Exploring the Water-Energy Nexus in California via an Integrative Modeling Approach

Yates, David; Mehta, Vishal K.; Huber-Lee, Annette; McCluskey, Alyssa; Purkey, David

doi:10.1061/(ASCE)WR.1943-5452.0001431

Open access

Technical Papers

Sep 29, 2021

Exploring the Water-Energy Nexus in California via an Integrative Modeling Approach

Authors: David Yates, Ph.D. https://orcid.org/0000-0002-0688-3460 [email protected], Vishal K. Mehta, Ph.D., Annette Huber-Lee, Ph.D. https://orcid.org/0000-0002-9897-5492, Alyssa McCluskey, Ph.D., and David Purkey, Ph.D.Author Affiliations

Publication: Journal of Water Resources Planning and Management

Volume 147, Issue 12

https://doi.org/10.1061/(ASCE)WR.1943-5452.0001431

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Abstract

A climatically driven, water system modeling framework that captures a full range of interacting components at necessary spatiotemporal scales was developed for the Southwestern United States, including a detailed representation of California’s water supply and demand network, within the Water Evaluation and Planning (WEAP) system. Credibility of the model as a tool for water-energy analysis was garnered through evaluation of the historic regional water balances, including metrics of water supply, water use, groundwater storage, environmental requirements, and the energy use associated with water. The ability of the model to address policy-relevant questions was demonstrated by evaluating the impacts of a future 30-year drought scenario that included a realistic 2°C warming trend through 2050. With drought, the total energy used to supply water decreases by about 3% compared to the historic use, and warming alone reduces overall energy use by a modest 1% due to overall reductions in water delivery. The strongest signal of persistent warming expresses itself in groundwater storage, which decreases by an additional 9% beyond the impact of drought alone. A water conservation scenario was used to demonstrate the response of the water sector to the types of conservation strategies being pursued at local and state levels, with conservation reducing water use by about 10% and energy use by 3% relative to historic levels. Such measures overcome the warming trend, but it is demonstrated that persistent drought challenges groundwater conservation strategies and reductions in energy use by the water sector, leading to further overdraft of aquifers and increased energy use.

Introduction

The Water-Energy-Food Nexus (hereafter simply, the nexus) has been defined by Endo et al. (2017) as “the complex interactions of tradeoffs and potential conflicts among water, energy, and food resources” and argues the need for frameworks that result in indices, models, and economic assessment methods that can integrate its interdisciplinary, multisector, and multidimensionality nature. The development of new nexus modeling frameworks should overcome individual system analysis or analysis that is often applied at an aggregated scale (Hamiche et al. 2016; Shannak et al. 2018). Bazilian et al. (2011) point out the need to draw system boundaries wide enough to encompass the full scope of the interacting components that can be used to explore policy alternatives. Acknowledging these challenges, this study demonstrates how a climatically driven approach, which takes a bottom-up perspective of the regional water balance, can be used to explore appropriate spatiotemporal scales and evaluation metrics of complex water and energy systems and their end-to-end interactions, with a demonstration in the Southwestern United States that emphasizes California.

There have been several water-energy studies that explored the relationship between water use and energy in California (CEC 2005; Tidwell et al. 2014). The California Public Utilities Commission (CPUC 2010) developed a scenario-based modeling approach that explored energy use in supplying and using water, and while the study adopted a water balance approach, demand and supplies are prescriptive and thus tied to a particular historic condition (i.e., normal, dry, or wet year). A primary finding was that energy used in groundwater pumping is more significant than previously estimated. Escriva-Bou et al. (2018b) explored water-energy tradeoffs in California, developing a network optimization using a simplified representation of the surface and groundwater systems. Although their model includes precipitation-driven flow and groundwater modules, a lack of regional detail places into question the credibility of results given the heterogeneity of water supply and demand. Other water systems models, such as that of Draper et al. (2003) and Harou et al. (2010), provide details of the California water system, but alternative hydrologic states have to be generated from perturbed historic hydrologic records, so the impacts of climate variability and change on the water system are not direct. In addition, these studies did not include water-energy analysis. For California, Liu (2016) notes a lack of integrated models and data systems that make it difficult to measure and evaluate these complex interactions and quantify the range of benefits and tradeoffs between sectors, and again, Liu (2016) specifically calls out WEAP as such a tool to achieve this integration.

Harou et al. (2010) explored the consequences of a prolonged, 70-year mega-drought in California and showed that if California’s water supply were effectively cut in half, that a 25% reduction in agricultural water use, mostly in California’s Central Valley, would minimize the statewide cost of water scarcity and would reduce the magnitude of groundwater overdraft. This outcome is consistent with the findings of Lund et al. (2018), who showed that during a severe five-year drought from 2012 to 2016, there was fallowing of less profitable croplands and significant movement toward pumped groundwater, resulting in marginal economic costs to California due to its diverse economy.

In the context of these studies, this work advanced a climatically driven water system model of the entire Southwestern United States (hereafter, SwWEAP) built within the Water Evaluation and Planning (WEAP) decision support system (Yates et al. 2013a, b; Yates and Miller 2013; Sattler et al. 2012). The WEAP platform has been adopted by the State of California as a tool for exploring various regulatory environments, and as such, studies that describe and validate its application are warranted (Young et al. 2019). Questions that the framework might address include, “What are the water, energy and cost implications of conservation and prioritizing local and alternative sources to meet water demands in favor of transbasin diversions” (Gold et al. 2015; Brown et al. 2019; Klein 2006), “How do water conservation and land use transformations in California impact water and energy sustainability goals” (Perrone and Rohde 2016; Miro and Famiglietti 2019), and “What level of conservation and priority of source (local surface, groundwater, or transbasin) are needed to halt overdraft and bring groundwater basins into balance (CDWR 2018; Thomas 2019), within the broader goal of reducing energy use, and by proxy, reduce greenhouse gases” (CARB 2017)? Since the water-energy modeling framework presented here is driven by monthly climate at the watershed level, it is possible to credibly explore these questions within the context of climate variability and change.

To explore these types of questions, the model and its framework need to be evaluated for their credibility and relevancy, achieved here by (1) showing the reproduction of the historic water supply and water and energy use and; and (2) demonstrating, with a simple scenario-based approach, how the model can be used for policy analysis by exploring the impact of regional warming and drought and water conservation on water use, water supply, and the energy of its supply (Vicuna et al. 2007). While the analysis presented here is relatively simple, we contend that it is important to document WEAP’s ability to represent water-energy dynamics, thus lending credibility for its use in conducting a more detailed analysis within the context of formal stakeholder processes. Before describing and applying the SwWEAP model, a broad overview of the water systems of California and their energy uses are summarized to place the analysis in its wider context.

Water and Energy in California

California’s Mediterranean climate of wet, cool winters is in contrast with the timing of peak energy and water demand in the hot, dry summers. The state exhibits one of the most altered water landscapes in the world, with more than 1,400 dams, and thousands of kilometers of canals, aqueducts, and irrigation ditches (Lofman et al. 2002; Carle 2004). Annual agriculture and urban water use in California has averaged about 42 billion cubic meters (BCM) or 34 million acre-ft (MAF, where 1

acre - ft = 1,233 m^{3}

) and 11 BCM (9 MAF) from 1998 to 2016, respectively with groundwater supplying from 10 BCM to 23 BCM (8 to 19 MAF), depending on wet and dry years (CDWR 2013c). While surface water storage is critical for the seasonal water supply, it only represents about one year of water use (50 BCM or 40 MAF), whereas groundwater storage is the workhorse of mitigating interannual variability with estimates of 500 to 1,000 BCM (400 to 800 MAF) of storage.

This water is often energy-intensive, as it is pumped both across and into the state, to serve the largest population center in the United States and to irrigate tens of thousands of hectares of highly productive agricultural lands (CDFA 2018). Yet, climate variability often strains these systems (Swain and Tsaing 2014; Mann and Gleick 2015), as prolonged drought has worsened already over-drafted groundwater storage (Famiglietti et al. 2011; Scanlon et al. 2012). In many regions in California, groundwater levels have dropped to record depths, and domestic and farm wells have dried at an unprecedented pace, prompting the state to enact the Sustainable Groundwater Management Act (SGMA 2014; Harter and Dahlke 2014; Harter 2015; Kiparsky et al. 2017).

Faunt (2009) estimates that groundwater in the Sacramento-San Joaquin Valley has been depleted by about 1.85 BCM (1.5 MAF) per year on average since 1960 and estimates that the rate nearly doubles during drought. On average, about 40% of the water supply of the Central Valley has come from groundwater, ranging from about 30% during wet years and up to 70% during dry years. The Sustainable Groundwater Management Act (SGMA) of 2014 seeks to address groundwater-level declines, enhance groundwater storage, and reduce surface water depletion, with policies such as the Water Conservation Act (CDWR et al. 2017) serving to help achieve these goals by reducing urban per capita water use by 20% (Cahill and Lund 2013). Spang et al. (2018) note that during the drought of 2015, significant municipal water conservation had the mutual benefit of a decrease in energy use and a concurrent reduction in greenhouse gas emissions. At the same time, there are objectives to reduce transbasin water diversions in an attempt to restore flows. For example, the Trinity River Restoration Program implemented the 2000 Federal Record of Decision to restore fisheries that reduce transbasin diversions.

In contrast, California’s electricity consumption per capita at

6,500 kW \cdot h

is one of the lowest in the United States at about half the US average at

12,100 kW \cdot h

per person (CEC 2016). The greatest portion of the population is located near or along the Pacific Coast, where relatively moderate temperatures mean less energy is used for cooling and heating. The total electricity from in-state generation was about 200 terawatt-hours (

TW \cdot h

), with about

100 TW \cdot h

of imports over the period 2005 to 2015 (CEC 2016), dominated by natural gas. From 2010 to 2015, solar generation has increased more than 10-fold, while in-state wind generation has nearly doubled with 35% of total generation from zero-GHG generation sources in 2015 that includes nuclear and hydropower sources. For California, annual electricity consumption has been about

260 TW \cdot h

, with the water sector accounting for about 8% or 22%

TW \cdot h

for pumping and treatment but excluding end use (CEC 2016; Jones 2005; Klein 2006; Escriva-Bou et al. 2018a). The California State Water Project (SWP) is the single biggest electricity user, averaging between 6 and

10 TW \cdot h

per year, although the project also averages about

6 TW \cdot h

per year in hydropower generation (CDWR 2016).

Southwest WEAP Model

The SwWEAP is a climatically driven, spatially explicit, hydrology and water resources model capable of calculating the dynamics of the hydrologic cycle that include changes in climate conditions and human interventions such as reservoirs, canals, and tunnels; irrigation systems; urban water systems; and hydropower facilities, with details of the initial model available in Yates et al. (2013a) and new capabilities presented here. The primary external drivers include monthly precipitation and temperature data, land use, energy and water use rates, and water supply unit costs. The data for these factors are taken from state government reports (CDOF 2018), municipal development strategy studies (CLA 2017a, b), state development goals (CDWR 2013b, a, 2016, 2018), climate data archives (Maurer et al. 2002; Reclamation 2013). Regarding run time, a 30-year scenario on a desktop PC with an Intel Core i7 at

180 GHz

and with 32 GB of RAM takes approximately

10 \min

to run (Intel Corporation, Santa Clara, California). Individual scenarios can be run simultaneously on separate processors; thus, an 8 processor machine could complete 800 30-year scenarios in a 24-h period.

The main external drivers include climate, population, agricultural and municipal water use intensity, energy demand intensities of the water supply, and water supply costs. The model is driven by a monthly climate forcing dataset that includes monthly total precipitation and average temperature for the period 1950 through 2010, with 1971 through 2010 used during the model calibration/validation period (Yates and Miller 2013; Reclamation 2013).

The geographic scope of the SwWEAP model is essentially the Southwestern United States bordered by the South Platte River to the east, Rio Grande and Colorado Rivers in the South Central, Pacific Ocean to the west, Klamath Basin to the northwest, Bear and Green Rivers to the Northeast, and critical Central Valley and Southern Coast regions of California (Fig. 1). This large extent reflects the interconnection between these river systems, with the many transbasin diversions, reservoirs, canals, and tunnels that move water for municipal and agricultural uses throughout the region.

Fig. 1. SwWEAP domain, with the small, blue circles representing the municipal, industrial, and energy demands; the brown circles representing the groundwater basins, the dark triangles representing the reservoirs, and the red stars representing locations with environmental or obligated flow requirements.

Water Supply and Hydropower Capacity in SwWEAP

The SwWEAP model includes a representation of the primary water infrastructure such as the Central Valley Project (CVP), the SWP, Colorado River Projects, and others (Yates and Miller 2013; TRRP 2004) and their yearly volumetric limits. Their energy requirements for water are based on the height for which they each must lift water. The model includes 55 reservoirs with a total storage capacity of 136 BCM (110 MAF), with 22 of the largest reservoirs included in California, with a modeled storage capacity of 40 BCM (32 MAF). Choy et al. (2014) and the California Department of Water Resource (CDWR) estimate California reservoir storage at just under 50 BCM (40 MAF). California has more than 250 hydroelectric facilities with an installed capacity of 14 Gigawatts (GW), with the SwWEAP representing about 10 GWs of this installed capacity. Lake Mead and Lake Powell provide electricity to California, with 2.0 and 1.3 GW of installed capacity, respectively.

There are 20 represented groundwater systems in the SwWEAP with an assumed initial storage of 2,000 BCM (1,600 MAF) overall and 500 BCM (405 MAF) in California (Choy et al. 2014). The main groundwater aquifer regions considered in the SwWEAP model for California are those in the Central Valley, including the Upper and Lower Sacramento, the San Joaquin and Tulare Basins, and local groundwater systems of the South Coast region (Fig. 2). These aquifers typically supply between 30% and 40% of the total water supply and between 50% and 80% of the total urban water demand. The SwWEAP model internalizes groundwater recharge, because a portion of water diverted for beneficial use in a given month can then recharge an aquifer or return to the river to be used downstream (Yates et al. 2005, 2013a).

Fig. 2. (1) The Sacramento Basin; (2) The San Joaquin and Tulare Basins; (3) The North and Central Coast; (4) The South Coast; and (5) The Owens and Imperial Valleys. Regions outside these primary zones are included in “All California” summaries.

Two desalination sources in Southern California include the active Carlsbad facility near San Diego at 50 million gallons per day (MGD), and an additional source was assumed in the scenario analysis in anticipation of new capacity at 150 MGD (Cooley et al. 2012). Municipal indoor water that is not consumed is either treated and returned to a river, groundwater, or ocean outflow; while a fraction can also be reused to meet municipal outdoor uses, with the reuse fraction assumed to be 8% in 2010 (Pacific Institute 2014; Mini et al. 2014). It is assumed that local surface sources have the highest supply priority, followed by local groundwater, then imported water, and desalination, while reclaimed water is assumed to be used up to its capacity to meet outdoor municipal and industrial demand.

Water Use in SwWEAP

There are over 100 water demand nodes in the SwWEAP model that represent municipal and industrial use (separated by indoor and outdoor), irrigated agriculture, and environmental/obligated flow requirements. The WEAP soil moisture model is used to solve the hydrologic water balance by means of lumped catchment hydrology using a simplified conceptual model based on a two-bucket root and deep zone approach (Yates et al. 2005). Irrigation for both agriculture and urban use are expressed as the irrigation depth required to maintain the water storage in the upper zone within defined upper and lower thresholds (Allen et al. 1998).

Population and land use are used to compute indoor municipal, commercial, and industrial use and outdoor water demand. Indoor water use is on a per-capita basis, prescribed at 415 liters per-capita per day (lpcpd) or 110 gallons per-capita per day (gpcpd), which is then multiplied by the population estimate in each region to estimate monthly municipal indoor water use. Population data by state and region were estimated from the US Census and California Department of Finance (US Census Bureau 2019; CDOF 2018). Land use was defined by the National Land Cover Database 2011 (USGS 2011) and is used to approximate irrigated agricultural areas and urban areas that have turf and amenity water demands. The estimates of municipal irrigated turf and amenity areas are simply a fraction of the total urban area in each of the urban regions represented in the model (Hanak and Neumark 2006). Since the agriculture sector is assumed efficient, any reductions in water use are primarily achieved through fallowing and land transformation, resulting in a reduction in net water use.

Energy Use by the Water Sector in SwWEAP

Energy use by the water sector is estimated in SwWEAP based on electricity intensity by activity multiplied by monthly water use for each activity. Table 1 summarizes the range of energy intensities for various water uses, where intensities are assumed constant except for groundwater, which varies with depth to pumping (NC 2006; CEC 2005; CPUC 2010).

Table 1. Energy intensities for key water sector categories

Activity	$kW \cdot h / m^{3}$	Description
Groundwater pumping^a	0.18	Electricity use is a function of water pumped and depth to groundwater; $α$ $(depth) \times (kW \cdot h / m^{3})$ .
Agriculture end-use	0.06	Electricity use associated with on-farm activities and transport.
Conveyance^a	0.003	Electricity use related to lifting and conveyance of surface water primarily for large-system irrigation use ( $kW \cdot h / m^{3}$ per meter of lift).
Reuse	0.12	Electricity use associated with reclaimed wastewater.
Desalinization	2.8	Electricity use for desalinization for potable water supply.
Municipal treatment and distribution	0.6	Electricity use for supplying water to municipalities (indoor and outdoor water uses).
Wastewater treatment	1.0	Electricity use associated with wastewater treatment.

Sources: Data from CEC (2005); NC (2006); CPUC (2010).

a

Energy use associated with groundwater pumping varies by depth to groundwater and conveyance with lift height.

SwWEAP Credibility through Historic Validation

Metrics of water and energy are derived from SwWEAP results and compared to historical observations. These include water supply delivered to the various demands and the sources of those supplies, Sacramento-San Joaquin Delta outflow, hydropower generation, and water sector energy use. Further details on model calibration are summarized in Yates et al. (2013a), with additional evaluation included here in support of the more detailed analysis of the water-energy nexus in California. Fig. 2 can be used as a reference to particular regions summarized in the subsequent plots.

Water Supply Delivered to California

The water demands in SwWEAP include irrigated agriculture, separate municipal indoor and outdoor uses, and environmental or obligated flow requirements. The simulated annual delivery of water by region, use type, and source of supply can be compared to historical averages derived from CDWR estimates for the available period 1998 to 2010 given as the dark squares in Fig. 3. For urban use, the North region includes the Sacramento-San Joaquin Basins and the North and Central coasts, while the South region includes the South Coast, Owens Valley, and remainder of California [Fig. 3(a)]. The SwWEAP simulation of municipal annual water use was 10.5 BCM (8.5 MAF) over this period, with outdoor use roughly 40% of indoor use, while historic annual municipal use was 10.4 BCM (8.4 MAF). The CDWR does not provide a split between indoor and outdoor use.

Fig. 3. Summary of (a) *SwWeap* simulated annual urban water use; and (b) agricultural water use by region, and regional water supply delivered by source in (c) Northern California; and (d) Southern California for the period 1981 through 2010. North are regions 1, 2, and 3; while South are regions 4 and 5 from Fig. 2. The black marks in each figure are the CDWRs estimates. Colors suggest categorical grouping (e.g., outdoor vs. indoor use; transbasin)

The applied irrigated agricultural water ranged from 36 BCM (30 MAF) to 52 BCM (42 MAF) with a mean of 42 BCM (34 MAF), which is consistent with the historic delivery for the period 1998 through 2010 [Fig. 3(b)]. The largest share of agricultural water use is in the San Joaquin and Tulare Basins, where the greatest amount of groundwater is also used. Note that the South use and supply include the Imperial Irrigation District, which gets the majority of its water from the Colorado River and represents the majority of the agricultural demand in the region.

Since supplied water in SwWEAP is strictly counted by sources to meet demands, a one-to-one comparison with modeled estimates is difficult. The State of California reports water by project, considers reused water within the basin, includes a “Wild and Scenic River” use and supply, and makes other accounting assumptions that are not consistent with the accounting in SwWEAP. In the modeled accounting, water transported via a WEAP transmission link and used outside its basin is counted as transbasin, and local water is primarily surface supplies that originate from within the basin. In the state’s accounting, project water can be used within the basins such as the Sacramento Basin’s use of water from the CVP. Generally, the reporting of supplied water is greater than the SwWEAP due to its accounting of reuse water and the inclusion of environmental water [Figs. 3(c and d)].

Fig. 3(c) includes the supply of groundwater for the North region that includes Sacramento and San Joaquin and shows total annual delivery of about 18 BCM (14.6 MAF). Table 2 shows the change in storage over this historic period that compares the historical estimate with the SwWEAP estimate of groundwater drawdown by geographic region, which is in general agreement. Hanak et al. (2017) estimate that in the San Joaquin and Tulare Basins, which represent nearly 90% of the state’s net water use, that groundwater drawdown has been as high as 2.5 BCM (2 MAF) per year. The SwWEAP average annual groundwater overdraft from 1981 to 2010 was about 2.5 BCM (2 MAF) for all of California and 1.5 BCM (1.2 MAF) for the San Joaquin and Tulare Basins.

Table 2. Total change in groundwater storage in millions of cubic meters (2005–2010)

Region	Decrease in GW storage^a	Modeled decrease in groundwater storage
San Joaquin	11,100	8,500
Sacramento	2,100	1,800
Tulare	Range of 4,441 to 10,860	5,830
South Coast	Changes not documented	1,250

a

Data from CDWR (2013b).

Environmental Flow Requirements

The SwWEAP model includes some of the critical environmental and obligated flow requirements of the rivers throughout the Southwestern United States. For example, there is a representation of the deliveries of the Colorado River to Mexico and the discharge of flows through the Sacramento–San Joaquin Delta. At the Delta, an environmental flow requirement constrains the SWP and the CVP exports, where the minimum monthly Delta exports are conditioned on the water year type as specified in the 2008 Biological Opinion (USDIFWS 2008; State Water Board Revised Water Right Decision 1641) and generally range from 80 to 130 cubic meters per second (cms) from September through December based on water year type; and while there are new efforts to modify delta export requirements, we assume that previous guidelines are applied in this analysis.

Two critical and integrating hydrologic points of interest are used to assess the skill of the SwWEAP model to adequately represent hydrologic processes. These include the Sacramento–San Joaquin Delta exports and the Colorado River above Lake Powell. Fig. 4 shows the simulated and observed annual [Fig. 4(a and c)] and monthly flows [Fig. 4(b and d), given as flow duration curves] flows for these points. These results demonstrate the model is skillful in capturing both the inter and intra annual variability. The model mutes the intensity of a few of the peak flows but is generally in good agreement at both monthly and annual timescales, and, thus, it provides an additional measure of validation of the model as a tool for water-energy nexus analysis.

Fig. 4. Comparison of the simulated vs observed (a and b) Sacramento-San Joaquin Delta outflows; and (c and d) inflows to the Lake Powell on the Colorado River; (a and c) for the annual; and (b and d) monthly average flows. The inset for the monthly flows shows goodness of fit summaries for the Sacramento-San Joaquin Delta (b) and the Colorado Inflows at Lake Powell (d). FDC = flow duration curve. The statistics include the root means square error ( $m^{3} / s$ ), normalized root mean square error, percent bias, and Nash–Sutcliffe efficiency (a value of 1.0 is perfect agreement).

Reflecting on water delivery, Fig. 5 shows the total annual and monthly average Delta exports from the SWP and the CVP to the San Joaquin Valley that includes the ultimate delivery to the South Coast. Note that annual correlation skill is low (not shown), because the model imposes a fixed set of diversion rules based on water year type. As an example, in a wet year like 1983, while water was legally and physically available for greater export, the historic deliveries were actually small in contrast to the SwWEAP simulated values. However, the model adequately captured the seasonal pattern of delivery, and most notably, the June through September period, when delta constraints are more binding.

Fig. 5. Comparison of SWP and CVP modeled vs. observed deliveries for the period 1971 through 2010, including (a) annual average; and (b) monthly average in cms. The blue lines are modeled, and black lines are the observations.

Energy Use by the Water Sector and Hydropower Generation in California

Fig. 6 summarizes the SwWEAP simulated annual electric energy use by each water subsector, including the municipal potable supply, wastewater treatment and reuse, transbasin conveyance, and the agriculture supply. The total annual SwWEAP estimated energy used to supply water across California for the historic period was about

27 TW \cdot h

, above the

22 TW \cdot h

net estimate, but hydropower generation by the SWP and others offset some of this energy use.

Fig. 6. (a) *SwWEAP* simulated annual electric energy use for the water supply of California by type; and (b) annual hydropower generated for the period 1971 through 2010 for California and Lakes Mead and Powell.

Hydropower generation is modeled in the SwWEAP for the largest reservoirs and as the aggregation of the smaller reservoir and run-of-river systems and their hydropower generating capacity. Historic hydropower generation in California from 1981 to 2010 compared with generation output from the SwWEAP model was within 10% of observed values, with a historic mean and standard deviation of 36 and

10 TW \cdot h

, respectively (Fig. 6). The annual average generation from Lake Powell and Mead was

7.5 TW \cdot h

, which is also within 10% of observed net generation estimates (Harpman and Douglas 2005). With a calibrated and validated SwWeap model, an analysis of the modeling tool for conducting water-energy analysis is presented in the next section.

Water and Energy in the Context of Regional Drying and Warming

A relatively simple set of scenarios were conceived to demonstrate the utility of the SwWeap model to address water policy questions such as those posed in the introduction. The scenarios assumed a 30-year period out to 2050, applying population trends in which California grows to about 47 million people, while the rest of the Southwestern United States grows to 49 million people (US Census Bureau 2019). The environmental requirements, such as delta outflows, transbasin water deliveries, and the transnational water delivery to Mexico from the Colorado River are held at their historical agreement levels.

Given the uncertainty of the future climate over the Southwestern United States, but a strong case for regional warming and drying (Pagán et al. 2016; Williams et al. 2020), a climate sequence was generated that corresponds to an extended, 30-year dry period. This was based on an analysis of the historic Eight-River Index (ERI) that blends the historical flows of the Sacramento and San Joaquin Rivers for the period 1906 through 2019 (Dettinger and Cayan 1995). The driest 30 years of the ERI correspond to the years 1919 through 1948. Since the meteorological record used in this study begins in 1949, analog years were used to generate a statistically similar set of years for this dry sequence (Fig. 7). In addition, we have imposed a regional warming trend based on the monthly temperature anomaly of the median climate projection from the CDWR (2015) and applied this anomaly to the monthly mean temperature for each catchment in the SwWeap model. Generally, the annual mean anomaly is 2°C in California and 2.5°C in the interior west by 2050.

Fig. 7. The 30-year ERI for the dry sequence and their water year types as Critical (C), Dry (D), Below Normal (BN), Above Normal (AN), and Wet (W). The dashed line is the 1971 to 2010 historic average of the ERI.

For the entire southwest, this dry sequence has a modest three percent reduction in precipitation when compared with the historic mean, while for the Northern Sierra of California, the dry sequence is about 12% drier. Relative to the historic period, the hydrologic implications of the future warming is an approximate 9% reduction in the simulated runoff for the Upper Colorado River, with greater flow reductions later in the period and runoff coming earlier due to warming [Figs. 8(a and b)]. For the Northern Sierra tributaries of California, temperature effects on simulated runoff are modest since the region is dominated by mid-to-late winter precipitation when temperature impacts on runoff are not as pronounced [Fig. 8(c); Vicuna and Dracup 2007; Huang et al. 2018]. In contrast, the drier conditions over the 30-year period lead to a 16% reduction in total simulated runoff volume relative to the historic period [Fig. 8(d)].

Fig. 8. The *SwWeap* simulated flows for 2020 to 2050 for the *Dry* and *Warm&Dry* scenarios (a) for the Colorado River above Lake Powell; and (b) for the Northern Sierras given as the sum of the Feather, Yuba, and American Rivers. Includes (c) annual average; and (d) monthly average (right) flows in cms. Light gray lines are the annual and monthly averages for the historic period (1971–2010).

Taking into account water conservation goals and the possible evolution of the agricultural sector in California, we used the SwWEAP to assess the response of the water system to both the dry (Dry) and the warm-dry (Warm&Dry) assumptions and then explored how conservation measures (conservation) could counteract these impacts and their water and energy implications. For agriculture, Thompson (2009) and Johnson and Cody (2015) speculate that prime California agricultural land of statewide importance will be reduced by about one million acres by around 2050 due to urban expansion and the fallowing of less desirable tracts. Likewise, there are goals to reduce indoor and amenity outdoor water use in California (e.g., CLA 2017a, b). To demonstrate how these types of future objectives can be explored within the modeling framework, a conservation scenario assumed the following:

•

Average per-capita indoor water use rate decreases to 190 lpcpd (50 gpcpd) by 2050 (CDWR et al. 2017).

•

Irrigated urban area decreases at a rate of 1.0% per year to 2050 (CLA 2017a; Mini et al. 2014).

•

Prime irrigated agricultural area is reduced at a rate of 0.5% per year to 2050 or from about 10 million to 8.5 million acres.

For purposes of demonstration, these assumptions were applied uniformly across California. Note that WEAP readily accommodates the ability to apply assumptions in a spatially distributed manner across the modeling domain, as would be the case in a formal stakeholder engagement process.

Water and Energy Savings through Conservation

Performance metrics used in the analysis include water delivered to end-use by type and source, energy use for water (excluding end-use), groundwater storage, and hydropower. Because the analysis assumes trends in both climate and population, results are generally presented as time series. Fig. 9 shows water use for the municipal and industrial sector and the agriculture sector for the three scenarios. Note that the supply delivered to the end-uses is fairly constant from year to year, because shortages are again generally made up by groundwater sources. The Dry and Warm&Dry scenarios generally deliver the same or more water over the 30-year period, as drier and warmer conditions mean more potential consumptive demand to counter soil moisture deficits, especially in the Warm&Dry scenario. Fig. 10 shows increased groundwater supply delivered during greater shortages, with the signal of the transbasin supply generally opposite that of groundwater. Warming slightly reduces transbasin water, with groundwater making up those shortages, and with local sources more time-varying. Conservation allows for less reliance on groundwater and local sources, as transbasin deliveries are maintained or even increased slightly with warming.

Fig. 9. Supply delivered for (a) urban uses; and (b) agriculture use, for the *Dry*, *Warm&Dry*, and *Conservation* scenarios. The gray line is the average for the historic period for both annual and monthly series.

Fig. 10. Annual water supply for 2020 to 2050 for each of the three scenarios by source for (a) the transbasin water supply, given as the sum of the CVP, the SWP, and the Colorado Aqueduct; (b) the local water supply sources to both agriculture and municipal uses; and (c) the groundwater supply to both agricultural and municipal uses (billions of cubic meters-BCM).

Fig. 11 summarizes electric energy use for water by the agriculture and urban sectors and the energy for transbasin diversion for the three primary conveyance systems including the Colorado Aqueduct, CVP, and SWP for the three scenarios. Drying and warming increase the overall energy use above the historic average, because groundwater pumping intensification increases the total energy use that is somewhat offset by a decrease in energy use associated with water conveyance. The agriculture sector exhibits an upward trend in energy use toward the end of the analysis period, primarily due to more intensive groundwater use. For the conservation scenario, there is a decrease in energy use in the municipal and industrial sector and for agriculture, corresponding to decreased water use and groundwater pumping, respectively (Spang et al. 2018).

Fig. 11. (a) Energy use by the Agriculture sector; (b) Municipal and Industrial sector; and (c) energy used for transbasin water delivery for all three scenarios (in gigawatt hours- $GW \cdot h$ ).

Fig. 12 shows the change in groundwater storage across all of California for the three scenarios. Statewide groundwater storage decreases by about 3.5 BCM per year (4.3 million acre-ft per year) for the drying scenarios, with an intensification of groundwater use toward the end of the simulation period due to intensive drought (Fig. 8) that results in enhanced drawdown. For the Warm&Dry scenario, statewide groundwater over drafting is even more pronounced with a warming trend of 2°C, resulting in an 8% increase in depleted groundwater storage. Through conservation, groundwater depletion is not as pronounced, but the prescribed level of conservation is not great enough to stabilize groundwater levels in the face of drought and warming.

Fig. 12. (a) Change in total groundwater storage throughout California for the 2020 to 2050 period and the three scenarios (BCM); and (b) the total electric energy use for groundwater pumping ( $GW \cdot h$ ).

Fig. 13 shows hydropower generation for California and Lake Mead and Powell. The warming associated with the Warm&Dry scenario when compared with the Dry scenario shows a 10% reduction in total hydropower generation in California, while warming in the interior west results in a more than 30% reduction in total hydropower generated from Lake Mead and Lake Powell. Interestingly, water conservation in California aids hydropower generation by Powell and Mead by keeping more water in storage through reduced deliveries of Colorado River water to California, while the benefits to hydropower from conservation in California are much more modest.

Fig. 13. Annual total hydropower for 2020 through 2050 for the three scenarios, including (a) California; and (b) Lakes Mead and Powell in $GW \cdot h$ .

Conclusion and Discussion

This paper validates and demonstrates a detailed, climatically driven, integrated water resources management model that can be used to quantify the relationship between water and energy, with a demonstration of the model in the southwestern United States and a focus on California water and electric energy use for water. Key contributions of the approach include an explicit representation of the integrated, physical hydrologic processes within the extensive, intertied, and flexible water systems of the Southwestern United States and their heterogeneous water supplies and demands that can respond to water stress and surplus, all while guaranteeing the conservation of mass. The configuration, calibration, and validation of the SwWEAP model for historic conditions demonstrate that it is a credible tool for conducting water-energy policy analysis in a meaningful manner, and the flexibility of the modeling environment allows for the exploration of a diverse set of assumptions that can be envisioned by the analyst and readily implemented and evaluated. The study demonstrates the need for a flexible modeling framework that can be used to quantify the types of water policy interventions being envisioned to achieve water and energy conservation targets.

The SwWeap model was applied under the assumptions of future drying and warming conditions, with results consistent with observed drought responses. For example, Lund et al. (2018) note that in the 2012 to 2016 drought, surface water available for agriculture statewide was reduced by 30%, with about two-thirds of that replaced by groundwater pumping. Our analysis shows similar results with regard to the response of the water supply to the shortage. The modeling framework was then used to explore the relative impact of warming, given as a spatial and time-varying trend across the region, on the water-energy nexus. The long-term expression of this climate signal was declining groundwater storage and increased energy use.

The limitations of this study include the fact that the model is, of course, an imperfect representation of very complex systems. From the physical representation of the contributing watersheds and the parameters used to simulate runoff fluxes such as soil and land use properties to gross simplifications of the multifaceted water management constructs, the model is an imperfect representation of these systems. As in all modeling efforts, there are boundary questions about which components should be included or excluded and limitations of imperfect and simplified data and assumptions. For example, only the energy used in delivering and treating the water supply are included and make use of relatively simple intensity estimates. In reality, these are highly variable in both space and time. The model has made simplifying assumptions about per-capita water use, outdoor water use, and water use by the agriculture sector in terms of their spatial heterogeneity, and the model is not considering individual water contracts and their seniority in terms of their water right. Additionally, the use of the simple dry and warm-dry climate projections does not consider potentially more extreme climate conditions as suggested by other climate change and paleoclimate studies (CDWR 2015; Harou et al. 2010). However, the study demonstrates within the context of a chronicled prolonged drought the merits of an integrated, disaggregated approach to explore the water-energy nexus of complex water systems.

Data Availability Statement

All of the data, models, or code that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgments

This research was partially funded by the US Department of Energy, Office of Science; A Framework for Improving Analysis and Modeling of Earth System and Intersectoral Dynamics at Regional Scales, DE-SC0016605; the US Department of Energy, US-China Clean Energy Research Center for Water and Energy Technologies; and the National Center for Atmospheric Research, supported by the National Science Foundation. The authors thank three anonymous reviewers for their comments that have helped improve the manuscript.

References

Allen, R. G., L. S. Pereira, D. Raes, and M. Smith. 1998. Crop evapotranspiration—Guidelines for computing crop water requirements: FAO Irrigation and drainage paper 56. Rome: Food and Agriculture Organization.

Abstract

Introduction

Water and Energy in California

Southwest WEAP Model

Water Supply and Hydropower Capacity in SwWEAP

Water Use in SwWEAP

Energy Use by the Water Sector in SwWEAP

SwWEAP Credibility through Historic Validation

Water Supply Delivered to California

Environmental Flow Requirements

Energy Use by the Water Sector and Hydropower Generation in California

Water and Energy in the Context of Regional Drying and Warming

Water and Energy Savings through Conservation

Conclusion and Discussion

Data Availability Statement

Acknowledgments

References

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