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Scientific American -
"A masterful portrait of how water shaped the American West. . . . Part detective story, part call to action, this book offers vital advice on how to fix the West's looming water crisis."
From Drought to Deluge
"NORMAL" CLIMATE IN THE WEST
And it never failed that during the dry years the people forgot about the rich years, and during the wet years they lost all memory of the dry years. It was always that way.
JOHN STEINBECK, East of Eden
HUMANS TEND TO PERCEIVE CLIMATE AS A FORCE of nature, one to be measured, classified, and ultimately conquered. John Steinbeck's quote speaks to another human characteristic: the tendency to forget. This seems especially true when it comes to the periods of wet and dry over the past century or so in the American West.
Perhaps, however, we can be sympathetic to this tendency when we realize that to "measure the climate" at any given point is really a measurement of the current weather. To understand complex climatic patterns requires a much longer timescale. Thus, in our lifetimes, we experience only a small sampling of climate effects—not the full range—in the West. Imagine a performance of a Beethoven symphony with only the first chords, played over and over again. If that were all we heard of the symphony, we would believe it was beautiful—but simple. We could hum along and predict what would come after the initial chord sequence. However, as we know, Beethoven wrote complex and intricate symphonies, often full of surprises. These may be pleasurable in musical compositions, but surprises in climate can have catastrophic results for societies. We can remove the element of surprise by understanding as much about our climate as possible.
In this book, we focus primarily on those aspects of climate that have an impact on water resources in the West. To further our understanding of those resources, imagine "flying over" North America using Google Earth. Traveling from east to west, we see obvious contrasts: shades of green that dominate the eastern portion of the country abruptly give way to vast areas of brown, tan, and yellow. The color contrasts are due, of course, to moisture differences. The humid East supports more forests and, moving westward, wide prairies. Western North America is more constrained by water availability, especially in the southwestern deserts.
If we zoom in a bit closer and fly across this western region from west to east, we begin to see a relationship between the lay of the land and the colors of the land—a relationship that is largely based on water. Passing above the Pacific coast, we observe mountains, especially along the northern coastline, with lush rainforests that give way quickly to grasslands and then to deserts. Within this region, it is clear where the water falls: the Pacific Northwest and Northern California appear deep green, as does the central and southern Sierra Nevada mountain range (see figure 2). Much of Southern California and the states of Nevada, Utah, Arizona, and New Mexico appear in varying shades of brown, tan, and red, reflecting the vegetation and the composition of the soils—both related in large part to the lower amounts of rain and snow that fall there each year (see figure 3).
Precipitation over the American West is spatially irregular because it depends on the complex behavior of the Pacific Ocean and its exchange of water and energy with the atmosphere. The atmosphere receives moisture from the ocean through the process of evaporation, and some of that moisture is eventually carried away from the ocean and delivered on the continent as rain or snow. Just how and when that evaporated seawater drops as rain or snow over the West depends on the patterns of ocean currents and atmospheric winds and on the interaction of moisture-laden air with the landscape as it advances inland.
Weather satellite images of the West Coast vividly show the movement of the atmosphere over the Pacific Ocean: large, counterclockwise swirls of white, moisture-laden air that drift toward the coast and finally over the land. The topography of this region, especially the high mountain ranges, disrupts this flow of air, forcing it to rise up the slopes. As the air moves to higher altitudes, the moisture condenses, because rising cooling air cannot hold as much water vapor. The major mountain chains of the West—the Cascades in the northwest, the Sierra Nevada in California, and the Rockies farther east—are oriented north-to-south or northwest-to-southeast, which are the ideal orientations for intercepting the Pacific winter storms that flow from west to east. As these storms are forced up and over these mountain chains, much of the moisture is wrung out on the windward slopes, leaving the leeward slopes and places to the east relatively dry (see figure 4). This explains the relatively lush western slopes of the mountains in the West compared to the drier and sparsely vegetated eastern slopes and valleys beyond (see figure 5). In this way, the landscapes of the American West both have an influence on the region's weather and are formed by it.
California spans nearly ten degrees of latitude, which results in a wide spectrum of natural plant communities, determined largely by the rainfall regime. Storm systems entering California first encounter the Coast Ranges and then pass through the Central Valley, move up the high mountain ranges of the Sierra Nevada, and continue into the deserts of the Great Basin. Northern California is the southern extension of the lush temperate rainforests of the Pacific Northwest, whereas Southern California is part of the deserts of the semiarid to arid Southwest. Nevertheless, California's moisture regime follows strict—if somewhat unusual—patterns. The Department of Geography at the University of California, Berkeley, has a long history of researchers interested in climate, including Jim Parsons, who, in a personal communication with this book's authors, noted that the state's climate is unique because of its arbitrarily conceived boundaries, which outline the only area of winter rain and summer drought in North America. This so-called Mediterranean climate—warm, dry summers and mild, wet winters—is a common occurrence on the west coasts of continents at about 35 degrees latitude. During the summer months, the atmosphere over California is dominated by the Pacific High, an area of warm, dry air that prevents storms from entering the state. Thus, California's climate is characterized by two main seasons: one wet, the other dry.
As described above, Pacific storms in the winter drop much of their rain and snow along the western mountain slopes, concentrated in a relatively short wet season. In fact, California has the highest year-to-year variability in precipitation of all the western states. This variability is the result of a relatively small number of Pacific storms—on average five or six—that enter the state between the late fall and early spring. California's precarious water situation is apparent in the fact that a shortfall of just one or two major storms can mean the difference between a "normal" water year and a dry year. As a result, water management is particularly challenging in this state.
In addition to the year-to-year variations in precipitation, of course, California's climate changes on longer timescales, from decade to decade, century to century, and millennium to millennium. The patterns of these longer-term variations are discussed in more detail later in this book.
THE IMPORTANCE OF SNOW
In the higher mountain elevations of the West, precipitation often falls as snow. This snow provides a crucial amount of water, filling streams, lakes, and reservoirs to be tapped during the long, dry summer. Across the West, the melting of the winter snowpack provides a significant proportion—50 to 80 percent—of the annual water supply for ecosystems as well as human consumption.
Water managers throughout the West rely on early forecasts of snowpack to plan for each water year. These forecasts are based on measurements of the mountain snowpack, obtained by hardy snow surveyors who head out on skis into the mountains to determine how much meltwater will be available in the coming year. Each year in California, over 300 snow courses are checked, each 1,000 or more feet long with measurements made every 100 feet on average. The practice has been a part of the state's water management program since the early 1900s, and the data collected provide valuable planning information for farmers, utility companies, municipal water managers, and flood control managers.
During the 1950s, 1960s, and early 1970s, another researcher in the UC Berkeley Department of Geography, Douglas Powell, spent his winters on skis taking part in the annual state snow surveys. Clear, sunny fall days may be welcome to most Californians, but Powell was always happier when the storm clouds arrived early, heralding a wet year that would replenish the state's water supplies. He would often remind his summer-session students, with a hint of wistful optimism, that storms—even thunderstorms—could arrive as early as September. Moreover, autumn blizzards in the Sierra Nevada range are not unheard of: in 1846, a series of October blizzards dropped ten feet of snow, trapping the Donner Party during their ill-fated trek across the Sierra.
Few of Powell's colleagues or students (this book's authors among them) have experienced the linkages between landscape and climate as intimately as he did. For several weeks each year, he and his team strapped on cross-country skis and backpacks loaded with log and field notebooks, pens, and special snow collection tubes to determine the depth, water content, and density of the snow he would trek across each day. They would measure the water content of the snow by filling a special tube with snow, weighing it, and weighing the empty tube. One ounce of snow translates into one inch of water content of the snow, so the snowpack can be converted into the amount of water that will be generated when that snow melts (see figure 6). Powell could think of no better way to spend his winter months than in the high Sierra Nevada, measuring the precious gift that the mountains held in reserve for the long, dry summer and fall months of the West.
Snow surveyors are still important today, but, since 1977, a considerable amount of data is collected by a network of instruments that have been installed across the mountains of the West to forecast water supply. These instruments automatically transmit the snow and climate information they measure to a centralized computer system by radio telemetry. Changes in daily snowpack are monitored, along with temperature, precipitation, stream flow, reservoir and groundwater levels, and soil moisture—all part of an early warning system for floods and droughts in the western United States.
As we will discuss further in chapter 13, studies of changes in the snowpack over the past century reveal that the average snowpack has steadily decreased by about 10 percent over that period, most likely due to warmer temperatures. Climate modeling predicts that, by the end of the twenty-first century, the snowpack could shrink by another 40 to 80 percent.
CLIMATE: THE EXTREMES
Scientists began closely monitoring the West's climate after the nineteenth century, recording the region's weather variables: temperature, atmospheric pressure, precipitation, and wind speed and direction. These measurements provided monthly and yearly averages, defining the region's climate. Climatologists have also examined extreme deviations from the mean in order to understand the causes of these deviations and perhaps learn how to predict them. The measuring and monitoring of climate over the past century has allowed climate scientists to assemble the pieces into a larger picture that begins to explain what drives climate, including the critical interactions between the Pacific Ocean and its overlying atmosphere. As geographically varied as the West is, climate in this region has been fairly consistent over the past 150 years and, by and large, suitable for human habitation. This period of time would seem to be long enough for us to know what to expect from our climate, both in normal years and in "extreme" years. However, as we shall see later in the book, this is not the case.
Extreme events have struck every few decades over the past century and a half, some with catastrophic results. Even more extreme events have occurred with some regularity in the distant past, though the intervals between them often lasted a century or more. Understanding the causes of these events and how they affected the West in the past can help current residents anticipate the effects when such events recur in the future—which they surely will.
On one end of the climate spectrum is drought, when the wet season fails partially or altogether. Nothing brings more fear to the hearts of water planners in the American West than drought. Droughts lack the visual and visceral images of other natural disasters; instead, they creep up on the landscape with no clear beginning or end. Meteorologists define drought rather vaguely as an abnormally long period of insufficient rainfall that adversely affects growing or living conditions. Such a bland definition belies the devastation wrought by these unique natural disasters. In human history, whole civilizations, even while at their peak, have toppled in the face of prolonged drought. For humans, drought means failed crops, desiccated landscapes, water rationing, decimated livestock, and, in severe cases, water wars, famine, and mass migration.
For the more distant past, drought is defined in relative terms, such as when precipitation, runoff, and lake levels fall below the long-term average by at least one standard deviation, or when the growth rings of trees become narrower than average by a certain amount. In drought-prone regions, native plants and animals usually employ adaptations to survive through periods of lower than average precipitation; such adaptations have evolved over very long periods of time. Humans in the West today, in contrast, have flourished only by manipulating the natural hydrology using modern engineering and technology.
The Driest Single Year in the West: 1976–1977
The worst single year of drought in recorded history over much of the American West occurred during the winter of 1976–77. The drought was widespread, extending across the entire state of California and to the midwestern states and north into the Canadian prairie region north of Montana. Even the Pacific Northwest experienced its worst drought on record, leading to shortfalls in crop yields and region-wide water rationing. Record low flows were measured on the Columbia River, leading to restrictions on power consumption from hydroelectric plants. The snow pack in mountains from California to Colorado reached historic lows, and ski resorts from the Sierra Nevada to the Rockies remained closed for much of the season.
The 1976–77 drought was particularly devastating in California. Precipitation levels that year dropped to less than half of the average level throughout the state (see figure 7). Northern California was especially hard-hit, with rainfall reaching only 15 percent of the average in some regions. Runoff and river flow in California fell to a quarter of their normal levels, and storage reservoirs saw dramatic declines, falling to about a third of their normal levels. In the Sierra Nevada, Lake Tahoe dropped below its outlet at the Truckee River (near Tahoe City), causing the Truckee River to cease flowing, which in turn caused Pyramid Lake in northwestern Nevada to drop as well. Aquatic habitats were severely affected, with the worst impacts in the lower to mid-elevations of the Coast Ranges and the western slope of the Sierra Nevada. Low river flows, combined with higher water temperatures, severely disrupted the migration and spawning of migratory fish species, including all species of salmon and trout.
Excerpted from The West without Water by B. Lynn Ingram, Frances Malamud-Roam. Copyright © 2013 The Regents of the University of California. Excerpted by permission of UNIVERSITY OF CALIFORNIA PRESS.
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