Note: This work is under development but has been released temporarily for classroom testing purposes. Look for the full series of TREX (Tree Ring Expeditions) labs in the fall of 2017.

Part 2 - Case Studies: Why is the Demand for Water Increasing?

In the 1920s, almost all of the water drawn from the Colorado River and its tributaries was used to irrigate crops. As time passed and as population in the Southwest grew at an astonishing rate, (Phoenix had a population of 30,000 in 1922, 106,00 in 1950, and 4.5 million residents in 2016.) demand for water increased. Besides the obvious needs of human beings, industrial concerns, and farmers who wish to use the water carried by the Colorado, other factors are at work that reduce the amount of available water in the system over time. In this activity, you will investigate what geographic and environmental factors are ramping up the demand for water in the Colorado River basin.

Instructions

1. Agriculture

One of the primary uses of the Colorado River in the arid west is for agriculture. 63% of water used in Upper Basin is for agriculture. 80% of Arizona's allocation is used for irrigating agriculture, and almost California's entire portion is for growing crops in the Imperial Valley.

In many cases, canals carry water from the Colorado River to regions in the desert where crops are growing. The All-American Canal is an 82-mile (130 km) long aqueduct, located in southeastern California. It conveys water from the Colorado River into the Imperial Valley and to nine cities. It is the Imperial Valley's only water source. The Imperial Dam, about 30 miles (48 km) northeast of Yuma, Arizona on the Colorado River, diverts water into the All-American Canal, which runs to just west of Calexico, California before its last branch heads mostly north into the Imperial Valley. Right click on the link below to take a look at the All-American canal in a new window.

The All-American Canal

The Imperial Valley and the Coachella Valley are some of the most productive agriculture areas in the world with nearly 500,000 acres being irrigated producing nearly $1 billion in crops annually. One out of every three jobs in the valley is dependent on the agriculture industry. Since 1942, the valley has received its water through the 82-mile long All-American Canal that carries water from the Colorado River in Arizona along the Mexico-California border to the California agricultural valleys.

The Imperial Valley sits in the middle of the blazing-hot Sonoran Desert, with no water-trapping mountains anywhere nearby. It receives only 3 inches of precipitation per year on average. The sole source of water in the Imperial Valley is the Colorado River. Imperial Valley's farms gets 3.1 million acre-feet annually—more than half of California's total allotment and more than any other state draws from the river besides Colorado. It's an amount of water equivalent to more than four times what Los Angeles uses in a year, according to figures from the Pacific Institute. To see what the valley used to look like, right click on the button below and open it in a new window.

Desert in the Imperial Valley

Imperial Valley churns out about two-thirds of the vegetables eaten by Americans during the winter. Major crops include broccoli, cabbage, carrots, cauliflower, and, most famously, lettuce and salad mix. Also enough alfalfa is grown to feed 350,000 cows.

2. Our Cities

There are other users that compete with farmers for their share of the river water. As less water flows down the river, competition becomes fiercer. Is there anything more important than guaranteeing water for our food supply?

Western water law is governed by the Law of Prior Appropriationis the legal doctrine that the first person to take a quantity of water from a water source for "beneficial use"—agricultural, industrial or household —has the right to continue to use that quantity of water for that purpose. Another way of putting it is first in time, first in right or first come, first served. The history of the country is told by its farmers. The landscape was a checkerboard of crops before big urban centers were built with their exploded suburbs. This means that the first water rights in the area belonged to the farmers and are still held as part of grower's cooperatives. Municipal claims for water are often susceptible to having their water cut off to assure that the farmers' claims are met first.

Western cities like Las Vegas, Nevada and Phoenix, Arizona have experienced tremendous growth in the years since the 1922 Colorado River Compact was signed. As you can see in the above graph, population in these urban centers will certainly continue in the future, adding more pressure on the Colorado River to supply drinking water to the area.

3. Energy

We can't talk about water use without addressing energy and energy production. Tremendous amounts of water is needed to produce the energy needed for cities, towns, and farms. More urban centers with increasing populations mean more of a demand for electricity. For the Upper Basin, that means more water used in extracting oil from the ground to be used for gas for cars or burned in plants for electricity. Most power plants use water as a coolant. The steam from the water is then pressurized and pushed through a turbine, which creates electricity. Whether it is a nuclear power, coal, oil, natural gas, or even thermal solar power plant, they all need water as part of the energy production process.

A 500-megawatt (MW) coal plant uses 2.2 billion gallons of water a year. To see how energy is generated in your state, go to the US Energy Information Administration site and click on your state. Read the summary below the map of resources in your state. What kind of energy production is taking place in or near your community?

Alternatives to coal power production in the area include hydroelectric, solar thermal and photovoltaic arrays. While these are clean sources of power production do not increase the impact of climate change in the area, both hydroelectric and solar thermal power plants have an ongoing demand for water.

4. Climate Change

Along with higher demands for water from agricultural, industrial, and cities in basin, climate change has affected the amount of water available for use and will have a potentially high impact in the future. The nation's two largest reservoirs, Lake Mead on the Arizona/Nevada border and Lake Powell on the Arizona/Utah border, were brim full in the year 2000. Four short years later, they had lost enough water to supply California its legally apportioned share of Colorado River water for more than five years. As of 2017, they still have not recovered.

This ongoing, unprecedented event threatens water supplies to Los Angeles, San Diego, Phoenix, Tucson, Denver, Salt Lake City, Albuquerque and some of the most productive agricultural lands anywhere in the world. It is critical to understand what is causing it so water managers can make realistic water use and conservation plans.

March of 2017 was the warmest March in Colorado history, with temperatures a stunning 8.8°F above normal. Snowpack and expected runoff declined substantially in the face of this record warmth. Clearly, climate change in the Colorado River Basin is here, it is serious and it requires attention for water mangers and users in the region.

While overuse has played a part in reduced water flow, a significant portion of the severe reservoir decline in the region is due to an ongoing drought, which started in 2000 and has led to substantial reductions in river flows. Most droughts are caused by a lack of precipitation. However, published research by Brad Udall and Jonathan Overpeck at the University of Colorado (Climate Change is Shrinking the Colorado River), shows that about one-third of the flow decline was likely due to higher temperatures in the Colorado River's Upper Basin, a direct result of climate change.

This distinction matters because climate change is causing long-term warming that will continue for centuries. As the current "hot drought" shows, climate change-induced warming has the potential to make all droughts more serious, turning what would have been modest droughts into severe ones, and severe ones into unprecedented ones.

In Udall and Overpeck's research study of the region, they found the period from 2000 to 2014 to be the worst 15-year drought since 1906, when official flow measurements of the Colorado River began. Look at the graph above. Notice that during these years, annual flows in the Colorado River averaged 19 percent below the 20th-century average. And, during a similar 15-year drought in the 1950s, annual flows declined by 18 percent. But during that drought, the region was drier—rainfall decreased by about 6 percent, compared to 4.5 percent between 2000 and 2014. Why, then, is the recent drought the most severe on record?

The answer is simple: higher temperatures. From 2000 to 2014, temperatures in the Upper Basin, where most of the runoff that feeds the Colorado River is produced, were 1.6 degrees Fahrenheit higher than the 20th-century average (see the chart below). This is why this event a called a hot drought. High temperatures continued in 2015 and 2016, as did less-than-average flows. Runoff in 2017 is expected to be above average, but this will only modestly improve reservoir volumes.

High temperatures affect river levels in many ways. Coupled with earlier snow melt, they lead to a longer growing season, which means more days of water demand from plants. Higher temperatures also increase daily plant water use and evaporation from water bodies and soils. So as it warms, the atmosphere draws more water, up to 4 percent more per degree Fahrenheit from all available sources, so less water flows into the river.

Knowing the relationship between warming and river flow, scientists can project how the Colorado River basin will be affected by future climate change. Temperature projections from climate models are robust scientific findings based on well-tested physics. In the basin, temperatures are projected to warm by 5°F, compared to the 20th-century average, by mid century in scenarios that assume either modest or high greenhouse gas emissions. By the end of this century, the region would be 9.5°F warmer if global greenhouse gas emissions are not reduced.

Using simple but strong relationships derived from hydrology models, which were confirmed by observations, scientists have calculated how river flows are affected by higher temperatures. They found that Colorado River flows decline by about 4 percent per each degree Fahrenheit increase. Given this fact, warming could reduce water flow in the Colorado by 20 percent or more below the 20th-century average by mid century, and by as much as 40 percent by the end of the century. Emission reductions could ease the magnitude of warming by 2100 from 9.5°F to 6.5°F, which would reduce river flow by approximately 25 percent.

Large precipitation increases could counteract the declines, however, that these all-but-certain future temperature increases will cause. But for that to happen, precipitation would have to increase by an average of 8 percent at mid century and 15 percent by 2100. But climate models do not agree on whether future precipitation in the Colorado Basin will increase or decrease, let alone by how much. Rain gauge measurements indicate that there has not been any significant long-term change in precipitation in the Upper Basin of the Colorado since 1896, which makes substantial increases in the future doubtful.

Megadroughts, which last anywhere from 20 to 50 years or more, provide yet another reason to avoid putting too much faith in precipitation increases. We know from tree-ring studies going back to A.D. 800 that megadroughts have occurred previously in the basin. Several new studies indicate that with warmer temperatures, the likelihood of megadroughts skyrockets in the 21st century, to a point where the odds of one occurring are better than 80 percent. So while we might have periods with average or above-average precipitation, it also seems likely that we will have decades with less flow than normal.

Planning for lower flows

It takes years to implement new water agreements, so states, cities and major water users need to plan now for significant temperature-induced flow declines. With the Southwest's ample renewable energy resources and low costs for producing solar power, the region could lead the way in reducing greenhouse gas emissions, and convince other regions to do the same. Failing to act on climate change means accepting the very high risk that the Colorado River Basin will continue to dry up into the future.

If you would like to read the full text of the Udall and Overpeck research paper, click here .