Population and Water Use

Who uses the Colorado River’s waters now? Who will in the future? Population and demographics change over time. These changes can significantly influence future water use and demand. The Colorado River Basin itself is rather sparsely populated. The Upper Basin contains an average of 10 people per square mile, and the Lower Colorado contains about 75 persons per square mile. Most people relying on Colorado River water live outside the Basin, served by trans-Basin diversions Definition such as canals, aqueducts, or pipelines. The Colorado River Aqueduct service area, for example, which contains both Los Angeles and San Diego, has a population density of 3,200 persons per square
mile,¹ though they get municipal water from a variety of sources in addition to the Colorado River. The interactive map shows a satellite image of the CRBAAS Definition at night and displays the population distribution and settlement patterns in the West.

Population growth is among the most important factors influencing future water demand (Boberg 2005 Citation). Therefore, understanding how the population relying on water from the Colorado River Basin has changed and is predicted to change is critical to understanding future water needs. In 2012, the US Bureau of Reclamation published a Supply and Demand Study for the Colorado River Basin. It stated that “about 40 million people living in seven western states within the hydrologic boundaries of the Basin plus adjacent areas outside the Basin rely on the Colorado River or its tributaries for some, if not all, of their municipal needs.” This number has been repeated countless times over the last decade; but there are several reasons to examine it more carefully and determine how much that number has changed in the past 10 years.

Wsing the WorldPop 2020 gridded population dataset, we estimated that the population of the CRBAAS study area differs slightly from the Bureau of Reclamation’s 2012 Colorado River Basin and adjacent areas supplied with Colorado River Water study area². We found that the population increased from 39.7 million in 2015 to 43.5 million in 2020, an increase of about 9% across the CRBAAS.³ However, determining how many of those people rely on or rely solely on Colorado River water is complicated. For example, the Metropolitan Water District of Southern California (MWD) serves about 1.5 billion gallons of water per day to about 19 million customers in 26 member water agencies.⁴ Colorado River water enters the MWD system through the Colorado River Aqueduct and comprises about 25% of the MWD’s water supply, though significant areas and populations (perhaps up to six million people) within the MWD service area do not receive Colorado River Aqueduct water and utilize a combination of local sources and water from California’s State Water Project. In Utah, water from the Duchesne and Strawberry Rivers, tributaries to the Colorado on the west side of the Wasatch Range, are diverted into the Great Salt Lake watershed via tunnels transferring water into the Provo and Spanish Fork Rivers. The Colorado River contributes a small amount to the water supplies in the Salt Lake City metropolitan area, but it’s most important during drought years. The same is true for the public water supplies of Denver, Fort Collins, Colorado Springs, and other communities on the Front Range Definition of Colorado, which use a mix of local sources (the South Platte and Arkansas Rivers and their tributaries) and imported Colorado River waters, with lesser contributions from the latter. The amount of water diverted into the adjacent areas changes depending on a variety of factors, so gauging how many people use water from the Colorado River Basin at any given time is very difficult. Conservatively, the highest number of people relying directly on water from the Colorado River Basin is 43.5 million. The population of the CRBAAS is significant because it’s the number of people whose communities would be most acutely affected by Colorado River Basin water shortages.

Far more than 43.5 million people living in the CRBAAS rely indirectly on the Colorado River via embedded water Definition (i.e., water used to grow food, feed and raise livestock, generate electricity, and produce commodities). Only about 12% of Colorado River water is used for domestic water supply, while 79% irrigates crops that comprise about 15% of U.S. food production (Richter et al. 2020 Citation; Solander et al. 2018 Citation). Roughly 4% of Colorado River water is used to generate power (Richter et al. 2020 Citation). The food, energy, and commodities produced with Colorado River water flow across the CRBAAS boundary and wind up not only across the nation but also in global supply chains. To understand more about the inflows and outflows of water, and water embedded in commodities, energy, and food, we encourage you to explore Northern Arizona University’s FEWSION project and its associated tools. FEWSION data has been used for a variety of applications including examining supply chains’ resilience and vulnerability to disruptions and natural disasters.⁵

On a finer scale, the fastest population growth from 2015 to 2020 occurred in the Lower Basin of the Colorado River, where the population increased by 16%. Of our 37 indicator communities, the unincorporated town of Enterprise in Clark County, Nevada, experienced the most rapid growth when its population more than doubled from around 100,000 to 220,000 in 10 years (2010–2020). Other communities grew by more than 15% in that decade, including Queen Creek, AZ (122%); Farmington, NM (48%); Flagstaff, AZ (17%); Mesquite, NV (34%); Buckeye, AZ (39%); and Avondale, AZ (17%). The Upper Basin had several very fast-growing communities as well, including St. George, UT (31%); Durango, CO (13%); Grand Junction, CO (11%); and Vernal, UT (11%), though the Upper Basin grew slower on average than the Lower Basin or the Adjacent Areas Served (AAS Definition). The lowest population growth (3% from 2015 to 2020) occurred in the area served by the Colorado River near Cheyenne, Wyoming. For a detailed look at the population change in different parts of the CRBAAS, examine the interactive dashboard below.

Understanding future water use and demand requires understanding if the growth of 2015–2020 is expected to continue. Projected growth rates for counties within the CRBAAS are broad—while some populations are expected to decrease, at least nine counties are expected to grow by 20% to 40% in the next eight years, nearly all of them in the Adjacent Areas Served in Utah and Colorado: parts of the Salt Lake City metro and the Colorado Front Range Urban Corridor. Assuming a middle-of-the-road growth scenario, known as shared socioeconomic pathway 2 (SSP2) Definition, the combined population of CRBAAS counties in the United States is expected to grow by an average of about 10.4% in the next eight years,⁶ reaching 47.6 million by 2030 and 51.6 million by 2040 (19.6% increase relative to 2020 numbers) (Hauer 2021 Citation). This means more people will rely on the shrinking supply of Colorado River water. Many communities are getting ahead of this curve, however, by working with the Sonoran Institute, Western Resource Advocates, the Babbitt Center, and other partners to push toward Growing Water Smart.

Typically, water is purchased, consumed, and billed by households, not by individuals. Therefore, the number of households and individuals per household are important considerations for outdoor water use. Per capita water use Definition has been shown to decrease as household size increases, though total household water use increases with household size (Lenzen 2002 Citation). Certainly, in larger households, more people will be taking showers, flushing toilets, and cleaning dishes, but they will also be sharing irrigated outdoor spaces, reducing the per capita use. Importantly, outdoor water use is consumptive Definition and on average represents 50% of residential water use. Therefore, we are tracking the size and number of households in the CRBAAS through time.

The total number of households in CRBAAS counties ⁷ is projected to increase from 14.5 million to 15.2 million, an increase of about 5%, which is much higher than the national prediction of just 0.8%, from 124 million to 129 million households, from 2018 to 2023.

Average household size (2.6 people)⁸ for counties in the CRBAAS is projected to change minimally between 2018 and 2023. This is quite close to the national average of 2.5 persons per household, which is also expected to change minimally from 2018 to 2023. Of the 120 counties in the CRBAAS, 20 are expected to have mean households of more than three people in 2023, and 10 of those 20 are in Utah. All 20 of those counties have median ages lower than the national median (38.1), suggesting they are home to younger families. While the size and number of households will remain an important indicator of water use, one of the most popular water use metrics is gallons per capita per day
(GPCD Definition).

Per capita water use declined between 2005 and 2015 in all Colorado River Basin states, from an average of 223 GPCD in 2005 to 179 GPCD in 2015.⁹ The Basin states all had higher per capita use than the national average of 82 GPCD in 2015, partly due to the region’s aridity. Wyoming and Utah had the highest per capita water use in 2015, as shown in the tables below. One interesting trend is the decrease in water use in Nevada, which had the highest per capita water use in the CRBAAS in 2005 and was in the middle of the pack by 2015. This may be driven by decreases in per capita use (from 335 to 203 GPCD) in Clark County, where most of Nevada’s population lives and where strict water conservation measures have been put in place to meet the goal of reaching 86 GPCD by 2035 (SNWA 2024 Citation). The US Geological Survey’s anticipated 2023 release of updated water use estimates through 2020 will help us determine whether these trends will continue or if new ones will emerge.

Per capita water use declined in 79 out of 120 counties in the CRBAAS between 2005 and 2015, as indicated in the table Estimated Water Use by State above. The most significant declines were in Piute County, UT, and White Pine County, NV, which declined from 1,275 to 525 GPCD, and 679 to 129 GPCD, respectively. However, as these two examples illustrate, while per capita water efficiency has improved in many CRBAAS counties, most are still well above the national average of 82 GPCD. The dry climate has pushed CRBAAS’ GPCD higher than levels in the rest of the US, since irrigation is required to maintain nonnative outdoor vegetation. Yet, within the region, some counties are much closer to the national average than others. The differences generally follow the rural-urban distribution. The counties with the highest per capita water efficiencies contain large, relatively dense urban areas such as the cities of Las Vegas in Clark County, NV; Phoenix in Maricopa County, AZ; and Denver in Denver County, CO.

Summary

While population is fundamentally linked to municipal water demand, the relationship is not always linear. Aggressively implemented conservation measures can decrease water use even while the population grows. In fact, a study of large cities in the Colorado Basin showed that states reduced their total water use while increasing their population. For example, total municipal water use in Albuquerque decreased by 14% and per capita use fell significantly (216 GPCD to 121 GPCD) between 2000 and 2020, while the city’s population increased by 26% (from about 450,000 to 560,000). Other cities, such as Phoenix, had significant population growth (22%), and per capita use decreased from 215 to 155 GPCD. However, total municipal water use in Phoenix increased by about 5% during that time (Colby and Hansen 2022 Citation), showing that although conservation measures can effectively reduce per capita water use, sustained growth still raises total water use. These kinds of trends will play out differently from place to place because of variables such as water availability and price, income, climate, cultural values and attitudes, landscape ordinances and zoning, and local conservation efforts. Water is typically purchased and used by households, rather than individuals, and the number and makeup of those households can make a significant difference in water use and demand. We will continue to monitor changes in population, growth rates, and water use across the Basin and expand our indicators in future reports.