Guest commentary: Reservoir Re-Operation – The Sleeping Giant in Water Supply Development

The following commentary was written by Robert Shibatani:

The current water shortage in California has generated broad discussion on a number of water topics. Many, if not most of these topics have focused on a variety of water supply solutions. None, however, are actually new. As is often the case during water short periods, the call for new supply development escalates and traditional alternatives are once again brought into the limelight. Abetted by their advocates, the virtues of long-standing supply solutions like new dams, inter-State transfers, desalination, and groundwater exploitation are revisited. Other, perhaps lesser impassioned alternatives such as rainwater harvesting, stormwater retention, fog irrigation, and atmospheric water generation also receive attention. Not to be forgotten, water conservation rounds out the roster of potentially viable solutions, albeit as the only solution focusing on demand reduction.

Unfortunately, none of these solutions in and of themselves is a silver bullet and none can be expected to entirely shoulder the responsibility for California’s long-term water supply needs.

Across the State, several ongoing efforts are aimed at addressing long-term water supply development, supply/demand sustainability, and the climatic resiliency of both. One of the more high profile efforts is that captured by the Delta Stewardship Council in its recent Draft “Principles for Water Conveyance in the Delta, Storage Systems and for the Operation of Both”. Discussions of these principles and, specifically, the contemporary and urgent issues facing new water supply development within the vital Delta watershed were recently held on August 28, 2015.

Perhaps one of the most overlooked and yet obvious solutions involves reservoir re-operation. Without the need for any new construction, land conversion, or, in fact, any on-ground environmental impacts, significant new water supplies could be developed through re-operation of existing storage facilities (e.g., reservoirs). This would involve re-designing the operating rule curves under which dam operators currently balance the intricate needs of flood control, water supply, and hydropower, among others. The most significant group of reservoirs where this would apply would be those behind dams operated under S.7 of the federal Flood Control Act; through U.S. Army Corps of Engineers authority. The majority of the State’s significant storage reservoirs fall into this category (e.g., CVP and SWP facilities).

Because of the significant influence of the U.S. Army Corps of Engineers (through its flood control mandate), any reservoir re-operation must address the flood risk and flood damage reduction priorities for which the original operational rules were developed. But is reservoir re-operation only a flood management issue? The answer depends on who you ask. Moreover, it depends on a number of factors including whether the facilities are multi-purpose, whether the jurisdiction genuinely practices co-equality across the various beneficial uses, and whether water supply is expected to be developed on a broad system-wide basis (e.g., CVP and SWP yield) from these facilities.

Most reservoirs do provide a water supply function; it is hard to avoid that responsibility, especially in densely populated areas or, where agricultural needs are a vital part of the regional economy. Supply enhancement, despite a reservoir’s many other potential functions, therefore, remains a legitimate need. Today, developing additional water supplies through new storage projects represents a high priority goal; the voter approved Prop 1 funding for new water storage attests to that reality.

But what if we could achieve that goal, only without the huge capital costs associated with massive new infrastructure? Is that even possible? Reservoir re-operation, by definition, alters storage at any given moment in time. Managed through a series of rule curves or operating levels, it prescribes where water levels (i.e., storage) should be based on two fundamental principles; first, the anticipated yield and temporal inflow pattern of the draining catchment above the reservoir and, secondly, the assumed priorities placed on water retention within the reservoir (e.g., flood withholding, water supply yield, thermal assets, instream habitat protection assets, water quality dilution, etc.). Re-assessing these operational rules provides the opportunity to change that storage and so, affect the ability to meet any of the aforementioned water use priorities. In other words, it can directly affect new water supply development.

But what motivation would there be to change these rule curves? Especially, since there has been virtually no attempt at doing so collectively since most reservoirs were first created. The answer lies in climate change. More specifically, in the now widely accepted view that our hydrologic baselines are shifting.

To most California water practitioners the general notion regarding the State’s changing hydrologic baseline is well known. Moreover, the implications of such shifting baselines to water operations are also not new. Accepting this reality and, in view of our understanding of reservoir operations, one is compelled to then ask the question,

“If all reservoirs were to reassess and reconfigure their operating rules because of changing climatic conditions, how much additional yield (e.g., water supply) could be gained using only our existing facilities; in other words, how much additional water supply is potentially available without the need for any new dams or reservoirs?”

Let’s follow this thinking through and explore the reasons why a shifting climate would affect current reservoir operations.

A warming climate does a couple of things. For one, it provides the opportunity for increased atmospheric saturation (e.g., more water can be held in warmer air) and secondly, it can change precipitation form; from snow/ice to rain. Having more water in the atmosphere and what that means to eventual precipitation landfall might appear obvious. Certainly the now well-known threat of atmospheric rivers (ARs) attests to the graveness of this hazard. But it is not as simple as that. The complex ocean-atmosphere interactions that define global circulatory processes and their intricate teleconnected relationships (e.g., PDO, ENSO, NAO), can have significant effects on resulting water availability on land. Pacific weather systems approaching the California coast under newly forced circulation conditions can, depending on the robustness of the system and the circulation conditions at the time, exist as coastal decaying, inland penetrating, or interior penetrating ARs. Depending on their landfall trajectories, this could mean the difference between a prolonged series of highly intense storms, scattered showers, or nothing at all.   From this depiction, one can appreciate the importance of spatiality (e.g., location) and at the same time, also recognize how storm character, their intensity, magnitude, and frequency, will likely play an increasing role in determining water availability, relative to current and past conditions.

Total annual incident precipitation at any one specific point on the ground may increase under a warming atmosphere. By the same token, it may decrease or indeed, may experience little change at all. But even if projected precipitation totals decline or stay the same, changing storm characterizations can have a significant effect on water availability. In California, the bulk of the annual precipitation (around 80%) falls in approximately 4 or 5 distinct storm events every year. If the number of storms decreases, but their intensities and magnitudes increase, the implications to water management and, water supply security, become quite important. Those who recite a drier future, but rely only on projected annual precipitation totals, and do not look at the extreme event conditions or, perhaps more importantly, future storm characterizations, run the risk of missing the complete hydrologic picture.

Secondly, a warming climate changes precipitation form. This can have significant implications to those regions where, traditional hydrology is influenced by snow accumulation and subsequent melt. In California, this threat has long been known. What this means from a catchment hydrology perspective is that the temporally significant winter storage ‘reservoir’, the snowpack, will gradually disappear. Without this attenuated water storage capability, any precipitation that does fall, will be directly translated into available runoff. A more direct rainfall-runoff relationship under these conditions will have the effect of “flattening” the annual catchment hydrograph. No longer will there be the momentous peak flow in late spring/early summer. Water inflow into reservoirs will be more measured, following more closely the actual precipitation events. Again, this is not a new revelation. In fact, we have continually observed the Q₅₀ and mass centroid of the annual hydrographs in snow dominated Sierra Nevada catchments progressively ‘migrate’ earlier into the season.

What does that all mean for California reservoirs? Actually, quite a bit.   As noted previously, all reservoir rule curves were originally based on both the catchment yield and the then established priorities for storage space usage. Based on historic record trends, these reservoirs held open empty space in order to retain as much of the winter rains as possible and, most significantly, the springmelt floods. California’s Mediterranean climate meant that the most critical period for empty space was the late spring. This is when snowmelt (with or without rain-on-snow induced melting) typically produced the highest river flows. Reservoir rules curves were intended to accommodate these annual and extreme potentialities.

Flood operations in California begin on October 15; at which point, reservoirs begin their annual drawdown. While mid-winter rain events do represent challenges, it is not until later in spring where the real test emerges. Keeping reservoirs ‘empty’ in order to capture or retain the spring freshet is perfectly acceptable from a flood management perspective. However, if refill (following the winter flood season) does not occur, as what happened in 2007 (despite forecasts for significant snowmelt runoff based on late season snow surveys), then water managers are left short for the remainder of the year. Unlike humid temperate environments, the precipitation season in California is discrete; we cannot rely on any appreciable precipitation later in the summer or fall. Accordingly, if the reservoir operators do not retain enough late spring rain/melt, then no additional water is available for the rest of the year. This can, and often does, pose real issues to water supply security and exemplifies the narrow margin under which reservoir operators manage these facilities. The critical constraints to reservoir operators to refill the reservoirs each year are the rule curves.

Under a changing climate, however, the need to retain late season empty space becomes significantly reduced. As the annual snowpack diminishes, the uncertainty of how this natural storage reservoir (i.e., the snowpack) will contribute to late season runoff also lessens. No longer is there the need to maintain the same volume of late season empty space. What this means is that the continued risk of falling short on vital refill is reduced as we can more easily re-establish reservoir storage by managing only for rain. Reservoirs can relax their late season empty space requirements and operate more to direct rainfall-runoff.

If the traditional refill threats are reduced and the need to ensure late spring empty space also becomes significantly reduced, how much additional yield can be generated in any given water year by adjusting the rule curves? No one knows. But with over 1,000 reservoirs in the State, the majority of which operate under older, antiquated, operational rule curves, the prospects for gaining new storage are enticing. Let us not forget, that most large reservoirs have a positive yield/storage ratio (e.g., 1.2 to 1.5:1). Some, like Folsom Reservoir, have a yield/storage ratio closer to 2.4:1. This means that the catchment hydrology is positive, relative to storage capacity; in other words, these reservoirs will easily fill in most years. The only constraint has been that because of the flood sensitized rule curves, they are not always allowed to do that. This has long represented an operational inhibition, not a climatic one.

So why hasn’t this opportunity been assessed, let alone implemented? A key reason why reservoir re-operation has not gained favor is largely due to our complex, and often parochial, water governance structure. As noted previously, for S.7 reservoirs, it is the U.S. Army Corps of Engineers who hold authority over reservoir rule curves. Water supply, as one of many beneficial water uses, while acknowledged as important, is not the primary focus of the U.S. Army Corps of Engineers. Yet it is they who control how, when, or even if, reservoir rule curves are to be changed. Without the motivation to engage and compel a broader look into these potential possibilities, few agencies are willing to undertake such an exercise involving elements that are clearly outside of their jurisdiction.

As noted at our August 28 Delta Stewardship Council meeting, this represents a pervasive concern in California water resources management. Despite all the best efforts at inter-agency integration and planning, major new changes in water operations, infrastructure, or regulatory governance can rarely breach the institutional silos that exist across the water resources spectrum. Water management priorities, use preferences, and indeed even inherent ideologies are not the same between the various federal, State, and local agencies. We like to think that they are, but the reality is that they are not.

Currently, there exists no water project in California that effectively addresses all of the intricate, dynamic, and very real potentialities regarding climatic forcing on reservoir operations and, in particular, how reservoir re-operation could (or even should) be implemented given our complex regulatory and institutional environments. In the midst of the ongoing dialogue on new water supply development and the fervent advocacy for many long-standing new reservoir projects, it seems that a cost effective alternative solution might actually be staring us right in the face. To be sure, the costs of pursuing this solution would be several orders of magnitude less than traditional construction efforts associated with new infrastructure. However, so long as our focus on a water solution or, WaterFix, remains preoccupied on this seeming urge to build something, anything, the quantifiable benefits of system reservoir re-operation and what this could mean as a potential new water supply will likely remain a Sleeping Giant.

About the Author:  Robert Shibatani 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 Vice Chair of the Sacramento EWRI/ASCE. Robert has been a hydrologist for the past 30-years, working with organizations such as UNESCO, Atomic Energy of Canada, Ontario Ministry of the Environment and Climate Change, various universities, and consulting firms. 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.

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