Uncertainty in Bottom-Up Vulnerability Assessments of Water Supply Systems due to Regional Streamflow Generation under Changing Conditions

Changing natural streamflow conditions apply pressure on water supply systems globally. Understanding potential vulnerabilities using IPCC-endorsed top-down impact assessments, however, is limited due to uncertainties in climate and/or hydrological models. In recent years, bottom-up stress tests have been proposed to avoid some of the uncertainties in top-down assessments, but the uncertainty in bottom-up approaches and its impact on vulnerability assessments are poorly understood. Here, we aim at addressing uncertainties that originate from synthetic realizations of regional streamflow with which the system vulnerability is mapped and assessed. Four regional streamflow generation schemes are used to form alternative hypotheses for performing a bottom-up impact assessment in a large-scale water supply system under changing conditions. Our findings suggest that despite having different levels of realism, none of the schemes can dominate others in terms of reproducing all historical streamflow characteristics considered. There can also be significant differences in the results of impact assessments, particularly in terms of variability in long-term streamflow characteristics and system performance. These differences cause uncertainty in assessing risk in system performance and stress-response relationships under changing conditions.