Guest commentary written by Robert Shibatani
In the often volatile arena that is California water, the focus on river flows has once again taken center stage. On September 15, 2016, the State Water Resources Control Board (SWRCB) released the draft revised Substitute Environmental Document (SED) in support of potential changes to San Joaquin River flow and southern Delta water quality objectives and program of implementation included in the Water Quality Control Plan for the San Francisco Bay/Sacramento-San Joaquin Delta Estuary (Bay-Delta Plan).
As expected, there have been passionate opinions expressed on both sides on what the SWRCB has proposed and, as always, the SWRCB is caught in the middle. The Governor’s recent letter provides a pragmatic offer for conciliation, a step welcomed by SWRCB Chair Marcus. It is a good step. But the solution is much more complicated than simply having competing beneficial users agree to a voluntary sharing plan, regardless of its comprehensiveness. As most practitioners in California water know, a much larger and pervasive problem has always existed. And while the economic consequences of the proposed flow prescriptions have been zealously ascribed, it is often instructive to refocus; turning instead to the fundamental precepts of operational hydrology to see if what is being proposed is conceptually supportable. So, let’s step back for a moment and look at this latest example from both a hydrological and system operational perspective.
River flows are categorized in one of two ways; that flow generated by carryover releases, and flows that are unimpaired, run-of-the-river. In regions dependent on managed carryover storage, like California’s Central Valley, a whole suite of complex and integrated factors affect how robustly (and effectively) this annual water yield ends up (or, can end up) as downstream flows. Such factors include reservoir operations (e.g., flood encroachment curves, seasonally skewed empty-space requirements, refill assumptions, etc.), as well as power scheduling priorities, reservoir abstractions/diversions based existing rights and contracted agreements, in-reservoir temperature management, downstream thermal refugia targets and, of course, downstream flow, habitat, ecosystem, and water quality regulation.
This carryover is essential in that it represents the managed yield capable of dispensing value across the entire range of beneficial users and approved uses. If we accept that it is yield that drives the ability to maintain downstream river flows, it begs the question, “Is it possible to regulate the latter without first addressing the former?” This gets at the heart of one of California’s longest entrenched water challenges. We often attempt to manage (read, regulate) one specific element of the hydrologic cycle, for example, instream flows, without perhaps appreciating its intricate tie to other prerequisites.
Currently, we can say that we have a good handle on projecting annual carryover. Hydrologically, we know for example, that precipitation, meltwater fluxes, reservoir inflow, natural losses such as deep seepage, vadose storage, ET/E, as well as direct diversions, all play a role in reservoir mass balance. In carryover managed systems, however, the operational rules of the impounding reservoirs are just as important. This is where the long-standing fill, spill, and refill edicts of reservoir operations come into play. Our long standing recognition of the prioritized dual functionality of reservoir operations; namely, storage for beneficial use carryover and, of course, flood retention, complicate yield planning.
This is especially true where known externalities, like climate change, are acknowledged. So, in order for our long-term yield projections to be accurate, we must know whether the operational balancing between these two functional necessities (e.g., fill and spill) accurately represent what we can expect and how we would inevitably prioritize between the two in the future. To date, however, this has not been done confidently or consistently.
As noted above, added to these inherent complexities, is the increasing effect imparted by a continually and a rapidly changing climate. Changing baseline hydrology, especially in snow-dominated regions, increasingly alters reservoir inflow timing and magnitudes across all Sierra Nevada and lower Cascade reservoirs. Such hydrologic shifts must be recognized and incorporated into any carryover analysis. After all, what’s the use of adhering, for example, to a 1921-2000 reservoir inflow regime, when all of the climatic data show that the temporal inflow hydrograph for say, the 2020-2050 period, looks quite different?
Changing reservoir inflow, static operational reservoir rules, reservoir capacity limitations, increasing demands being placed on existing beneficial uses; all of these collectively affect the “available” yield in our existing terminal reservoirs. Accordingly, any instream flow standard(s), downstream of these reservoirs, must focus carefully on all of these factors, if they are to have confidence that the baseline hydrology (i.e., accrued carryover) can support the instream flow rates under consideration.
Once the climate-sensitized yield is established, the question then turns to what kind of flows are being advocated? In the 70s and 80’s, maintaining unimpaired flows was thought of as an important means of helping replicate natural river hydrographs, in response to the massive dam building that was occurring at the time. Even then, however, it was a rather narrow interpretation of how best to regulate riverine systems that had changed so significantly.
But that was then, and this is now. Today, it is largely accepted that unimpaired flows represent a remnant of a bygone era. Climatic shifting, especially in snow-dominated regions like much of the source areas (e.g., high elevation zones) responsible for generating the annual water yield for the Central Valley and export destinations, has all but eliminated the importance of unimpaired flows.
So, regardless of whether the unimpaired reservation is 40%, 60% or any other percentage, one is compelled to look at the shifting available yield in those source reservoirs to determine whether such releases are indeed supportable over the months prescribed. This involves determining what the likely projected future carryover storage will look like, how that seasonally skewed inflow will be affected by existing operational rules, how existing depletions will affect that yield, and how the assumed future reservoir operations one chooses may
temper overall yield retention.
At the end of the day, the SWRCB proposed flow prescriptions may, or may not be proven justified. At this point, it not possible to say without a detailed review of the all the assumptions and analytical methods used (some of which were alluded to here). In spite of this uncertainty, what we do know is that contemporary instream flow prescriptions, especially in managed carryover-derived systems, must include a thorough investigation of upstream hydrology and reservoir operations, both strongly influenced by known climatic
forcings, as well as any anticipated changes to those elements.
While the current focus may be downstream… it all starts upstream…
About the Author:
Robert Shibatani, a physical hydrologist, is an international expert witness on reservoir-operations, climate change hydrology, flood damage reduction, and water supply development. He is on the Editorial Board of the Journal of Water & Climate (UK), International Water Association (IWA) Climate Change Task Force, and past Vice Chair of the Sacramento EWRI/ASCE. He is the Managing Partner & Principal Hydrologist of The
SHIBATANI GROUP International and is the Founder and Chair of the international consortium, “Implementable Dam and Reservoir Development” (IMDAM) with corporate partners in the UK, Germany, New Zealand, Australia, Canada, and across the U.S. Robert resides in Sacramento, California.
Photo of San Joaquin River headwaters by Justin Gaerlan
