% of grey VW net exports occurred in the north of severe water scarcity and water pollution levels. The analysis highlights the impact of the crop supply chain on water quantity and water quality in the consumption of agricultural inputs, the importance of step-by-step supply chain analysis for regional crop water conservation management, and the urgent need for integrated management of agricultural and industrial water resources., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Afforestation plays a crucial role in the remission of water and soil erosion, adsorption of heavy metals, and protection of soil microbial community structure for mining areas. However, soil drought, the variability of precipitation, and low rainwater use efficiency severely limit the early survival rate of trees. A new small-scale system of rainwater harvesting combined with irrigation (RWHI) for afforestation in mining areas was established, which consisted of a rainwater catching board, storage tanks, and ceramic emitters. A daily water balance model under variable water supply was presented and experimentally verified to confirm the optimum catchment area, the storage capacity of tanks, and the rated discharge of ceramic emitters. Taking the Wuda mining area in Wuhai, China, as a case study, three representative years, including dry, normal, and wet years were selected by analysing local rainfall features. The results showed that the soil water content in the root zone maintained a suitable range (between field capacity and wilting coefficient) using the RWHI system. With the aim of the maximum system operating reliability for various weather conditions and tree species, it was recommended that the rated discharge of the ceramic emitter of 6 ml h -1 , storage tanks of 25 L, the catchment area of 1 m 2 , and filling water schedule of twice a year were employed. A generalised equation for the estimation of the filling water amount was given. The vegetation coverage of the mine was significantly improved, and the survival rate of trees (Murraya paniculate) exceeded 90% using the RWHI system. It was indicated that the RWHI system has a broad application prospect in the afforestation of the mining areas., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Jointly influenced by the natural factors and the artificial protection measures, the ecological environment of Loess Plateau has been significantly improved in recent years, but which has already brought about some water-related problems. To maintain the balance between precipitation and water consumption is an important foundation for sustainable development of the ecology remediation. This study used Budyko Framework to simulate the actual water consumption of 161 sub-basins from 1990 to 2014. Based on the simulation results, the research also analyzed the evolution characteristics of water balance in Loess Plateau from 1990 to 2014. Results show that, with the increase of vegetation coverage, the regional precipitation and actual evapotranspiration were both showing a significant increasing trend, and the increasing rate of precipitation was 1.91mm/a on average, which was greater than the increasing rate of actual evapotranspiration of 1.34mm/a. To further demonstrate the water balance regime in Loess Plateau, the evapotranspiration coefficient (ECC) was used to quantitatively indicate the ratio of the vegetation water consumption and the total precipitation. The average values of ECC were 0.868, 0.863, 0.851 and 0.837 respectively in four sub-periods of 1990-1999, 2000-2004, 2005-2009 and 2010-2014. The above analyses indicate that with the vegetation recovery and ecological restoration, the percentage of evapotranspiration in the total precipitation is keeping decreasing and in turn the percentage of water yield in the total precipitation is keeping increasing. Consequently, it seems more sustainable for vegetation water use in most areas of Loess Plateau currently., (Copyright © 2016. Published by Elsevier B.V.)

Irrigation plays a major role in Chinese agricultural production, as China is experiencing water and food scarcity. Assessing water use (WU) and water productivity (WP) will contribute to regional water management and grain production improvement. This paper quantifies the water use and productivity in grain production for 31 Chinese provinces, autonomous regions and municipalities (PAMs) by distinguishing between irrigated and rain-fed farmland. An indicator of marginal productivity of blue water (MWPb) is established and calculated to evaluate irrigation profits. National water use (WUt) for grain cultivation from 1998 to 2010 was approximately 689.04 Gm(3) (42.26% blue water and 57.74% green water). The productive water proportions for irrigated and total croplands were 65.57% and 76.85%, respectively. Water use compositions from both blue-green and productive-unproductive perspectives changed slightly over time. The water use productivity (WPU) and water consumption productivity (WPC) for integrated grain products of China during the study period were 0.747 and 0.972 kg/m(3), respectively. The spatial distribution patterns of irrigated WPs (WPUI, WPCI) were consistent with those for total cropland. China has achieved sufficient food supply without increasing water use. The national MWPb was estimated to be 0.673 kg/m(3), revealing a higher increase in crop yield on irrigated land compared to rain-fed land. The northeast provinces urgently need to improve irrigation efficiency, and the North China Plain PAMs should promote rain-fed crop yield to increase grain production and control water use in the future., (Copyright © 2015 Elsevier B.V. All rights reserved.)

The integration of water footprints and virtual water flows allows the mapping of the links between production, trade, and consumption and could potentially help to alleviate water scarcity and improve water management. We evaluated the water footprints and virtual water flows of crop production, consumption, and trade and their influencing factors in the Hetao irrigation district in China for 1960-2010. The water footprint of crop production and the export of virtual water fluctuated but tended to increase during this period and were influenced mainly by agricultural factors such as crop yield, irrigation efficiency, and area sown. The water footprint of crop consumption and the import of virtual water increased during 1960-1979 and decreased during 1980-2010 and were influenced by socio-economic factors such as total population, the retail-price index, and the proportion of the population in urban areas. Most of the water footprint of production was exported to other areas, which added to the pressure on local water systems. The import of virtual water led to a saving of water for the Hetao irrigation district, while its share of the water footprint of consumption has decreased significantly since 1977. An increase in irrigation efficiency can alleviate water scarcity, and its application should be coupled with measures that constrain the continued expansion of agriculture. Full-cost pricing of irrigation water was an effective policy tool for its management. Re-shaping regional water-production and water-trade nexuses by changing crop structures could provide alternative opportunities for addressing the problems of local water scarcity, but the trade-offs involved should first be assessed., (Copyright © 2014 Elsevier B.V. All rights reserved.)

A core goal of sustainable agricultural water resources management is to implement a lower water footprint (WF), i.e. higher water productivity, and to maximize economic benefits in crop production. However, previous studies mostly focused on crop water productivity from a single physical perspective. Little attention is paid to synergies and trade-offs between water consumption and economic value creation of crop production. Distinguishing between blue and green water composition, grain and cash crops, and irrigation and rainfed production modes in China, this study calculates the production-based WF (PWF) and derives the economic value-based WF (EWF) of 14 major crops in 31 provinces for each year over 2001–2016. The synergy evaluation index (SI) of PWF and EWF is proposed to reveal the synergies and trade-offs of crop water productivity and its economic value from the WF perspective. Results show that both the PWF and EWF of most considered crops in China decreased with the increase in crop yield and prices. The high (low) values of both the PWF and EWF of grain crops tended to cluster obviously in space and there existed a huge difference between blue and green water in economic value creation. Moreover, the SI revealed a serious incongruity between PWFs and EWFs both in grain and cash crops. Negative SI values occurred mostly in north-west China for grain crops, and overall more often and with lower values for cash crops. Unreasonable regional planting structure and crop prices resulted in this incongruity, suggesting the need to promote regional coordinated development to adjust the planting structure according to local conditions and to regulate crop prices rationally. [ABSTRACT FROM AUTHOR]

There have been significant recent advances in understanding the ecohydrology of deep soil. However, the links between root development and water usage in the deep critical zone remains poorly understood. To clarify the interaction between water use and root development in deep soil, we investigated soil water and root profiles beyond maximum rooting depth in five apple orchards planted on farmland with stand ages of 8, 11, 15, 18, and 22 years in a subhumid region on the Chinese Loess Plateau. Apple trees rooted progressively deeper for water with increasing stand age and reached 23.2 ± 0.8 m for the 22‐year‐old trees. Soil water deficit in deep soil increased with tree age and was 1,530 ± 43 mm for a stand age of 22 years. Measured root deepening rate was far great than the reported pore water velocity, which demonstrated that trees are mining resident old water. The deficits are not replenished during the life‐span of the orchard, showing a one‐way mining of the critical zone water. The one‐way root water mining may have changed the fine root profile from an exponential pattern in the 8‐year‐old orchard to a relative uniform distribution in older orchards. Our findings enhance our understanding of water‐root interaction in deep soil and reveal the unintended consequences of critical zone dewatering during the lifespan of apple trees. [ABSTRACT FROM AUTHOR]

Groundwater recharge (GR) is a key component of regional and global water cycles and is a critical flux for water resource management. However, recharge estimates are difficult to obtain at regional scales due to the lack of an accurate measurement method. Here, we estimate GR using Gravity Recovery and Climate Experiment (GRACE) and Global Land Data Assimilation System (GLDAS) data. The regional-scale GR rate is calculated based on the groundwater storage fluctuation, which is, in turn, calculated from the difference between GRACE and root zone soil water storage from GLDAS data. We estimated GR in the Ordos Basin of the Chinese Loess Plateau from 2002 to 2012. There was no obvious long-term trend in GR, but the annual recharge varies greatly from 30.8 to 66.5 mm year−1, 42% of which can be explained by the variability in the annual precipitation. The average GR rate over the 11-year period from GRACE data was 48.3 mm year−1, which did not differ significantly from the long-term average recharge estimate of 39.9 mm year−1 from the environmental tracer methods and one-dimensional models. Moreover, the standard deviation of the 11-year average GR is 16.0 mm year−1, with a coefficient of variation (CV) of 33.1%, which is, in most cases, comparable to or smaller than estimates from other GR methods. The improved method could provide critically needed, regional-scale GR estimates for groundwater management and may eventually lead to a sustainable use of groundwater resources. [ABSTRACT FROM AUTHOR]

International food trade entails virtual water flows across trading partners. It has been proposed to attenuate regional water scarcity by importing water‐intensive commodities from water‐abundant regions. In addition to alleviating water scarcity in virtual water importing countries, existing studies have reported that food trade also generates global water savings. However, little is known how these global water savings may alleviate water scarcity, which is more relevant to the sustainable use of water resources than only assessing the savings. In this paper, we conducted a comprehensive review on studies of water savings and losses associated with food trade on different spatial scales. We found that the concept of global water savings is built on the disparities in water productivity across countries, whereas the regional water savings measure the inflows of virtual water trade. The significance of water savings is dimmed by the fact that the savings are often not driven by water scarcity. Meanwhile, lacking policy relevance impairs the usefulness of water saving accounting. Future studies should link water savings to alleviating water scarcity at various levels. The water saving accounting needs to go to finer scale, for example, to subnational and river basin scales, to support real water resource management. In the meantime, interdisciplinary efforts are necessary to enhance the water savings as a holistic measure for addressing water scarcity on regional and global scales. This article is categorized under: Engineering Water > Planning WaterHuman Water > Value of WaterScience of Water > Water Quality Water savings and losses related to food trade highlight the in‐ and out‐flow of virtual water and differences in water productivities between food importers and exporters. Challenges in accounting for water savings and losses remain and should be carefully addressed. [ABSTRACT FROM AUTHOR]

Crop residues and energy crops grown on marginal lands can be sources of advanced bioethanol without interfering with agricultural land for food, and can replace traditional fuels with a neutral carbon cycle, but demand additional water for crop growth and energy production. However, existing bioethanol potential assessments ignore the constraints of limited water resources. Here we quantify the bioethanol potential from twelve crops' residues and two energy crops (sweet sorghum and cassava) grown on marginal lands under both water and land constraints in China, based on bioethanol water footprint accounting, local water scarcity degree and marginal land identification. Results show that the bioethanol potential from crop residues is up to 68.5 Mt (∼1.8 EJ calorific value), approximately 49% lower than the theoretical potential without considering water limits. The suitable bioethanol potentials from sweet sorghum and cassava are in ranges 18.7–32.7 Mt (∼0.5–0.9 EJ) and 14.0–52.4 Mt (∼0.4–1.4 EJ), respectively. Northeast China are more suitable for the development of bioethanol due to their greater productive potential and lower water scarcity, whereas South and Southwest China have advantages in terms of growing energy crops on marginal lands and should be the focus of additional attention. • Bioethanol potential under water limits is evaluated by water footprint assessment. • Suitable bioethanol potential here means no effects on water and land pressures. • Suitable bioethanol potentials from crop residues and energy crops are shown in China. • Estimated bioethanol potential accounts for up to 27–56% of China's gasoline usage. [ABSTRACT FROM AUTHOR]

• Machine learning models of crop water footprint are evaluated in time and space. • Optimal model types differ by crops, water supply scenarios and spatial scales. • The feature variables of spatial and temporal categories are more important. Crop water footprint (WF) calculation via physical process-based crop or hydrological models has basic requirements in terms of cost, time, and knowledge threshold. It is unsuitable for laypersons such as policymakers or cross-disciplinary researchers. Here, machine learning (ML) is an effective solution owing to low cost, high speed, and low processing difficulty. However, the feasibility and applicability of ML modeling for crop WFs at multiple spatial scales remain unverified. Here, we quantitatively evaluated the applicability, optimal model parameters and importance of the feature variables of four ML methods, including random forest (RF), light gradient boosting machine (LightGBM), generalized additive model (GAM), and artificial neural network (ANN), for constructing WF models for crop production at site and provincial scales. The models were tested through the case study for major grain crops (wheat, maize, soybean, and rice) in China over 2000–2019, distinguishing between irrigated and rainfed crops and between different irrigation methods. The findings show that the WF simulation accuracy for the irrigated scenario was higher than that for the rainfed scenario. The type of feasible ML model varied with crop types, variable combinations, water supply scenarios, and spatial scales. At the site scale, the RF model was generally suitable for simulating WF of maize, soybean, and rice, with R2 values ranging from 0.7 to 0.78 under different irrigation scenarios; the LightGBM was more suitable for simulating wheat with R2 values ranging from 0.76 to 0.82. The LightGBM was deemed to be the best model at the provincial scale, with some regions presenting a 50% lower root mean square error in the crop WF simulation than in RF. Variables related to spatio-temporal class feature were found to be the most important in both the RF and LightGBM simulations. This study provides a global methodological reference for different countries or regions to establish methods for rapid WF assessments in crop production with cross-disciplinary researchers or macro policymakers as the target users. [ABSTRACT FROM AUTHOR]

Blue water (irrigation water) and green water (soil moisture), as natural bases in crop production, are vulnerable to natural and socioeconomic disturbances. However, quantitative resilience assessments of blue and green water for crop production are still lacking. Here we constructed a social-ecological based general water resilience assessment framework that distinguishes blue and green water for crop production in four dimensions, including resistance, absorptive capacity, adaptive capacity and transformative capacity. The framework was applied to quantify the spatiotemporal characteristics of water resource resilience for three staple crops (rice, wheat, and maize) at provincial level in mainland China over 2000–2017. Results indicate that water resilience related to crop production of China as a whole was at the lower-middle level over the study period. The green water resilience had higher initial performance, while the blue water resilience had a faster growth rate (>50%). The performance of adaptive and transformative capacity was 8.9∼153.7% better than resistance and absorptive capacity. There were significant heterogeneities in the water resilience score among crops and in time and space. Wheat had the highest resilience of green water, while its blue water resilience was the lowest among the three crops. The water resilience for crop production varied by up to 2.4-fold across provinces. Synergies and trade-offs among the four dimensions of water resilience were further evaluated. The current study provides an effective approach for assessing the state and progress of resilience of regional water resources for crop production. [Display omitted] • Spatiotemporal resilience of blue and green water for crop production was evaluated. • Crop-related water resilience in China was at the lower-middle level. • Blue water resilience increased by over 50% over 2000–2017. • Adaptive and transformative capacity exceeded resistance and absorptive capacity. [ABSTRACT FROM AUTHOR]

Scarcity of water resources is one of the major challenges in the world, particularly for the main water consumer, agriculture. Virtual water flow (VWF) promotes water redistribution geographically and provides a new solution for resolving regional water shortage and improving water use efficiency in the world. Virtual water transfer among regions will have a significant influence on the water systems in both grain export and import regions. In order to assess the impacts of VWF related grain transfer on regional water resources conditions, the study takes mainland China as study area for a comprehensive evaluation of virtual water flow on regional water resources stress. Results show that Northeast China and Huang-Huai-Hai region are the major grain production regions as well as the major virtual water export regions. National water savings related to grain VWF was about 58 Gm 3 , with 48 Gm 3 blue water and 10 Gm 3 green water. VWF changes the original water distribution and has a significant effect on water resources in both virtual water import and export regions. Grain VWF significantly increased water stress in grain export regions and alleviated water stress in grain import regions. Water stress index (WSI) of Heilongjiang and Inner Mongolia has been increased by 138% and 129% due to grain export. Stress from water shortages is generally severe in export regions, and issues with the sustainability of grain production and VWF pattern are worthy of further exploration. [ABSTRACT FROM AUTHOR]

Summary Fresh water is a scarce and critical resource in both natural and socioeconomic systems. Increasing populations combined with an increasing demand for water resources have led to water shortages worldwide. Current water management strategies may not be sustainable, and comprehensive action should be taken to minimize the water budget deficit. Sustainable water resources management is essential because it ensures the integration of social, economic, and environmental issues into all stages of water resources management. This paper establishes the indicators to evaluate the sustainability of water utilization based on the Drive-Pressure-Status-Impact-Response (DPSIR) model. Based on the analytic hierarchy process (AHP) method, a comprehensive assessment of changes to the sustainability of the water resource system in the city of Bayannur was conducted using these indicators. The results indicate that there is an increase in the driving force of local water consumption due to changes in society, economic development, and the consumption structure of residents. The pressure on the water system increased, whereas the status of the water resources continued to decrease over the study period due to the increasing drive indicators. The local government adopted a series of response measures to relieve the decreasing water resources and alleviate the negative effects of the increasing driver in demand. The response measures improved the efficiency of water usage to a large extent, but the large-scale expansion in demands brought a rebounding effect, known as “Jevons paradox” At the same time, the increasing emissions of industrial and agriculture pollutants brought huge pressures to the regional water resources environment, which caused a decrease in the sustainability of regional water resources. Changing medium and short-term factors, such as regional economic pattern, technological levels, and water utilization practices, can contribute to the sustainable utilization of regional water resources. [ABSTRACT FROM AUTHOR]

Owing to increasingly limited water resources, evaluating large-scale regional crop water footprints (WFs) within acceptable errors is the basis for efficiency and productivity improvements for sustainable agriculture. However, quantitative sensitivity analysis of outputs to inputs and parameters of advanced and widely used crop WF accounting models are still lacking. In this study, we evaluated the spatial and temporal sensitivity of the blue and green WFs of crops to key input variables based on year-by-year simulations for wheat, rice, maize, and soybean production in China from 2010 to 2019. We employed one of the most widely used AquaCrop models to test different water supply and irrigation scenarios. Nine input variables or model parameters were selected, including reference evapotranspiration (ET o), crop transpiration coefficient (Kc Tr), soil evaporation coefficient (K E), maximum canopy cover (MCC), precipitation (PR), canopy decline coefficient (CDC), planting density of the crop (DC), reference harvest index (HI 0), and normalised water productivity (WP*). The results showed that crop WFs are generally more sensitive to ET o among the input variables and Kc Tr and K E among the model parameters. We also found that the blue WF was more sensitive than the green WF. In most cases, WFs in rainfed scenarios were more sensitive to changes in input variables compared to WFs of irrigated crops. Of the four studied crops, the WFs of maize and wheat were more sensitive than the WFs of rice and soybean. Wheat, maize, and soybean also showed a visible spatial variation in the sensitivity of the WF to each input variable. Overall, the WF sensitivity was influenced by crop type, water supply, irrigation method, and regional climate features. The current analysis provides a feasible approach to comprehensively identify the key inputs and parameters to be calibrated for accurate crop WF accounting. [Display omitted] • Sensitivities were tested for crop water footprint simulation by AquaCrop model. • Water footprints were generally most sensitive to reference evapotranspiration. • Sensitivities of water footprints differ by crop types and by irrigation methods. • Sensitivities of water footprints of a same crop varied in time and space. [ABSTRACT FROM AUTHOR]

• The flow pattern of grain virtual water in Chinese Mainland was shown. • Constructing a virtual water flow trade gravity model based on socio-hydrology theory. • Quantifying the driving force of grain virtual water flow using trade gravity model. • Analyzing the response trajectory of virtual water flow under the shock of variable fluctuation. • Research deficiencies and future research directions were given. To ensure regional grain security, the grain is transferred from grain surplus provinces to grain deficient provinces in China, then water embedded in grain (virtual water) flows accordingly, which promotes the spatial redistribution of water resources. To explore the influencing factors of virtual water flow, this study first obtained the inter-provincial virtual water flow pattern, and then constructed a virtual water flow trade gravity model based on socio-hydrology theory and estimated the model with multiple regression methods. And the impulse response function was used to analyze the response trajectory of virtual water flow under the shock of other endogenous variable fluctuation. The results show that from 1997 to 2014, the quantity of inter-provincial grain virtual water flow increased from 73 billion m3to 124.64 billion m3. The positive driving effect of the population and per capita GDP in virtual water inflow areas is significant, and the negative suppression effect of the grain output per unit in inflow-areas and the population in outflow-areas is significant. Heilongjiang is the province with the largest outflow of virtual water, and the outflow is more sensitive to the shock of local per capita water resources fluctuation. Guangdong is the province with the largest inflow, and the inflow is more sensitive to the local per capita GDP fluctuation. Chinese Mainland's grain virtual water flow pattern and trend will continue to exist in the future. In addition to the endowment of water resources, social and economic factors also need to be considered during the process of water resources management. [ABSTRACT FROM AUTHOR]

Water consumption in grain production consists of blue (i.e., surface water and groundwater) and green water (i.e., rainwater), however, is rarely distinguished in existing emergy analysis. Taking four major grain crop production systems (rice, wheat, maize, and soybean) by provinces in mainland China over the period 2001–2016 as the study case, in this study the blue and green water were differentiated, for the first time, in the emergy analysis for crop production systems based on water footprint assessment. Three indicators were estimated per crop per province per year: water resource emergy input (WEI), water resource emergy contribution ratio (WECR), and water resource emergy use efficiency (WEUE). Results show that during the study period, the total WEI into the major grain crop production systems increased by 16.61%; blue and green WEI accounted for 52.64% and 47.36%, respectively, by 2016. Although the blue WECR was higher than green WECR, it decreased significantly over the study period. The annual average green WEUE was 10.99 sej sej−1, 7.7 times the blue; moreover, it increased by 27.28% over the study period. The annual average total WECRs in nine provinces were higher than the national average, but the corresponding total WEUEs were lower than the national average; of these, four were distributed in arid and rainwater-deficient areas of northwest China. The emergy analysis of blue and green water separately provides information for a more precise and efficient water allocation as well as better cropping structures given corresponding water endowments. [Display omitted] • Blue and green water were distinguished in water emergy analysis for crop production. • Three emergy based indicators were estimated to measure the water use efficiencies. • Visible spatial and temporal heterogeneities were shown in crop-related water emergy. • Blue water and green water have different emergy in crop production systems. [ABSTRACT FROM AUTHOR]

With fast development of economy and trade globalization, the water resources system has undergone profound changes due to the overconsumption of physical water and the complex virtual water flow. China is poor in water and rich in arable land for the north, but rich in water and poor in arable land for the south. The spatial mismatch of water and food production has posed a great threat to sustainable development. To quantify the virtual water flow with grain trade and evaluate its influence on the regional water resources system, this study simulated the interprovincial virtual water flow embedded in grain transportation based on a linear optimization model and assessed the water pressure induced by virtual water flow. Results show that the annual virtual water flow (as much as 73.46 billion m3 in 2012) associated with interprovincial grain trade is mainly from the North to the South. The grain output areas are mainly located in the Northeast, Northwest, and the North China Plain, while the grain input areas are mainly distributed in the eastern and southern coastal provinces with well-developed economies. According to the estimation, when China's food demand reaches a peak of 650 million tons in 2030, the virtual water flow will further increase and the volume of virtual water flowing out of Northeast China will be up to 50 billion m3, accounting for 90% of the total virtual water flow. The increasing the virtual water outflow in North China will severely exceed the local water resources capacity in the arid and semi-arid areas and have an obvious negative impact on sustainable development in Northern China. Further improving the water use efficiency, adjusting the structure of spatial grain production distribution and physical-virtual water coupled management are effective countermeasures to ensure the coordination safety of food and water in China. Image 1 • There is a mismatch between water resources and cultivated land resources in China. • Anticipated population increases will exacerbate this problem in the future. • This study simulated interprovincial virtual water flow embedded in grain transportation. • Annual virtual water flow associated with grain trade is mainly from north to south. • Virtual water outflow in much of North China will exceed local water capacity. [ABSTRACT FROM AUTHOR]

• Blue and green water resources, footprints and scarcities were quantified. • Effects of irrigation on blue and green water resources were considered. • Middle reaches faced both blue and green water scarcities at least 3 months a year. Blue water (surface and ground water) and green water (water stored in unsaturated soil layer and canopy evapotranspiration from rainfall) are the two sources of water generated from precipitation and communicating vessels that define the limits of water resources (WRs) for both human activities and ecosystems. However, the blue and green evapotranspiration in irrigated fields and their contribution to blue and green water flows have not been identified in studies conducted on blue and green WRs. In addition, information on intra-annual variations in green water footprints (WFs) and water scarcities is limited. In particular, there is a lack of information on water consumption obtained from hydrological model-based blue and green water assessments at the basin scales. This study quantified, taking the Yellow River Basin (YRB) over 2010–2018 as the study case, the inter- and intra-annual variations in blue and green WRs, WFs and water scarcities at sub-basin levels. WRs and WFs were simulated using the Soil and Water Assessment Tool (SWAT) model. The results revealed that the annual average blue and green WRs of the YRB were 119.33 × 109 m3 yr−1 and 296.94 × 109 m3 yr−1, respectively, over the study period. The blue and green WFs of the crops in the middle reach were significantly larger than those of the crops in the upper and lower reaches. There were visible spatial heterogeneities in water scarcities across the basin. Several areas in the middle reaches were subject to both blue and green water scarcities (at least modest level) for a minimum of three months a year. Furthermore, the norther region of the YRB was subject to significant and severe blue water scarcity for the whole year. [ABSTRACT FROM AUTHOR]

• China's inter-provincial crop-related inefficient inter-provincial VWFs reduced. • Inter-provincial unsustainable blue VWFs increased by 8% over 2004–2013. • Trade-offs between efficiency and sustainability of VWFs existed in five provinces. The phenomenon of virtual water flows (VWFs) reveals the remote pressure on local water resources on one hand, and provides the possibility of global water saving if VWFs from water-rich places of higher water use efficiencies to water-poor or lower water use efficiencies regions on the other hand. A VWF is marked as efficient when at least half of the water footprint (WF) in producing the traded products is below the corresponding WF benchmark. Whereas the VWF is marked as sustainable when flowing from the places where the water consumption did not exceed local sustainable renewable water availability. However, most attention has been taken to the sustainability of VWFs, ignoring their efficiency and spatial-temporal variations. Here we, taking inter-provincial crop-related VWFs of twenty major crops in the mainland China over 2004–2013 as a study case, analysing the efficiency of VWFs and the sustainability of blue VWFs, as well as their mutual relationship. Results show that over the study period, although inter-provincial crop-related VWFs increased slightly by 2%, the North to South VWFs and blue VWFs increased by 41% and 2.5 times, respectively. The weight of inefficient inter-provincial VWFs reduced to 75% in 2013 from 94% in 2004, whereas the unsustainable blue VWFs increased by 8%. Trade-offs between the efficiency and sustainability of VWFs existed in five provinces. Wheat, rice, maize, and cotton were the main contributors to inefficient and unsustainable VWFs. Assessments in VWFs should account for local water resources conditions, considering both efficiency and sustainability to determine hotspots and critical products and allow appropriate management of crop production and water allocation. Image, graphical abstract [ABSTRACT FROM AUTHOR]

In this study, the water footprint (blue, green and grey WF) and virtual water theory were used to uniform measure the new challenges (population growth, population urbanization, dietary structure change, energy industry development, grain trade and climate change) of food security in Northwest China. Moreover, this study quantified the demand for new challenges to water resources from 2000 to 2016, and then evaluated their impact on water resources and food security in Northwest China. The results showed that in 2000–2016, population growth caused the food consumption WF to increase from 153.8 Gm3 to 159.6 Gm3, with an average annual growth rate of 0.4%. The ratio of per capita consumption of WF of urban residents to rural areas has increased from 80.3% to 120%. The per capita food consumption in the region increased by 1.3% annually due to changes in dietary structure. However, with the increase of water use efficiency, the WF decreased by 0.3% per year. Among them, the total consumption WF of food rations decreased by 51.9%, with an average annual decrease of 4.4%, and that of meat, dairy products and aquatic products increased by 2.4%, 10.8% and 3.0% per year, respectively. From the economic point of view, the development of the energy industry has increased the competition index of energy-grain to water resources from 0.22 to 0.49. Due to climate change, although the precipitation increased at a rate of 3.2 mm/yr, the increase in ET 0 was 3.3 mm/yr, and thus the demand for water resources in agricultural production increased. Based on the results, this paper suggests to carry out measures such as optimizes crop planting structure, adopts effective biological, agricultural technologies, guides healthy food consumption structure, strengthens international food trade and biofuel use and so on to reduce the WF of grain crops and energy industry. Ultimately, the goal of reducing regional water stress and ensuring food security is achieved. Image 1 • Analysis of its impact on food security from the perspective of six factors. • The water footprint (WF) and virtual water measure the above indexes uniformly. • The WF of energy and animal products increased by 400.0% and 47.9% in 2000–2016. • Virtual water output is increasing in the Northwest (2.2 G m3/yr). [ABSTRACT FROM AUTHOR]