**1. Introduction**

Transition to a low carbon and sustainable society will involve multi-sectoral and multi-disciplinary approaches to support decision-making. Reducing the emissions associated with energy consumption is an obvious component of any robust greenhouse gas mitigation plan, and the water sector is one of the largest energy loads in most municipal regions, making it a valuable opportunity for greenhouse gas reductions [1]. Energy is needed to source, convey, treat and distribute water to residential, commercial and industrial users. Energy is also consumed for wastewater collection and treatment, in order to ensure the safe discharge of treated wastewater effluent into the environment [2]. Based on a 2013 study, about 69 billion kWh or 2% of total electricity consumption in the U.S. was consumed for drinking water supply and wastewater managemen<sup>t</sup> systems [3]. In some water-stressed regions where local freshwater is not abundant, water systems can consume much more energy than average through large pumping projects or advanced treatment of degraded water sources. California, for example, depends on large pumping networks to deliver raw water from where it originates to large water demand regions in Southern California. Consequently, in California, roughly 7.7% of total electricity was consumed in the water sector in year 2001 based on a study published in 2010 [4].

The energy requirements of water supplies are expected to increase in regions with expected population growth, stricter environmental regulations, increasing water scarcity, growing groundwater depletion, and higher dependence on long-distance inter-basin transfers [5], particularly in areas facing extreme and prolonged droughts [6,7]. Several water-stressed regions and cities have formed initiatives to address the rising challenges of reliable water supply and climate resiliency. In arid and semi-arid parts of the U.S. (such as California and Arizona) and across the world (such as in Israel, Australia, and Saudi Arabia), water utilities have promoted programs to expand water use efficiency, water conservation, water reuse, and other alternative supply options in efforts to mitigate water stress [8–11]. The literature underscores the importance of evaluating the energy tradeoffs of these strategies, particularly in densely populated urban areas. For instance, a multi-sector systems analysis by Bartos and Chester [12] found that water conservation policies in Arizona could reduce statewide electricity demand up to 3%. In another case study for Mumbai [13], a scenario-based approach was used to evaluate the residential water-energy nexus for achieving the Sustainable Development Goals over the time frame of 2011–2050. This study found that the interactions between water and energy during end use (i.e., a change in energy consumption prompts a change in water consumption and visa versa) significantly affected water demand and therefore, the electricity consumption for water supply and wastewater systems [13]. Another study used a cost-abatement curve method to analyze energy and water efficiency opportunities across household appliances, and found that an average U.S. household could annually save 7600 kWh of energy (electricity and natural gas) and 39,600 gallons of water if baseline appliances were replaced by energy and water-efficient appliances [14].

Quantifying the energy and emissions footprint of alternative water supply options has also been studied. A set of key performance indicators were used to compare the performance of six decentralised and three centralised water reuse configurations for the cities of San Francisco del Rincon and Purisima del Rincon in Mexico [15]. The results indicated that decentralised water reuse strategies performed the best in terms of water conservation, greenhouse gas emissions, and eutrophication indicators; however, almost negligible energy savings were reported [15]. Two studies estimated the future energy requirements of urban water managemen<sup>t</sup> for the City of Los Angeles [16] and Los Angeles county [17]. Both studies highlighted that conservation and alternative supply options could reduce the overall energy consumed for water while an increased reliance on long-distance transfers could exacerbate future energy needs. Another study used a spatially explicit life-cycle assessment method to estimate the emissions associated with different water sources for Los Angeles and concluded that the greenhouse gas emissions footprint of water recycling could be as high as water supplied from some imported sources [18].

This paper builds on the prior literature analyzing the energy trade-offs of the City of Los Angeles's water supply portfolio by estimating the energy and emissions trade-offs of LA's future water supply trajectories through 2050 for a variety scenarios, including those that significantly increase locally sourced water supplies. We analyze how electricity demand and emissions are shifted in time and space across electricity serving utilities in California. We first provide an overview of the city's baseline water supply, as well as a series of projected business-as-usual and local water supply trajectories; second, we estimate the electricity demanded for water across the time frame extending from 2020 through 2050 and identify the main driving factors that affect water-related electricity demand for each trajectory; third, we spatially disaggregate each electricity demand estimate according to the utility delivering electricity for sourcing and/or treating each water source; and finally, we discuss the energy and emissions burden of future water supply trajectories. Our analytical framework is applied to reveal the potential tensions that could arise in efforts to simultaneously increase Southern California's local water supply, while ramping up efforts to decrease greenhouse gas mitigation strategies.

#### **2. Water Supply System of the Los Angeles Department of Water and Power**

The water supply system in Los Angeles was engineered in the early twentieth century [19,20]. The Los Angeles Department of Water and Power (LADWP) manages the City's water supply and is the

second largest municipal water utility in the U.S. [21], delivering water to nearly four million people living in its service territory [7]. Approximately 560 million cubic meters of water is consumed annually by over 680,000 residential and business water service connections [21]. Much of LADWP's water supply is imported from sources outside the City, as local water supplies are limited and precipitation averages only about 12–15 inches per year [20]. Therefore, a large fraction of LADWP's water portfolio has historically been purchased from Metropolitan Water District (MWD), which pumps water hundreds of miles from the Colorado River (via Colorado River Aqueduct or CRA) and northern parts of the state through California Aqueduct in the State Water Project (SWP-East branch and SWP-West branch). In addition, the City of Los Angeles owns the gravity-fed Los Angeles Aqueduct (LAA), which conveys water from the Owens River in the Eastern Sierra Nevada Mountains to Los Angeles. These aqueducts are shown in Figure 1. During 2012–2016, these three major sources (LAA, SWP, and CRA) collectively served about 84% of LADWP's consumed water [21]. Local groundwater makes up most of the remaining supply. Recycled water in the past few years has offset some non-potable water demand (i.e., for industrial and irrigation uses). Efficiency and conservation have also been major priorities for LADWP because of the limits of its local water supply commensurate with its population. In fact, based on a comparative study [22], LA's success with conservation measures has led to constant reduction in LA's daily per person water use even to levels less than many other major cities in the U.S. and across the world. Additionally during drought periods, LADWP has used mandatory water conservation ordinances to ease water shortages [20,23].

**Figure 1.** Water supply sources for the Los Angeles Department of Water and Power (LADWP). The areas shown in red and green represent the LADWP and the CAISO (the California Independent System Operator) regions, respectively. The color of each block in the bottom illustration represents the energy intensity of its respective water supply source, where the darkest blue corresponds to the highest energy intensity source and the lightest blue, the lowest.

For Los Angeles, a reliable water supply has been a grand challenge given the region's historical experience with multi-year drought events. Thus, the city seeks alternative sources of water to expand local water availability and to support water supply reliability. Hence, the City of Los Angeles has policy initiatives in its sustainability plan to increase the utilization of local water supplies [24]. These initiatives include:


Shifting LADWPs water portfolio will also shift the energy required for its water supply. New water recycling projects can be as energy intensive as MWD imports. (See Figure 1 for the relative energy intensities of LA's local and imported water sources.) Water recycling projects, including non-potable reuse (NPR) and indirect potable reuse (IPR) (via groundwater recharge), are important elements of Los Angeles's plan to increase local water supplies, but they have different energy needs and potential/capacity limitations. Non-potable reuse primarily offsets industrial and irrigation demands [25] (e.g., for agriculture, landscapes, parks, schools, golf courses) and, therefore has limited potential in replacing potable water demands. Furthermore, some industrial facilities have applications that require water that is of higher quality than non-potable water quality (i.e., typically tertiary-level treated) and/or might not have access to recycled water distribution networks. LADWP currently has four recycled water service areas with separate distribution networks that collectively delivered about 45 million cubic meters of NPR in fiscal year 2014/2015, from which approximately 84% was consumed for environmental uses (e.g., for dust control, seawater barriers, and other environmental uses), 14% for irrigation, and 1.6% for industrial applications [7].

IPR via groundwater recharge has higher potential in terms of offsetting urban potable water demands [26]. Requirements for indirect potable categories of recycled water use are different from NPR. IPR requires advanced treatment techniques such as microfiltration, reverse osmosis, ozone, biological activated carbon, and/or advanced oxidation that are often more energy intensive than tertiary treatment and disinfection required for NPR applications [27,28]. In addition, groundwater recharge projects need energy for pumping recycled water from its water treatment location to a groundwater spreading basin (i.e., for injecting water into groundwater aquifers), as well as for pumping water back up from an aquifer and transferring it to potable water distribution network. Since new recycling projects within LADWP's service network are still in their planning stages, there is grea<sup>t</sup> uncertainty about their energy footprint and water recovery rates. These factors will depend highly on regional topography, existing land use, the distance between recycled water production and spreading basins [29,30], as well as the type of treatment technology and scale of treatment capacity [31,32]. IPR has a large potential for expansion. The largest wastewater treatment plant in Los Angeles (i.e., Hyperion plant) treats about 363 million cubic meters of wastewater annually. Hyperion currently discharges nearly 83% of its treated wastewater effluent to the Pacific Ocean, which could otherwise be treated to a higher quality to produce recycled water [7]. In addition, there are three smaller wastewater treatment facilities in the city and a few others in neighboring cities that could either produce some amount of recycled water now or be retrofitted to do so. In regards to spreading ground capacity to store recycled water, one study estimated that there are about 30 existing spreading basins in the metropolitan Los Angeles region that are generally underutilized outside the winter months (i.e., approximately 12% of their theoretical infiltration capacity is used) [30]. Stormwater runoff from urban areas is another underutilized local water resource that can be used for groundwater recharge or direct use for landscape irrigation. Stormwater generally requires less intensive water treatment than water treated to IPR standards, and hence, requires less energy. Several centralized and distributed rainwater harvesting projects being pursued by LADWP are estimated to have a total volumetric potential between 163 and 178 million cubic meters by 2035 based on conservative and aggressive scenarios, respectively [7].

The trade-offs between water availability, water supply potential, and the energy requirements of different water supply sources challenge the sustainability of long-term water supply plans; thus, these trade-offs must be accounted in the decision-making process. LADWP's Urban Water Management Plan (UWMP) is a comprehensive water managemen<sup>t</sup> planning document (mandated by the California Department of Water Resources for every urban water supplier that annually delivers over 3.7 million cubic meters (or 3000 acre-feet) of water annually, or serves more than 3000 urban connections [33]) and is updated in every five years. Although the electricity use in California's water sector is substantial, it is generally voluntary for the water agencies to report water-related energy consumption. LADWP reports information about water supply-related electricity use for historical years in its 2015 UWMP [7], and briefly describes electricity demand trajectories for its future water supply plans. However, there are no energy projections to estimate the consequences of the City of Los Angeles' latest water sustainability goals, which are not ye<sup>t</sup> reflected in LADWP's UWMP. This study addresses this knowledge gap by analyzing the factors that are most likely to drive shifts in the electricity needed for future water supply options.
