Stress Testing California's Water System using an Exploratory Ensemble Analysis Conditioned on the Late Renaissance Megadrought and Climate Change

In this study, we introduce a novel stochastic exploratory modeling framework to investigate how scenarios conditioned on the Late Renaissance Megadrought and plausible climate changes yield high consequence impacts that propagate throughout California’s complex water system. California has experienced significant cycles of drought extremes over the last century but these extremes do not fully encompass the variability that exists in the paleo record. Moreover, climate change is projected to make California's drought extremes more severe and frequent. The ultimate impacts to system users will be shaped by how climate change co-evolves with natural climate variability and the system’s complex institutional framework that governs water deliveries throughout the state.

This study utilizes a stochastic weather generator, conditioned on tree-ring based weather regime dynamics, to develop a large ensemble of high-resolution, daily weather sequences that capture the extreme drought conditions associated with the Late Renaissance Megadrought (1550-1580 CE). Plausible regional climate changes are superimposed on the weather sequences and then used to force hydrologic models of twelve watersheds that drain into key system reservoirs. The resulting streamflow ensembles are used to force the California Food-Energy-Water System model (CALFEWS), which simulates water storage and conveyance throughout California, to create a stress testing framework that explores user vulnerabilities under megadrought and climate change conditions.

Our results demonstrate that persistent low inflows associated with the megadrought lead to critically low storages at key reservoirs, multi-year periods of curtailed water deliveries, and complete drawdowns of groundwater assets for junior and senior water rights holders. When plausible climate changes are considered, there is an increased frequency of reservoir levels hitting dead pool and complete curtailment of water deliveries. To our knowledge, this is the first stress testing framework that explores the asymmetries in risk faced by California’s two main water projects.