Climate change evaluations can be done at a variety of levels ranging from qualitative regional descriptions of temperature and precipitation changes as in the IPCC reports (e.g., 2014b) and U.S. National Climate Assessments (e.g., Walsh et al. 2014) to quantitative daily climate change projections of streamflow at a specific gage location (e.g., Vano et al. 2010a,b). It is, therefore, important to first understand what information is needed to answer the climate change questions posed, what is possible, and the tradeoffs between the required effort and detail. It may also be important to balance investments across various aspects of a study, for instance, considering how a water system is vulnerable to both climate and non-climate risk factors.

Generally, it is better to start with an understanding of big-picture changes and then decide what details are needed to help inform decisions, so they can be explored most effectively (Willows and Connell 2003; Brekke et al. 2009). Region-based inquiries and qualitative analysis are usually relatively simple and cost-effective (Reclamation 2014a) and can be a good starting point even if more involved analysis is desired. Willows and Connell (2003) describes this as a tiered approach - by first studying the problem in a broad, holistic way, risks can be characterized qualitatively and then prioritized, which allows the most significant risks to be assessed first.

The required level of detail depends on the decisions being informed. Climate change information has a wide range of applications in water resource planning, for example the information can be used to modify system operations, to make decisions on new or improved infrastructure, to establish long-term planning objectives, and to plan river restoration (Reclamation 2014a). In all decisions, identifying the minimum level of information required to alter a decision can help. For example, some decisions can be made by just knowing a direction of change (e.g., summer temperature increases). Other decisions require a better understanding of the magnitude of change for one or more variables. Still others require an investigation of relative differences (e.g., identifying stream reaches more vulnerable to temperature increases for endangered species protection (Mantua et al. 2010; Isaak et al. 2015)).

The level of certainty must also be considered. Reclamation guidance (2014a) recommends considering both relevance and certainty when determining the appropriate level of climate change analysis. The report suggests that climate change information should be: (1) included if changes are well supported and relevant, (2) explored through sensitivity analysis if changes are highly uncertain, but still relevant, and (3) excluded if changes are irrelevant or too uncertain. If too uncertain, one should consider more carefully the costs and risks involved and consider planning for more severe scenarios or other contingency-based planning (e.g., Observational Method example).

Fortunately, many resources, tools, and techniques already exist. The questions below, included in the UK Climate Impacts Programme’s report Climate adaptation: Risk, uncertainty and decision-making (Willows and Connell 2003), can be used to help evaluate appropriate tools or techniques for specific situations:

• How much will it cost? (tool development, staff time, expert assistance)

• How long will it take? (no matter how useful the tool, it is of little use if it cannot make the decision deadline)

• To what extent will the analysis improve the decision? (what information is required to make a different decision)

• Can appropriate data and information be obtained? (if not, reconsider costs and timeline)

• Who will undertake the analysis? (in-house, external expert)