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Environmental Sciences
The measurement of water consumption by trees is fundamental for detecting potential opportunities to mitigate water resource depletion. The water footprint (WF) is a tool to address the environmental effects related to water use, identifying ways to reduce overall water consumption.
- life cycle assessment
- water consumption
- water nexus
1. Introduction
The current challenge of global water scarcity is addressed in Goal 6 of the 2030 Agenda for Sustainable Development (‘Ensure availability and sustainable management of water and sanitation for all’) aiming to increase global water use efficiency and guarantee sustainable freshwater withdrawals and supply [1], with a huge grade of interconnections with other Sustainable Development Goals such as Goals 7, 13 and 15 [2,3,4].
Forest and orchard trees play a relevant and fundamental role in human life, such as in providing raw materials and goods (e.g., wood, lumber, pulp, paper, fuel, firewood), food, and ecosystem services (e.g., habitat and biotic preservation, hydrological cycle regulation, watershed protection, erosion control, and climate change adaptation and mitigation) [5,6,7]. However, the agriculture and forestry sectors are highly dependent on the sustainable use and management of water resources, which can have different origins. Trees can be rain-fed, use groundwater, or be irrigated. Blue water includes surface and groundwater, i.e., water in freshwater lakes, rivers, and aquifers, while green water corresponds to rainwater on land that does not run off or recharge the groundwater but is stored in the topsoil, incorporated into the vegetation, or temporarily stays on the top of the soil or vegetation and that is evaporated and transpired by plants [8]. Disturbances in tree management affect evapotranspiration (ET) and, consequently, atmospheric moisture transport and freshwater availability [9].
The increasing water demand from human activities exacerbated by current climate change trends (e.g., temperature rise, and changes in rainfall patterns), and the rapid depletion of groundwater will put further pressure on water supplies [10,11]. This can be an issue to sustain the capacity of producing enough food to feed a world population of 11 billion by the end of the century [12]. In regions that are already experiencing increasing water scarcity, such as South Africa, at least 90% of the orchard tree plantations are dependent on irrigation [13]. Tree water consumption is a complex issue that depends on tree type, soil characteristics, climate conditions, water availability, irrigation system, and land management practices, among other edaphoclimatic conditions [9,14,15]. The water consumption issue of trees widely researched and discussed and still attracts debate. Some authors state that trees use more water than shorter types of vegetation [9,16], but other studies state there is no evidence that trees have an impact on water availability (e.g., [17,18,19]).
Overall, trees consume more water than shorted vegetation mainly due to the interception of rainwater by their aerodynamically rougher canopies and deeper rooting that increases transpiration rates. Globally, evaporation from tree interception represents about 11% of ET [20]. Nonetheless, depending on environmental conditions and leaf area, interception can be greater such as in boreal forests, where interception gets around 40% of the total ET [21]. Nisbet [22] has found that, in the United Kingdom, the forest loses between 10 and 45% of annual rainfall by interception and 300–400 mm per year by transpiration. In addition, water use by trees varies greatly throughout the year. Nisbet [22] stated that the maximum daily transpiration loss for large individual trees can vary between 500 and 2000 L on a hot summer day. In addition, the effects of reforestation on water availability are still unclear [23,24]. The measurement and reporting of water consumption by trees are fundamental for detecting potential opportunities to reduce water consumption and consequently ease the pressure on water resources. In this context, the water footprint (WF) is a tool to quantify water consumption from a life cycle perspective that also evaluates the environmental burdens on natural resources. Thus, it allows the improvement of water resources management, identification of strategies to reduce overall water consumption, and reduction of water-related scarcity impacts associated with products [9,25,26]. It also allows to consider the effects of climate change on regional and local water availability [25,27,28,29] and, consequently, can support decision makers in establishing measures to fight against climate change effects on water resources. There are two main approaches for estimating the WF of a product: (1) the WF developed by the Water Footprint Network (WFN) [8] to quantitatively depict the volume of water use along the life cycle and to assess its relevance in water resources management, and (2) the WF approach based on Life Cycle Assessment (LCA), according to ISO 14046 [20] (from now on referred to as LCA-based WF).
The first approach has been applied for water management by quantifying the WF as the volume of water consumed via a Water Footprint Assessment, with the idea of optimizing water use and productivity at a global level, while considering different parameters such as total water available at the watershed and water scarcity when interpreting the sustainability assessment [8]. The LCA-based WF approach quantifies the potential environmental impacts related to water. Therefore, the water scarcity footprint addresses not only the volume of water consumed but also the quantification of water scarcity from an environmental impact perspective, including the identification of the potential contributions to water scarcity from blue and green water consumption [9,30,31,32]. Recently the WF has been also used to address the water node of a food or forest-energy-water nexus. A nexus approach can support a transition to agricultural and forestry sustainable systems and can be used to achieve stronger integrated management of the nexus nodes implicated (water-food/forest-energy resources) by cross-sector dialogue and coordination of stakeholders involved.
2. WF Approaches
This approach was applied in 26 case studies (74%), of which 2 address a nexus concept. The remaining 9 case studies (26%) applied the LCA-based WF approach, including one focusing on a nexus concept. Regarding the mixed approaches, 2 adopted the WFN approach and the other adopted the LCA-based WF approach.
3. Goal and Scope
An essential phase of a WF study is the definition of the goal and scope of the study. In general, the main goal of the reviewed studies is to analyse how forest and orchard trees relate to issues of water scarcity and to find out how they can become more environmentally sustainable from a water perspective. The product under study and its specificities, the geographical coverage, and spatial and temporal resolutions for evaluating water availability and consumption should be also defined in the goal and scope.
Regarding the tree type, each case study and mixed approach may involve several tree types (for instance, Chouchane et al. [53] considered almond, date palm, olive, and orange, which were expressed as “occurrences”). Therefore, case studies and mixed approaches encompass 64 and 12 occurrences, respectively, concerning tree type.
Most of the occurrences refer to the global scale (28%), followed by Greece (12%), Italy (12%), and Turkey (8%). The latter are countries from the Mediterranean area where water resources availability and management are important issues, which can explain the higher interest in WF studies. In such arid and semi-arid environments, the success of agricultural production largely depends on adequate irrigation [82,83]. Olive and citrus, which were the most evaluated trees, have been studied in 7 and 6 locations, respectively. Studies on olive were carried out mainly for the Mediterranean area where this tree type and water scarcity are relevant, with a higher number of occurrences from Italy and Spain. Studies for citrus cover a wider geographical area including not only countries from the Mediterranean area but also countries from Asia (Iran and China) and South America (Argentina). Studies on oil palm, which is the third most studied tree type, have been conducted in Asian countries.
Spatial differentiation was considered in 86% of case studies and mixed approaches, but only 18% clearly indicated the spatial resolution of WF results. The temporal differentiation of the WF results was considered in only 33% of the case studies and mixed approaches. Most of these studies considered a yearly temporal resolution, i.e., they present WF results for different years. Only one case study, conducted by Chiarelli et al. [84], adopted a higher temporal resolution (monthly).
4. Accounting/Inventory
The accounting/inventory phase of a WF study involves the compilation and quantification of the blue and green water consumption of the trees under analysis. Given that some studies address several tree types and quantify both WF components, each tree type and the corresponding WF component were considered as one occurrence, resulting in a total of 131 occurrences (118 in case studies and 13 in mixed approaches). In the case studies, the blue water component was assessed in 53% of the occurrences, while the green water component corresponds to 47% of the occurrences. In the mixed approaches, 85% of the occurrences assessed the blue water component and only 15% assessed the green water component.
Globally, 56% of the occurrences refer to the blue water component and 44% to the green water component, which demonstrates that green water is often excluded from the WF assessment. However, some studies on forest species, for which irrigation is not performed, address only the green water component. This is the case of May et al. [59] for pine and eucalyptus, Quinteiro et al. [14] for eucalyptus, and Schyns et al. [74] for forest trees not specified.
5. Sustainability Assessment/Impact Assessment
The sustainability assessment phase of the WF approach consists of evaluating whether the water estimated during the accounting phase is sustainable from an environmental, social, and economic point of view. The impact assessment phase of the LCA-based approach consists of the assessment of the magnitude of the potential environmental impacts related to water consumption (e.g., water scarcity footprint). However, only 13 studies (34%) were classified as case studies and mixed approaches included these phases. 69% of the case studies (24 studies) assessed the results only from the accounting or inventory phases and only 33% (11 studies) performed sustainability assessment or impact assessment. In the mixed approaches, one study presented results at the accounting level [38], one study performed a sustainability assessment [61], and also one study performed an impact assessment [14].
In the sustainability assessment or impact assessment phase, all 11 case studies focused on blue water, while 1 mixed approach addressed green water and 1 study evaluated the blue water component. It is noteworthy that the study that included the green water at the impact assessment level was exclusively analysing that component. On the other hand, there are 6 case studies that accounted for or inventoried both blue and green water components but at the sustainability assessment or impact assessment phase have only considered the blue water component.
Although the first case study on WF of trees dates from 2008, the first studies that include this phase are from 2015, which demonstrates the time elapsed until the development of sustainability assessment and impact assessment methods and, consequently, a full operationalisation of the WF concept. The WFN is the most used approach (5 studies), covering, however, only the environmental sustainability assessment component, as the methodology to evaluate social and economic dimensions is still not sufficiently developed to be operationalised [85,86]. Within the LCA-based approach, different impact assessment methods to quantify the blue water scarcity footprint of forest and orchard trees have been applied as a likely consequence of an evolution of the methods that took place in the last years [40,87]. In 2015 and 2016, the method applied was the one developed by Pfister et al. [26], in 2018 and 2020 it was applied the method developed by Ridoutt and Pfister [88], whereas the AWARE method [40] has been applied since 2019. The AWARE method was recommended by the UNEP-SETAC Life Cycle Initiative Flagship Project [89]. For the green water scarcity footprint based on LCA, only one method [14] has been applied to forest and orchard trees.
4. WF Accounting/Inventory Results
Some studies were not included in Figure 1 because water volumes are expressed in different units. Regardless of the WF approach followed, the WF consumption can vary greatly for the same tree species depending on several aspects that include local edaphoclimatic conditions and methodological choices. Blue and green water consumption depends on planting dates, system management practices, soil properties and water holding capacity, rainfall levels, temperature, and irrigation requirements [9]. In addition, it is important to note that even when considering the same WF approach, different methodological choices can be adopted, such as addressing only one water component, using different methods to estimate the ET of trees, and using a different spatial resolution to calculate the water accounting/inventory dataset. Therefore, a comparison of WF results between different studies should be performed with caution, considering all these aspects.
Figure 1. WF results obtained in case studies and mixed approaches for orchard trees at the accounting or inventory phase. Notes: (*) refers to results following the LCA-based approach; numbers (1–3) refer to different occurrences within a study due to different scenarios studied.
Regarding the selection of different methods to estimate the ET of trees, for instance, Zotou et al. [81] compared the use of the equation of Penman–Monteith modified by FAO [81,90,91] and the modified Blaney–Criddle equation [92] to estimate the reference monthly ET of blue and green water of several trees (almond, apple, lemon, orange, olive, peach, and pear) in Greece. They found that higher values of ET were estimated by the modified Blaney–Criddle equation (except in almond), mainly due to an overestimation of the ET during the summer. According to these authors, the equation of Penman–Monteith provides more reliable results as it uses a larger climate dataset, while the modified Blaney–Criddle equation is easier to apply and more conservative.
Concerning the effect of different management practices, for instance, Pellegrini et al. [67] compared the WF consumption of different agronomic cropping systems of olive trees in Italy: traditional (<200 trees per hectare), intensive (250–500 trees per hectare) and high-density plantations (1200 trees per hectare). The high-density plantations allow to maximise production yields compared to the traditional ones. The study concluded that intensive plantations had the highest consumption of blue water followed, in this order, by high-density and traditional plantations. Regarding green water, the traditional plantation had the highest demand followed, in this order, by intensive and high-density plantations.
The effect of tree age and soil type on water use was analysed by Safitri et al. [70] for oil palm fresh fruit bunch in Indonesia through monitoring soil moisture, rainfall, and water table throughout the tree’s growth. The water requirement of trees was almost 100% supplied by green water as green water from rainfall on the upper oil palm root zone delivered the highest contribution to oil palm root water uptake in comparison to the blue water on the bottom layer root zone. They concluded that within the same soil type, younger trees have a higher water consumption. They also found variations depending on the soil type, with lower values of water consumption for spodosol soil types than for inceptisol and ultisol soil types.
This entry is adapted from the peer-reviewed paper 10.3390/w14172709
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