Review and reframing of the operationalisation of the water planetary boundary for river basin planning

The Planetary Boundaries framework defines a Safe Operating Space for humanity by setting biophysical boundaries to critical Earth System components, including the water cycle. This framework is a promising paradigm to address the challenges posed by increasing anthropogenic pressures and climate change impacts on water systems. Nevertheless, the Water Planetary Boundary framework has not yet been systematically integrated into basin-scale water management planning. We address this challenge by first reviewing the development of the global Water Planetary Boundary concept and summarising the approaches used to apply this concept at sub-global scales, while highlighting its merits and gaps. Building on this review, we argue that existing research on operationalising the Safe Operating Space for water at sub-global scales often examines limited variables, neglecting a comprehensive basin-scale approach and failing to provide adaptable frameworks for other regions or future scenarios. At the same time, the global Water Planetary Boundary framework, although setting aspirational goals, lacks actionable implementation strategies. We propose a novel framework for operationalising the Water Planetary Boundary principles at the basin scale, integrating socio-ecological functions into the assessment of the basin-scale water boundary. Our framework is based on the assumption that operationalising the Water Planetary Boundary principles at the basin level requires prioritising Earth’s stability within a Holocene-like state and social equity. This involves preserving water functions critical to ecosystem services and ensuring the resilience of water systems for current and future generations. The framework's application can enhance the understanding of how human activities interfere with the water cycle and highlight critical issues that must be addressed to maintain the water system within a Holocene-like state at the basin scale.