Metropolitan water district of southern california map

The Metropolitan Water District of Southern California (Metropolitan), a water wholesaler, produces an Integrated Water Resources Plan (IRP) about every five years that lays out a strategy for meeting future water demands through imported supplies and local sources and conservation (see Figure 1). Although the IRPs have always accounted for hydrologic variability, over the past ten years, Metropolitan has worked to account for how uncertainties beyond historical hydrologic variation could affect its investment needs. Beginning with the 2010 IRP [PDF], Metropolitan has used methods for Decisionmaking Under Deep Uncertainty (DMDU) to stress-test its long-term plan for a broader range of uncertainties, including climate change, and inform an adaptive management strategy.

This case study describes an unpublished RAND Corporation study, funded by Metropolitan and completed in early 2020, that used Robust Decision Making (RDM) to evaluate the robustness of Metropolitan’s 2015 IRP [PDF] to a wide range of uncertain future trends and conditions. The study team used these findings to describe a nascent monitoring approach for Metropolitan to inform adaptation of the IRP to future conditions.

This case study provides the reader the opportunity to review the steps of an advanced RDM-style vulnerability analysis and explore a monitoring framework that can guide the adaptation of a long-term robust water management strategy.

Figure 1. Metropolitan Water District of Southern California Service Area

SOURCE: Courtesy Metropolitan Water District of Southern California. Used with permission.

Background

Metropolitan is a water wholesaler that provides water to 26 Southern California water agencies that together serve about 19 million customers. Metropolitan imports water from Northern California via the State Water Project (SWP) and from the Colorado River via the Colorado River Aqueduct (CRA). The City of Los Angeles, which is within the Metropolitan service area, additionally imports water from the Owens Valley in Eastern California via the Los Angeles Aqueduct (LAA). Metropolitan also supports the development of local resources and conservation throughout the region.

Metropolitan’s 2015 IRP presents estimates of future retail water demands by its member agencies based on demographic and economic projections, historical local climate conditions, and expected levels of conservation. It then combines these retail demands with estimates of local supplies (reflecting historical climate variability) to determine the demand over time for water from Metropolitan.

The IRP defines an adaptive management strategy to guide the development of new supplies and conservation to meet its targets. Such targets include (1) support and financial resources for new infrastructure in the Bay-Delta known at the time as the California WaterFix, (2) maintenance of supplies from the Colorado River in the face of significant regional challenges (see the Colorado River Basin Case Study), (3) additional conservation savings, and (4) development and protection of local water supplies. The strategy also describes a set of conceptual Future Supply Actions, which includes actions related to public outreach, legislation and regulation, technical studies and support, and land and resources acquisition.

Following publication of the 2015 IRP, Metropolitan asked RAND researchers to use RDM methods to evaluate the IRP’s vulnerabilities to a broader range of uncertainties, including climate change, demographic changes, and the implementation of improvements to the Bay-Delta system. Figure 2 highlights the key RDM steps from this case study. The RAND study did not formally develop alternative strategies to compare, but instead described a monitoring approach for the IRP’s currently defined investments, which is a key component of an adaptive, robust plan.

Figure 2. Robust Decision Making Steps Used in the Southern California Case Study

1. Decision Framing
2. Evaluate Strategies Across Futures
3. Vulnerability Analysis
4. Tradeoff Analysis
5. New Futures and Strategies

RDM Steps in This Case Study

• Decision framing: The Metropolitan RDM analysis was an internal study, and the decision framing step was performed by a small group of water planners and RAND researchers. The study was designed to evaluate the IRP under a broader set of uncertain futures and perform a vulnerability analysis to identify signposts for adaptation.
• Evaluate strategies across futures: The study team expanded Metropolitan’s modeling framework to account for additional uncertainties and evaluate the IRP across hundreds of futures. These results were summarized and presented using interactive visualizations.
• Vulnerability analysis: An RDM-style vulnerability analysis was used to analyze how well the IRP would perform across the results of the hundreds of simulations modeled in the previous step. This process identified three simple decision-relevant scenarios (i.e., descriptions of future conditions that would require significant management changes, investments, or both).
• New futures and strategies: This study proposes a framework for monitoring future conditions to provide indications for how the IRP might need to address pending vulnerabilities.

RDM Step: Decision Framing

The RAND RDM study convened Metropolitan staff and water managers from outside agencies to develop the scope of the analysis.1 Using the results of this convening and subsequent discussion, the study team framed the analysis as summarized in Table 1.

Table 1. XLRM Matrix for Metropolitan Water District of Southern California Case Study

The following subsections describe the key uncertainties, performance metrics, and modeling framework used in the RDM study. Unlike the RDM analyses described in the other case studies, the Metropolitan RDM study evaluated only the current strategy described in the 2015 IRP.

Uncertainties (X)

There are various longer-term trends and factors that could affect Southern California water management needs in the future. The Metropolitan RDM study focused on uncertainty about future demographics that affect demand, climate conditions that affect demand and supplies, groundwater yields, Bay-Delta operating conditions, and the implementation of Bay-Delta conveyance improvements.

Demographics and Water-Use Rates

Changes in Southern California housing patterns and water-use rates will affect how much water is needed in the Metropolitan service area. According to the IRP, the population served by Metropolitan is expected to grow from around 1.9 million people in 2018 to between 22 million people and 23 million people by 2050, but this projection is uncertain. How much water each person uses is also highly uncertain. Per capita water use has declined since 2008 in response to more-efficient new housing, replacement of old water-using devices with more-efficient models, replacement of landscapes, and behavioral response to past droughts. But it is difficult to predict how this trend will evolve going forward. For Metropolitan, assuming high population growth and reversal of declining water use would suggest significantly more water demand in Southern California. Preparing for this future yet experiencing one in which population growth slows and per capita demands continue to decline could lead to significant overinvestments. Conversely, preparing for too little demand would lead to costly future shortages.

The Metropolitan RDM study considered the IRP projections of households and per-household demand along with three other projections developed by Metropolitan for the RDM study—Balanced Growth, High Growth, and Periurban. Visualization 1 shows how total occupied housing and per-household demand combine to yield estimates of water demand. The top figure shows how per-household demand in acre-feet per year (vertical axis) and total occupied housing units (horizontal axis) have varied from 1990 to 2017 (brown line) and Metropolitan’s projections going forward from 2015 to 2050 (blue, green, orange, and purple lines). The lower figure shows the corresponding demand projection over time. This visualization shows that all four projections anticipate significant increases in the number of housing units and declining per-household demand, and these changes all translate into increasing demand. Mouse over the different projected demographic and water use conditions (top) to see how they translate into demand (below).

Visualization 1. Water Demand and Drivers for Historical Period and Four Future Projections

NOTE: AF = acre-feet.

Climate Conditions

Climatic conditions, including temperature and precipitation patterns, affect demand for water in Metropolitan’s service area and supplies from the various source regions. Climatic changes, however, are not predictable. Global climate models (GCMs), for example, project a wide range of temperature and precipitation trends across Metropolitan’s source basins (Upper Colorado River, Northern California, Eastern Sierras, and Southern California). To illustrate this point, Visualization 2 shows projected trends in temperature and precipitation across the different basins that supply Metropolitan. Trends are based on the change in temperature and precipitation from a historical baseline period (1950–2000) to the decade between 2041 and 2050. You can adjust the global emissions scenario (or representative concentration pathway [RCP]) used to drive the GCMs and select different GCMs to see how temperature and precipitation could change through 2050 across the basins.

Visualization 2. Projected Trends in Temperature and Precipitation Across Source Basins

To summarize these changes for the purposes of the analysis, the RAND RDM study team calculated temperature and precipitation trends averaged over each basin for each GCM simulation. Visualization 3 shows the ten-year trends through the 2050s for the set of GCM runs available for each of the four source basins’ respective climate regions. From this visualization, one can see how uncertain the projections are within a single region and across the regions by cycling through the climate regions using the radio buttons in the upper right. Hovering over the symbols shows the climate trends over time.

Visualization 3. Climate Trends for the Selected Source Basins

Groundwater Yields

The IRP assumes constant Southern California groundwater availability for all years based on best estimates of the basins’ sustainable yield. For the RDM analysis, however, the study team considered how Southern California precipitation trends could affect groundwater sustainable yields. The team specified three levels of change—no change, 10-percent change by 2050, and 20-percent change by 2050—and assumed a negative change for cases in which the precipitation trend was negative and a positive change for cases in which the precipitation trend was positive.

Regulations on Bay-Delta Exports

Supplies from the SWP are further influenced by San Francisco Bay-Delta regulations that affect how and when water can be pumped from the Delta into the SWP aqueduct. Two flow conditions—existing conveyance high outflow (ECHO)2 and existing conveyance low outflow (ECLO)3—are evaluated.

Implementation of Bay-Delta Conveyance Improvements

Another major element of the IRP is the implementation of a Bay-Delta conveyance project that would replace Delta water diversions from the southern part of the Delta with a diversion point from the Sacramento River, north of the Delta. At the time of the 2015 IRP and the subsequent RDM study, different proposals for conveyance were under consideration and being evaluated as part of the California WaterFix, a project designed to increase the reliability of exports that was expected to be constructed and implemented by the early 2030s.

Recognizing the uncertainty in implementation of this expensive and contentious project, researchers evaluated futures in which the WaterFix would be implemented in the 2030s and futures in which it would be delayed past the time horizon of the study. The permit application for the WaterFix has since been withdrawn because of both political and scientific concerns, and another proposal is under development.

Performance Metrics (M)

The RDM study evaluates the performance of the IRP for each future year based on the estimated regional net water balance over a set of 91 different hydrologic conditions that reflect historical variability. A positive net balance implies that the region is able to use or store water available to it, and a negative net balance indicates that the region is unable to meet all customer demands (i.e., a shortage). The team considered shortages that would be greater than 10 TAF, 10 percent of the time, to be of concern.

Relationships or Systems Model (R)

To address climate and other uncertainties in Southern California and additional source regions in a consistent way, the RDM study team augmented Metropolitan’s standard planning model (IRPsim) with additional modules that translate deviations in temperature and precipitation for the source regions into changes in supplies from the CRA, SWP, and LAA and account for uncertainties about Bay-Delta conditions. Details about IRPsim can be found in the 2015 IRP Technical Appendix 2011 [PDF].

RDM Step: New Futures and Strategies—Developing a Monitoring Strategy

Although the Metropolitan RDM study did not specifically define an alternative strategy to the IRP, it used the results from the vulnerability analysis to describe a possible monitoring approach to guide its adaptive management process.

Specifically, the study team presented an approach for how Metropolitan could use insights from the vulnerability analysis to develop a framework for monitoring demographic and climate conditions to anticipate the conditions that would require IRP augmentation. We illustrate the framework by applying it to the second IRP-relevant vulnerability scenario (moderate demographic growth, with declining precipitation and warming trends). The framework incorporates three elements, which we describe in Table 4.

Table 4. Elements in the Framework for Monitoring Demographic and Climate Conditions Requiring IRP Augmentation

Visualization 6 brings these elements together. The first tab shows historical precipitation (top) and temperature (bottom) data for the user-selected basin. A linear trend is fit through these data and projected through 2055. The second tab shows the same information but also includes the projected precipitation and temperature in 2055 based on the GCM projections. Hover over the symbols to see the specifics of each GCM projection. The third through sixth tabs indicate which 2055 conditions would contribute to a vulnerability for each of the four basins. They are preset to the thresholds defined for vulnerability 2 (see Table 2). By using the slider bars on the right, the user can adjust these vulnerabilities. These visualizations enable a comparison between the current trend derived from the historical data and indicated by the dotted line and the future trends that would contribute to a vulnerability. The user can also plot an additional ten years of hypothetical observations to see how future hypothetical data could change the calculated trends.

Summary and Conclusions

In this case study, we describe a climate vulnerability analysis of Metropolitan’s IRP. Using RDM tools that are similar to those used for the CRBS, we define conditions under which the IRP would not meet Metropolitan goals and would need to be modified to adapt to those conditions. These conditions are the IRP-relevant scenarios. We call them decision-relevant because they are relevant for Metropolitan’s decisionmaking regarding the IRP. Second, we describe a framework for monitoring for those conditions and assessing whether such adaptations should be implemented. This information can help prepare Metropolitan for potential adjustments it will need to make and qualitatively indicates how likely those adjustments might be.

The analysis identified several key conditions to which Metropolitan would need to adapt. The monitoring framework shows that historical demographic, temperature, and precipitation trends from the past 40 years are consistent with one of these IRP-relevant scenarios. If these trends continue, Metropolitan will need to adjust its IRP in the coming decades. However, if demographic patterns lead to lower demands because of higher water use efficiency or slower increase in the number of households, then current climate trends might not trigger the need for adaptations.

The framework for monitoring decision-relevant conditions is a novel but natural extension of the traditional RDM process. It helps make strategies agile and responsive to the future as it unfolds. Such agility to changing conditions is an important way to inject robustness into a strategy.

The monitoring approach described in this case study echoes techniques used in other DMDU methods, such as Assumption-Based Planning (ABP). ABP is a qualitative method for assessing conditions under which a plan is vulnerable and identifying signposts that would signal that those conditions are indeed coming to pass.

The next case study on Monterrey, Mexico, builds on this work and takes it further by developing a fully adaptive, robust strategy. In particular, the case study uses optimization to identify no-regrets, near-term policies and defines adaptive pathways along with signposts to guide future policy changes.

Case study authors: David Groves and James Syme

Acknowledgments: The authors would like to thank Brandon Goshi, Jenny McCarthy, Mike Ti, and Jennifer Nevills of Metropolitan for data and information for this case study and for their helpful review and suggestions.