摘要：Increased water demand and overexploitation of limited freshwater resources lead to water scarcity, economic downturn, and conflicts over water in many places around the world. A sensible policy measure to bridle humanity's water footprint, then, is to set local and time‐specific water footprint caps, to ensure that water appropriation for human uses remains within ecological boundaries. This study estimates—for all river basins in the world—monthly blue water flows that can be allocated to human uses, while explicitly earmarking water for nature. Addressing some implications of temporal variability, we quantify trade‐offs between potentially violating environmental flow requirements versus underutilizing available flow—a trade‐off that is particularly pronounced in basins with a high seasonal and interannual variability. We discuss several limitations and challenges that need to be overcome if setting water footprint caps is to become a practically applicable policy instrument, including the need (for policy makers) to reach agreement on which specific capping procedure to follow. We conclude by relating local and time‐specific water footprint caps to the planetary boundary for freshwater use. Plain Language Abstract Around the world, people are using too much water from rivers, lakes, and streams. As a result, rivers run dry, lake levels drop, and fish and other animals and plants that depend on freshwater are being harmed. We therefore need to strike a better balance between how much water we use for ourselves and how much water we leave in rivers and lakes for nature. In this study, we calculate this ceiling to water use by people, for every watershed in the world. We call this ceiling a water footprint cap. We first looked at how much water is available in total and subtracted what is needed for nature. What remains can be sustainably used by people, for example, to irrigate crop fields, to use at home, or to use in factories. While it makes perfect sense to not use more water than there is available, it is not so straightforward for policy makers to just make a rule that says: “Let's not use more water than this cap.” The reason is that the amount of water that is available changes from one year to the other and also from month to month. This means, for instance, that authorities cannot give out water use permits to farmers based on average water availability levels. After all, if the year or month turns out to be dry, there will not be enough water to meet the permit. We therefore discuss some steps policy makers should think of when planning to use a water footprint cap.
