Historically, with differing perspectives on the future of water management in California, one of the largest agricultural water districts and a leading environmental organization have completed a joint study that shows how both interests can benefit from operating major storage reservoirs  in conjunction with groundwater basins.

In the view of this study, healthy rivers are not just environmentally valuable, they also are central to ensuring reliable, sustainable water supplies.  That is because water supply systems that work in concert with the environment are less likely to be encumbered by court orders, water rights hearings, and other restrictions that can have drastic effects on water supplies for farming and human needs.

Beginning in 2006, the Glenn Colusa Irrigation District (GCID) and the Natural Heritage Institute (NHI) jointly embarked on an investigation of the potential benefits from changing the operations of the two largest reservoirs in California so that they can capture a larger fraction of the annual rainfall and snowmelt.   These are Shasta reservoir, the largest in the Federal Central Valley Project (CVP), and Oroville reservoir, the only water storage facility for the State Water Project. Both reservoirs are located in the Sacramento Valley, and control water flows on the Sacramento River and the Feather River, respectively. 

The study shows that water yield could be increased in these reservoirs by re-operating them to release additional water to meet irrigation demands and for ecosystem enhancement, including salmon runs.  But this creates a risk that the reservoir will not refill during the following winter and spring.  To eliminate this risk, the study looked at the possibility of drawing upon the groundwater aquifers in the Sacramento Valley to supplement the deliveries from the reservoirs. The study found that if this technique had been used during the 82 years during which records have been kept, it would have been necessary to turn to the groundwater system to assure full deliveries in only 4 of these years for the federal reservoir and 6 years for the state reservoir, and that the groundwater levels would have rebounded during the following precipitation season, or soon thereafter. 

According to Gregory Thomas, the CEO of NHI, the environmental partner, “this is surely the most rigorous study to date on the potential for optimizing the operations of existing water infrastructure in California to produce benefits for both the environment and water supply that are complementary rather than competitive.  We know from this study what will work and what may work even better by more fully integrating the management of existing reservoirs and groundwater systems and by physically interconnecting them. This work strongly suggests that alleviating the conflicts in the delta through isolated conveyance could provide substantial improvements in environmental conditions in the upstream rivers as well as within the delta itself. “

The investigation began with the expectation that surplus water generated through the re‐operation of these reservoirs could be banked in the groundwater aquifers in the Sacramento Valley, like other conjunctive use programs in the San Joaquin Valley of California, with water put into groundwater storage in wet years and extracted in dry years. However, initial assessment and site screening revealed that conditions in the Sacramento Valley are not conducive to this approach primarily because groundwater aquifers generally refill completely during each precipitation season. What emerged instead was a conjunctive management approach based on reservoir re‐operation backstopped by additional groundwater extractions to supplement reservoir storage when refill is insufficient.  The results show that this technique can provide environmental flow benefits while also improving water supply reliability, reducing flood risks, and buffering the effects of climate change.

Explaining results

The study explains these results as follows:  Reservoirs that have dual water supply and flood control functions, like the CVP and SWP reservoirs, are typically operated under conservative rules designed to maximize water supply while avoiding flood risks.  This results in relative high levels of water storage going into the winter season when the reservoirs refill, but frequent “spills” of water during the rainy season to create sufficient flood storage capacity as necessary to prevent flood damage in the downstream floodplain. These spills are a portion of the water endowment that is not controlled and therefore not appropriated for beneficial use under California water law. To capture and manage this water would require creating additional storage capacity. One way to do that without enlarging the reservoirs, or constructing additional ones, is to lower the water storage levels going into the refill period, thereby creating more reservoir capacity to capture high flows. Storage levels can be lowered by delivering additional water to meet new water supply objectives, including enhancing flows for healthier rivers and augmenting water supplies for agriculture or cities.  

While the peak flood control releases do provide some environmental benefits, more focused and quantifiable benefits can be achieved by capturing and releasing them in a controlled pattern that is tailored to the needs of target species, such as salmon.  In a sense, this is a strategy to use limited environmental water supplies in a more efficient manner.   However, more aggressive exercise of the reservoirs to improve flow conditions for ecosystem enhancement may entail a greater risk of depleting cold water reserves needed for downstream temperature maintenance.   The study is explicit about these potential environmental trade-offs.

The analysis of impacts to existing groundwater users  shows that roughly 3 percent of existing domestic wells would likely be affected because these wells tend to be shallow.  The report also describes measures to hold other groundwater users harmless during those years when additional groundwater extractions would be necessary to “pay back” the reservoirs.   The study assumes that any such project would adhere to a "good neighbor" principle and design its mitigation plan to assure no appreciable harm to existing groundwater users. For instance, the study recommends deepening of impacted wells to and considers that cost to be a project expense.  Impacts to the operability and yields of existing irrigation wells would be negligible.   Modeling of the surface and groundwater systems reveals that some streamflow reductions would occur in tributary streams but would not significantly affect the primary spawning habitat for Spring‐run Chinook salmon or Central Valley steelhead trout in Butte Creek, the tributary for which sufficient data are available for conclusive analysis. 

Self-financing

If implemented, the project would also be designed to generate net economic benefits so that the program can be self‐financing.  To do this, the project must be able to generate revenues that more than offset the expenditures associated with project implementation, including construction, operation, maintenance and any mitigation costs.  The economic analysis did not assign a monetary value to the environmental benefits that would accrue, even though the potential increase in salmon productivity could be quite substantial.   Rather, the economic analysis was conducted to determine whether the revenue from the sale of water generated by the project alone would be large enough to pay for the capital and operational costs.  The net economic benefit varies depending primarily on where the water generated by the project can be sold and, to some extent, on whether new wells are constructed or the project is operated using primarily existing wells.

GCID’s General Manager Thaddeus Bettner, the agricultural district partner, explains that, “this detailed study and investigation has shown that there is the capacity to reoperate existing surface water reservoirs and generate new yield to the system to benefit the environment and water users with minimal risk.  What we have learned is that with every decision we make with our current infrastructure is that there are trade-offs that require real time decision making to identify the best benefits when re-operating a system.  We are hopeful that this study will help policy makers, project operators, water users, and our neighbors understand the balance of these trade-offs and benefits.”

The potential benefits relative to risks revealed through this investigation suggest that further efforts to develop and implement conjunctive water management in the Sacramento Valley are warranted.  A number of specific recommendations for further development and refinement of Sacramento Valley conjunctive water management are provided in the report, including evaluating the effects of climate change, reconciling tradeoffs among different types of environmental water uses, conducting more detailed water temperature modeling,  and  options for minimizing or eliminating reliance on Sacramento Valley aquifers to pay back the reservoirs. 

The entire report can be downloaded at: 

D:\Dropbox\NHI Documents for Sharing\NSVCWMP Report Final_20120925.pdf

 

Or

http://www.gcid.net/documents/NSVCWMP%20Report%20Final_20120925.pdf