The water supply for Los Angeles County (LAC) has long been dependent on imported out of basin surface water sourced from distant sources. LAC’s dependence on imported water exposes it to potential shortages during severe or prolonged drought conditions that reduce availability of imports. Recognizing this vulnerability, there have been concerted efforts to reduce LAC’s reliance on out of basin surface water that include a combination of aggressive demand management, expansion of water reuse, and optimizing use of local surface water and groundwater resources. However, the highly fragmented nature of LACs public water utilities, with over 80 water providers supplying water to over 9.5 million people, adds to the complexity of evaluating water supply vulnerability under scenarios of changing supply and demand. Water planning, in particular how to prioritize efforts within provider regions, can be aided by a LAC-wide representation of water supply that considers a wide range of plausible future demand and supply scenarios. In this study we use a network flow optimization model, building off the existing Artes model of LAC’s water supply system, to interrogate supply reliability of LAC’s water system. The water supply network is resolved using a network of 342 nodes representing sources of supply or demand, and 717 links that define node topology and link-specific conveyance capacities and runs at monthly resolution. This work conducts an initial sensitivity analysis of how uncertainties in population growth, evolution of urban form and water use efficiency, local climate, and reliability and sustainability of existing water supply sources collectively influence water supply vulnerability. Modeling results can help diagnose the main drivers of uncertainty in supply reliability, both at the provider-level and collectively across LAC, and identify consequential scenario spaces driving supply vulnerability.