Mitigating impacts of climate change on flow regimes through management of small dams and abstractions

• River flow regimes are key to ecosystem health. • Rivers without major storages can still be affected by other anthropogenic threats. • Impacts of climate change and small abstractions were modelled for over 500 sites. • Managing consumptive use could mitigate climate change during low flow months only. • In contrast, climate change impacts in high flow months were too large to mitigate. Unregulated ‘free-flowing’ streams are essential in maintaining the ecological health of river systems within basins that are increasingly regulated by large dams. However, flow regimes in ‘free-flowing’ streams can nonetheless be impacted by smaller distributed (diffuse) forms of regulation such as small dams and pumped extractions, and climate change is expected to exacerbate these impacts. Research into these diffuse sources of regulation is rare. This study examines the relative impacts of climate change, small dams, and small extractions on streamflow. We assess whether active management of small dams and small extractions represents a potential option for mitigating reductions in flow due to climate change. Monthly rainfall-runoff models were used to characterise baseline (without human impact) flow regimes for sites across Victoria, Australia. From this baseline, the effects of climate change, small dams, and small extractions were simulated and systematically compared in various scenario combinations. We found that the projected impact of climate change far exceeded the impacts of consumptive use across most of the region during higher flow months. During lower flow months, however, the relative impacts of climate change were more significant. Importantly, our analysis shows that these impacts can be at least partly mitigated by managing the flow regime impacts of small dams and abstractions in many locations. Our approach thus not only identifies streams ‘at risk’ from climate change but also streams where mitigation options exist. These findings are likely to be broadly similar in other areas with seasonal flow regimes where the future is expected to be drier. Managing small dams and extractions in this way will require shifts in current water policy and management, noting that further research will be needed in ’at risk’ waterways to more fully understand the risks as well as potential costs and benefits of this management approach at a local scale.