The Sacramento Valley Conjunctive Water Management Program is a collaborative planning effort by the Glenn Colusa Irrigation District (GCID) and the Natural Heritage Institute (NHI). The project objective is to examine whether and how operation of groundwater aquifers in the Sacramento Valley can be integrated with operation of existing surface water reservoirs to produce additional firm water supplies. New supplies could be used to satisfy unmet in-Valley agricultural demands, contribute to meeting environmental flow targets in the Sacramento and FeatherRivers, produce additional water supply for the state, and buffer effects of climate change. The project is funded by the United States Bureau of Reclamation and the California Department of Water Resources.
Technical Analysis & Modeling
Part of the project involves planning-level technical analysis and modeling of: 1) how conjunctive management projects may operate, 2) how additional water supplies may be developed, and 3) what effects the projects may have on both the surface and groundwater systems in the SacramentoValley. GCID and NHI are carrying out this analysis with the assistance of MBK Engineers and CH2M Hill. Two sites are being modeled in the Sacramento Valley: the Glenn-Colusa Irrigation District connected to Central Valley Project (CVP) at Shasta and the Butte Basin connected to State Water Project (SWP) at Oroville. The project team has prepared careful requirements for the analysis and modeling. For example, it will honor existing CVP and SWP operations and account for stream-aquifer interaction and impacts to existing pumpers in the Valley. An overarching principle of the project is that existing water users, at a minimum, will not be adversely impacted and preferably will benefit from conjunctive operations.
The team is utilizing two separate surface water and groundwater models. The surface water model was used in gaming sessions and by members of the project team to understand benefits, risks, and limitations of various conjunctive management configurations - including environmental objectives and agricultural water demands. The groundwater model was used to investigate differences in aquifer drawdown and changes in stream-aquifer interaction for different pumping capacities, seasons, and well fields.
Based on an understanding of annual aquifer drawdown and recovery, the team developed a conjunctive operation configuration that relies primarily on re-operation of existing reservoirs to achieve the project objectives, drawing on groundwater as a “backstop”. Groundwater provides an additional source of water to protect surface water reservoirs from being excessively drawdown. This type of operation offers different opportunities and challenges than conventional groundwater banking. Surface water is not banked in the aquifer in wet years and recovered during dry years. Instead, additional water is released from surface reservoirs for delivery to meet project objectives (unmet local irrigation demands and environmental flow targets). These releases result in lower end-of-year reservoir storage levels and more reservoir space available to capture winter runoff.
The goal of this operation is to capture additional water supply by refilling reservoir space vacated by additional project releases with surplus surface water that otherwise is released for flood control. In years when refill is not complete from surplus winter flows, groundwater pumping is used in project areas in lieu of surface water deliveries that are otherwise made from reservoirs. This allows an equivalent volume of water to remain in reservoir storage to recover from prior year project releases.
Environmental Restoration
Another important component of the project is to identify an environmental flow regime for the Sacramento and the Feather Rivers in order to:
- Test the feasibility of reoperating terminal reservoirs in the Sacramento River Basin without diverting additional water away from agriculture,
- Develop a comprehensive hypothesis regarding the range of flows that may be necessary to restore ecological processes to the Sacramento River, and
- Use the environmental flow targets to inform and guide conjunctive use scenarios.
A holistic approach is being applied in the project to identify an environmental flow regime. It consists of the following 6-step process:
1. Identify specific environmental objectives (i.e., target species, aquatic and riparian communities, and desired ecological conditions that are flow dependent).
2. Approximate the timing, magnitude, frequency, and duration (TMDF) of flows necessary to support target species, communities and desired ecological processes.
3. Compare existing vs. historical hydrology to understand natural hydrologic patterns and how they have been altered.
4. Identify obvious gaps between objective flow requirements and existing flows.
5. Develop an environmental flow hydrograph to achieve ecological objectives based upon a clear understanding of historical and existing hydrologic patterns, and identify key hypotheses and uncertainties regarding the relationship between flow patterns and environmental objectives.
6. Design an adaptive management program to further test and refine environmental flows.
The project will model the Environmental Flow/Threshold objectives; prioritize the them by year type and recurrence interval; determine water cost by comparison with existing flow conditions; and compare water cost with project assets to determine release (assets function of reservoir storage and GW pumping capacity).
