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was less than 10% for the 1950s, comparable to emigration during wet decades
(Bowden et al. 1981 ) . The ranching (Kelton 1984 ) and dry farming (Rautman 1994 )
economies of the Southwest were hard-hit, but the postwar economy of the United
States was booming and the drought had little economic impact at the national level.
The 'Sun Belt miracle' of Southwestern population growth and intensive water and
energy demand had yet to occur. However, Mexico experienced 'national drought'
during the 1950s that had severe impacts on rural farming and ranching, highlighting
international differences in the economic environments in which this severe regional
drought developed (Florescano 1980 ) .
The first major drought of the twenty-first century (Seager 2007 ) , which began in
1999 and currently afflicts much of the western United States and northern Mexico,
already has had major environmental and human consequences. Severe precipitation
shortfalls have caused crop failures and cutbacks in both small- and large-scale dry-
land farming. Irrigation agriculture, municipal water supplies, and the generation of
electricity have been threatened by unprecedented low water levels in Southwestern
reservoirs. All this has occurred at a time when skyrocketing human population is
vastly increasing demand for water and electricity and driving fierce competition
among the affected groups, cities, and states. Massive fires have consumed more
than a million acres of desiccated forests, fueled by living trees with moisture lev-
els below that of kiln-dried lumber. These catastrophic fires have been linked with
drought and regional climate change (Westerling and Swetnam 2003 ; Westerling
et al. 2006 ) and have displaced burned-out communities, destroyed watersheds, and
ravaged the tourist and lumber industries. Millions of moisture-stressed conifers
(especially pinyon) have succumbed to the drought, or insect infestations, or the
lethal combination of both, leaving barren landscapes exceptionally vulnerable to
fire and erosion. The current forest dieback appears to exceed the mortality asso-
ciated with the 1950s drought (Breshears et al. 2005 ) , which had major ecological
consequences across the Southwest (Swetnam and Betancourt 1998 ) . Tree-ring data
suggest that other major forest mortality events may have occurred during the
droughts of the late thirteenth and sixteenth centuries (Swetnam and Betancourt
1998 , Fig. 15), but the extent to which the current dieback is related simply to
drought or may also reflect other human impacts on Western woodlands has not
been determined.
Wet climate extremes may also have significant long-term socioeconomic con-
sequences, as was illustrated from 1905 through 1917 during the early twentieth-
century pluvial (Fye et al. 2003 ) . The most recent assessment of the available
tree-ring data for the western United States indicates that the first two decades of the
twentieth century was the wettest multiyear episode in the past 1200 years (Cook
et al. 2004 ) . The tree-ring-reconstructed Palmer Drought Severity Indices (PDSIs;
defined in Fig. 10.2 ) during the wettest decade of the twentieth-century pluvial
(1907-1916) indicate prolonged wetness from Baja California across the Rockies
to the Canadian border (Fig. 10.3 ) . In fact, Stockton ( 1975 ) reconstructed Colorado
River streamflow at Lees Ferry, Arizona, to arrive at perhaps the most famous
number ever calculated with tree-ring data, a long-term mean annual flow of only
13
10 6
10 6
×
acre feet/year compared with 16.4
×
acre feet/year estimated by
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