Three-dimensional water scarcity assessment by considering water quantity, water quality, and environmental flow requirements: Review and prospect

Abstract

The increasing population, improving living standards, and expanding economic activities are responsible for rising water demands. In recent decades, the imbalance between water supply and demand has become increasingly prominent. Coupled with the unreasonable use of water resources, this has led to serious water scarcity problems that affect the sustainable development of modern society. Under this background, water scarcity has become an important environmental issue for our sustainable planet. Water scarcity is affected by regional water resource endowments and the ways in which water resources are developed and utilized by human beings. Quantity-induced water scarcity occurs when the quantity of water resources is insufficient. Meanwhile, pollution can cause water scarcity as the services provided by polluted water are not equivalent to that of clean water. Quality-induced water scarcity occurs when the pollutants exceed the environmental carrying capacity. Since Marlin Falkenmark first proposed the concept and assessment method of water scarcity in the 1980s, water scarcity assessment has developed for nearly 40 years. With the development of new theories, progress has been made, such as development of different approaches for assessing water scarcity, identification of influencing factors of water scarcity, revealing the formation mechanism, and exploring strategies to cope with water scarcity. As a result, water scarcity assessment has experienced an evolutionary pathway from a one-dimensional model emphasizing only quantity-induced water scarcity to a two-dimensional model (considering both quantity-induced and quality-induced water scarcity), toward a three-dimensional (3D) model (or 3D water scarcity theory) that considers water quantity, water quality, and environmental flows simultaneous. Based on a systematic review of water scarcity assessment in the literature, this study demonstrates the 3D water scarcity theory and elaborates on its basic concepts, principles, and core methods. The 3D water scarcity theory was first proposed by Chinese scholars and then accepted by international scholars. The quantity-induced water scarcity is assessed based on the ratio of water consumption to water availability, and the quality-induced water scarcity is quantified by comparing gray water footprint (the amount of water required to dilute pollutants in wastewater sufficiently to meet environmental water quality standards) with local available water resources. Environmental flow requirements are quantified based on the characteristics of climatic and hydrological conditions rather than the traditionally adopted simple approach by assuming that they are equal to 80% of water resources. Hence, the 3D water scarcity theory and assessment method can consider water quantity, water quality, and environmental flow requirements comprehensively. Finally, four main future research directions of water scarcity assessment are summarized: The further development and improvement of the theoretical framework of 3D water scarcity, applications of the assessment methods on multiple spatial scales, explicit consideration of the impacts of physical and virtual water flow, and mechanism of the evolution of water scarcity. 3D water scarcity is a key to achieving sustainable and harmonious development between humanity and water resources, and its assessment framework can break through the limitations of the traditional one- or two-dimensional models. Water scarcity theory provides methodological support for measuring the state of water resources on global, national, and regional scales, and it will help formulate appropriate water mitigation measures for integrated water resources management.