How Climate Change Is Drying up the Western US

   

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When I was a kid, I didn’t think much about where our water came from. I’d turn the tap on and fill up to my heart’s content. Later, I learned that the water which flowed so reliably through my tap was thanks to one of the most intricately engineered water systems in the US, capturing snowmelt in our mountainous regions and rerouting rivers into concrete canals and aqueducts. Water in the Bay Area? That’s from snowmelt in the Sierra Nevada. In LA? That water made its way from the snowmelt of the Rockies into the Colorado River, to then be pumped nearly 250 miles in the Colorado River Aqueduct to reach the thirsty Southern California fields and the sprawling city of Los Angeles.

Water defines so much of our lives: what we plant, where we build, how we spend our leisure time. Here in the Western US, water is precious, as its supply has always been limited. Because of this, huge amounts of time, money, and labor have been poured into increasing water availability, resulting in aqueducts and canals that bring water from mountainous regions to otherwise dry regions for agriculture or cities, and massive dams that store water for the hot and dry months.

The often dramatic history of water management in the Western US has been a central part of our story, inspiring stories like the film Chinatown. But just like the movie, our waterways have aged, and perhaps not aged well. They were designed in an era of more plentiful water. Today, we face a very different reality. In recent decades, due to increasingly warmer temperatures leading to drier years, fossil-fueled climate change has brought in an era of even greater limitations to our water.

Our biggest reservoir: the mountain snowpack

While we do get water from some local sources in the western US (e.g. local groundwater or reservoirs), around half of runoff comes from winter snowpack. In mountainous regions like the Sierra Nevada or Cascades, it can be up to 75%. Except for the US Southwest—which gets summer rains carried in by the North American Monsoon—most of our precipitation comes in the cold, winter months. In California, most of our precipitation has historically come in just a handful of winter storms, some of which carry so much water that they can act as ‘drought busters.’

This fact is crucial to understanding our water supply: when that precipitation falls in our high mountain regions, it’s cold enough that it falls as snow. As such, our winter snowpack functions as our largest reservoir. Whereas rain requires an actual reservoir to hold that water for summer months, snowpack holds onto its water content until it gets warm enough to melt. That water content, or Snow Water Equivalent (SWE), generally peaks around April 1, after which, as winter gives way to spring, snow begins to melt, running off into our rivers and streams, meaning snowpack acts as a bridge between when the moisture comes in (winter), holding onto water until it’s needed (spring-summer).

We get in trouble when our snowpack fails, as we saw this year. Across much of the western US, our snowpack was abysmal—from California to the Colorado Basin to the Pacific Northwest, we experienced our lowest to second-lowest April 1 snowpack on record. This record-low snow drought was made 4 times more likely by climate change. Our warm winter meant that precipitation came in heavy and warm, at times falling as rain on top of the snowpack. This was followed by a warmer-than-average spring, including a record-breaking heatwave, leading to the rapid melt of the already small snowpack. While this year was remarkable, spring temperatures have been warming significantly over the past two decades, with an increase in these types of ‘snow eater’ heatwaves.

Dramatic reductions in snowpack mean we have a longer window of dry conditions, with huge implications for the health and wellbeing of our ecosystems, human health, and infrastructure. We’ve seen what this means when it comes to fire. Our forests are historically adapted to plentiful water supplied by a late-melting snowpack. When the snowpack fails to accumulate, or melts early, forests can become bone-dry, ready to burn on a hot windy day. Moreover, when wet winters are followed by hot summers, dry grasses become tinder. We saw how this can play out in Spokane, Washington, earlier this month.

And a low snowpack, crucially, spells trouble for water resources. Lake Mead just broke records by reaching its lowest level ever recorded. And the hot, dry season is when agriculture, which sucks up the most water, comes online.

Our biggest water user: agriculture

I learned from a young age to conserve water by taking shorter showers and turning off the tap while brushing my teeth. While these things matter, they are such a small slice of the (water) pie: Western US water is mostly used for industrialized, irrigated agriculture, ranging from 74% of developed water in the Colorado Basin to 80% in Washington to more than 90% in California’s San Joaquin Valley. This multi-billion-dollar agricultural industry produces crops consumed from the East Coast to China to Saudi Arabia.

In fact, much of the water infrastructure across the Western US was designed to bring agriculture to regions that were historically water-poor but have plenty of sun and fertile soils. Today, however, many basins in the western US are already overallocated, operating at the tail end of what they were designed for. And when those surface water supplies fail to materialize or when local demand surges, groundwater has been used to fill the gap.

In the agricultural-intensive San Joaquin Valley, so much groundwater has been used for the past century that groundwater declined by ~12mm/year during 1962-2021, leading to dramatic land subsidence (or sinking) of up to 30 feet in some regions.

Since the onset of the megadrought in the 2000s, groundwater decline in California has accelerated. This has been in part due to lower snowpack, and in part due to greater water demand from irrigation due to warmer temperatures from climate change.

Irrigation comes online in spring with the start of the growing season, and extends into summer until harvest time. As you may remember from biology class, plants need water to photosynthesize. They ‘breathe’ in carbon dioxide, then store that carbon and emit oxygen. As part of this process, plants move water up from the soil then out through their leaves through tiny pores called stomata. On a hot, dry day (i.e. low humidity), more water evaporates into the air due to a strong difference, or gradient, between the water content on the leaf and the water content in the air. This process—evapotranspiration—is strongly related to the temperature and water content of the atmosphere. The problem is that this demand for water or ‘thirstiness’ of the atmosphere rises exponentially with temperature, in fact by 7% per degree Celsius.

To visualize this process, think of the air as a giant sponge. When the sponge is dry (i.e. the humidity is low), it sucks up water from the ground below. Warmer air can hold more water, so as temperatures increase due to climate change, that sponge gets bigger and can draw even more water out of the land surface—including plants.

Therefore, as climate change increases temperatures—especially in the warmer spring and summer months—it also increases that thirstiness, which translates to greater demand for crop irrigation. That water has to come from somewhere, and if there isn’t enough surface water being delivered from snowmelt, farmers turn to groundwater.

Climate change is largely due to the combustion of fossil fuels, including coal, oil, and gas. Ironically, in places like the San Joaquin Valley, there are oil wells peppered throughout agricultural fields. In these places, the two industries compete for water: agriculture pumps water for irrigation while oil pumps use water to extract even more oil. Rural communities bear the brunt of these impacts since residential wells are shallower than industry’s wells and hence dry up first, and these communities then breathe in and drink up polluted air and water.

The full picture

This all adds up to a water system pushed to the brink. Declining snowpack means less water is available while a thirstier atmosphere means water demand will only increase. So what do we do? This is a problem with no easy fixes.

California Governor Newsom’s proposed to expand the state’s reservoirs, but reservoirs can increase downstream flood risk during wet years, which we expect to see more of with climate change, and even dispossess tribes of culturally important lands.

Other proposals tackle multiple issues at once, such as placing solar panels over canals or in agricultural fields (“agrivoltaics”), which both reduce evaporation of water by shading the area as well as help move us away from greenhouse-gas emitting energy sources.

Efforts to pursue climate accountability from major emitters, through litigation or climate superfund laws, also play an important role in ensuring that companies that have contributed the most to climate change pay their fair share.

Any effective measures to protect our water will require systemic changes and cooperation that aim much higher than our individual efforts to take shorter showers. However, water conservation on its own isn’t going to cut it if we don’t address climate change. It’s past time for our decisionmakers to be realistic about what our changing climate means for equitable management of and access to water.

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