Employing the agricultural water footprint concept to enhance the sustainable management of water resources: a review

Water is an essential component of sustainability. The use of water by the population results in multiple environmental and social impacts. The different patterns of consumption and economic development, as well as increasing population growth, lead to stress on water sources that are already depleted (Fridman et al. 2021). The technologies for delivering water services are of concern to policymakers, as access to clean water is considered one of the key goals of sustainable development, while 2.1 billion people still lack access to an improved water source (UNICEF & hygiene 2017). Moreover, the achievement of other sustainable development goals (SDGs) depends on the achievement of water sustainability goals (Parkinson et al. 2019). It is estimated that accomplishing the goals bordered by the United Nations SDGs will require an additional annual expenditure of between 1.5 and 2.5% of global gross domestic product (GDP) (Schmidt-Traub & Shah 2015).

Globally, agriculture is the most important driver of land-use change, with the agricultural sector withdrawing about 70% of freshwater for irrigation (Liu et al. 2020). This has led to the rapid depletion of groundwater and surface water resources in some areas. In this context, international grain trade is considered to be the main driver of growing irrigation demand. Global food exports consume 11% of non-renewable groundwater. Furthermore, these export regions often suffer from water scarcity, threatening local and global water and food security (Dalin et al. 2017). The agricultural sector including livestock farming and crop cultivation also plays a vital role in meeting the world's growing food needs in the face of a growing population However, increased agricultural activity also requires agricultural productivity and upgraded overall proficiency to decrease water consumption (Zhai et al. 2020).

To face these challenges, the water resources management strategy included policies and action plans to manage water demand and supply considering the protection of its water resources. According to the concept of water footprint (WF), a more advanced perspective is introduced to study the relation between a commodity consumer or producer and the use of freshwater systems, WF could be used as a useful indicator to support the evaluation of water supply processes. The agricultural WF is an estimation of the water consumed by crops and livestock in a given area from all types of blue, green, and gray water. The agriculture sector consumes 2,500 km³ of water for irrigation purposes annually (Fanzo 2019).

The difference between traditional water statistics and the WF is clear, traditional water takes into account the amount of water withdrawn to produce a product from generally used surface water resources, while WF concerns the sum of all water used in the production processes of the commodity throughout its supply chain. The WF is an extension of the virtual water perspective, including an explicit spatial and temporal indicator of freshwater use (ElFetyany et al. 2021). Although numerous water-saving initiatives, particularly in the agricultural sector, are being adopted to ease the situation, their efficiency and effectiveness in terms of long-term sustainability have been the primary focus in developing solutions (Lovarelli et al. 2018). The objective of involving the sustainability aspect is not only to utilize natural resources without harming the water or land quality but also to ensure that both current and future water needs are met.

Most previous studies focused on reviewing the concept of the WF application at the national level. At the same time, there are no reviews in specialized sectors, nighter nor links with water management practices. Therefore, this study aimed to review the agricultural WF with considerations of sustainability in managing water sources, the methodologies used in an assessment, and recent application levels as well as discuss the pros and cons of each and provide organization and interpretation of existing literature in the light of recent developments in the field to achieve consistency in the knowledge and contribute in the optimal solutions in agriculture practice and enhance the sustainability.

The WF was introduced by (Chapagain & Hoekstra 2004) as a consumption-based indicator of water consumption, measuring all direct and indirect freshwater consumption in the supply chain of a product, service, or production process. This indicator defines each contribution to water consumption geographically and temporally, showing the amount of water consumed by source and the amount of pollutants by type of pollution. It is a volumetric measure of water use and pollution (Aldaya et al. 2011).

WF differs from traditional ‘water withdrawal’ measurements in three ways: First, bluewater consumption is not included in the calculation as this water returns from where it came from. On the other hand, it is not limited to direct water consumption but also takes into account indirect water consumption. The total WF is the sum of three inputs, not just the blue water use: the blue, green, and gray WF (RECANATI 2013).

The Components of WF contain three different categories, the colors blue and green refer to the different water sources from which the water is consumed, and the color gray to the polluted water as described in Hoekstra (2011) and Herath (2013):

• Blue WF discusses the consumption of blue water resources in both surface and groundwater along the entire product chain. ‘Consumption’ is when water evaporates, returns to another watershed or sea, or is incorporated into a product. Blue water has more than one alternative in use and needs a cost of supply therefore the cost of its operation is high.

• Green FW refers to the consumption of green water resources, i.e. rainwater without runoff and groundwater supply water is preserved in the soil, in the form of water moisture, and it is consumed by evaporation and transmission during the production process, in addition to what is collected from harvested rainwater.

• Gray FW refers to the amount of water used due to pollution: It is defined as the quantity of fresh water required to absorb the pollution load, with concern about water quality standards in the region and natural contextual concentration.

With its components, the WF thus offers a complete description of the relationship system between the various producers or consumers and freshwater applications. Instead, it is a volumetric measure and does not measure the severity of local environmental impacts related to water abstraction and pollution, as these depend on the sensitivity of the local water system and on the intensity of the consumption and pollution processes and economic (Recanati 2013).

The actual water needs of the population in terms of agriculture are the focus of this paper. it is not measured in the traditional way, which expresses the volume of water extraction in the country, because some of these needs are completed through virtual water. A measure has been developed that takes into account virtual water throughout the world. This measure includes the water that is included in imported goods and is consumed within the country (Chapagain 2006).

According to Mekonnen & Hoekstra (2011a, 2011b), the agricultural production contributes 92% to the total global footprint, and the global WF in the period 1996–2005 was 9,087 Gm³/year (74% green, 11% blue, 15% gray) Additionally, 20% of the global WF relates to production for export while the global water saving as a result of trade in agricultural products in the same period was 369 Gm³/year. Spain and Mexico have the largest national blue water savings as a result of trade. The WF of the global average consumer in the period 1996–2005 was 1,385 m³/year. 5% to the consumption of industrial goods, and 4% to domestic water use and all remaining consumed in the agriculture sector. The average consumer in the US has a WF of 2,842 m³/year, while the average citizen in China is 1,071 m³/year and in India is 1,089 m³/year according to a study by Mekonnen & Hoekstra (2011a, 2011b).

A country's WF determination depends on four factors. These are consumption volume; consumption pattern; climate and agricultural practice. Also, it is divided into two main components: the internal WF and the external WF (Chapagain & Hoekstra 2008). The internal WF refers to the domestic freshwater used to produce goods and services consumed by the inhabitants of a particular part of the country. An external WF refers to fresh water used to produce goods and services outside a region and then imported for consumption within a given region. When analyzing the footprint of a defined area, such as a country, the distinction between internal and external water footprints is crucial. Prior to the introduction of the WF concept, policymakers had limited understanding of the characteristics of agricultural production and supply chains that strongly influence water consumption and water pollution (Ali 2019).

Water problems are not limited to its abundance and sufficiency, but rather to its uneven distribution in the world. Some countries have water abundance and others suffer from scarcity. Therefore, importing water to arid regions through food products and services in the form of virtual water instead of producing it locally can be an effective solution (Ali 2019).

In 1993, the concept of virtual water arose through Allan as an alternative option for countries that suffer from severe water scarcity, and it was defined as the volume of water required to produce a good or service along their supply chains, but then it evolved to include the total volume of fresh water used to produce a product or service, including water consumed in production and not actually present in the product. It expresses the direct and indirect use of water (Ridoutt et al. 2009; Qasemipour et al. 2020).

According to Horlemann & Neubert (2007), 80% of virtual water trade across the world is agricultural goods and the remaining 20% by all other goods. An increase in levels of trade has been encouraged in order to help mitigate the water crisis in several parts of the world, particularly in arid areas.

Hoekstra & Chapagain (2011) argued that domestic water resources can be conserved by trading virtual water between countries. For example, Japan saves 94 billion cubic meters of water annually from its domestic water resources, Mexico 65 Gm³ annually, and Italy 59 Gm³ annually. A closer look if Mexico moved to growing wheat, maize, and sorghum instead of importing them from the US, it would lose a lot. These products require 7.1 Gm³ of water per year in the US compared to 15.6 Gm³/year in Mexico. Thus, at the national level, 15.6 billion cubic meters of water annually are saved because nothing grows, while internationally 8.5 Gm³/year are saved.

Horlemann & Neubert (2007) showed how virtual water trading can save water in a global context. In Egypt, it takes 1,100 L of water to produce 1 kg of corn, while in France almost half of that is needed, resulting in an overall water saving when Egypt imports corn from France.

Another example is provided by Mekonnen & Hoekstra (2010) who points out that Morocco saves 3.77 Gm³ of domestic water resources by annually importing 906,000 tons of wheat from France. Since wheat production in France uses only 600 million cubic meters of water per year, the annual grain trade volume between Morocco and France saves 3.17 Gm³ worldwide.

Saving water at the global or national level is not just a question of a water deficit for domestic needs, other factors also play an important role in the allocation of water resources. One such factor is plant productivity. For example, Japan imports corn from the United States, although corn in Japan requires 367 million m³ during the growing season, compared to 411 million m³ in the United States. However, the corn yield in the United States is about three times that in Japan (Chapagain et al. 2005).

An additional example illustrating this is provided by Wichelns (2010) who argues that the importance of efficient water management in agriculture can be easily demonstrated due to the importance of agriculture and research dealing with agricultural phenomena. For example, South Africa in the SADC exports more corn than any other country in the region despite having relatively little water; in fact, Zambia and Zimbabwe are the main importers of this maize, both of which are characterized by significantly higher rainfall.

De Fraiture & Wichelns (2010) reported that researchers at the International Water Management Institute have elucidated the national and international implications of virtual water trading. They showed that in addition to the 433 km³ of water needed to produce 215 million tons of wheat in 1995, 178 km³ of irrigation water would have been added if the wheat had been produced in importing countries.

Based on the availability of trade data and the need for national water security, most WF applications have been done at the national scale. However, the number of regional and urban scale applications is progressively increasing (Konar et al. 2012; Chen & Chen 2013).

At the national scale, Hung (2002) conducted the initial global virtual water estimates assessing international crop trade adding to the UNESCO-IHE report series, which makes available updated national and global WF. Hoekstra & Mekonnen (2012) analyzed the blue, green, and gray WF of humanity and found that 20% of the WF of global production was for exported goods, thus highlighting the global role played by virtual water flows. The study evaluated the temporal evolution of the network statistics of global virtual water trade, showing that countries increased, to a larger extent, their virtual water trade and savings. Economic factors are the main driving force in the international virtual water flows while it is not strongly connected with domestic water resources (Dalin et al. 2012). To justify this, WF analysis at the urban scale is required.

With regard to the urban and subnational scale, Table 1 presents the most prominent studies that dealt with assessing the WF at this level. Some studies indicated that the reality is at the level of the subnational dimension, that areas rich in water are imported from areas with limited water, as in the United States of America, as well as in the United Kingdom, where it was found that some areas affected by drought in the southeast require more water for agricultural purposes than from the northwest, which is rich in water, and the same thing is repeated in India and northern China, and this confirms the necessity of conducting an analysis of the WF with different spatial borders.

An assessment of the WF was conducted at the urban level in Milan, Beijing, and London. In addition, a case study was conducted in the German capital, Berlin, Lagos, Nigeria, and Delhi in India. These studies can be summarized that the assessment of the WF at the urban and regional levels provides an insight into water consumption and is more accurate compared to the national level. This has a clear impact on urban water management.

Given that cities are diverse and have multiple characteristics in terms of size, infrastructure, population, and dietary pattern, it may be necessary to go to the local areas and obtain accurate data for the city, which constitutes one of the challenges related to the analysis of the urban WF.

Hoekstra (2017) stated that the advances in WF assessment expand our understanding of how different roles can contribute to forms of water governance that integrate the important principles of environmental sustainability, social justice, economic effectiveness, and supply security.

In general, WF analyses have focused on blue and green water components, with uncommon studies dealing the gray water. WF analysis methods can commonly be divided into bottom-up (product-level) and top-down (sector-level) approaches (Daniels et al. 2011; Chenoweth et al. 2014). Due to the widespread review of general WF methods, we highlight three main approaches used in regional or urban studies: (1) water footprint assessment (WFA), often used at the product/commodity level; (2) environmental extension inputs output (EEIO), which uses economic input–output tables, thereby accounting for sector-level data; (3) life cycle analysis (LCA), which relies massively on standardization and databases to Environmental (including water) and health effects. Hybrid approaches of these mechanisms have additionally been used to combine scales and available datasets (Paterson et al. 2015).

Hoekstra (2011) has created a WF network method, which can be applied to any sector, product, and scope, and through it determines the amount of freshwater consumption. This method is based on the WaterState database. This method is applied in agricultural commodities as well as livestock because there is a large proportion of agricultural production that depends directly on fresh water and there is a global focus on green water. Moreover, we can apply the hydrological model effectively in estimating the WF. There have been studies where it used CROPWAT as in a study (Ma et al. 2006; Mubako & Lant 2013), as well as applying the Global Crop Water Model (GCWM) as in a study (Siebert & Döll 2010; Hoff et al. 2014). With the update of the global hydrological models consisting of two sides, the human side, and the hydrological side, we were able to simulate what the main agricultural crops contain virtual water. This model is called H08 as mentioned in the study (Hanasaki et al. 2010; Dalin et al. 2014).

The network methodology provides a mathematical framework for the integrated modeling on which the WF system is based, for example, the consumer, the producer, the transfer, etc. One of the drawbacks to this method is that several countries do not have accurate data, and this approach cannot be followed from top to bottom (Ruddell et al. 2014).

The environmentally extended input–output (EEIO) method works as an econometric tool to analyze interrelationships between sectors by monitoring financial flows in the supply chain and their subsequent association with environmental consumption transactions (Paterson et al. 2015).

In the context of the WF, the default water quantity is determined in units of water volume per dollar of commodity volume. Therefore, it is required to determine the amount of water consumed in each sector in the IO tables. The study was implemented in a single region as in a study done by Velázquez (2006), as well as it was carried out between inter regions as in a study by Mubako et al. (2013a, 2013b). A study by Lenzen (2009) and Vadis (2011) showed in their studies, The using of multiple regional analyses is more efficient (MRIO) because commercial data are more easily available.

Life cycle assessment (LCA) is a tool that systematically describes the use of water resources and pollution related to the production of a specific product or service. It includes an environmental assessment based on resource depletion and pollution by studying the supply chain from its inception, from raw material to production. The product life cycle refers to the stages of life from the beginning as raw material through using it all the way to production at the end of the chain (Arnøy 2012). The studies conducted by Berger & Finkbeiner (2010) and Hester & Little (2013) presented the implication of life cycle assessment in evaluating water use in sectors and products.

The International Organization for Standardization (ISO) relied on general principles and developed a framework for conducting a life cycle assessment. It standardized these conditions and guided its implementation (Standardization 2006). The generally used databases are GaBi and Quantis linked to tracking freshwater consumptive use besides Ecoinvent data currently track withdrawals (Kounina et al. 2013). The software used to conduct LCA is SimaPro. It is used by consulting societies and international databases are integrated in this program. It permits to conduct LCA and build models of production methods or services. The core application is the environmental effects of products and services (Recanati 2013).

Table 2 outlines the main advantages and disadvantages of using these methods at the city scale.

In recent years, there have been several efforts to assess global water consumption in agriculture at high spatial resolution. The earlier evaluations focus on the estimation of blue water withdrawal and irrigation water needs. More recently, a few studies have divided global water consumption for crop production into green and blue water and made a global estimation of agricultural green and blue water consumption with a spatial resolution (Mekonnen & Hoekstra 2011a, 2011b).

Estimating the agricultural WF requires gathering fitting data for the studied crops. Assessment of the blue WF involves the component of blue water consumption and consistent yield for that crop. Calculation of the green WF needs control of evapotranspiration and crop coefficient, together with the crop yield and green water elements. As well as, the estimation of gray WF is essential to take into consideration the leaching runoff coefficient data, fertilizer application rate, and acceptable level of nitrogen in fresh water besides the natural rate of nitrogen inside water bodies. These data are attained from Food and Agriculture Organization Corporate Statistical Database (FAOSTAT), Global Crops (2019) also blue water consumption typical for crops from the Bureau of Statistics as mentioned in Hoekstra (2011).

The blue water component of water usage by crops is the quantity of provided water that evaporates into crops without reversing to the original source. Estimation of the blue WF necessitates data on provided water to crops.

Hossain et al. (2021) calculated the crop evapotranspiration and attained the data from the Agriculture Victoria Organisation (AGO) owing to the reference evapotranspiration value required. It calculated the values for some parts of Victoria for a typical crop-growing cycle. The crop-growing season is conducted during the ninth month. These data have been measured over a 7-year and averaged to progress precision. The coefficients specify water use interrelated to crop type. It is noted that K_c values have been calculated by FAO. However, the values can be modified for the local condition.

The sum of all control evapotranspiration is between d = day 1 of the growth cycle and lgp is the total growth cycle in days.

The gray water component dealings the amount of freshwater that is gained from a source and used to eliminate pollutants instigated in production. For agriculture, fertilizer is generally the leading pollutant that requires adjustment. Data on fertilizer application rates have been taken from the literature to assess a gray water component in the WF for crops.

The application rate is expressed in kg ha⁻¹. C_max is the allowable maximum level of nitrogen in freshwater (mg L⁻¹) subsequent to the World Health Organization (WHO) standardization. Region-specific authorities may have a different value for C_max and therefore should be consulted. C_nat is the natural level of nitrogen within water bodies in that region. For example, Australian waterbodies are considered pristine and therefore C_nat must be taken as 0.37 Y is the crop yield rate (ton ha⁻¹) provided by FAO (Mekonnen & Hoekstra 2011a).

As stated in the study carried out by Hoekstra et al. (2009), the WF within a geographically delineated area (WF area) is calculated as the summation of all WF processes in the area. The WF of the consumers in a nation (WF_cons,nat) has two components: the internal WF and the external WF. The internal WF of national consumption (WF_cons,nat,int) is the sum of the WF within the nation (WF_area,nat) minus the volume of virtual water export to other nations insofar as related to the export of products produced with domestic water resources (V_e,d). The external WF of national consumption (WF_cons,nat,ext) is defined as the volume of water resources used in other nations to produce goods and services consumed by the population in the nation considered. It is equal to the virtual water import into the nation (V_i) minus the volume of virtual water export to other nations as a result of the re-export of imported products (V_e,r). The virtual water export (V_e) from a nation consists of exported water of domestic origin (V_e,d) and re-exported water of foreign origin (V_e,r). The virtual water import into a nation will partly be consumed, thus constituting the external WF of national consumption (WF_cons,nat,ext), and partly be re-exported (V_e,r).

Economic and social progress must have access to a steady supply of freshwater. As a result, we must sustainably handle our freshwater sources. Sustainable water resource systems involve three interconnected elements, which are the natural environment, the socio-economic situation, and the management system. The goal of this integrated system is not only to utilize natural resources without harming the water or land quality but also to ensure that both current and future water needs are met regardless of changing circumstances (Ghabayen et al. 2016).

When managing water resources sustainably, it is important to take into account both current conditions and future demand and supply without causing harm to the system. Signs of unsustainability can include disruptions in the supply and demand of water, a continuous decline in groundwater resources, and a lack of environmental flow. In order to tackle this issue, governments often create macro-level policies. However, it is crucial to evaluate these policies before implementing them, as they may be ineffective in addressing existing challenges or even lead to contradictory outcomes or negative consequences (Raeisi et al. 2019).

Before implementing policies, it is recommended to conduct a quantitative assessment. This approach not only provides policymakers with more understanding of the consequences of their decisions but also helps in evaluating the effectiveness and sustainability of water policies. Several frameworks are available for this purpose, such as the lifecycle assessment framework as elucidated in the study of Matohlang Mohlotsane et al. (2018), environmental impact analysis, sustainability standards as in Jamshidi (2019), and the WF accounting framework (Hoekstra 2019). These frameworks have been mentioned in various studies and have gained attention for their contributions to evaluating water policies.

The WFA framework, as mentioned by Muratoglu et al. (2022) has the ability to uncover the total amount of water used during the production of goods or services. It can also be implemented for a thorough evaluation of water and agricultural systems. By utilizing this framework along with the available water, valuable insights can be gained regarding water consumption, productivity, and sustainability, as stated by Hoekstra (2019). Additionally, the WFA framework takes into consideration the interaction between blue and green water. This aspect is crucial for assessing the effectiveness of policies implemented upstream of a basin in managing green water and observing its impact on blue water downstream as highlighted in Zarezadeh et al. (2023).

Hoekstra (2009) was one of the pioneers in exploring the sustainability aspect of WF by comparing it to the availability of freshwater. According to Hoekstra, conducting WF analysis offers a more comprehensive method of evaluating the sustainable usage of freshwater resources by humans. Additionally, Hoekstra and his colleagues (2011) proposed a four-step method for assessing WF sustainability, which includes determining sustainability criteria, identifying WF sustainability hotspots, quantifying the main impacts of these hotspots, and analyzing the secondary impacts they cause (Hoekstra 2011).

In a study conducted by Zeng et al. (2012), it was discovered that the environmental sustainability of the blue WF in the Heihe River Basin, which is situated in the arid and semi-arid regions of Northwest China, was in jeopardy. This was due to the blue WF surpassing the amount of available blue water in the basin for a duration of 8 months each year.

In their study, Lathuillière et al. (2018) examined the sustainability of agriculture in Southern Amazonia in terms of the availability of green and blue water. Another study by scrutinized the environmental sustainability of blue WF in the major river basins globally and discovered that approximately four billion individuals experience significant water scarcity for at least 1 month annually.

Evaluation the environmental sustainability of WF (including blue, green, and gray WF) for 31 provinces in China in the years 2002, 2007, and 2012. Given that China faces significant challenges regarding water quantity and quality, it is an ideal location to study the environmental sustainability of WF. This study introduces innovative methods and presents the first analysis of the environmental sustainability of blue, green, and gray WF across different provinces and time periods. The findings of this study will contribute to policymaking regarding sustainable water resource management (Liu et al. 2020).

In a specific region in Chile, the sustainability of agricultural water usage in this area is influenced by factors such as runoff, surface water availability, water consumption, and water quality. The current management practices at the basin level have made food production reliant on the type of irrigation used and the climate conditions. This raises two important questions: Is the WF blue (WF) of the crops sustainable considering the availability of surface water and variability in climate? And, during which periods and sections does the WF blue of agriculture become unsustainable? This study assessed the sustainability of the WF in agricultural practices in Chile (Novoa et al. 2019).

A comprehensive assessment of the WF of crop production on a large scale was conducted, specifically focusing on a river basin. This assessment will evaluate the sustainability of agricultural activities by considering both the quantity and quality of water used. The main goal of the study is to identify potential strategies that can enhance the sustainable use of water and land in regions where water is limited. To gather accurate data on human activities and agricultural practices, a thorough survey was carried out in high detail over the course of a year. Additionally, interviews were conducted with farmers to gather reliable information. Measurements of streamflow and nitrogen concentrations in surface waters were also recorded (D'Ambrosio et al. 2018).

The WF indicators' critical input is consumption, which is a key aspect in analyzing and altering water-related policies. Overloading from rising water demand has an impact on the long-term viability of water resource systems, particularly in watersheds with extensive farmed regions (Lovarelli et al. 2018).

Manzardo et al. (2016) used WF as an innovative strategy to reveal the state of freshwater resources by using indicators of blue, gray, and green water. As a result, water policies focused on green water (e.g., watershed management or rain-fed areas) can be notable from those based on blue water (e.g., adjusting irrigation systems or cropping forms) or gray water.

Several studies utilized The Soil and Water Assessment Tool (SWAT) model to evaluate agriculture policies and their impacts on water saving. It examines how altering crop patterns and converting surface irrigation systems to pressurized ones affected water productivity and the enhancement of environmental flows and the impacts of different land-use changes as well as investigated strategies to manage soil moisture deficiency and plant water requirements in addition also assessed the vulnerability of water resources and crop yields, specifically winter wheat and sunflower by implementing adaptation activities using the SWAT model (Ahmadzadeh et al. 2014; Maier & Dietrich 2016; Brouziyne et al. 2018).

The implementation of any management measure without assessing its impact on the long-term sustainability of the system can result in irreversible consequences. Therefore, it is recommended to conduct a pre-evaluation using an integrated framework and relevant indicators to achieve more effective and efficient outcomes from management measures at a lower cost. Certain activities related to Iran's main plans for agriculture were examined, such as the adoption of modern irrigation systems, watershed management, rain-fed orchards on sloping lands, and the reduction of water-intensive crops (Nalbandan et al. 2023).

Four instruction-level documents that have had the highest effect on water and agriculture policies are selected in Iran. It included (1) general Iranian policies for the agricultural sector (2012), (2) general policies for a resilient economy (2013), (3) agricultural sector policies for crisis and disaster management, and (4) the sixth five-year plan for socio-economic and cultural development (2017–2021). It is noted that is important to evaluate the effectiveness of policies in conserving water, enhancing environmental flow, and restoring groundwater within a comprehensive approach. This means these policies should not harm each other and should work together positively (Zarezadeh et al. 2023).

The success of water-saving initiatives at a larger scale, such as investing in advanced irrigation systems, changing which crops are grown, and using greenhouses, relies on making sure water depletion is minimized. This means reducing the amount of water lost through evapotranspiration. Additionally, it is crucial to effectively manage the ‘rebound effect,’ which refers to the potential increase in water usage that can occur if water-saving measures are not implemented properly. This requires better management of water allocation, including the development of relevant infrastructure and institutional arrangements. Failing to address these issues can actually lead to higher water consumption. When defining water productivity and its indices, it is essential to prioritize the element of reducing depletion. Relying solely on reducing withdrawal without considering other factors can result in incorrect choices. In order to improve the deficiencies found in the higher-level documents, it is important to prioritize the integration of key strategic concepts, such as ‘water resources carrying capacity’ and ‘sustainability,’ when developing water policies (Zarezadeh et al. 2023).

Using the suggested integrated modeling method and its ability to monitor various aspects of the water budget and simulate hydrological processes on a smaller and larger scale, is a sensible way to evaluate policy effectiveness and potential negative impacts. This approach helps avoid wasting resources on ineffective policies and ensures the long-term sustainability of water resources.

Water-related problems pose a significant challenge to China's sustainable economic and social development, primarily due to its large population, inadequate water and soil resources, and imbalanced water supply and demand. China has acknowledged the importance of water resources management through initiatives such as the implementation of the ‘Tenth Five-Year Plan’ in 2001, the introduction of the ‘Three Red Lines’ in water resources management, and its commitment to the ‘2030 Agenda for Sustainable Development.’ As green development gains prominence, effective water resource management goes beyond enhancing economic benefits and encompasses improving water-saving capacity and production. Consequently, it is crucial to study China's sustainable utilization of water resources and provide a foundation for policymakers to develop context-specific water management strategies (Wu et al. 2022).

The sustainability assessment of a WF according to the Global Standard allows for the comparison of local water footprints with local sustainable water availability levels. Hydrological studies determine the amount and location of available water, while ecologists determine the amount of water that should be preserved for nature. Any remaining water can be sustainably used by humans.

If the water footprints exceed the maximum sustainable water availability levels, water scarcity occurs. Additionally, if more pollutants are added to a water body than it can handle, water pollution occurs. The concept of the WF helps identify the causes and contributions of specific human activities toward these water challenges. This information can be used by producers, consumers, and policymakers to effectively target reduction efforts (Hogeboom 2020).

To ensure sustainability, the WF in a certain catchment must fulfill specific requirements. Sustainability encompasses not only the environmental aspect but also the social and economic aspects as presented in Hoekstra (2011).

Environmental sustainability entails maintaining water quality and ensuring that it meets agreed-upon standards. It is also important to regulate river and groundwater flows to prevent disruptions to ecosystems and the livelihoods of those who rely on them. These regulations, known as environmental flow requirements, are similar to the boundaries set for pollution through water quality standards. Similarly, for green water, the environmental green water requirements establish limits for human use.

Social sustainability pertains to ensuring a sufficient quantity of freshwater is accessible distribution of resources should prioritize essential human requirements, including the minimum amount of water needed for drinking, cleaning, and cooking domestically and the minimum amount of water needed for food production to ensure an adequate food supply for everyone. This means that only the portion of available fresh water that remains after subtracting the water needed for the environment and basic human needs can be used for non-essential items. It is important to guarantee a minimum domestic water supply at the catchment or river basin level for drinking, cleaning, and cooking. At a global level, a minimum amount of water needs to be allocated to food production since communities in river basins may not have enough food on their own, as long as food security is ensured through imports (Hoekstra et al. 2011).

Economic sustainability: it is important to manage and utilize water in an economically efficient manner. The advantages of using water for a specific purpose (whether it is labeled green, blue, or gray) should outweigh the total expenses linked to its usage, including external costs, missed opportunities, and the cost of scarcity. If this is not the situation, the WF is considered unsustainable.

If the green, blue, or gray WF in a specific area fails to meet the requirements of environmental, social, or economic sustainability, it cannot be considered as being sustainable from a geographical perspective.

Agricultural activities are a major cause of water stress worldwide, but previous studies have not connected water scarcity in specific locations to the water usage for growing specific crops and evaluating the sustainability of each crop's production. However, there has been a recent rise in studies that assess the sustainability of the blue WF for crop production. When the blue WF surpasses the available renewable blue water, it becomes unsustainable and has negative consequences for the environment, such as depleting groundwater and not meeting environmental flow standards (Mekonnen & Hoekstra 2020).

Novoa et al. (2019) assessed the sustainability of water consumption in agriculture by analyzing the amount of water used for key crops in the Cachapoal River basin. The study considered different climate scenarios (dry, wet, and normal years) to understand how variations in weather patterns can impact water usage. By analyzing the data, the researchers aimed to identify instances where water extraction for agriculture becomes unsustainable. This analysis was conducted using two indicators, namely WFS blue and WFS gray, to determine the sustainability of water usage in agriculture.

Several research studies have utilized sustainability assessment to determine the level of blue water scarcity on a global level as mentioned in Lathuillière et al. (2018). However, there has been only one study so far that has tried to measure green water scarcity. As a result, there is a need for additional studies that employ the concept of green water scarcity in order to fully validate WF assessments. The WF sustainability assessment was analyzed in different scenarios for the years 2030–2031 and 2050–2051 to develop a strategy for managing water resources by considering previous and future decisions regarding land and water usage. By considering changes in land use, climate patterns, and agricultural production, the assessment provides valuable information for water resources management and the wider WF community. This knowledge can be used to make informed decisions about regional production processes (Lathuillière et al. 2018).

Mekonnen & Hoekstra (2020) examined the sustainability of crop production's global blue WF, specifically focusing on each of the 146 primary crops from 1996 to 2005 that contribute to the unsustainable portion of the blue WF. Furthermore, it explored the potential for water savings by lowering the WF to a benchmark level. This research enhances previous studies by providing a more comprehensive understanding of the individual contributions of crops to the unsustainable blue WF and evaluating the potential water savings achieved by reducing crop WF to a benchmark level.

The significant point to note is that there is a pressing need to promptly decrease the pressure that humanity places on freshwater systems. This necessity is not due to a total lack of freshwater globally, but rather because of the current imbalanced and strained use of freshwater, especially in vulnerable watersheds.

The solution to reducing humanity's WF primarily lies in changing how we produce, consume, and invest in goods and services. This requires implementing supportive policies and reporting frameworks that incentivize businesses to make necessary changes while minimizing the risk of being at a competitive disadvantage compared to companies that do not take action. Giving proper value to water and preventing negative outcomes caused by market distortions that result in excessive depletion of resources in low- and middle-income nations are also important objectives (Ridoutt & Pfister 2010).

By considering the factors outlined by A. Chapagain et al. (2005), a nation can decrease its WF. One method to achieve this is by implementing production methods that use less water per unit of output, resulting in more products for every unit of water utilized. For example, this can be done by employing advanced technology in irrigation systems and enhancing rainwater storage systems for additional irrigation purposes.

One alternative is to switch from consuming products that require a lot of water to those that require less, like reducing meat intake. To make this happen, it is necessary to increase awareness among consumers and planners and alter consumption habits. Water pricing could be an efficient method to motivate people to change their consumption patterns. Furthermore, transfers the manufacturing process from nations that have low water efficiency to nations that have high water efficiency. An illustration of this is Jordan, which has effectively outsourced its water consumption by relying on imports of rice and wheat from the USA, a country with greater water productivity (Ali 2019).

With climate change and worldwide demographic shifts happening, taking action to decrease the blue, green, and gray water footprints becomes essential. In order to address the two important issues of ensuring enough food for everyone and protecting the environment, we need to consider water as a global resource and change the traditional methods used in agriculture that have harmed the environment. We should carefully evaluate the competition between land and water for producing food, feed, and bio-energy. Using a particular product more effectively in regions with abundant water leads to higher output using the same amount of water. This could potentially lessen the WF in areas where water is scarce, as fewer imports of virtual water would be needed. Additionally, it allows for the allocation of water toward the production of other goods (Hoekstra 2011).

Increasing agricultural production through the use of green water resources can lead to the depletion and degradation of these resources. It is crucial to decrease the dependence on green water and find alternative, more sustainable methods of increasing food production. Vanham & Bidoglio (2013) illustrated various methods to decrease the WF components, including a reduction in the domestic WF, achieving higher productivity of agricultural products through improved use of water resources (both surface water and groundwater), and closing the gap between potential and actual yields within the European Union. This can be accomplished by increasing the efficiency of irrigation practices. However, it is important to prioritize sustainable intensification by learning from organic farming methods and adopting precision agriculture techniques. In line with this, the European Parliament has urged for immediate action to cut food wastage in the European Union in half by the year 2025. Additionally, citizens across the 28 member states of the EU need to make adjustments in their consumption habits. Especially reducing the consumption of animal products, particularly meat, would significantly reduce the WF of the EU28. This is because more than 50% of cereal production is utilized as animal feed, with additional feed being imported (Vanham & Bidoglio 2013).

This paper provided further understanding and knowledge about the WF and its applications as an indicator of water resource sustainability. The paper focused on the agricultural sector, which is a water-intensive sector. It was demonstrated that water scarcity and WF cannot be viewed in terms of size alone, but rather are linked to political, institutional, and social processes. Therefore, a detailed explanation of the concept of WF and its stages of development was provided, and virtual water was discussed as a concept linked to the WF. It is important to estimate the water requirements of agricultural crops to control water scarcity and improve blue water consumption by employing advanced technology in irrigation systems, modifying the agricultural structure, and replacing water-depleting crops, within an integrated process to rationalize water consumption. The paper also discussed the three components of the WF (blue, green, and gray), their applications, and the results achieved. It referred in detail to the approaches used to assess and estimate the WF at the agricultural level using the Hoekstra methodology, and provided a summary of the countries that apply the WF and its impact on water sustainability, in addition to making a comparison between its pros and cons by reviewing previous studies. Ultimately, this research discusses the strategies and policies for sustainable water resources that previous works have investigated through the WF, and showed a clear reduction in water consumption in many countries, and these results helped improve and enhance the efficiency of the water sector. Through this research, the scientific gaps in the aspects of green WF sustainability were explored, and this needs further future studies as there are no studies that cover this path in depth as well. It can be noted that building on this study can improve agricultural practices and extract the best ways and methodologies to apply it according to the nature of the climate and geographical area, and it can be recommended to conduct detailed research on irrigation practices in order to improve the agricultural WF. These results have shown the effectiveness of the WF concept in helping policy makers develop the agricultural structure and enhance the sustainable management of water resources.

Agricultural water footprint: This is an estimation of the water consumed by crops and livestock in a given area from all types of blue, green, and gray water.

Virtual water: The total volume of fresh water used to produce a product or service, including water consumed in production and not actually present in the product. It expresses the direct and indirect use of water.

Sustainable water resource: This utilizes natural resources without harming the water or land quality and ensures that both current and future water needs are met regardless of changing circumstances.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.