Linking exploratory scenarios to process-informed insights in climate vulnerability assessments

<p>The threat that climate change poses to water resource systems has led to a significant and growing number of impact studies. These studies tend to follow two general methodological approaches: (1) top-down, process-based studies driven by projections of future climate change supplied by downscaled general circulation models (GCMs), and (2) bottom-up, vulnerability-based studies driven by exploratory scenarios. Top-down studies generate realistic climate scenarios, but computational burdens limit the ensemble size. As a result, critical vulnerabilities may be left unexplored. Bottom-up approaches make it possible to assess a wide range of scenarios, but usually without connection to physically plausible climate processes, limiting their utility in adaptive planning. This study develops process-informed exploratory scenarios that bridge the gap between top-down and bottom-up methods. This hybrid approach yields several advantages. First, emerging vulnerabilities associated with non-linear hydrologic changes are linked to thermodynamic and dynamic climate drivers modeled in the GCMs with differential likelihoods and plausible ranges of change. This provides a transparent link between stakeholder defined vulnerabilities and climate processes that is often missing in bottom-up assessments. Second, non-linear shifts in vulnerability are directly linked to specific climate drivers, through the systematic perturbation of process informed climate variables. Making this connection in top-down assessments is difficult since the climate response to an emissions scenario is modeled as part of an endogenous process. The hybrid approach developed by this study is presented with a case study in the Tuolumne River watershed; through which thermodynamic and dyanamically guided climate scenarios were created by a process-informed stochastic weather generator to evaluate flood and drought related performance vulnerabilities at the New Don Pedro Dam near the watershed&#8217;s outlet. This case study finds that flood and drought performance at the dam is more sensitive to process-informed climate drivers than less theoretically grounded delta shifts precipitation, and non-linear system responses to climate drivers are revealed through the systematic perturbation process-informed climate variables.</p>