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Drought in the western Great Plains, 1845-56. Impacts and implications

Authors:
  • Lukas Climate Research and Consulting

Abstract and Figures

A sustained mid-nineteenth-century drought in the western Great Plains has been indicated by a tree-ring analysis of trees flanking the western Great Plains, and in tree-ring reconstructions of drought and streamflow for eastern Colorado and the Colorado Front Range. The development of new tree-ring chronologies for the western Great Plains, in combination with existing chronologies, now enables a more detailed assessment of the spatial and temporal characteristics of this drought. The analysis of a set of drought-sensitive tree-ring chronologies ranging from the northwestern Great Plains to central New Mexico indicates a core area of drought from south-central Wyoming to northeastern New Mexico for the years 1845-56. Drought was particutarly severe in the years 1845-48, 1851, and 1854-56, contracting and affecting smaller regions in intervening years. The impact of this drought on natural ecosystems and human activities is difficult to gauge because of the paucity of historical documents and the confounding effects of land use changes occurring over the same period. However, it is probable that this drought played a role in the decimation of bison herds in the second hall of the nineteenth century. Were it to occur today, this relatively small but persistent drought would have significant impacts on the Colorado Front Range metropolitan area and the agricultural regions of eastern Colorado.
Locations of tree-ring chronologies. Shaded area indicates core drought region for 1845-56. Chronology sites are numbered and correspond to those listed in Fig. 3. Chronologies selected were from species known to be sensitive to drought (ponderosa pine, Pinus ponderosa; Douglas-fir, Pseudotsuga menziesii; pinyon pine, Pinus edulis; and post oak, Quercus stellata), and were taken to be proxies of drought (generally winter/spring in the south grading to spring/early summer in the north). All but the three Montana chronologies (courtesy of D. Meko) were obtained or are now available from the World Data Center for Paleoclimatology's International Tree-Ring Data Bank (ITRDB; available online at www.ngdc.noaa.gov/paleo/treering.html). In all but three cases (the Montana sites, where raw data were not available), raw ring-width measurements were used to generate treering chronologies (ARSTAN; Cook 1985) to ensure that the same standardization process and conservative detrending methods were used for all chronologies. Also included were 11 newly generated chronologies from isolated ponderosa pine, Douglas-fir, and pinyon pine woodlands growing in the Great Plains in Nebraska, eastern and central Colorado, and northeastern New Mexico (Woodhouse and Brown 2001). Except for the three Montana chronologies, residual chronologies, from which low-order autocorrelation presumed to be biological in origin has been removed (Fritts 1976), were used for this study. Also shown are locations of gridpoint PDSI reconstructions used in Fig. 4 (circled X symbols).
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1485OCTOBER 2002AMERICAN METEOROLOGICAL SOCIETY |
The environment and culture of the western Great
Plains changed significantly over the course of
the nineteenth century. While Native Americans
were displaced and Euro-American settlers moved
across the region, the bison population, which at one
time numbered tens of millions (Flores 1991), suf-
fered an astoundingly abrupt decimation. The cause
of this near extinction has been hotly debated and at-
tributed to a range of factors that include the impacts
of both Native American and Euro-American land use
and hunting (Robbins 1999; West 1995, and refer-
ences within). Drought has been cited as a possible
contributing factor as well, but it is difficult to unravel
the roles played by each of these factors. In this pa-
per, we describe a mid-nineteenth-century drought
in part of the western plains—recorded by tree
rings—that appears to have persisted for more than a
decade in some parts of this region. We examine evi-
dence for this drought in historical accounts and its
contribution to the bison population’s demise. We
end by discussing the impacts such a drought might
have in the future.
The droughts of the 1930s and 1950s have long
served as benchmarks for severe and sustained
drought in the United States. Societal and ecological
impacts of these droughts were prolonged and well
documented. Although the spatial dimensions of the
two droughts were different, both had severe impacts
on the high plains of Kansas and Colorado (McGregor
1985). While this region is recognized as drought
prone (Karl and Koscielny 1982), the limited length
of instrumental records (100 yr or less) precludes a
full evaluation of the rarity of these droughts.
However, paleoclimatic records provide evidence of
climate for years prior to the keeping of instrumen-
tal records, and can be used to gauge the severity of
droughts in the twentieth century as well as for prior
centuries. In recent work, two new tree-ring-based
hydroclimatic reconstructions have been produced
for the eastern half of Colorado: a streamflow recon-
struction for the Colorado Front Range (Woodhouse
2001) and a Palmer Drought Severity Index (PDSI;
Palmer 1965) reconstruction for eastern Colorado
(Woodhouse and Brown 2001; Fig. 1). These recon-
DROUGHT IN THE WESTERN
GREAT PLAINS, 1845–56
Impacts and Implications
BY CONNIE A. WOODHOUSE, JEFFREY J. LUKAS, AND PETER M. BROWN
A relatively small, but severe and persistent drought occurred in the western Great Plains
during the mid-19th century, and may have contributed to the decimation of bison herds.
AFFILIATIONS: WOODHOUSE—NOAA Paleoclimatology Program,
and Institute of Arctic and Alpine Research, University of Colorado,
Boulder, Colorado; LUKAS—Institute of Arctic and Alpine Research,
University of Colorado, Boulder, Colorado; BROWN—Rocky
Mountain Tree-Ring Research, Fort Collins, Colorado
CORRESPONDING AUTHOR: Connie Woodhouse, NOAA
Paleoclimatology Program, National Geophysical Data Center, 325
Broadway, Boulder, CO 80305
E-mail: Connie.Woodhouse@noaa.gov
DOI: 10.1175/BAMS-83-10-1485
In final form 2 May 2002
© 2002 American Meteorological Society
1486 OCTOBER 2002
|
structions indicate a period of remarkably sustained
drought and low streamflow lasting from approximately
1845 to 1856 that matched or exceeded the severity of
the droughts in this area during the 1930s and 1950s.
Another indication of western Great Plains drought
about this time is seen in the analysis of tree-ring chro-
nologies flanking the Great Plains (Meko 1992).
This period of drought occurred just prior to the
establishment of permanent weather-recording sta-
tions in the western Great Plains and thus is not docu-
mented in instrumental records. Although there have
been a number of tree-ring reconstructions of
drought for areas that include the Great Plains, a pe-
riod of drought as severe and persistent as indicated
in the Front Range and eastern Colorado reconstruc-
tions is not evident in large regional reconstructions
of the central United States (Fritts 1965, 1983; Stock-
ton and Meko 1983; Cook et al. 1996, 1997, 1999;
Woodhouse and Overpeck 1998). One reason it has
not been recognized in these reconstructions may be
an absence of high-resolution paleoclimatic data for
the eastern plains of Colorado. For the most part,
these past reconstructions utilized trees adjacent to,
but not in, the Great Plains to reconstruct climate, a
justifiable practice because tree growth typically re-
flects regional climate variability, and trees suitable for
climate reconstructions are rare in the Great Plains.
A comparison of a gridpoint reconstruction of
drought for eastern Colorado, which is part of a larger
reconstruction network, with the more recent east-
ern Colorado PDSI reconstruction indicates an im-
proved regional reconstruc-
tion with the inclusion of
Great Plains tree-ring chro-
nologies (Cook et al. 2002;
Woodhouse and Brown 2001).
Another reason is related to
the scale of drought. Studies of
large spatial patterns of drought
(Cook et al. 1996, 1997, 1999;
Woodhouse and Overpeck
1998) indicate discontinuous
periods of widespread, severe
drought during the 1840s and
1850s. In particular, the years
1845–47 and 1855–56 have
been reconstructed as severe
drought years for large areas of
the western and central United
States (Cook et al. 2002), but
the persistent drought seen in
the new Front Range stream-
flow and eastern Colorado
drought reconstructions is not seen in these large-
scale reconstructions. Studies focusing on regional
droughts in the eastern and southern Great Plains
(Iowa, Arkansas, Texas; Blasing and Duvick 1984;
Stahle et al. 1985; Stahle and Cleaveland 1988;
Cleaveland and Duvick 1992) do not show severe
drought conditions occurring from the mid-1840s to
the mid-1850s with the same consistency as seen in
the Colorado reconstructions either, suggesting this
period of persistent drought was limited in spatial
extent. Widespread drought conditions are indicated
somewhat later, overlapping with the Colorado
drought, in the decade centered around 1860 in re-
constructions for the central and southern plains
(Fritts 1983; Blasing et al. 1988; Stahle and Cleaveland
1988). In contrast, severe and sustained drought con-
ditions in the Front Range and eastern Colorado
abated after 1856 with hydroclimatic conditions re-
maining near or above average until 1861
(Woodhouse 2001; Woodhouse and Brown 2001).
DEFINING THE DROUGHT. To identify in
greater detail the spatial and temporal characteristics
of the mid-nineteenth-century drought seen in the
two Colorado reconstructions, we examined a set of
60 tree-ring chronologies that range from eastern
Montana and western North Dakota, across the west-
ern Great Plains/Colorado Front Range to western
Colorado, central New Mexico, and Oklahoma, in-
cluding 11 newly generated tree-ring chronologies
from isolated woodlands growing in the western
FIG. 1. (a) Reconstruction of Middle Boulder Creek’s mean annual flow in
(cms) m3 s
1, 1710–1987 (Woodhouse 2001). (b) Reconstruction of spring/
summer droughts (May–Jun–Jul PDSI) for eastern Colorado, 1552–1995
(Woodhouse and Brown 2001). Series have been smoothed with a five-
weight binomial filter. The nineteenth-century drought in both series is
indicated by the striped bar. The 1930s and 1950s droughts are shaded for
comparison of duration and severity to the nineteenth-century drought.
1487OCTOBER 2002AMERICAN METEOROLOGICAL SOCIETY |
Great Plains (Woodhouse and
Brown 2001; Fig. 2). Annual ring-
width values for each of the 60 chro-
nologies were ranked for the 285-yr
period, 1680–1964. Rankings were
evaluated for 1840–60, the years that
bracket the drought documented in
the Colorado Front Range stream-
flow and eastern Colorado drought
reconstructions. The years in each
chronology that fell within the low-
est 50th, 25th, and 10th percentiles
of growth were highlighted (Fig. 3).
The tree-ring chronology rankings
show a core area of low growth that
ranges from southern Wyoming to
southeastern Colorado/northeastern
New Mexico for the years 1845–56
(Figs. 2 and 3). Outside of this re-
gion, low growth occurred in subsets
of these years in western Colorado
(1845–48, 1851), the west/central
Dakotas and Nebraska (1845–48,
1855–56), and central New Mexico
(1841–43, 1845–48, 1851). In this
core area (here called eastern Colo-
rado, but as noted, with extensions
into southern Wyoming and north-
eastern New Mexico), severe
drought in 1842 was followed by wet
years in 1843 and 1844. The period
of sustained drought began in 1845
and was severe and widespread
throughout the core region in 1845–
48, 1851, and 1855–56. The extent of
low growth/drought was less in the
intervening years, 1849–50, 1852,
and 1854, but smaller core areas of
extreme low growth (25th percentile
or less) persisted in the northern and
central Front Range. The year 1853
was the least severe, with below-
median growth persisting only in
scattered sites, but with two sites still
exhibiting growth below the 25th
percentile. By 1857, above-median
tree growth returned to many areas,
and to virtually all areas by 1858. We
interpret this break in widespread
low growth in the eastern Colorado
region as an end to this period of
persistent drought. Below-median
tree growth in 1859 indicated a re-
FIG. 2. Locations of tree-ring chronologies. Shaded area indicates core
drought region for 1845–56. Chronology sites are numbered and cor-
respond to those listed in Fig. 3. Chronologies selected were from
species known to be sensitive to drought (ponderosa pine, Pinus pon-
derosa; Douglas-fir, Pseudotsuga menziesii; pinyon pine, Pinus edulis; and
post oak, Quercus stellata), and were taken to be proxies of drought
(generally winter/spring in the south grading to spring/early summer
in the north). All but the three Montana chronologies (courtesy of D.
Meko) were obtained or are now available from the World Data
Center for Paleoclimatology’s International Tree-Ring Data Bank
(ITRDB; available online at www.ngdc.noaa.gov/paleo/treering.html).
In all but three cases (the Montana sites, where raw data were not
available), raw ring-width measurements were used to generate tree-
ring chronologies (ARSTAN; Cook 1985) to ensure that the same
standardization process and conservative detrending methods were
used for all chronologies. Also included were 11 newly generated chro-
nologies from isolated ponderosa pine, Douglas-fir, and pinyon pine
woodlands growing in the Great Plains in Nebraska, eastern and cen-
tral Colorado, and northeastern New Mexico (Woodhouse and Brown
2001). Except for the three Montana chronologies, residual chronolo-
gies, from which low-order autocorrelation presumed to be biologi-
cal in origin has been removed (Fritts 1976), were used for this study.
Also shown are locations of gridpoint PDSI reconstructions used in
Fig. 4 (circled X symbols).
1488 OCTOBER 2002
|
turn to dry conditions in parts of Colorado. Drought
prevailed again in eastern Colorado for three more
years (1861–63), but dry conditions were scattered
and only intermittent in the Front Range during these
years.
The extent to which this mid-nineteenth-century
drought spread eastward into the central Great Plains
is unknown. Trees are scarce in this region, and tree-
ring collections in central Kansas have yielded mostly
young trees or samples that are difficult to date due
to numerous interannual rings (Woodhouse and
Brown 2001). Tree-ring chronologies from the south-
ern and eastern flanks of the Great Plains do exist and
have been used to reconstruct precipitation and
drought (Blasing and Duvick 1984; Blasing et al.
1988; Stahle and Cleaveland 1988; Stahle et al. 1985;
Cleaveland and Duvick 1992). Reconstructions for
Texas, Oklahoma, and Arkansas show severe and
prolonged drought peaking about 1860 (Blasing et al.
1988; Stahle and Cleaveland 1988; Stahle et al. 1985).
Drought starts in the mid-1850s in this area, over-
lapping with the drought identified in eastern Colo-
rado, but because of the difference in timing and lo-
cation, this period of drought appears to be of a
different nature than the 1845–56 Colorado
drought. Reconstructions for Iowa and Illinois also
show below-average moisture conditions in the
mid-nineteenth century but conditions are only
slightly below average during this period (Blasing and
Duvick 1984; Cleaveland and Duvick 1992). To illus-
trate the differences in timing and/or magnitude of
drought conditions suggested in the eastern and
southern plains reconstructions described above, the
gridpoint PDSI reconstructions from Cook et al.
(2002) for the eastern and southern Great Plains,
which use many of the same tree-ring data as well as
data from Kansas, were examined (locations shown
in Fig. 2, circled X symbols). A comparison of
gridpoint reconstructions for the eastern (southwest-
ern Iowa, northeast Kansas, the eastern Kansas and
Oklahoma border, eastern Oklahoma, and northeast-
ern Texas) and southern (central Oklahoma, north-
ern Texas) plains and the two eastern Colorado re-
constructions (eastern Colorado PDSI and Front
Range streamflow; Woodhouse and Brown 2001;
Woodhouse 2001) shows a difference in timing of
drought conditions (Fig. 4). Although there is evi-
dence of regional overlap, the Colorado drought is
more strongly centered on the late 1840s, while the
southern and eastern plains period of drought is cen-
tered on about 1860.
Although early instrumental records were kept at
forts in Kansas and Nebraska, most records are few
and fragmented until about 1858 (Mock 1991). Two
longer records exist from Fort Leavenworth and Fort
Scott in eastern Kansas and show several dry years in
the 1840s and 1850s, but no period of sustained
drought [the National Oceanic and Atmospheric
FIG. 3. Spatial distribution of low-ranking (narrow ring
widths) years for 60 tree-ring chronologies, 1840–70.
The chronologies were arranged by geographic region
to illustrate patterns of low growth, a proxy for
drought. The chronologies are grouped by region, from
roughly north to south: the northern and central Great
Plains, Colorado Front Range, eastern Colorado and
northeastern New Mexico, western Colorado, central
New Mexico, and Oklahoma. Annual ring-width indi-
ces for each of the 60 chronologies were ranked for
the 285 yr 1680–1964. Rankings are shown for 1840–
60 (major drought years outlined in black), the years
that bracket the drought documented in the Colorado
Front Range streamflow and eastern Colorado drought
reconstructions, and for the 1860s central Great Plains
drought, for comparison. The years in each chronology
that fell within the lowest 50th, 25th, and 10th percen-
tiles of growth were highlighted by red for a ranking
below the 10th percentile, orange for the 10th–24th
percentiles, yellow for the 25th–50th percentiles, and
white for the above-median ranking.
1489OCTOBER 2002AMERICAN METEOROLOGICAL SOCIETY |
Administration’s (NOAA’s) Nineteenth-Century U.S.
Climate Data Set Project; available online at www.ncdc.
noaa.gov/onlinedata/forts/forts.html]. Other limited
historical documents exist in the form of written ac-
counts by early explorers traveling across the Great
Plains in the eighteenth and nineteenth centuries. A
review of reports documenting blowing sand from the
Nebraska Sand Hills southward to northern Texas
indicates multiple observations of eolian activity from
about 1840 to 1865. It is, however, difficult to attribute
activation of sand dunes to a specific drought year or
set of years (Muhs and Holliday 1995). Along with the
tree-ring data from the southern and central flanks
of the Great Plains, these scant historical documents
suggest an eastern limit of drought conditions dur-
ing the period of sustained drought in eastern Colo-
rado, 1845–56.
DROUGHT AND BISON. Bison (Bison bison)
have been present in the Great Plains since at least the
last glacial period, and evolved under a climate regime
that included extensive periods of drought. Some re-
search suggests a decline or even absence of bison
remains in parts of the Great Plains during the mark-
edly dry mid-Holocene, from about 8000 to 5000 yr
B.P. (Dillehay 1974). (For the reader not familiar, B.P.
refers to “before the present.”) Other research clearly
establishes that bison persisted during a part of the late
Holocene that was far drier than the nineteenth cen-
tury as demonstrated by numerous hoofprints in sand
found in buried sand dune sediments in the west-
central Great Plains (Muhs 2000; D. H. Muhs 2001,
personal communication). Survival of the species dic-
tated the development of behavioral adaptations to
mitigate the impacts of drought, and likely included
some degree of migration to areas less affected by
drought (Flores 1991). A mass migration out of the
Great Plains is not indicated during the dry mid-
Holocene (Graham and Lundelius 1994), but we
speculate that bison likely migrated to locally moister
regions along riparian corridors.
In the last decade, the prevailing view that the
abrupt near extinction of the bison in the nineteenth
century was caused largely if not entirely by Euro-
American market hunting after the Civil War has
been challenged by some historians who have argued
that the decline began in the 1840s and resulted from
multiple interacting factors, including drought
(Flores 1991; West 1995; Isenberg 2000). Given the
survival of the species through periods of aridity last-
ing thousands of years, it is hard to believe a decadal-
length drought in the nineteenth century had much
of an impact on bison populations. However, the en-
vironment to which bison had long been adapted was
systematically disrupted by human activities in the
nineteenth century (Bamforth 1987).
Flores (1991), using older tree-ring data from
Nebraska, southern Wyoming, and the Colorado
Front Range that indicated generally dry conditions
in the southern and central Great Plains from 1846
to 1855 (Weakly 1943; Schulman 1956), proposed that
drought was one of several causes of the collapse in
bison populations across the Great Plains. This hy-
pothesis was developed further by West (1995), also
employing previous older dendroclimatological stud-
ies as well as historical accounts, who argued that gen-
FIG. 4. Tree-ring reconstructions for drought (PDSI) and
streamflow, 1800–99 with annual (black line) and
smoothed (five-weight binomial filter, red line) values.
Vertical shaded bars indicate eastern Colorado drought
(1845–56, dark gray) and southern/eastern plains
drought (1855–65, light gray line). (a) Colorado recon-
structions (Woodhouse 2001; Woodhouse and Brown
2001), (b) eastern Great Plains gridpoint drought re-
constructions from Cook et al. (2002), Nos. 91, 92, 93,
94, 95. (c) Southern Great Plains gridpoint drought re-
constructions from Cook et al. (2002), Nos. 82, 83.
1490 OCTOBER 2002
|
erally reduced rainfall in the 1840s and 1850s, com-
bined with increased grazing by both emigrants and
Indians, severely impaired the forage resources of the
uplands. Isenberg (2000) also attributed the bison
population demise to a combination of cultural and
environmental factors, including drought. The rela-
tively lush and wooded river corridors through the
western Great Plains had historically served as shel-
tered winter habitat and were likely year-round ref-
ugium for bison during drought as well (Flores 1991;
West 1995). Beginning in the early 1840s, large cara-
vans of both U.S. Army forces and Euro-American
settlers, with thousands of horses and other livestock,
traveled these corridors, severely reducing both for-
age and woodlands (West 1995). At the same time,
the Native American populations of the western Great
Plains, many of whom were relatively newcomers to
this area, with their numerous horse herds, were in-
creasing their usage of these same riparian corridors
in response to the regional bison hide market created
by newly established trading posts along the rivers
(West 1995; Isenberg 2000). As a result, bison would
have found much poorer conditions for subsistence
during a period when these riparian areas would have
been most critical. Studies during the 1930s and 1950s
found that grass cover in the shortgrass uplands was
dramatically reduced by drought (in one case from
90% to 20%) even in ungrazed areas (Malin 1947;
Tomanek and Hulett 1970); during the drought in the
mid-nineteenth century, conditions in the uplands
may well have been worse. Poor upland range condi-
tions and competition for riparian forage must have
exacerbated the impacts of what was, in the context
of the Holocene, a relatively minor drought. Thus,
even though bison had persisted through much worse
droughts in the mid- and late Holocene, human im-
pacts on the Great Plains environment likely altered
the bison’s ability to cope with drought in the nine-
teenth century (West 1995).
The concurrent timing of the drought and the de-
cline of the bison population lend further credence
to drought as a possible pivotal factor. Reports of the
reduction of bison numbers coincide, for the most
part, with drought years documented by tree rings.
The initial decline in the bison population was noted
as early as 1844 (see footnote 40 of Flores 1991). This
first report may have had less to do with drought than
with a reportedly “epic” spring snowstorm in eastern
Colorado in the spring of 1844 that apparently caused
a local die-off of bison and other ungulates (Benedict
1999). However, by the late 1840s, anecdotal evidence
from Kiowa painted robe calendars indicated few or
no bison for the years 1849–52 (Flores 1991), and
other historical accounts reflect a continued decline
through the 1850s (West 1995).
Although the cause for the decline in bison in the
nineteenth century remains a complex and much
debated subject, the drought conditions reflected in
the tree-ring records probably contributed to the de-
mise of Great Plains bison. Our study, by more clearly
describing the drought conditions at this time and by
locating a core of drought along and east of the Front
Range where bison populations apparently declined
first (West 1995), reinforces the work of Flores (1991),
West (1995), and Isenberg (2000) and adds even
greater support to the idea that drought contributed
to the bison population decline.
IMPORTANCE OF REGIONAL DROUGHT.
This relatively small nineteenth-century drought
would have had a very limited effect on the bison
population if human activities had not been a factor.
However, the severity and duration of this drought in
eastern Colorado qualifies it as a major drought for
this particular region. Although the size and length
of this drought have been matched and exceeded by
droughts in other areas, this drought is unique in
terms of its impact on this region, having been un-
matched here in at least the last three centuries. This
drought, were it to occur today, would have consid-
erable impacts now that the area includes a major,
rapidly expanding metropolitan area as well as large-
scale crop and livestock production. Large-scale
droughts have obvious social, economic, and ecologi-
cal impacts, but smaller-scale droughts may have sig-
nificant impacts as well, which may be aggravated by
location and timing.
An examination of small-scale droughts and their
relationships to periods of more widespread drought
is important for understanding why and where they
may be likely to occur. While drought conditions
were widespread during some of the years of this
nineteenth-century drought, they appeared to con-
tract into and persist in a core region in intervening
years. Major droughts in the twentieth century, while
more severe over larger areas, have displayed similar
episodic fluctuations. The 1930s drought period had
four distinct episodes of widespread dryness, and
similar episodes occurred in the 1950s drought
(Riebsame et al. 1991), with drought shrinking back
to core regions between years of expansion
(McGregor 1985). It is important to identify core ar-
eas within widespread droughts in order to assess pos-
sible significant regional impacts—and areas of drought
susceptibility—that are not noted in larger-scale analy-
ses. The core drought areas for both the mid-nine-
1491OCTOBER 2002AMERICAN METEOROLOGICAL SOCIETY |
teenth-century drought described in this paper and
the 1930s drought included southeastern Colorado.
In the western Great Plains, late spring and sum-
mer are the most important seasons with regard to
drought, since this is when most of the annual pre-
cipitation occurs (Bryson 1966; Fritsch et al. 1986;
Helfand and Schubert 1995; Mock 1996). Rainfall
during this period can result from several different
circulation mechanisms, including frontal systems
drawing moisture from the Gulf of Mexico in the
spring (Hirschboeck 1991); and in summer, the Great
Plains nocturnal low-level jet (Tang and Reiter 1984;
Helfand and Schubert 1995; Higgins et al. 1997);
mesoscale convective complexes (Fritsch et al. 1986);
and less commonly, synoptic-scale upper-level distur-
bances (Helfand and Schubert 1995; Mock 1996).
Research suggests that conditions in both the Pacific
and Atlantic Oceans can lead to drought in the Great
Plains, directly or indirectly, by inducing perturba-
tions in patterns of atmospheric circulation and the
transport of moisture (Trenberth et al. 1988; Palmer
and Brankoviƒ 1989; Trenberth and Guillemot 1996;
Ting and Wang 1997). Although studies have linked
equatorial and northern Pacific conditions with
spring and summer precipitation in the Great Plains,
this relationship likely has more to do with Pacific sea
surface temperature influences on circulation than
direct transport of Pacific moisture (Ting and Wang
1997). On the other hand, drought in the Great Plains
is strongly linked to the flow of moisture from the
Gulf of Mexico in spring and summer, which is in-
fluenced by conditions in the Atlantic (Oglesby 1991;
Helfand and Schubert 1995). Enfield et al. (2001) sug-
gest that decadal-scale fluctuations in North Atlantic
sea surface temperatures (SSTs) are related to drought
in the central United States and also interact with
ENSO variability. Thus, drought in the Great Plains
may be caused by a number of different, but possibly
interrelated factors over an area that included both the
Atlantic and Pacific Oceans.
There is evidence to suggest that the drought con-
ditions in eastern Colorado in the mid-nineteenth
century were the result of several different overlap-
ping droughts, each related to different circulation
mechanisms. Recent work, using tree-ring and coral-
proxy climate data, suggests that the severe drought
centered around 1860 (roughly 1855–65) was linked
to an unusually long, cold ENSO event, possibly en-
hanced by low-frequency variations in the extratro-
pical Pacific (Cole et al. 2002). This event may have
been a cause of the drought centered around 1860
seen in reconstructions for the southern Great Plains.
However, the patterns of drought in the 1840s and
early 1850s reconstructed from tree rings are less con-
sistently representative of an ENSO cold event–
drought pattern (Cole et al. 2002; also see PDSI re-
constructions in Cook et al. 2002). This suggests that
while ENSO may have been a factor in the latter years
of the drought, other factors may have been respon-
sible in earlier years of the drought. Unfortunately,
no proxy records of Atlantic SSTs are yet available to
test relationships with regional drought during the
nineteenth century, but it is likely that a combination
of circulation patterns, including those influenced by
slowly varying conditions in both the Atlantic and
Pacific Oceans, was responsible for mid-nineteenth-
century drought conditions in the western Great
Plains. Further investigation of the associated sea sur-
face and atmospheric conditions for this time period
through an analysis of independent proxy data, such
as historical documents and other tree-ring data,
could yield more information about possible causes
of this mid-nineteenth-century drought.
CONCLUSIONS. This relatively small but persis-
tent nineteenth-century drought in the west-central
Great Plains likely influenced the cultural and ecologi-
cal history of this region. If a drought of this dura-
tion and severity were to occur here today or in the
future, impacts would be very different, but poten-
tially as significant. The state of Colorado, and south-
eastern Colorado in particular, has experienced sus-
tained, above-average precipitation for much of the
last two decades (McKee et al. 1999), a period also
characterized by rapid growth along the Front Range
urban corridor. In the Colorado Front Range, water
supply systems are commonly designed to handle the
“drought of record,” the most severe hydrologic event
in the instrumental record (Howe et al. 1994).
Although results of recent studies indicate that water
managers from a variety of Front Range municipali-
ties have considerable confidence in the reliability of
their current water supplies (Howe and Smith 1993),
how well would these cities and adjacent rural agri-
cultural areas endure a decade-long drought?
The importance of identifying and understanding
regional-scale drought should not be overlooked.
Since instrumental records exist for only the past 100
years in many areas, the potential to study regional
droughts based on instrumental data is limited. This
study points to the continued need for filling in spa-
tial gaps in high-resolution paleoclimatic data. The
lack of a finescale network of paleoclimatic data can
preclude detailed spatial analysis of past climate, but
in this study, new data and a regionally focused analy-
sis allow the identification of this regionally persistent
1492 OCTOBER 2002
|
drought. Global and large regional area reconstruc-
tions of past climate are very important for obtaining
an understanding of patterns of past climate and for
investigating possible large-scale controls. However,
studies of past climate at smaller regional scales are
of utmost importance especially in areas such as the
western Great Plains, where the cultural and ecologi-
cal history are entangled and disputed issues exist, and
the Colorado Front Range where an event the mag-
nitude of the mid-nineteenth-century drought would
have major societal, economic, and ecological impacts
were it to occur today.
ACKNOWLEDGMENTS. This project was supported
by NSF Grant ATM-9729751. We thank David Meko for
the Montana chronologies, and all contributors to the In-
ternational Tree-Ring Data Base who made this analysis
possible. Special thanks to Patricia Limerick, Elliott West,
and Dan Muhs for stimulating discussions on the role of
drought in the bison population decline. Thanks also to
Dan Muhs and David Stahle for their thorough and insight-
ful reviews of this paper.
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Tree ring indices from an expanded network of 17 white oak (Quercus alba) sites in eastern and central Iowa were used to reconstruct state average July Palmer hydrological drought index (PHDI), annual precipitation (previous August to current July), and other climate variables for 1640-1982. We removed nonclimatic variance trends caused by changing sample size and senescent growth. July PHDI correlated better with tree growth than annual precipitation. Occurrence of prolonged droughts throughout the reconstruction suggests that decades like the 1930s occur about twice per century in Iowa. Iowa climate is correlated with the Southern Oscillation Index (SOI) from June in the year of El Nino onset (Yr0) through the next February (Yr + 1), with negative SOI (El Nino) associated with wetter conditions. When the June (Yr0) to February (Yr + 1) average SOI reaches extremes greater-than-or-equal-to +1.0 or less-than-or-equal-to -1.0, it correlates significantly with observed and reconstructed July PHDI (r = -0. 37 and -0.56, respectively). Climate during solar cycles centered on sunspot minima alternates between wet and dry regimes that differ by an average of 1.21 units of observed July PHDI and 46.7 mm of annual precipitation for 1877-1982. The solar relationship has been stable since 1640. Combining solar and SOI influences in forecasts may improve prediction of Iowa climate.
Article
The Palmer Index was used to compare the droughts of the 1930s and the 1950s in the central United States. A series of maps was produced to compare the droughts in terms of location, severity, and duration. The results indicated the 1930s drought lasted longer and consisted of three distinct waves. The 1950s drought, although shorter in overall duration, was actually more intense over a larger area for a longer period. There were changes in the size and configuration of the drought-affected area, but both droughts were especially persistent in a core region on the High Plains. A simple index of overall drought impact was developed based on the drought-affected area and the Palmer value from each climatic division. The probability of drought occurrence was calculated for each of the 76 climatic divisions in the study area. [Key words: drought, Palmer Index, Great Plains.]