Review of Quantitative Applications of the Concept of the Water Planetary Boundary at Different Spatial Scales.

The water planetary boundary (PB) quantifies a safe operating space for human water consumption and is understood as a control‐variable set at a safe distance from a selected threshold (or dangerous level) based on precautionary principles. The water PB is useful for guiding sustainable development, and various multiscale quantitative assessments of water PB have been conducted. To improve our understanding and guide future applications, we conduct a comprehensive review of all studies published since the concept's introduction. We identify a total of 288 relevant papers in the literature and perform a systematic review and meta‐analysis of 28 papers providing 488 data records at various spatial scales. The analyses mainly involve the relevant quantification indicators, quantification methods, comparisons of the water PB with the current state of water consumption at different scales, and the identification of associated problems. Finally, we propose 10 outlooks for important future research. This review indicates that the theoretical water‐PB framework, as an early‐stage research area, has gradually developed, despite the few, imperfect available methods. To address the limitations of previous studies, we propose two improvement frameworks for guiding future research: the further improvement of traditional popular approaches to quantifying the multiscale water PB within a consistent scientific framework and an improved framework for quantifying the water PB at various scales to obtain better‐fitting results for different research purposes and study areas. Based on this review, we point out some important issues that have received little attention, including the quantification of safe distances and the linkages between multiscale water PBs. Key Points: The first systematic review and meta‐analysis on the quantification of water planetary boundary at various spatial scales is developedExisting studies have developed three types of quantification indicators with corresponding quantification methodsWe proposed 10 outlooks and two future improvements with specific guidance for implementation [ABSTRACT FROM AUTHOR]