Toward Integrated Hydrologic Modeling for Climate Adaptation and River Operations

Abstract

California’s water management faces a growing challenge of balancing rising demands across different sectors, ensuring groundwater sustainability, meeting environmental flow requirements, and allocating water fairly amid intensifying drought and climate change. Historically, these challenges were addressed using fragmented tools and models that treated groundwater, surface water, water use estimation, and reservoir operations separately. This approach occurred within a legally divided governance framework, where surface water was regulated by the State Water Resources Control Board, while groundwater remained under local control for many years. The legal and hydrologic separation is now being reconciled through the Sustainable Groundwater Management Act (SGMA), which mandates locally driven groundwater sustainability planning supported by the Department of Water Resources and enforced by the State Water Board. SGMA requires consideration of interconnected surface water (ISW), groundwater, and groundwater-dependent ecosystems (GDEs) as part of the groundwater sustainability plan (GSP) development. As analysis of water systems grows more holistic, the need for integrated modeling tools that simulate nonlinear feedback across legal and hydrologic boundaries is becoming clear. Integrated hydrologic models offer a path forward by simulating surface water, groundwater, water use, land use changes, and water operations within a single framework,
as fragmented models fail to realistically simulate complex interactions among different legal and hydrologic processes. Such integrated models allow decision-makers to evaluate trade-offs, test climate adaptation strategies, evaluate management scenarios, and coordinate water use more effectively. They can also be used to support near-real-time reservoir operations in systems that use the paradigm of Forecast-Informed Reservoir Operations (FIRO; Delaney et al. 2020). This article highlights a case study that employed the integrated GSFLOW-MODSIM modeling framework to simulate interconnected surface and subsurface hydrology, agricultural water demands driven by soil zone moisture dynamics, and managed river-reservoir operations. The modeling system was applied to simulate the Santa Rosa integrated hydrologic system and explore diverse management scenarios to balance water demand and environmental minimum flow requirements.