Watershed-relevant climate change scenarios can provide information useful in assessing how the system is vulnerable to climate change and help identify adaptation options. To generate climate change information at the global, planetary scale and make it relevant to local watersheds, many methodological choices must be made by both information producers (on how to generate the datasets) and users (on how to apply the climate data to their decision). In the U.S., for example, the U.S. Army Corps of Engineers has 21 regional reports (http://www.corpsclimate.us/rccciareport.cfm) and Appendix A in Bureau of Reclamation’s (Reclamation) Literature Synthesis on Climate Change Implications for Water and Environmental Resources (Reclamation 2013) lists over 300 papers that could be leveraged as examples. In Europe, the Service for Water Indicators in Climate Change Adaptation (SWICCA) currently provides 15 case studies (http://swicca.climate.copernicus.eu).

Models, including global and regional climate models, as well as watershed models, are used to explicitly characterize possible futures as well as historical and current conditions. These simulated futures, often referred to as projections, when used together with simulated historical conditions, can then be used to assess potential changes. More specifically, evaluating relative differences (modeled historical vs. modeled future) in system performance over time can provide improved perspectives on potential improvements as well as risks . In this, it is important to recognize that model outputs are not intended to be predictions, and should be treated instead as possible future ‘scenarios’ which can complement existing monitoring and performance evaluation systems. They provide an opportunity to explore how natural and managed systems may respond to and influence future changes and to investigate uncertainties (Weaver et al. 2013; IPCC 2014b; Milly et al. 2015; Reclamation 2016). Scenarios can be viewed as narratives that can be used to stress-test water systems and infrastructure (Moss et al. 2010; Weaver et al. 2013). As such, a single stress test can be misleading when viewed in isolation; multiple stress tests, especially when they span a range of possible stresses, are preferred and can be added to as resources and time permit.

In performing these stress tests, current approaches often capitalize on “ensembles of opportunity” – that is, collections of available datasets – to evaluate the range of future impacts and their uncertainties. This may be the most appropriate path forward at present; although as the field of climate change impacts advances and computing capacity improves, it will be possible to better understand and quantify underlying uncertainties (Harding et al. 2012; Gutmann et al. 2014; Clark et al. 2016), evaluate and account for model dependencies (Knutti 2010b; Knutti et al. 2013; Bishop and Abramowitz 2013), and improve how models are selected for use including ensuring they capture features that make them appropriate for particular uses (Knutti 2010a; Tebaldi et al. 2011; Sanderson et al. 2015). The DOS AND DON'TS review important considerations when designing studies so models can be useful tools in exploring future change.

Text orginally published in Vano et al., Climate Services, 2018

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