2. Evaluation Tools and Models
Sustainable systems are especially apt to compare favorably with conventional systems when the comparison includes a full cost accounting of the environmental and public health harms and benefits of each system
.—Horrigan et al.
SWM evaluation requires accounting for real costs, opportunity costs, and competing requirements among and within water use sectors. These sometimes include vague political and socio-economic components, which often do not translate easily to the quantitative values necessary for planning, decision-making, and rigorous monitoring and assessment. Evaluation models range from generic indices used to compare management practices across multiple agencies and environments, to site-specific modeling analyses that enable individual managers and governments to assess progress toward or away from sustainable practices [63
Inputs to the evaluation should be quantifiable, independent from each other, unambiguous, and representative. Quantitative evaluations allow for assessing conditions and comparing management options. Following the definitions for sustainable development, many tools aim to evaluate environmental, social, and economic aspects of the system [64
]. While many such tools quantify environmental impact and resource utilization, we continue to lack robust quantitative evaluation methods for social-cultural criteria, and the interconnected impacts of social, biological, and physical components of complex systems [66
In addition to having a quantitative framework that bridges a complex system, evaluation models must account for the time and place dependent conditions of sustainability. Time is relevant within the lifecycle of a specific action or management practice, and also in the subsequent impacts on the environment. The spatial scale of evaluation governs what resources are assumed to be available for a given location, ranging from household to village to watershed to country to multinational region. The spatial extents of the region, including quantity of available water resources and competing users, will influence analysis of existing and potential development. Accounting for local conditions and perspectives in the evaluation method has particular utility when assessing policy impacts over time or when advocating for specific policy changes [25
There are three primary evaluation methods for SWM: (1) indicators and indices; (2) product related assessment; and (3) integrated assessment [64
]. Water indicators and indices should simplify, quantify, and communicate information [68
]. The classification, weighting, and method of index aggregation varies by model [68
], making the processes somewhat arbitrary. However, the advantage of the indicator method is a simple numerical result that provides comparative capability across cases. Common indices which address water in sustainable development include the Water Poverty Index [69
], the Canadian Water Sustainability Index [70
], the Environmental Performance Index [71
], and the Watershed Sustainability Index [72
]. Water management indicators typically include water infrastructure, environmental quality, economics and finance, institutions and society, human health, and technology [4
]. Other indices which specifically address some of the socio-economic dilemmas mentioned previously include accessibility of data, institutional schemes to resolve water conflict, and democratic water-related decision making [3
Product related assessments, or life cycle assessments (LCA), can provide information about land, water, and energy requirements for a physical system or supply chain. The LCA framework can be used to inventory a set of sustainability indicators across the supply chain of a water system. Examples such as the Ecological Footprint [73
] and its hydrologic corollary, the Water Footprint [74
], translate biophysical assets into progress measurements. These tools are resource accounting mechanisms that enable integrated quantitative assessments of land and water resources in terms of current and potential demand, and regenerative probability at national and global scales.
Integrated assessments tend to be holistic assessments completed using system dynamics models, risk analysis, cost-benefit analysis, and impact assessments. Integrated assessments provide a systems perspective, often incorporating more robust quantification than the indicator method alone. Information theory can be used to study how efficiently human systems are using resources, and the resilience of those systems [75
]. A systems dynamics modeling approach can be used with a framework based on viability loops to monitor water system acceptance, use, and economics [76
]. Chung and Lee [77
] demonstrate the value of coupling a hydrologic model and a multi-criteria decision model (MCDM) to evaluate alternatives for sustainable development. Many of the integrated assessment methods can include a Monte Carlo method to test for uncertainty and sensitivity.
Beyond the three typical evaluation types, groundwater use is often evaluated by comparing extraction to the aquifer safe yield. Historically, groundwater extraction sustainability was based only on groundwater recharge, which over-simplified subsurface dynamics. Early definitions of safe yield preclude pumping that is “dangerous” [78
] or produces an “undesirable result” [79
], including rapid declines in groundwater levels. More recent studies have called into question both the value and the sustainability of safe yield [80
]. Evaluations of groundwater development sustainability account for natural groundwater recharge rates, as well as capture which includes induced recharge and decreases in natural discharge [5
]. Advances in numerical and statistical models are improving our estimates and projections of groundwater use sustainability.
The diversity of evaluation methods described above provides water users and managers a wide array of tools to assess the sustainability of water use and allocation in specific water systems at a variety of scales. These tools reflect the complexity of evaluating sustainability within the physical and social world. Local variations, data availability, and socio-political objectives may lead to selection of different water quantity sustainability evaluation methods. This is particularly relevant for setting estimates of thresholds for sustainable use and allocation. Future work will improve evaluation frameworks in terms of measuring and assessing SWM including spatial and temporal efficiency, supply longevity, and equitable distribution.
4.1. Areas of Greatest Improvement in Urban and Agricultural Systems
4.1.1. Urban Systems: All Water is a (Re)Usable Resource
Meeting the challenges of water resources sustainability increasingly involves … applying innovative approaches to conjunctive use of groundwater, surface water, artificial recharge, and water reuse.
—Alley and Leake [80
Managing water resource sustainability requires considering water in all states and forms as potential resources for use and reuse. Improving use efficiency, capture, and reuse of these non-traditional water resources is more critical in water stressed regions, and those which are expected to become stressed due to climate change or population growth. Treating wastewater is a key part of solving water scarcity [16
]. As climate change makes dry regions drier [112
], the need for water capture and reuse intensifies in areas with increasing water stress.
There are two key points when considering all water as resources; first, not all applications require the same quality water, and second, not all “used” water requires the same level of treatment before it can be reused. Treatment before and treatment after of the combined water stream uses unnecessary amounts of energy and effort [91
]. Incentivizing a selective system of treatment and reuse requires that water be priced appropriately. Water must be considered an economic good to account for its competing uses [85
], where the price depends on availability and quality. To encourage treatment and reuse, the value of water should equal the cost of treating source water to necessary standards.
Green infrastructure and stormwater capture are not fully utilized in both developing and developed regions. Excess water during rain events can be harvested and stored for use in dry periods [91
]. Depending on the level of existing infrastructure and water application purposes, the scale of water capture and distribution can range from household to neighborhood to city. In many developing regions, small scale capture is recommended at the household level using storage tanks or infiltration ponds in conjunction with hand pumps for recovery [114
]. Increased capture and use of all available water resources will significantly reduce water stress, especially during dry seasons and periods of drought.
4.1.2. Agricultural Systems: Crop Water Productivity
Improvements in crop water productivity can result in commensurately large decreases in water use because agriculture accounts for the largest quantity of water use. With water use efficiency ranging between 10% and 30% for rainfed and between 40% and 95% for irrigated agriculture [22
], there is nearly always opportunity for improvement. Methods for improved on-farm agricultural water management include supplementing rainfed crops, irrigation scheduling, and efficient irrigation methods [117
]. At the national or global scale, agricultural water use efficiency can be improved by growing more food in high water productivity regions and exporting to less productive regions [118
]. Irrigation (or electricity) subsidies should target regions with sustainable water sources, or should couple incentives for high efficiency irrigation systems and low water-requirement crops. As water demands increase in the developing world, irrigation reliability is expect to decline from 0.79 (out of 1.0) in 2005 to 0.71 in 2025 [119
]. In areas where groundwater use is unsustainable, improving efficiency (and decreasing total extraction) allows production to continue longer into the future [120
The use of technology to inform irrigation scheduling can save water, and also increase crop yields compared to over-irrigation [121
]. The estimated benefits of irrigation scheduling will vary by method and location. Methods for irrigation scheduling include using soil moisture sensors and incorporating weather forecast data. Soil moisture sensors or tensiometers indicate soil wetness conditions, and can be compared to plant moisture requirements. Sensor informed agriculture water savings range from 18% to 50% [122
]. Crop water production can be further improved by combining irrigation scheduling with farm management techniques including mulching, reducing soil hydrophobicity, and the use of wastewater [123
]. Instrument cost is a primary barrier in both developing and developed regions. The technology must be designed within the budget of the intended farmers, and/or should be subsidized by the government.
In regions where water is the limiting factor to production instead of land, increases in water use efficiency may allow farmers to irrigate more land. While this is arguably not a SWM solution because it does not reduce the total amount of water used for irrigation [124
], it does increase the crop water productivity of the region. In cases were over-irrigation results in runoff and water supply for downstream users, irrigation reduction may in fact reduce water availability to these users, and should be considered in the overall management strategy [125
4.2. Relevance of Country Development Status
This review highlights several differences as well as similarities between developing and developed nation SWM objectives (Table 2
), challenges, and solutions (Table 3
and Table 4
). The differences lie in the context and level of development, and not in the definition of sustainability. The model selected for SWM evaluation typically varies to accommodate local infrastructure and economic conditions, but still maintains the objectives of sustainable development.
For urban water systems, equitable and reliable supply is the objective in developed and developing regions. Water stress and aging infrastructure are challenges faced around the world. Developing nations may face additional challenges including intermittent electricity and disparities in access to water delivery or built infrastructure. Indeed, the focus of SWM in developing countries is on providing equitable and reliable water supply, while developed nations may focus on water reuse and system longevity (Table 2
). In addition, many developed regions now strive to have water systems that mimic natural ecosystems [75
]. Water system evaluation may also include a demand management component in developed regions. Conversely, this metric is irrelevant in regions where people are not yet receiving the recommended amount of water, for example more than half of the population of the Middle East, as determined by the Islamic Network on Water Resources Development and Management [16
Evaluation models and opportunities in agriculture are most sensitive to whether the farm is rainfed or irrigated, and what technology and information the farmer has access to. In developing nations, increasing crop productivity and equitable water allocation is critical. However, as we have learned from the Green Revolution in South Asia, increasing productivity at the expense of natural resources is not a sustainable solution. In rainfed agriculture, methods for improving yields should include supplemental irrigation during critical growth periods, and on-farm management practices that improve soil moisture holding capacity. In developed nations with high levels of food security, increasing resource use efficiency should be a priority. The use of soil and plant sensors to inform irrigation scheduling must be part of the solution, as well as switching to more efficient irrigation technologies.
The literature on environmental water management is dominated by developed region case studies and models where water management needs have been met and sustainability emerges as a priority. We note the presence of environmental objectives in both urban and agricultural development (Table 2
). Conversely, in developing regions, the objectives for environmental water management are presented by two contrasting sides. The first follows the order of the Hierarchy of Water Management Needs (Table 1
), asserting that until individual and community water needs are met, environmental sustainability is not a priority. Larsen and Gujer [8
] stated “sustainable development is only possible in the absence of extreme poverty … In areas with a lack of safe drinking water, biological diversity and other ecosystem requirements will not be given any priority.
” In alignment with this thinking, meaningful protection of the environment is generally not integrated into developing nation policy because it is considered anti-development.
It is irresponsible to allow damage to resources that will ultimately be required for a population to continue developing. The second perspective, which is held by communities who depend heavily on the natural environment for their livelihoods, prioritizes sustainable resource use and protection of the environment regardless of economic development level. The Southern African Development Community is an example of a progressive group regarding environmental sustainability. They maintain that poverty reduction does not need to compromise environmental health and services. This perspective will grow as further evidence of economic and social development coupled with environmental protection is successfully documented. All regions can have the intent of SWM, while practices may vary with geography and economic capabilities.
Limitations of this study include: (1) the exclusive scope of urban, agricultural, and natural systems; (2) the challenge of obtaining municipal and other types of non-peer reviewed documents; and (3) omission of relevance of country geography, climate, and other factors. While urban and agricultural systems account for over 80% of global water consumption, other water uses such as industrial and recreational can be significant in some countries or local regions. Future reviews of other water uses would be complementary to this one, and provide value in improving water management practices across sectors. Much of the relevant literature, especially for urban water management, may include white papers, municipal reports, and other documents which are not typically available in academic databases. This review likely overlooked numerous documents describing SWM assessments, practices, challenges, and solutions. Future reviews would benefit from a more thorough search for these reports. Lastly, the scope of this paper did not include a synthesis of how country geography, climate, and other factors affect SWM practices. Future studies may focus on a number of factors to illustrate why SWM practices can differ in proximal nations or be similar in distant nations; relevant endogenous factors may include in-country distribution of wealth and resources, socio-cultural traditions, and political stability, while exogenous factors may include climate, geology, and a more detailed look at historical and present inter-country conflict.
SWM of urban, agricultural, and environmental systems is integral to continued development. Numerous models and metrics exist for evaluating sustainable management practices. Improvements to these methods should focus on the interconnectedness of social and physical systems using robust quantitative metrics. Urban water management in developing regions faces challenges of equitable delivery, especially under rapid urban population growth. Sustainable management plans should focus on continued improvements in stakeholder involvement and infrastructure in developing regions, and on water reclamation and reuse in developed regions. Water reuse will reduce stress during drought periods, though technology adoption cost and risks are still barriers in both developing and developed nations. Improvements to crop water productivity can benefit all sectors of water users discussed in this article by reducing competition between the agricultural sector and urban and environmental users. Crop water production in irrigated areas can be improved with changes in crop water allocation and adoption of efficient irrigation and on-farm technologies, while rainfed agricultural areas will benefit from supplemental irrigation. Maintaining sustainable water supply in natural systems can be seen to conflict with development practices if only looking at the near-term future. Long-term economic development is clearly linked to environmental system health, evidenced by developed country focus on restoration and protection of water resources. In application, decisions informed by the estimated value of ecosystem services may be used to set thresholds for environmental degradation, in the context of social and economic development goals. SWM will vary with geography and economic capabilities, though all regions can manage water resources in a way that supports sustainable social, economic, and environmental development.