Relationship between water withdrawals and freshwater ecosystem water scarcity quantified at multiple scales for a Great Lakes watershed

Even in relatively water-rich regions, withdrawal and consumption of water has the potential to create instream freshwater ecosystem water scarcity, especially at seasonal and local scales. Water resource policy must balance consumptive uses of water against corresponding ecosystem impacts of flow depletion. In this study, the concept of an adverse resource impact threshold, as established by the Michigan Water Withdrawal Assessment Process, is applied in conjunction with a water use database to identify the cause, location, and scale in space and time of instream freshwater ecosystem water scarcity caused by consumptive uses of water. The study results show that there is a strong multiscalar linear relationship between freshwater consumption, adverse resource impact ecological flow thresholds, and spatial scale. On average and at the wholewatershed scale, water scarcity does not exist in this watershed, but water scarcity does occur on a localized basis, especially in the summer and at small watershed scales of less than 300 km², because of a combination of irrigation withdrawals, concentrated urban withdrawals, and low ecological flow thresholds. The aggregated effects of localized flow depletion also affect 800-2,000 km² scale catchments. Management of water scarcity in water-rich areas should therefore focus on the spatio-temporal locations where the impacts occur and where an average pattern of water abundance yields to localized scarcity, in this case during late summer months in subwatersheds smaller than 300 km². This analysis informs integrated water resources management approaches, contributes to a better understanding of the relationship between scale and environmental impact as water is shared among competing uses, and sheds light on the use of adverse resource impact ecological flow thresholds to define water scarcity in relatively water-rich regions. These results may be generalized to inform the implementation of the Michigan Water Withdrawal Assessment Process and similar processes throughout the Great Lakes region and in water-rich locations around the world where water is generally abundant but localized water scarcity is becoming an increasingly important issue.