Freshwater flow to the San Francisco Bay-Delta estuary over nine decades (Part 2): Change attribution

Abstract

This paper explains observed trends in freshwater flow to the San Francisco Bay-Delta estuary as reported in a companion paper (Hutton, Rath, & Roy,). We employ a historical hydrologic record spanning nine decades and define a set of idealized flow scenarios to identify drivers of change in delta outflow and consequent salinity regime. Flow changes are measured against a baseline scenario representing 1920-level land use and water management conditions. Additional scenarios are defined to represent the system absent state and federal water project reservoir and export operations, absent key non-project reservoir operations, and absent historically-observed sea level rise. These scenarios, in conjunction with the principle of superposition, are used to ascribe outflow and salinity trends to different anthropogenic and natural causes. We find that project and non-project water management are attributed similar responsibility for decreasing outflow trends in April and May and consequent increasing spring salinity trends. In contrast, we find that increasing July and August outflow trends (and lagged decreasing salinity trends) are attributed to flow contributions from project water management; these contributions more than fully attenuate impacts associated with non-project water management.