Minimizing Cost and Overdraft in California's Multi‐Reservoir Conjunctive Use System With Limited Hydrologic Foresight

Hydroeconomic modeling yields many insights for water management, including integration of supply and demand management, reservoir operations and conjunctive management, and management under changed conditions. Hydroeconomic models of large, conjunctively operated water systems with many reservoirs operated for over‐year storage commonly assume perfect hydrologic foresight, a simplification that misrepresents how hydrologic variability affects operational rules and economic values. To address this gap, this study implements linearized quadratic carryover storage value functions on 26 surface reservoirs and linear groundwater storage penalties on 32 groundwater reservoirs to optimize California's inter‐regional seasonal and inter‐annual water system operations with limited hydrologic foresight. A tradeoff between total system cost and groundwater overdraft is developed through multi‐objective evolutionary optimization of inter‐annual carryover storage penalties. The resulting over‐year storage valuations reveal a wide range of near‐optimal reservoir operations, suggesting substantial system flexibility and adaptability when managed in an integrated way. Compared to carryover operations based on a prescriptive simulation model, the performance of the near‐optimal carryover value‐driven reservoir operations suggests little room for improvement in currently prescribed reservoir control rules and emphasizes the role of contractual requirements and environmental constraints in system water management. Overall, this study demonstrates the practicability and utility for understanding large‐scale integrated multi‐reservoir conjunctive use systems using limited foresight with explicit carryover storage values.