**5. Discussion**

#### *5.1. Energy Trade-Offs of Local Water Supply Options*

As water sources dependent on snowpack are vulnerable to drought (e.g., water imports from the Sierra Nevada and the Colorado River), there are large benefits to expanding local water supplies, such as stormwater and recycled water, for mitigating against water shortages during drought. However, increasing recycled water supplies that are treated to potable water quality standards have electricity demand ramifications. Treating water to the quality acceptable for IPR (for groundwater recharge) is generally more energy intensive than treating surface water supplied from distant sources [16], and therefore, there might not be considerable benefits in terms of overall electricity demand for water, and in some cases there may actually be overall energy increases as water recycling projects may need extensive pumping in addition to treatment [6]. In our analysis, we assumed large groundwater recharge projects will be implemented by 2035 in the S2 scenario, when the local water supply percentage will increase substantially at the cost of increased electricity demand in 2035 compared to the previous projection year 2030.

Water demand managemen<sup>t</sup> strategies, namely water conservation, also have energy impacts. Saving water saves energy that would otherwise be consumed to supply the amount of water saved. The energy savings associated with each unit of saved water is not equal given LADWP's diverse water portfolio. To reflect LADWP's goals for managing its water supply portfolio over time, we made an underlying assumption that water supply is prioritized from local sources first (which are limited by factors such as treatment and distribution infrastructure for water recycling), then water imports from LAA (which are limited by environmental regulations and hydrology conditions), followed by imports from MWD. Hence, the energy savings of water conservation depends on the marginal supply source that conservation is otherwise avoiding. In this sense, the energy benefits of water conservation are equal to the energy that would otherwise be used importing from MWD, when these water supplies are the marginal source. After the need for MWD purchases is eliminated, the LAA supply becomes the next marginal imported water source, and therefore, the further reduction in water use has lower energy benefits since water from LAA is the least energy intensive source. In other words, the energy benefits of saving water is higher when imports of water from SWP and CRA are the water volumes being displaced, compared to the case in which LAA water is the only source of water imports (i.e., in S2).

Water conservation strategies are not just important for drought resiliency. In lieu of expected growth in population, a continuous commitment to conservation programs is necessary to maintain or even decrease daily per person water use volumes further. Water conservation strategies are not just important for drought resiliency. In lieu of expected growth in population, a continuous commitment to conservation programs is necessary to maintain or even decrease daily per person water use volumes further. Our decomposition analysis suggests that conservation in the average weather scenarios (i.e., S1 and S2) also tends to mitigate increases in energy for water that are driven by higher water demand projections in the year 2035 (compared to the historical average year).

#### *5.2. Spatial Shift in Electricity Demand for Water*

Increasing the usage of local water sources in Southern California (and reducing imports from large pumping projects) will cause a dramatic shift in the locations where the water-related electricity demands occur across the state (see Table 3). The large pumping energy requirements of conveying water from the SWP and CRA mostly occur in regions outside of LADWP's electricity service territory; hence, reducing those imports translates in reductions in energy usage by CAISO investor owned utilities including Pacific Gas and Electric and Southern California Edison. On the other hand, the energy loads incurred from the pumping and treatment of local water sources, including groundwater, stromwater and water recycling, are majorly located within LADWP's electricity service territory. Accordingly, moving away from MWD imports and towards local supply sources will shift the energy footprint of water from outside the city into LADWP region. Our spatial disaggregation of electricity demand reveals that transitioning to a local water supply might vastly decrease the electricity demand for LADWP's water supply in the CAISO region. This shift in electricity demand might increase the percentage share of water-related electricity demand from LADWP's total system load from 0.8% in the reference year to about 4% in 2035. This increase in electric load is equivalent to the annual electricity use of over 22,000 average households in California (based on data from [44]). This additional electricity demand in the city will add to its carbon footprint and other upstream environmental externalities associated with electricity generation under current electric grid conditions (although the externalites associated with decreased CAISO generation would be reduced in other regions). The magnitude of environmental externalities will depend on the success of decarbonizing the energy system by moving away from coal and natural gas fuels towards cleaner sources of energy. If the energy transition happens fast enough, the increased electricity demand for water might be insignificant in terms of its carbon footprint. However, with the current electric grid fuel mix, the city's water-related emissions will increase dramatically when aggressive local water supply plans are implemented.

## *5.3. Achieving Water-Related Sustainability Targets*

For comparison, the performance indicators calculated for all four scenarios are summarized in Table 5. The estimated ranges of electricity demand intensity values in 2035 for S1 are lower than the reference historical average, while the other three scenarios show either a similar or a higher range. The emissions intensity of water depends on the assumed average emissions factor of the future electric grid and the electricity demand intensity of the water supply mix. The combination of these two factors show that the emissions intensity could be much lower for a cleaner grid; however, under the current electric grid fuel mix, the emissions intensity of water could be higher in S2–S4 than the historical value (0.3 kgCO2/m3).

Comparing the performance of studied scenarios with the water targets of the Los Angeles City plan [24] highlights a few points:



**Table 5.** Summary of performance of studied scenarios in achieving LA's sustainability targets, as well as electricity demand intensity and emissions intensity of water for year 2035 of each scenario.

1 For the reference year, the value is calculated only based on emissions intensity of California's 2018 electric grid; therefore, no range is provided. The other ranges use two different emissions intensities that are applied to average electricity demand estimates (i.e., 225 and 75 kgCO2/MWh representing California's 2018 [39] and a future decarbonized electric grid [40], respectively).

## *5.4. Policy and Planning Implications*

The results of this study demonstrate important implications for urban system planning with broader sustainability objectives. The case study of LADWP shows that water policies intended to increase the sustainability and resilience of its water system can be energy intensive and burden its electricity system, thus challenging other sustainability objectives such as greenhouse gas mitigation. Extensive coordination between the water and energy sectors is needed, so that transitions in the energy and water systems facilitate short-term and long-term sustainability priorities without exacerbating tensions between the two sectors.

We recommend that the energy trajectories of future water supply portfolios be incorporated into the city's long-term planning to facilitate holistic decision-making in regards to electricity demand planning, drought resiliency considerations, and ecosystem protections. Synergistic opportunities in water and energy systems open new areas of innovative solutions such as improving the energy efficiency of operations, coordination of water pumping operations with water storage, resource recovery, on-site energy generation, technological innovations, and water trading schemes that could benefit both water and energy sectors, and could reduce the energy needs of water systems [45–51]. Future water systems should be designed based on holistic systems' paradigms that are multipurpose and integrative to promote reliability, resilience and sustainability for the city's urban water system.
