Water Woes in the Western US
The Western United States has always conjured arid images of tumbleweeds blowing through dusty towns or the stark landscape of the Dust Bowl. At the same time, cities in the Western US are booming as young professionals and old retirees flock to Nevada, Arizona, and California.
A severe drought which began in 1998 has policy planners and scientists scrambling to learn more about the past and future of Western climate changes. What they have found is a grim reality: Western water supplies are already overused, and the situation will only grow worse in the near future.
How the West Was Won, and Now Wants for Water
People living in the West have always known that water is precious, mainly due to the extended periods during which water was scarce. The Holocene-era peoples who lived in North America since the last ice age often experienced harsh droughts on the Great Plains. The Southwest's Anasazi civilization, which constructed enormous canyon dwellings with up to 500 rooms, experienced a severe drought in the thirteenth century that coincided with its collapse.
But later settlers never bothered to inquire into the fate of these people. Until recently, neither did we.
The story of water and European-American settlement of the West seems to be a series of strange coincidences. From a climate standpoint, the history of the West has always been alternating between persistent droughts and persistent wet periods known as pluvials.
One of these pluvials occurred during the first two decades of the twentieth century, just preceding the 1922 signing of the Colorado River Compact that shared out water rights among seven states. As a result, the state delegates overestimated the river flow as 22 million acre feet per year. A recent University of Arizona study has shown the average annual flow is actually 14 million acre feet. Today, water users lay legal claims to more than 17.5 million acre feet of river flow.
Another pluvial occurred during the decades between 1977 and 1998, coinciding with a population boom in the Southwest states.
"There has been a huge increase in retirement communities in states like Arizona and New Mexico," says Richard Seager, Senior Research Scientist at the University of Columbia's Lamont-Doherty Earth Observatory. "The two decades between 1976 and 1998 were the wettest since European settlement of the West began in the mid nineteenth century, and may even be the wettest in the last 1000 years (according to tree ring studies). Even a return to more normal amounts of precipitation in the West would be bad news for people there, let alone persistent drought."
This is not to say that the West has not experienced periods of drought in modern times. Perhaps the most memorable was the Dust Bowl of the 1930s Great Depression era. The agricultural areas of Colorado, Kansas, Oklahoma, Texas, and New Mexico in particular were stricken. Drought-prone wheat crops dried up from lack of water, leaving valuable topsoil to be blown away in enormous dust storms which deposited dirt as far as Washington D.C.
Despite this and other droughts, the west continues to grow. Las Vegas topped the list of fastest growing US cities in the 1990s with a whopping 83% population spurt. According to US Census Bureau figures, Western cities continue to dominate the list of cities with fastest growing populations today.
The water-intensive agriculture of the West, the largest user of water by far, is one great cause for concern. Agriculture accounts for over 90% of water usage in California alone. This prolific usage is based on the faulty assumptions of past water rights agreements and policy planners.
"Most of the water infrastructure for agriculture, hydroelectric production, and other uses has been developed during the past 50-60 years -- a period during which we have not experienced prolonged drought," notes Dave Peterson of the U.S. Agriculture Department's Forest Service Pacific Northwest Research Station and the University of Washington's College of Forest Resources.
Peterson was involved in a recent study on the Columbia River Basin, which flows through the Pacific Northwest. By measuring tree rings, he and his colleagues found evidence for six multi-year droughts between 1750 and 1950, including one which stretched for 12 years. This confirms what scientists now know from examining the paleoclimate record: fluctuating periods of extreme drought are the norm for the Western US and have been for thousands of years.
Through Rain and Snow: Water and Climate
Water in the west is all part of an ongoing climate cycle. Precipitation accumulates through snowfall in the mountains, from the Rockies to the Sierra Nevada and the Cascades. This forms the annual "snowpack" which melts in the spring, feeding the rivers that supply people and agriculture from Colorado to California.
Recent climate modeling work at NASA Goddard Space Flight Center in Maryland and at Richard Seager's Lamont-Doherty Earth Observatory has shown that tropical Pacific sea surface temperature (SST) variations are responsible for unusually wet or dry conditions in the western US. The SST variations are determined by El Nino or La Nina conditions, which are part of the ocean-atmosphere cycle in the Pacific.
Under La Nina conditions, the tropical Pacific and the entire tropical atmosphere become colder than usual. This affects atmospheric circulation and storm systems so that air is forced into a descending motion in the mid-latitudes over the United States. Since rain and snow require an ascending motion, the overall effect is a drying that can produce a drought.
El Nino conditions tend to produce the opposite effect, though there are regional differences. The Pacific Northwest tends to become warmer and dryer, while California becomes cooler and wetter. This occurs every 2-7 years, making for fairly large variability. The effect is also increased when El Nino combines with the Pacific Decadal Oscillation, a climate process which is not well understood by scientists.
The effects of these ongoing processes are evident in the latest drought afflicting the West.
"The drought that began in the West in 1998 has been interrupted this (past) winter by heavy precipitation," explains Seager. "This may be related to a weak tropical Pacific warming (El Nino event) that has been going on but it is not yet clear. We are also unsure if La Nina conditions, and drought will return in the next few months. It is too soon to say the post 98 drought is over - even the Dust Bowl decade of the 1930s contained some wet years!"
Ruby Leung, a Laboratory Fellow at the Department of Energy's (DOE) Pacific Northwest National Laboratory in Richland, Washington, has used regional climate modeling to examine the effects of greenhouse warming on conditions in the West. Under the DOE's Accelerated Climate Prediction Initiative, she published a 2004 study in the journal Climatic Change on a "best case scenario" involving a loss of water stored as snow in the Western US of up to 70 percent over the next fifty years.
"Studies have suggested that greenhouse warming has significant impacts on water resources in the western U.S.," Leung said in a recent e-mail. "These impacts, however, are mainly due to the warming that leads to reduced snowpack in the mountains. Snowpack supplies 70-90% of water resources in our region. In terms of precipitation changes, different climate models project very different scenarios. However, regardless of precipitation changes, the snowpack reduction driven by warming seems to suggest major water resources issues."
However, Leung cautioned that climate models are not so much predictive tools as ways to understand what affects climate. She also expressed hope that regional climate models are becoming sophisticated enough to incorporate more factors such as groundwater, changing vegetation and aerosol processes. Thus models will be able to address even more climate and environmental issues which affect the West.
The Once and Future West
The effects of drought mean more than residents of Colorado only being able to water their lawns twice a week. They mean the increase in risk for brushfires across Arizona and California, losses for agriculture in the fruitbaskets of the Pacific Northwest, and increasing political conflict over access to water among Western communities.
"If there is a shift to less precipitation I would imagine there will be a fight for what there is between the politically powerful agriculture industry and the numerically vast citizen population.an interesting prospect," observes Seager.
Seager pointed out that there are considerable differences in how cities use water; for instance, per head water consumption in Tucson is only half of that in Las Vegas. Efficient use of water is certainly not impossible, as nations such as Israel have demonstrated through their water conservation methods.
But while Israel has developed water conservation from the start, Seager remarked that "profligacy is the rule" in US agriculture. He suggests one possible solution of reducing agricultural subsidies which prop up US agribusiness. Without agricultural subsidies in place, agribusinesses would have to pay the "real cost" for water, which would promote more efficient water usage.
Scientists and policymakers seem to agree that the problem is more of a policy and planning issue than a scientific issue. However, this hardly means that scientific challenges are lacking.
"In my opinion, the biggest unknown factor is decadal climate variability. We know very little about the decadal oscillations, and how precipitation is affected by them," Leung says. She commented that for longer time scales in which multiyear droughts play out, new factors such as groundwater need to be represented in climate models.
Seager and his colleagues at Lamont-Doherty are interested in examining future climate model simulations involving greenhouse gases.
"Our plan is to examine how the tropical ocean temperatures respond, how that effects the locations of convective storms in the tropical atmosphere as well as tropical atmosphere temperatures. We will then examine how these impact the global atmosphere circulation and precipitation over North America. This is just beginning."