Salinas Valley Operational Model: Interlake Tunnel and San Antonio Spillway Modification Project

Science Center Objects

An operational model for Salinas Valley is needed to evaluate and compare ecosystem, conservation, and water demands. This model must simulate current operations, assess benefits of and evaluate scenarios for tunnel operation and potential reservoir and spillway modifications.

Study area map Salinas Valley, Monterey County, California.

Study area map Salinas Valley, Monterey County, California

Groundwater and surface water are used for water supply in the Salinas Valley.Groundwater withdrawals, mainly for the irrigation of agricultural crops, have resulted in water-level declines and related seawater intrusion since the 1930s. Thus, understanding the effects of groundwater-surface-water interactions on water budgets is essential for sustainable future water supply development. To plan for sustainable future water use, it is important to develop tools necessary to evaluate the benefits of constructing an Interlake Tunnel from Nacimiento to San Antonio Reservoirs and increasing the spillway elevation of San Antonio Dam (Project) designed to provide supplemental water supply.

Monterey County Water Resources Agency (MCWRA) is currently assessing the potential benefits of the Project. This assessment will develop a greater understanding of the supplemental storage and flood control benefits that could be realized by construction of the Project and evaluate the potential changes in hydrologic effects of groundwater development in the Salinas Valley Groundwater Basin with the additional water provided by the Project, which would aid in potentially developing new strategies for basin management.

Throughout the Salinas Valley, there is significant exchange between the surface and groundwater systems in the basin. Moreover, reservoir operations are highly complex. The entire hydrologic system (e.g. Reservoir storage and operations, Salinas River, regional aquifers) needs to be considered to adequately assess the benefits.


The USGS is developing a reservoir operations framework that is integrated in the Salinas Valley Integrated Hydrologic Model. This operational framework incorporates all of the required reservoir operations rules for conservation and flood protection allowing for analysis of potential benefits of the Project. The model considers stream network gains and losses when simulating operations.

The consultant team for MCWRA will use this model to   evaluate the Project’s impact on reservoir storage, surface-water flows, land use and associated pumping demands, groundwater-level elevations, and the extent of seawater intrusion using a baseline of 2014 conditions with hydrology for the period 1967 - 2014.

Science Plan

The USGS has built a historical USGS MODFLOW-based One Water Hydrologic Flow Model of the Salinas Valley (SVIHM) and a hydrologic Simulation Program-Fortran (HSPF) Salinas Valley Watershed Model (SVWM) for the historical period from 1967 to 2014. The SVIHM Operational  Model will be used to evaluate the various alternatives of the Agency’s proposed Project on surface water and groundwater in the Salinas Valley Groundwater Basin and will include analysis of groundwater and surface-water conditions based on the historical hydrology and current (2014) land use. The differences relative to the baseline hydrologic model results will be analyzed by the MCWRA consultant team for potential changes in surface-water deliveries needed to meet all of the demands associated with reservoir operations, including diversions at the Salinas River Diversion Facility (SRDF), passage flows in the Salinas River for Steelhead migration, flood control, hydroelectric production, and recreation. The USGS will provide technical assistance to the Agency’s consultant with the application of the SVIHM Operational  Model. This includes assistance with model input structure and tools used for the evaluation of various model scenarios.

This study will benefit local water users by providing an improved understanding of the source, movement and use of surface water and groundwater subject to the potential increase in surface-water supply and changes to flood-control releases provided by the Project. Evaluating the Project with reference to 2014 baseline conditions will help to provide a frame of reference for additional analysis of changing land use and potential effects of surface-water and groundwater use. Benefits to the nation include quantifying the resources of a sole-source aquifer, through the development information and tools necessary to assess and manage a water resource used for irrigation, drinking-water supply, and support of critical habitat of a distinct population of federally listed, threatened South-Central California Coast Steelhead (Oncorhynchus mykiss).