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Fig. 10.2 Instrumental
summer (June-July-August,
JJA) Palmer Drought Severity
Index averaged at each of the
286 grid points for North
America(the2.5 ×
2.5
latitude/longitude grid from
Cook et al. 2004 , 2007 ) and
then mapped for the Dust
Bowl ( a , 1931-1940) and
1950s ( b , 1950-1957)
droughts. The PDSI is an
integration of monthly
precipitation and temperature
effects on available soil
moisture, and it has proved to
be a good model for the
effects of climate on tree
growth at moisture-limited
sites. The 10-year average
summer PDSI fell to 2.5
over the central Great Plains
and reached above +2.5 over
Mexico during the 1930s (a;
contour interval is 0.5 PDSI
units; dashed lines = negative
PDSI and dry; solid lines =
positive PDSI and wet). Note
the changing geographical
focus of decadal drought from
the 1930s through the 1950s
through the national economy, rather than the tidal wave of system collapse on the
entire western front of the Plains as in the 1890s' (Bowden et al. 1981 ) .
By contrast, the severe drought of the 1950s, which impacted the southern Plains,
Southwest, and Mexico (Fig. 10.2b ) , lasted nearly as long and impacted a region
nearly as large as the Dust Bowl drought (Fig. 10.2a ) , but did not produce a frac-
tion of the social consequences associated with the Dust Bowl in the United States
(e.g., Warrick and Bowden 1981 ) . Out migration from the hard-hit southern Plains
 
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