This doctrine has its roots in the Code Napoleon (1804) and English Common Law and has been applied primarily in states east of the Mississippi River. The basic provisions in the early 1800s were that:

1. so-called "Riparian water rights" extend to the center of a non-navigable water course;
2. navigable water courses belong in the public domain and cannot be obstructed (although it appears that access from privately owned stream banks could be denied);
3. mills or milldams could be developed by landowners with stream bank holdings and could be transferred upon sale of property;
4. beyond use for millraces, excess water could not be removed and water returned must be equivalent to that removed in quantity and quality; and
5. riparian landowners must be compensated for illegal capture of water by others. This last stipulation was interpreted by the U.S. Supreme Court (in 1827) in a way that gave riparian landowners (those with properties bordering a stream) the right to make "reasonable" use of water in a stream. However, they cannot claim ownership of the water, nor can they divert or dam a stream to the detriment of other riparian landowners.

All states (31 states) east of the Mississippi River have water allocation laws based on the Riparian Doctrine. Any waterway that can be used for navigation in its normal condition is considered navigable. If it is only used for intrastate commerce or transport, it is under control of that state. If used for interstate or foreign commerce or transport, it is under the control of the Federal government. There is no "water ownership" under the present Riparian Doctrine and principles of Reasonable Use and Correlative Rights are applied. Riparian landowners can use any quantity of water as long as it does not interfere with the rights of other landowners. They must also, therefore, share the total flow of stream water with other riparian landowners; for example, during a drought, restrictions on water extraction can be enacted to allow all owners (users) a reasonable share of the reduced flow in proportion to their ownership of stream bank property. During floods, riparian landowners can take exceptional action to protect their property, regardless of consequences for other landowners. In addition, the Riparian Doctrine is being altered in some states to allow permits to allocate water based on rates of use and other factors that can be changed by the state at any time. Courts or state water agency officials settle disputes over alleged injurious water use. The Riparian Doctrine works because water resources east of the Mississippi River are not, in general, limiting and irrigation for agriculture is not necessary.

This water law principle developed somewhat gradually in the western U.S. Many western streams had intermittent flows that were not amenable to the specifications of the Riparian Doctrine. Initially, the sparse settlement, general lack of competition for water resources, and seasonality of flow of western rivers allowed landowners to modify river channels to impound water for their use—first-come, first-served. Certainly, the Federal government did not anticipate widespread settlement of the West because it was so arid. By the early to mid-1800s, the influx of Mormon settlers in Utah required some solution to relatively sparse water resources in the face of increased agricultural activity. In response to the need and their religious principles, they established a water allocation system that favored shared use of that resource with a principle that favored beneficial use. However, the beneficial use philosophy was later replaced by that of the "Prior Appropriation Doctrine."

The Prior Appropriation Doctrine grew out of the California gold rush, and the need for gold miners to establish some system of mining claims and water use because of the limited water resources available. This is where the "first come, first served" aspect of water rights arose. California, which became a state in 1850, therefore adopted the Doctrine of Prior Appropriation that allowed diversion of water from a watercourse for use on non-riparian lands. In other words, if irrigation of crops or washing of mine tailings was required on lands with no direct stream access, these uses were permitted, with a priority (time of claim) basis. This doctrine established water rights, based on priority use, that could be sold or transferred as long as they did not interfere with another prior appropriation (" first in time, first in right" as long as this appropriation was properly filed). This doctrine prohibited "junior" (later claimants) users from using water if the resource was so limiting as to reduce that available to "senior" claimants below their allocation. Presently, the "California Doctrine" allows the application of both the Riparian Doctrine and the Doctrine of Prior Appropriation to operate (the so-called California Doctrine), depending on the availability of water resources (e.g., more water-rich northern California vs. arid southern California). Other states had somewhat different histories, but still made use of modified versions of the Doctrine of Prior Appropriation. Colorado, in particular, established the doctrine with respect to agricultural use for non-riparian lands. An interesting aspect of the Prior Appropriation Doctrine is the "use it or lose it" aspect. Once a claim is made, the water use must meet the stipulations of the claim annually, or, potentially, lose that claim. New claims relating to the expansion of irrigation, for example, are treated as "junior" claims that may or may not be honored, depending on the surface-water flow rate and other more senior claims.

Colorado, Alaska, Arizona, Idaho, Montana, Nevada, New Mexico, Utah, and Wyoming presently apply the strict Doctrine of Prior Appropriation as established in Colorado. California, Kansas, Nebraska, North Dakota, Oklahoma, Oregon, South Dakota, Texas, and Washington use the California Doctrine, whereas Hawaii applies its own version of priority depending on the water use.

Figure 3. A 38-year history of water use and source supply for the City of Los Angeles.

Source: Urban Water Management Plan, LA Department of Water and Power, 2020

The trend in total water use for the City of Los Angeles (Figs. 3 and 4) is interesting because, although the population has increased significantly since 1970, average demand has remained relatively constant between 600 and 700 million acre-ft per year and has even decreased to around 500 million acre-feet in the past few years. This is a testimony to the effects of conservation and reuse because of source limitations (competing uses, drought) and rising costs. Economic downturns may also play a role. Certainly, one way to conserve water in LA is through limiting outdoor water use (car washing, landscaping/lawns). It is estimated that watering landscaping for individual homes is about 38% of total water use. Perhaps, like Las Vegas, LA should further encourage xeriscaping and graywater use for irrigating lawns and golf courses, but more on solutions in Module 8, Part 2 next week.

Figure 4. A 50-year history of water use for the City of Los Angeles. Note that the population has grown over 1 million people during this interval (1970-2020), but the overall consumption has remained stable.

Source: Urban Water Management Plan, LA Department of Water and Power, 2010

In the mid-1800s, early settlers named the area "Las Vegas", Spanish for "the meadows", because the Valley, fed by the Las Vegas Springs, was lush, grassy, and green. The springs yielded approximately 5,000 acre-feet of water per year. As you may recall, this is about the amount of water needed today to support 5,000 families of four, or a population totaling around 20,000. With a plentiful natural water supply, Las Vegas became a key stop and hub for the railroads: first the San Pedro, LA, & Salt Lake City Railroad, and later the Union Pacific.

In the early 1900s, private wells drilled into the valley-fill confined aquifer became commonplace to augment the spring flows, as residents tried to turn the valley into productive farmland. Many of the wells were artesian but were left uncapped (Figure 7). By 1912, the 1000 residents of Las Vegas withdrew about 22,000 acre-feet of water per year from the springs and aquifer. By 1930, a combination of several dry years and increasing demand led to overdraft conditions. In the meantime, the Colorado River Compact of 1922 allocated a small amount of Colorado River water to Southern Nevada (see Sidebar: CO River Compact). However, Las Vegas continued to rely principally on groundwater, and aside from some industrial uses, the Colorado allotment went largely unused until the 1940s. (Note that Hoover Dam, the primary infrastructure that allows surface water storage and withdrawal for Clark County, was not completed until 1936.)

Figure 7. Photo of the Eglington Well in Las Vegas flowing at approximately 615 gallons/minute (ca. 1912). The well, like many others in the Las Vegas Valley, was artesian when first drilled and was left uncapped.

Source: U.S. Geological Survey Circular 1182

With a steadily growing population and water demand, withdrawals greatly exceeded natural recharge and overdraft of the aquifer worsened. In an effort to reduce groundwater extraction, the Las Vegas Valley Water District was created in 1947, in part to begin using the Colorado River allotment. Despite these efforts, by 1960 the valley’s population had swelled to over 110,000, and almost 50,000 acre-feet of water were extracted from the aquifer annually. The natural springs dried up in 1962, and sustained overdraft led the potentiometric surface to drop by a few feet per year on average. The pattern continued through 1971 until the Southern Nevada Water System began delivery of Colorado River water from Lake Mead for municipal supply – 24 years after the water district was created.

With a plentiful supply (300,000 acre-feet per year) of Colorado River Water ready for delivery and distribution, population growth accelerated, reaching almost 700,000 by 1990 (Figure 8), and about 2 million by 2012. Coincident with the shift to water supply from Lake Mead in 1971, dependence on groundwater gradually started to decline (Figure9). As discussed in more detail below, managed (induced) recharge of the groundwater system using surplus Colorado River water was begun on a small scale in the late 1980s; this “banking” of water in wet years or times of surplus is viewed as one strategy to cope with water shortages.

Figure 8. Map showing land use and the growth of the Las Vegas metropolitan area from 1972 to 2010.

Source: NASA Visualization Explorer

Figure 9. Water use in Southern Nevada from 1900 to 2008. Note that groundwater use started to decline and surface water use increased steadily from 1971 onward when the distribution of Colorado River water for municipal use began. Prior to that, CO River withdrawals were limited to a few industrial uses. Note that from around 1990 to present, withdrawals from Lake Mead have exceeded Nevada’s 300,000 acre-foot allotment (see text).

Source: SNWA water resource plan, 2009.

Currently, about 90 percent of Southern Nevada’s water comes from Lake Mead (the Colorado River) (Figure 9); the rest comes from groundwater. Because of the very limited natural recharge to the aquifer system, and the fact that no other surface water is available, Las Vegas depends almost exclusively on the Colorado River to sustain its population and economy. The city is essentially at the mercy of the Colorado River. When the Colorado River Compact was signed in 1922, the allotment of 300,000 acre-feet per year was viewed as generous for the sparsely populated state. However, as may sound like a familiar story, with a rapidly growing economy, combined with good weather and apparently plentiful water, population growth rapidly exceeded most projections (see Figure 5).

Of the water delivered by the Southern Nevada Water Authority, it may be surprising to note that most (almost 60%) goes to residential use (Figure 10). Of this, a large fraction is used consumptively for watering lawns. As discussed in detail in The Big Thirst, incentive programs for removal of turf from parks, common areas, and residences is one strategy to reduce water use. Golf courses and resorts, which are often the stereotypical poster children for water “waste” in Las Vegas, use about 14% combined.

The pie chart shown in Figure 10 provides the first blueprint for conservation efforts and potential re-use, by identifying the key water uses in the district. Moreover, there is also a recognition that not all water uses are “equal”: some require clean water (i.e. residential uses, many industrial uses, medical), whereas others do not (golf courses, parks). As a result, reclaimed and partly treated water may be used for many needs. In Las Vegas, water re-use – essentially getting two uses of the same water - is one part of a diverse strategy to maximize the limited allocation of Colorado River water (additional detail on treatment facilities and pricing for reclaimed water are described on the water district’s website.

Figure 10. Municipal water uses in Southern Nevada as of 2022.

Click for a text description

Residential (single-family): 43.3% Residential (multi-family): 16.3% Commercial/Industrial: 14.4% Golf Courses: 5.1% Resorts: 6.3% Common areas: 6.5% Schools/Government/Parks: 6.3% Other: 1.8%

Source: SNWA water resource plan, 2022.

Due to a decades-long drought in the Colorado River system (see Sidebar: CO River Compact), the water level in Lake Mead has dropped by almost 170 feet since 2000 (Figure11). This corresponds to a decrease from ~25 million acre-feet of stored water to around 10 million acre-feet. If the lake water level drops to 1075 feet (as of June 2022, it is 1043 feet!), a federal shortage would be declared, triggering a reduction in Nevada and Arizona's allocations. In June of 2022, the U.S. Bureau of Reclamation decarded an emergency request for Colorado River states to reduce use by 2-4 million acre-feet within 18 months.To make matters worse, the two intakes in Lake Mead that withdraw water for Las Vegas cannot function if the lake level drops below 1050 feet (intake #1) or 1000 feet (intake #2). With the possibility of continued dry conditions, and because of their near sole dependence on Colorado River water, Las Vegas has developed a multi-pronged strategy to hedge against uncertainty due to future climate change coupled with likely increased demand due to growth and development in Clark County.

Figure 11. Photo of Lake Mead, facing upstream, taken from the Arizona side of Hoover Dam (ca. 2009). The white “bathtub ring” is caused by bleaching of the rock, and marks the previous high water level approximately 140 feet higher than today.

Source: Demian Saffer

As you have read about in The Big Thirst: Dolphins in the Desert, Las Vegas has been aggressive in water conservation efforts. Part of these efforts focuses on simple reductions in household water use through education, regulation (i.e. watering restrictions), and incentivized removal of water-intensive landscaping. The city has also implemented GPS technology and pressure and acoustic sensors to monitor leaks in their pipelines to limit leaks and thus maintain high efficiency. As a result of these efforts, per capita, water use in Las Vegas has decreased substantially over the past 20 years or so, from over 340 gallons per day to less than 200 gallons (a 40% reduction!) (Figure 12). The SNWA has set a conservation target of 105 gallons per day fro 2035. As a result, Southern Nevada's total annual water use dropped by almost 90000 acre-feet (30 billion gallons) from 2002 to 2012, even as its population grew by 400,000.

Additionally, as noted above, Las Vegas treats wastewater for re-use, especially for applications that (a) don’t require high-quality water, like watering golf courses and parks; and (b) are consumptive. Re-use, incentivized by lower pricing, effectively allows the same water to be used twice, thus making the modest allotment of Colorado River water go further. Indeed, although Southern Nevada’s gross withdrawals from Lake Mead are almost 600,000 acre-feet per year (Figure 9), this is offset by the return of treated water to the Lake such that net withdrawals (consumptive use) remain at the 300,000 acre-feet limit.

Figure 12. Historical (blue) and projected (red) per capita water use in Southern Nevada.

Source: SNWA water conservation plan, 2014-2018.

Despite a history of overdraft in Las Vegas itself, Southern Nevada has recently turned its eyes back to the underground as an additional water source – but this time in sparsely populated valleys to the North and Northeast of Clark County (Figure13). The rationale for the SNWA’s “Groundwater Development Project” is that groundwater recharge is partly a function of the area over which infiltration occurs, so distributed withdrawals of groundwater from several large valleys fill aquifers outside of Las Vegas may be more sustainable than focused withdrawals from only the local aquifer system. Additionally, the targeted aquifers are in sparsely populated areas, with relatively small water demand.

Nonetheless, as you might imagine, there has been strong opposition to the plan from both environmental groups and ranchers and residents of these valleys, especially when considering past examples of the annexation of water rights for large cities (e.g., Los Angeles and the Owens Valley) and the negative outcomes for the local communities.

Figure 13. Map showing regional groundwater flow systems in Nevada and Utah.

Source: USGS Water Sources of the Basin and Range

As another hedge against water shortage and climate change, the Southern Nevada Water Authority has entered into a series of “Water Banking” agreements with other the Lower Basin Colorado River states, Arizona and California. In these agreements, Nevada pays the other Colorado River water rights holders to store unused water in times of surplus by injecting it into aquifers. Nevada then receives credits for the stored water; if the water is needed, Nevada uses the credits to draw the equivalent water from Lake Mead, and in exchange, the “banker” withdraws the same amount from the aquifer. Although pumping is energy-intensive, groundwater banking does not require the construction of large reservoirs, and the water is not subject to large evaporative losses.

In its water banking agreement with Arizona, the SNWA paid \$100 M initially and began making yearly \$23 M payments in 2009 that will continue indefinitely. The agreement allows the SNWA to withdraw up to 40,000 acre-feet per year. In 2004, SNWA also began a water banking agreement with the Southern California Metropolitan Water District (the water district that serves L.A.) in which some of Nevada’s surplus Colorado River water is stored in an aquifer in Southern California. The agreement allows the SNWA to withdraw up to 30,000 acre-feet per year, provided that they give 6 months notice. Since 1987, Southern Nevada has also been banking its own surplus water – when available - in the valley’s aquifer for later use if needed. In Nevada, about 333,000 acre-feet have been stored through 2022, and in Arizona’s aquifer, the SNWA has stored 614,000 acre-feet of the Colorado River’s water through 2023.

In 2005, faced with the specter of prolonged drought and projected Lake Mead water level declines, the SNWA board of directors approved construction of the so-called “Third Straw”, a new $812 M intake from Lake Mead that would allow Southern Nevada to physically extract water from the lake at water levels as low as 1000 feet above sea level (Figure 14). Construction of the intake involves boring a 23-foot diameter tunnel through 3 miles of rock, with much of its length beneath one of the Earth’s largest man-made reservoirs!

The new intake will intersect the lake at 860 feet above sea level but will share a pumping station with intake #2, so will only be able to operate at water levels of 1000 feet (the same as for intake #2). The primary purpose of the third straw is to maintain overall system capacity if Lake Mead falls below the 1050 ft water level limit for operation of intake #1. It also will access the deepest parts of Lake Mead, where water quality is highest. The initial plan for the third intake included a separate pumping facility but was removed to cut costs. It is always possible that the $200 million pumping station and pipelines could be added in the future, though if the Lake Mead water level were to drop much below 1000 feet, there would be much bigger problems throughout the lower Colorado River basin.

Figure 14. Figure showing projected water levels in Lake Mead as of 2023, with levels shown for federally declared shortage and operational limits of the SNWA intakes.

Source: SNWA Water Resource Plan, 2023.

Figure 14 shows the elevation of Lake Mead on the y-axis versus the year on the x-axis including different water conservation projects. The important take home from this figure is that as we near 2022 we see that water levels drop. But as different conservation projects (in green, pink, purple, etc.) grow, the rate at which water levels drop decreases. In the time series, the thick dashed line represents the hypothetical elevation of Lake Mead due without conservation projects, while the solid black line represents the actual water level. Although water level is still dropping as of 2022, conservation efforts play a large role in stabilizing Lake Mead water levels.

Just about anyone could do climate science. Agencies, particularly in the US and Europe, have made an immense amount of weather and climate data available and with a modest amount of training and software anyone could perform rudimentary analyses of temperature or precipitation trends (e.g., see ncdc.noaa.gov or weather.gov or prism.oregonstate.edu). Of course, such analyses don’t answer all the questions. Tens of thousands of highly trained, independent scientists around the world collect and analyze climate data and develop models of global or regional climate change, which are typically tested using historical data and projected into the future. To provide a forum for discussion and debate that could be synthesized to represent our best understanding of climate change, the United Nations Environment Program (UNEP) and the World Meteorological Organization (WMO) established the Intergovernmental Panel on Climate Change (IPCC) in 1988. Thousands of scientists contribute data, analyses, and model results to the IPCC and provide critical peer review of any climate-related research, all on a volunteer basis. Six major assessment reports have been generated by IPCC, with the most recent report released in 2023.

While it is not our intent in this module to explore this question in detail, it is worth pointing out that many human activities clearly affect the climate system. Most notably, emissions of greenhouse gases, especially carbon dioxide and methane, are causing more heat to be trapped within Earth’s atmosphere. This effect, called the greenhouse effect, has been well understood since it was discovered by Svante Arrhenius in 1896. Figure 1 below, taken from the 2023 IPCC Working Group 1 Technical Summary shows the relative amount of heating or cooling of the climate system that can be attributed to the various factors that have changed between 1750 and 2019. The anthropogenic modifications to the climate system, enumerated in panel (a) of the figure, show the breakdown of radiative forcing. Anthropogenic forcing greatly outweighs the changes due to natural changes in solar irradiance. Panel (b) shows the effect each emitted component has on global surface temperature. The IPCC is quite careful to note the level of confidence associated with any given piece of knowledge, seen here with the black error bars of 5-95%. They are also transparent and are quick to point out when new understanding has significantly changed estimates or predictions, as has happened with our understanding of stratospheric water vapor, which was thought to be a significant contributor to warming in the Fourth IPCC Assessment Report (AR4, released in 2007), but has been found to be less significant.

Figure 1. (IPCC Figure TS.15) Effective radiative forcing (ERF) of climate change since the Industrial Era. Panel (a) shows the various radiative forcing emitters and their effect on global surface temperature, seen in panel (b). Black bars represent uncertainty in the data.

Source: IPCC, 2019

So what does all this human-induced warming mean for the water cycle and water availability? Thinking back to module 2, you learned that warmer air can hold more water (i.e., warmer air has a higher saturation vapor pressure). Therefore it is reasonable to expect higher amounts of water vapor in the air. This is supported by observations that show a 3.5% increase in water vapor in the past 40 years as the climate has warmed about 0.5°C, with relative humidity remaining approximately constant.

Changes in precipitation are harder to measure (or predict) compared with changes in atmospheric water vapor content because of the immense temporal and spatial variability of precipitation. Nevertheless, patterns of precipitation change can readily be observed from historical records (Figure 2), with many areas seeing increases greater than 25 mm/year per decade (i.e., going from 300 mm/yr to 325 mm/yr over the course of a decade) and other places (particularly Africa and Southeast Asia) seeing decreases in precipitation at rates greater than 10 to 25 mm/year per decade. With increasing temperatures, it naturally follows that a greater proportion of precipitation would fall as rain, rather than snow, which has also been documented by the IPCC.

Figure 2. (IPCC Chapter 2, Figure 2.15) Maps of observed precipitation change over land from 1901-2019 (panels (a) and (b)) from the Climate Research Unit (CRU, top), Global Precipitation Climatology Center (GPCC, middle). Map of observed precipitation change over land from 1980-2019 (panels (d) and (e)) from same sources. Panel (c) shows global land precipitation anomalies and panel (f) shows globally complete precipitation data from the Global Precipitation Climatology Project (GPCP). More information is found in the supplemental section of chapter 2.

Source: IPCC, 2019

Salt Lake City: A case study in water development history and planning for the future

Salt Lake City (SLC) provides an interesting case study in terms of the history and future of water resource development. The first permanent settlers of Salt Lake Valley arrived in 1847 and immediately began diverting water from City Creek (northernmost of the four watersheds highlighted in blue in Figure 5). It is estimated that the early settlers hand-dug 1000 miles of ditches in the first few decades to distribute the water to agricultural fields, Salt Lake City and nearby settlements. By 1879 the population of Salt Lake County had grown to nearly 32,000 and the city authorized construction of the Jordan and Salt Lake City Canal, which was completed in 1882 with a capacity of 150 cubic feet per second, expected to provide enough water for 100,000 residents. The canal is still in use today. Several major dams were constructed as early as 1892 to 1907. Following a major water shortage in 1924, Mayor John Bowman proclaimed that ‘a city can never be greater than its water supply’ and initiated an ambitious water development program to supply reliable water for more than 400,000 residents. Several other large dams were constructed from the 1940s to as late as the 1990s to keep ahead of the rapidly growing population, but options for additional water storage via new reservoirs are now very limited.

Today Salt Lake City’s water supply is derived from several mountainous watersheds to the east of the city, in the Wasatch Front and western Uinta Mountains (Figure 5). About 50-60% of the water is derived from the four creeks just to the east of SLC (highlighted in blue), with the remaining portion delivered from the Weber, Provo, and Duchesne rivers via inter-basin transfers (tunnels, canals, and aqueducts shown as blue and white dashed lines in Figure 5) and extracted from groundwater. Around 70-80% of Salt Lake City’s water supply originates as snowmelt. Thus, the storage of water as snowpack, the timing of snowmelt, and water storage capacity within the system are all critical to ensuring reliable water supply.

Figure 5. Map of SLC water supply basins, including four proximate drainages (highlighted in blue), which comprise 50-60% of water supply and three more distant drainage basins (orange, pink, salmon), which deliver water to SLC via interbasin water transfers (blue and white dashed canals, tunnels, and aqueducts).

Figure from Open Access journal article: Bardsley, T., Wood, A., Hobbins, M., Kirkham, T., Briefer, L., Niermeyer, J., & Burian, S. (2013). Planning for an Uncertain Future: Climate Change Sensitivity Assessment toward Adaptation Planning for Public Water Supply. Earth Interactions, 17(23), 1-26.

Public utilities water use has remained relatively steady at 80,000 acre-feet of water per year since 1980. To put that number in perspective, imagine a tank of water an acre at its base and 80,000 feet (15 miles) tall, or the equivalent of a tank the size of Central Park in New York City flooded 100 feet deep. The fact that total public water use has remained steady over the past three decades is an impressive feat considering the population of Salt Lake County has nearly doubled from 620,000 in 1980 to nearly 1.1 million in 2014. Much of the Greater SLC area is populated by members of the Mormon religion, which has traditionally emphasized large families. More recently the size of families has decreased, but the population as a whole continues to grow.

Despite a growing population, total water use has started to decline in the past decade despite the fact that this time period includes three of the hottest summers on record, due to effective public education and water conservation campaigns (Figure 6).

Figure 6. Comparative water use trends by sector, measured in 100 cubic feet.

Source: Salt Lake City Department of Public Utilities 2009 Water Conservation Master Plan.