JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
AMERICAN WATER RESOURCES ASSOCIATION
VOL. 37, NO. 3
JUNE 2001
TOWARDS CHARACTERIZING AND PLANNING FOR DROUGHT
IN VERMONT - PART I: A CLIMATOLOGICAL PERSPECTWE'
Lesley-Ann Dupigny-Giroux2
ABSTRACT: In the Green Mountain state of Vermont, droughts of
one form or another and of varying intensities, seventies, and areal
extent are not uncommon occurrences. The 1990s were marked by
at least three drought events of which the 1998 to 1999 was the
most recent. In spite of this recurrence, ongoing drought monitoring and mitigative planning efforts are not as advanced as they
could be and no official drought plan exists for the state. This article is the first of two in this volume. It summarizes the cascade of
drought types that impacted the state during the 1998 to 1999
episode. From a number of precipitation statistics and drought
indices, fine spatial scales (county or better) were found to best capture the character of drought impacts, while the weekly time step is
recommended as the temporal unit around which to base planning
and monitoring efforts. The companion article outlines a possible
framework for drought planning efforts and highlights key constituencies to be included in the process.
(KEY TERMS: drought mitigation; drought indices; climatology;
agriculture; water resources; Vermont.)
INTRODUCTION
Drought is an inherent part of climatic variability
and can impact any geographic location including
humid continental regions like the New England state
of Vermont. Of the two hydrometeorological extremes,
drought monitoring and mitigative procedures are not
as well-developed as they are for flooding, the other
extreme. In fact, Vermont is one of approximately half
a dozen states for which no official plan exists to mon-
annually, tend to attract more attention because soci-
etal damages are more visible and sudden (Tase,
1976).
Despite the disparity between the frequencies of
occurrence of floods and droughts, however, it is not
uncommon for both hydrometeorological extremes to
be observed within the same calendar year. In Ver-
mont there is a saying that one extreme follows
another. For example, the Great Flood of November
1927, which resulted from copious amounts of rainfall
produced by the remnants of a tropical storm, was
actually preceded by drought-like conditions that lasted from at least April to October. Again in September
1938 rainfall associated with a decaying hurricane
brought much needed relief from dry conditions that
had persisted all year as part of the severe drought of
the 1930s. More recently, the statewide drought of the
spring/summer of 1995 was followed by flash flooding
in northern Vermont in August. In 1998, the ice storm
of January and statewide flooding in June/July finally
gave way to drought conditions as the year drew to a
close. These dry conditions continued through the
spring, summer and fall of 1999.
In order to initiate the process of drought planning
in Vermont it is first necessary to understand and
adequately quantify the temporal, spatial, and other
characteristics of dry conditions across the state. In a
climate that is best described as changeable, it is
sometimes challenging to interpret climate signals
from one season to the next and the drought signal
itor and combat drought on an ongoing basis. This
may be related to the relative rarity of severe
may be hidden by the inherent variability of the climate regime. On this humid, temperate landscape the
onset and termination of a drought are often difficult
to identify although the severity of its impacts on
droughts as a phenomenon across the entire state,
even though their occurrence often leads to the depletion of both physical and financial resources. By contrast, moderate to severe floods, which recur almost
lPaper No. 00023-I of the Journal of the American Water Resources Association. Discussions are open until February 1, 2002.
2Assistant Professor, Department of Geography, University of Vermont, 200 Old Mill Building, 94 University Place, P.O. Box 54170,
Burlington, Vermont 05405-4170 (E-Mail: [email protected]).
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such sectors as agriculture, forestry, and hydrologic
activities is easier to determine as the dry conditions
progress. The questions then become: What are the
biweekly, producing abrupt changes in temperature,
moisture, sunshine, wind direction, and speed. Some-
characteristics of drought in Vermont? Does this
unchanged for extended periods as was the case of the
change from season to season and/or is this dependent
on the time of year during which it began? Is one definition of drought sufficiently broad to quantiQy all of
the sectors being impacted? How are drought severity
and intensity quantified? What are the optimum spa-
tial unit and time frame of study best suited to capturing the emerging and progressing drought signal?
These questions will be explored within the context
of the recent drought of the fall 1998 to fall/winter
1999. A variety of hydrometeorological variables will
be examined to pinpoint which are best suited to monitoring changing moisture conditions across the landscape. This will, in turn, set the stage for identifying
the corresponding constituencies that should participate in the drought planning process in Vermont to be
explored in a subsequent article in this volume. The
latter will also include recommendations on the inter-
times, however, weather patterns can remain
summer rains of 1998 or the abundant fair weather
that often accompanies a drought. It is these blocking
episodes that become pivotal in the perpetuation of
dry conditions.
Vermont is known for its consistency of precipita-
tion receipt, although on an annual basis a slight
maximum is observed during the summer in the
northern and western sections. Very severe droughts
are rare. When they do occur, these multiyear events
(e.g., 1930-1936, 1939-1943, 1960-1969, 1980-1981,
1988-1989) tend to affect the entire state and are
often related to patterns in the larger atmospheric
dynamics that aid in the persistence of these
droughts. Less severe, shorter droughts are actually
quite frequent and more localized in extent. Within
the last decade (1989-1999), every year there have
been dry events lasting at least a month in one or
more climate divisions of Vermont. Prior to the 1998
to 1999 drought, the previous events to produce last-
disciplinary foci, spatial and temporal scales best
suited for characterizing drought conditions in Vermont.
ing impacts occurred in 1991 and 1994 to 1995.
Lake Champlain is the largest body of water in the
region. It is the mouth for 20 river basins including
the Missisquoi, Lamoille, Winooski, New Haven
CLIMATOLOGICAL CONTEXT
Rivers, and Otter Creek on the Vermont side, as well
as the Great Chazy, Ausable, Saranac Rivers, and
The climate of Vermont displays a high degree of
variability on all time scales (seasonal, annual, and
beyond). Climatic variations in space are also notable
and any generalizations should take into account such
factors as altitudinal differences, terrain variations
and distance from major water bodies (such as Lake
Champlain and the Atlantic Ocean). Climatologically,
the state is divided into three divisions: (1) Northeastern, (2) Western, and (3) Southeastern (3) as shown
on Figure la. The western division spans the entire
Putnam Creek in New York state. It can therefore be
considered an integrative representation of the hydrological conditions on the western side of the state. On
the eastern third, the Upper Connecticut River acts
as the equivalent hydrologic integrator given that it is
the mouth for the Passumpsic, Wells, Waits, White,
Ottauquechee, Black, West, and Williams Rivers.
Flow characteristics of these two major integrators
provided an important baseline by which to gauge the
cumulative drying of the environment.
north-south length of the state to the west of the
Green Mountains. It covers an area of 3,177 sq. miles
(8,228.4 sq. km) and is least affected by the influence
of the Atlantic Ocean. Division 1 is the largest of the
three at 4,854 sq. miles (12,571.9 sq. km) and covers
the northeast, north-central and east-central portions
of Vermont with the exception of a narrow segment
along the Connecticut River Valley in the east-central
region. This latter segment is included in the South-
DATA AND METHODOLOGY
In order to quantify the magnitude of the drought
in various aspects of the hydrometeorology and econo-
my of the state, the following variables and indices
were examined. Monthly precipitation totals at the
eastern division due to its lower elevation (NOAA,
state, climate division, county, and station levels were
1982).
used to differentiate the influence of spatial scale
on the timing and evolution of the drought. These
Vermont lies within the zone of the prevailing westerlies, with the large-scale flow being modified at the
data were acquired from the Northeast Regional Climate Center, National Climatic Data Center, North-
local scale by the existing topography such that in
many areas the winds blow parallel to a valley. A
variety of weather patterns, contrasting air masses
and low pressure systems move across the state
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east River Forecast Center, and National Weather
Service/Burlington International Airport, respectively.
Cumulative departures from normal for each county
506
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
Towards Characterizing and Planning for Drought in Vermont — Part I: A Climatological Perspective
a
GRAND
C
BURKE
Figure 1. Schematic of the (a) Climate Divisions, (b) Counties, and (c) Cooperative Stations in Vermont.
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
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Dupigny-Giroux
were generated for each of the 14 counties where normals refer to the 1961 to 1990 period. Twenty Cooper-
ative (COOP) Stations with long-term records and
normals for the same period were selected as index
sites. The resulting spatial distribution was not
Table 1 summarizes the monthly precipitation
totals, percent of normal and ranking relative to the
driest year on record for Vermont as a whole for the
September 1998 to December 1999 period. Unlike
many of the surrounding states in New England
entirely representative of the whole state (Figure lc).
Monthly precipitation totals at each COOP station
were used to generate pluviosities, which in this case
represent the ratio of precipitation depth to the mean
monthly value. The standard period normal could also
have been used to calculate the pluviosities. Ratios of
less than one (100 percent) indicate dry conditions,
while values greater than one denote wet periods
(where dry conditions were observed since at least the
summer of 1998) or the mid-Atlantic states (where
drought conditions persisted from 1997-1999), the
area-weighted state average for Vermont for September 1998 shows that above average precipitation was
received, following one of the wettest summers on
record for the state. Precipitation surpluses tend to
complicate the determination of the onset of the
(Beran and Rodier, 1985).
At the climate division scale, monthly drought
indices — Standardized Precipitation Index (SPI), and
modified Palmer Drought Severity Index (PMDI) —
were acquired from the National Drought Mitigation
Center/Western Regional Climate Center and
NOAVNCEP, respectively. The performance of these
indices relative to the actual observed drought characteristics was evaluated. Given the inherent drawbacks of the monthly PMDI (Alley, 1984; Guttman,
1998), weekly PMDI and Crop Moisture Index (CMI)
values were also examined to determine the influence
drought by enhancing the fact that surface moisture
supplies in both the soil and vegetation were often
adequate in the face of the initial precipitation
deficits (Ebert, 1997). The wet conditions in Septem-
ber 1998 were followed by three months of below
average conditions, culminating in December 1998
when the statewide precipitation receipt was only 32
percent of normal. This trend towards precipitation
shortfalls through the fall and into the winter was
interrupted by the receipt of above average precipitation in January 1999 (and to a lesser extent in March
1999), making pinpointing the exact date of the onset
of dry conditions at the state level somewhat ambigu-
of shorter time scales on drought monitoring and
assessment.
ous.
Ground water and Lake Champlain levels were
used to determine the lagged response to the precipitation shortfalls. Data from seven monitoring wells
affiliated with the U.S. Geologic Survey (USGS) as
measured at the end of a given month, were analyzed
Influence of Spatial Scale
The onset of the drought was also very much a
function of the spatial scale examined. Most of the
current drought data exist at the climate division
with respect to their quartile values. Finally, daily
levels of Lake Champlain were acquired from the
King Street Ferry Dock in Burlington, Vermont.
level. Figures 2, 3, and 4 are composites by (a) climate
division of the precipitation receipt, (b) monthly
PMDI, (c) one-month SPI, and (d) weekly PMDI and
CMI (d) over the period September 1998 to December
1999. It is immediately clear that the onset and pattern of precipitation deficits varied markedly over the
three divisions as well as by index or statistic used.
In focusing on the county level data, an even more
RESULTS
The Development of the 1998 to 1999 Drought
Drought is by nature a "creeping phenomenon" for
which an accurate determination of the onset or end
accurate representation of the onset and spatial
extent of the dry conditions is gleaned. Figure 5
is difficult (Wilhite and Glantz, 1985) and some
authors (e.g. Hershfield et al., 1973) consider the
shows the cumulative precipitation departure from
normal for each county, grouped by climate division.
While the global statewide data for September 1998
beginning of a drought can only be recognized after a
period of time. One way of identifying a drought's
indicate that above average precipitation was
onset is directly from precipitation statistics or
received, there were actually five counties in central
(Orange) and southern Vermont (Bennington, Rutland, Windham, and Windsor) with below normal precipitation. With the exception of Windham County,
the others displayed increasingly negative cumulative
departures over the entire period of record. It is not
surprising therefore, that the severity of the drought
manipulation thereof while another method is via a
drought index. In terms of the former, the origins of
the most recent drought can be traced to the fall of
1998 when precipitation shortfalls relative to normal
were first observed at a statewide level.
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JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
Thwards Characterizing and Planning for Drought in Vermont — Part I: A Climatological Perspective
TABLE 1. Statewide Precipitation Totals, Percent of Normal and Rank for Vermont
(1 is the driest and 105 the wettest year on record).
Month
Percent of
Precipitation
Normal
(mm)
Ranking
(1 = driest year
on record)
105.9
121
66
66.3
78
45
November 1998
66.5(5)
70
32
December 1998
26.9
32
4
January 1999
82.8
131
72
February 1999
March 1999
41.4
71
21
94.2
134
74
April 1999
42
2
May 1999
34.0(4)
89.2
95
57
June 1999
68.1
71
19
July 1999
96
99
52
August 1999
61.7
55
14
215.4
246
105
October 1999
88.4
104
65
November 1999
68.6
73
38
36.3
43
10
September 1998
October 1998
September1999
December 1999
(Data compiled by the Northeast Regional Climate Center — 1998 represents 104 years of record and 1999 represents 105 years of record.)
impacts was most striking in the southern portions of
the state. For many of the northern counties (with the
exception of Franklin and Essex Counties), precipitation deficits leading to negative departures from nor-
Although every drought is unique in its physical
characteristics, location, and areal extent within each
agreement with the coarser statewide data.
The 1998 to 1999 drought was originally categorized
as mild-moderate with most of the impacts being concentrated along the western and southern portions of
the state. By mid to late summer 1999, effects of the
serious drought, some localities are more severely
impacted than others. This spatial heterogeneity renders the areal quantification of a drought difficult.
mal were first observed in December 1998, in
Figure 6 shows the pluviosities at the 20 COOP
stations, augmenting the level of spatial detail found
at the county scale. It is interesting to note the latitu-
accumulated precipitation deficits were observed
dinal similarities betweenlamong stations that cut
statewide, with conditions in August being considered
as severe.
The identification of the end of a drought is simpler
or as elusive, again depending upon the way in which
drought is defined. Hydrologically, Beran and Rodier
(1985) consider the end of a drought to be more visible
and easier to determine than the commencement, par-
across county boundaries. For example, the southern
towns of Rutland, Chittenden, and Cavendish
received about 75 percent of normal precipitation at
the onset of the drought. Further north, the towns of
Chelsea, Bethel, South Newbury, and Union Village
Dam displayed marked similarities in the magnitude
of both the deficits (e.g., February 1999) as well as
ticularly when abundant rain saturates the ground,
increasing streamfiow and recharging ground water
surpluses (January and September 1999). On the
western side of the state, Burlington, Salisbury, and
Cornwall showed rapid declines in pluviosity at the
onset. Most of the towns that comprise the northern
reaches of Division 1 did not display any clear patterns. Mt. Mansfield stands out as a notable exception. At 1,339 m it is Vermont's highest peak and the
reserves. Following the winter shortfalls, the summer
of 1999 was not marked by abundant, prolonged pre-
cipitation. Instead, heavy downpours from severe
weather events over the course of a day or more alter-
nated with much longer periods of zero or trace
amounts of rainfall. Examples of such events included
above normal precipitation reflect enhanced oro-
a "conveyor belt system" on May 19, 1999, which
graphic effects.
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
brought substantial rainfall totals of 12.7 to 74.2 mm
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514
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
Towards Characterizing and Planning for Drought in Vermont — Part I: A Climatological Perspective
to locales in northern New England, including
Vermont; convective and frontal events at the end of
June and the beginning of July that produced similar
totals to the aforementioned event; and a series of
frontal systems that moved across the state in
August. The mesoscale convective system during the
week of July 4, 1999 produced electrical outages and
tree damage across Vermont, and even spawned a tor-
nado a few hundred kilometers to the north in the
province of Québec. Each of these episodes aided in
the temporary recharge of the upper soil profiles,
while ground water supplies continued to be stressed.
By the end of August a number of record low flows
were observed in both streams and ground water
monitoring wells around the state.
September 1999 was characterized by substantial
accumulations associated with Hurricane Dennis
(September 6-7), Tropical Storm Floyd (September 1617) and a series of cold fronts that moved across Ver-
mont in the subsequent period. Precipitation totals
related to Tropical Storm Floyd alone ranged from
76.2 to 127 mm statewide, with a maximum of 289
mm being recorded at Mt. Mansfield. Daily stream
discharges responded rapidly to the moisture inputs,
remaining above their respective mean daily flow values in the weeks following the storm. it is important
to note that under nondrought conditions, these large
precipitation accumulations would have produced
flash flooding, indicating the extent of the moisture
stress on the environment. September 1999 was the
second wettest month on record at the Burlington
International airport and the fourth wettest month on
record at St. Johnsbury. The rains of September and
October 1999 contributed significantly to recharging
both surface and subsurface moisture supplies, such
that groundwater conditions across the state were
also undermines the usefulness of consecutive rainfree days as a definition. Similarly, definitions that
rely on the number of consecutive days for which
streamflow discharge remained below a given thresh-
old, are incomplete because the steady decline
towards base flow conditions may be offset by these
large precipitation events. Thus, the duration of the
1998 to 1999 drought is related to the spatial scale in
question and the recovery of the entire landscape as a
unit. Using the finest spatial scale the drought began
as early as September 1998 in the southern part of
the state, persisted through September 1999 when a
reversal began such that by November 1999 ground
water levels had returned to normal.
Intensity and Severity — Impacts and Consequences
The intensity and severity of a drought are related,
but not equivalent. Drought intensity may be qualitatively described as the strength of a given episode, or
the degree to which the moisture is deficient. It can be
measured by the departure from normal of a climatic
index (Wilhite, 1983), low flows in rivers or groundwa-
ter levels or the effects on crops, flora and livestock
(Tase, 1976). Severity is related to the impacts caused
by the water deficit and depends upon the duration,
intensity and areal extent of a given drought episode,
as well as on the demands made by human activities
and vegetation on the water supply of a region (Wilhite and Glantz, 1985). Drought severity can be quantified by declines in soil moisture as well as rainfall
and runoff deficiencies (Hudson and Hazen, 1964).
The unevenness of the drought impacts over space
were initially a function of the spatial distribution of
the precipitation received. As the drought intensified
near their median quartile values (considered normal)
by the end of November 1999. The lower than normal
during the spring and summer of 1999, the initial
precipitation in the western part of the state during
November and December 1999, gave way to above
normal precipitation during the late winter/early
spring of 2000 effectively ending the 1998 to 1999
impacts of the deficits were evident in shallow rooted
vegetation such as grasses and flowers. These would
later be translated into economic ones such as forest
health issues, tourism, agricultural losses, and eventually hydrologic shortfalls that were intricately relat-
drought.
ed to the pedology, topography, and geology of a given
The duration of a drought is therefore intimately
linked with its starting and ending dates and is some-
locale. Thus, given that the intensity and, therefore,
impacts varied widely across the state and within a
times expressed in terms of the number of consecutive
rain free days. However, for a variety of reasons, an
absolute number of days or weeks of no or low precipi-
given climate division, statewide, and divisional
drought data are questionable for representing
drought severity as a function of areal extent.
tation may be inappropriate for quantifying the
length of the moisture deficit in the Vermont context.
For example, the severe precipitation shortfalls that
were observed in the early stages were interspersed
with above average precipitation receipt in January
and March 1999. The sporadic nature of the summer
convective and frontal activity previously described
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Agriculture
Agriculture and related activities account for $1.2
billion annually with prime sectors including dairy
farming and other animal husbandry, corn, apple, and
515
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Dupigny-Giroux
vegetable production. It was here that the impacts of
agricultural drought impacts could best be mapped as
a function of precipitation receipt and local conditions. Dairy farming suffered in three ways. Precipitation shortfalls led to the drying of pastures and low
yields of grass hay and alfalfa. The Farm Service
Agency estimated an almost $30 million loss in hay
and pasture statewide by the end of 1999. As a result,
dairy herd intake during the summer needed to be
supplemented with hay that is usually stored for the
subsequent winter. Secondly, the forage crops (alfalfa,
several strains of clover, birdsfoot trefoil, timothy, and
a variety of cool-season grasses; Sid Bosworth, 1999;
personal communication) used for this winter hay
feed were also affected due to their susceptibility to
drought. Alfalfa is a more drought-resistant crop than
grasses, but is negatively impacted by a lack of adequate snow cover in the winter. Four years ago (1995)
snowfall deficits were so extreme that many farmers
in Addison County replaced alfalfa with legumes and
grasses, after losing 75 percent of their alfalfa crop
that winter, which proved to be an unfortunate choice
Corn production losses in the western counties of
Addison, Chittenden, and Rutland were estimated at
$2,249,520, $29,586, and $664,290, respectively (Mike
Toussaint, 1999: personal communication). In these
regions, stunted plants, rolling leaves, and small ears
were evidence of the lack of precipitation at critical
stages of growth. As farmers began chopping the
standing stalks for silage for their animals, the concern shifted to that of the abnormally high concentrations of nitrates in the lower parts of the stalk. Under
dry conditions, the conversion of nitrate into protein
is interrupted leading to nitrate accumulations that
could prove toxic to animal health. The danger is
exacerbated when nitrate-tainted feed is combined
with nitrate-contaminated drinking water (Comstock,
1999). Apple loss estimates exceeded $1 million
statewide. For the Christmas tree industry with over
300 growers who earn $15 million annually, 50 to 100
percent of the seedlings or transplants set in 1999
were lost, although the growth rates and color of the
market-sized trees were acceptable (Jeff Carter, 1999;
personal communication).
given the summer of 1999 (Craig Miner, 1999; person-
al communication). By January 2000, at least 150
farmers prepared to request federal assistance in supplementing their feed stores (Golec, 2000). Finally,
livestock water supplies were also affected. By August
1999, the Vermont Emergency Management (VEM)
TABLE 2. Volume and Percentage of Carrying Capacity for the
Somerset and Harriman Reservoirs, June to December 1999.
had deployed the water buffalo of the Vermont
National Guard as shallow ponds and wells ran dry
on 292 farms statewide (Mike Toussaint, 1999; personal communication). VEM also possessed four 750
gpm (gallons per minute) pumps and three miles of
pipe for loan as needed (Louise Calderwood, 1999;
personal communication). Much of the emergency
Type of
Loss
Statewide
Loss
County
Hay
$25,192,794
Addison
Rutland
Wmdham
Corn
$2,935,400
water supplies needed to be stored in borrowed maple
sugar sap drums because subsurface water levels
were so depleted that filling ponds and wells resulted
in downward percolation to recharge the underground
aquifers (Debra Marckres, 1999; personal communica-
$1,390,000
Rutland
Water
$1,640,000
Franklin
Rutland
Addison
$395,000
$255,000
$250,000
Orleans
$600,000
(Source: Farm Service Agency, Vermont.)
Finally, some crops actually thrived in the warm,
dry conditions. Fruits like berries and grapes were
found to be larger, sweeter and ripened earlier (in the
case of the former). Hot weather crops like tomatoes,
cucumbers and non-silage corn also ripened early and
in abundance.
Soviet Union.
Substantial losses were also observed in a number
of nonhusbandry activities as summarized on Table 2.
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$2,249,520
$664,290
$29,586
$1,390,000
Livestock
$14,917,500
$4,646,425
$3,630,000
Rutland
Chittenden
Apple
tion).
By May 1999, agricultural drought conditions were
compounded by atmospheric drought, whereby the
abnormal dryness of the atmosphere produced a high
evaporative demand on vegetation and soil moisture
supplies. Daily maximum temperatures of at least
30CC were often accompanied by very low relative
humidities of 25 percent or less. The intensity of the
drying effect can be further exacerbated by moderate
to strong winds of higher than usual temperature, a
synergistic combination referred to as sukhovei in the
Addison
Loss by
County
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Water Resources
Apart from the effects of depleted soil moisture levels on crops and vegetation, other components of the
hydrologic cycle that were stressed as a result of the
precipitation shortfalls included snowpack accumulation, lake levels, river discharge, and ground water
supplies. As with agriculture, the timing and extent of
these impacts varied with location, magnitude of the
shortfalls and characteristics of the environment (e.g.,
soils, geology, and well depth).
At the onset of the drought in September/October
1998, Lake Champlain levels (Figure 7) were approximately 0.5 m above the expected low flow conditions
of early autumn. These levels fell through January
(contrary to the usual rise) before rising to about
0.15 m below normal in March 1999. The annual peak
in April was again 0.6 m below expected values, in
response to the below normal snowmelt that was
itself a consequence of below average snowpack, snow
density, and liquid water equivalent values. At the
same time, normal/above-normal conditions were
observed across the Upper Connecticut River and its
tributaries. By early to mid-September lake levels
had declined to their lowest values in 1998 to 1999
period. One of the factors behind the short lag time
between these low levels and the subsequent response
to the tropical storm remnants is the fact that most of
the rivers flowing into the lake peak within 24 hours
of a precipitation or snowmelt event (Shanley and
Denner, 1999).
The additional moisture stresses of the summer of
1999 exacerbated existing stressors of faulty dam outlets and an ever-increasing beaver population on Sunset Lake in Orange County. The 23.5 ha, 2 m deep
lake ran dry in September temporarily jeopardizing
the wetland and wildlife resource it offered (Duke,
1999).
By the end of June 1999 the monthly mean stream-
flow levels across the state had fallen below their
respective 25 percent quartile (Q25) values to levels
more typical of August and September. Record low
lows were observed along the Sleepers River near St.
Johnsbury in Caledonia County (James Shanley,
1999; personal communication). The USGS has moni-
tored this site as its index station for Vermont since
E
I0
I
Iw
ILi
1998
TIME (days)
1999
Figure 7. Lake Champlain Water Levels at the King Street Ferry Dock, Burlington, Vermont, September 1998 to
December 1999. Maximum and minimum values correspond to the record high and low values.
Data are missing on observed U.S. holidays.
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Dupigny-Giroux
1991. The monthly mean values for August showed
the greatest departure below the Q25 value for a number of other rivers including the Poultney River, Lam-
forestry sectors, further wildfire outbreaks did not
occur.
Northern hardwoods (maple, beech, and birch) predominate as a forest type across the state, with sugar
maple being the most abundant tree species (Teillon
and Wilmot, 1993), although beech and birch trees
were most affected by the drought (Sandy Wilmot,
2000; personal communication). Like the red maple,
sugar maples are susceptible to dry conditions and
leaf scorch was clearly observed in the urban trees
oille River, and Dog River.
Record low streamfiows coincided with record low
ground water levels at Hartland (Figure 8d) and Morriscw (Figure 8g), two of the seven wells monitored
by the USGS. Both wells are relatively deep (on the
order of 15 m) and have similar flow characteristics
even though their subsurface materials differ:
sand/gravel for Hartland and silty to fine/medium
sand for Morristown. If low flow durations are defined
as the length of time for which Q is exceeded, all
seven wells were lower than their median (Q50) level
from April to August 1999.The relationship between
ground water well depth and precipitation inputs is
not a linear one, although most wells seemed to lag
their respective county precipitation by a month. The
measurement of well depths at the end of a given
month makes it difficult to determine the precise
lagged response to a high magnitude precipitation
across Chittenden County during the summer of
1999. In many cases, the scorch affected the outer
edges of the leaves, reflecting a reduction in the surface area for transpiration (Ian Worley, 1999; personal
communication). In other species such as oak and
hickory, entire trees became so moisture-stressed that
total mortality was observed. These dead, brown trees
were striking markers especially during aerial data
collection over Chittenden and Addison Counties in
August to September 1999, prior to the stripping
away of the leaves by strong winds during Tropical
Storm Floyd, and the senescence of other species in
September and October 1999. The mortality of these
event. As Figure 8 shows, the relatively shallow wells
responded to the tropical storm remnants of September 1999 exceeding their respective Q75 levels, where-
and other species may mask an even more widespread
as deeper wells like Pittsford (Figure 80, Hartland,
and Morristown only rose above their corresponding
forest health issue should they fail to recuperate in
Q25 levels.
In all, 85,000 acres (34,425 ha) of forest were
affected with drought symptoms ranging from leaf
scorch, leaf yellowing, and browning to early leaf
colour and leaf drop (Sandy Wilmot, 2000; personal
the summer of 2000.
Forestry
communication).
Two main issues of concern to Vermont's forests
were the threat of wildfires and long-term forest
health. With the aforementioned, atmospheric
drought of the late spring/early summer posed the
Tourism
threat of wildfires. April 1999 was marked by brush
fires in Bennington and Windham Counties as the
precipitation shortfalls combined with gusty winds to
turn the underbrush into tinder (National Weather
Service, 1999; personal communication). During the
weekend of May 1 and 2, 40 to 50 wildfires were
reported across the state, in addition to the 54 fires
reported at the end of April (Sutoski, 1999). The outbreak of wildfires led to a restriction and eventual
rescinding of burn permits during the month of May.
The wildfire threat was also fueled by the presence of
large quantities of dry combustible material on the
ground. Much of this debris resulted from damages
caused by the ice storm of January 1998. For trees in
the northern part of the state that came under varying ice loadings during the storm, the ongoing issue of
forest health is a very pertinent one. It is interesting
to note that even though moisture deficits in the topsoil and duff conditions continued throughout the
summer of 1999, impacting both the agricultural and
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A large percentage of Vermont's $3.7 billion tourist
industry is intricately woven with its natural
resources. Three sectors impacted by the dry condi-
tions include the ski industry, fishing, and other
aquatic sports, as well as businesses that cater to fall
foliage visitors (also known as leaf peepers). The
American Skiing Company, which owns several
resorts, reported a $9.8 million loss for the period
November 1998 to July 1999 as a result of the lack of
snowfall and warmer than usual temperatures (Soga,
1999). It is interesting to note that in October 1999,
the Killington Ski Area was granted the right to
extract 362 million gallons of water annually from the
Woodward Reservoir in Windsor County for snow-
making activities. This decision was significant
because the resort is located in the drier part of the
state and more importantly, at the time of the ruling,
reservoir and stream levels were recuperating from a
lower than normal baseline following a 15-month
drought.
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Towards Characterizing and Planning for Drought in Vermont — Part I: A Climatological Perspective
d'
a
-
:
I
b
I
I
——
I I
I I
e
L--
—.__.4._
—--......
III
f
C
,.
— ._....:---I
*::;
I
g
:
Figure 8. Monthly Well Depths at the Seven USGS Monitoring Sites. Q25, Q50,
and Q values have also been plotted. Scales differ for deep vs. shallow wells.
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
519
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Dupigny-Giroux
In terms of aquatic life, both insect and fish populations were affected. Warm, dry conditions decreased
Figures 2(b) and 2(d), 3(b) and 3(d), and 4(b) and 4(d)
summarize each index over the period of interest.
the surface area used for breeding by mosquitoes,
black flies, and ticks and caused an acceleration in
their life cycles. This in turn created a nutritional
deficit for the fish populations that rely on waterborne insects. As the water temperature rises, the
metabolism of fish species such as trout and bass
slowed causing them to seek deeper, cooler water
either away from the banks where anglers are located, or in shallow poois where they were exposed to
For a given averaging period, the Standardized
Precipitation Index of McKee et al. (1993) represents
the deviation of the precipitation received from the
mean, as adjusted to a normal/gamma distribution.
Dry events are said to exist during the months for
which the SPI remains negative, with at least one
month having an SPI value of -1 or less (Guttman,
1998). The one-month SPI (hereafter referred to as
SPI-1) is analogous to the percent of normal precipita-
tion and reflects conditions over the short term
other types of predators (Buckley, 1999).
The acceleration in the life cycle was also observed
(National Drought Mitigation Center, 1999). SPI values at the three-month (SPI-3), six-month (SPI-6) and
nine-month (SPI-9) time steps (Figure 9) were also
evaluated. From these four averaging periods, the following picture emerges. The SPI-1 captures the dry
conditions at the onset in September/October 1998, as
well as throughout the summer of 1999. However, as
among wildflowers. Wildflower Farm in the town of
Charlotte, which is located between Chittenden and
Addison Counties, is known for its variety of blossoms
which bloom at varying intervals from March to October. However, the hot, dry weather caused most of the
flowers to blossom several months earlier than usual,
leaving few species in bloom by late August 1999.
Vermont is well-known for its brilliant fall foliage
and as summer faded into autumn there was speculation about the impact of the hot, dry conditions on the
1999 fall foliage season. Warm sunny days followed by
cool nights have been found to activate the decomposi-
expected, the SPI-1 is rather sensitive to larger
monthly totals, even if they were produced by one or
two events that did not reverse the drying trend. SPI3 mirrors the SPI-1 and captures the drought's onset
in Divisions 1 and 2, although its interpretation in
tion of chlorophyll so that the inherent carotenoids
(yellow, orange, and brown) and anthocyanins (red,
Division 3 is somewhat more problematic. The large
fluctuations of the SPI-3 values over time are a function of its sensitivity to short-term precipitation vari-
purple) emerge. By mid-August, some deciduous trees
had already begun changing color, partly in response
ations as well as to the combinations of disparate
counties within a climate division. For example in
to the cooler conditions and partly in response to
Division 2, Lamoille and Franklin Counties in which
ongoing stress of drought and shallow soils. Yellowing
the deficits were not marked prior to March 1999
and early fall foliage are both documented results of
moisture stress. Aided by the rains from the extrat-
were included with Addison, Rutland, and Bennington Counties where the drought signal can be traced
back to 1998. The longer-term SPI-6 is less sensitive
to the effect of these large precipitation events, making it a good representation of the overall cumulative
drying of the environment.
The Palmer Drought Severity Index was initially
ropical storms in September, fall foliage changes
appeared to be on schedule although color in the
northeastern-most portion of Vermont (Northeast
Kingdom) was described as "subdued" (Sandy Wilmot,
1999; personal communication). A month later, many
of the northern forests were past their peak color and
developed as a meteorological index, to quantify
those in the southern parts of the state were recover-
drought by the interplay between the magnitude and
duration of the moisture deficiency, as expressed by
the weighted differences between the actual precipitation and evapotranspiration (Walsh, 1987; Wilhite
ing enough to change color.
THE ROLE OF DROUGHT INDICES
and Glantz, 1985). The modified PDI (PMDI) as
implemented by the National Weather Service Cli-
Drought indices represent an operationalization of
drought and crop moisture information that is summarized and periodically disseminated on a regional
basis. The performance of three such indices (Stan-
mate Prediction Center, uses probabilities to weight
the average of the dry and wet index terms (Heddinghaus and Sabol, 1991). An examination of the PMDI
for Division 1 reveals the September 1998 to March
1999 period with varying levels of wetness and it is
dardized Precipitation Index, the modified Palmer
Drought Index, and the Crop Moisture Index) was
only in August 1999 that mild-moderate drought
becomes manifest. In the western division, again a
examined in terms of capturing the foregoing drought
characteristics. Each index is available at the climate
sequence of moist months is observed from September
1998 to March 1999, with mild to moderate drought
beginning in April 1999 and lasting through August
1999. The southeast division is unique in that mild to
division level at a monthly time step, with the PDI
and CMI also being calculated at weekly intervals.
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Towards Characterizing and Planning for Drought in Vermont — Part I: A Climatological Perspective
a
'U
C
z
Z
C
TIME (month,)
b
'U
C
—+-SPI.3
—.—spI.6
C
2
—I
-2
TIME (months)
C
-SPI.3
——SPI-6
-SPI.9
TIME (month.)
Figure 9. Standardized Precipitation Index Values for the Three-, Six-, and
Nine-Month Averaging Periods, for Climate Divisions 1(a), 2(b), and 3(c).
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
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Dupigny-Giroux
moderate drought existed from September 1998 to
December 1998 and in April 1999. From June to
August 1999, conditions across this division deteriorated from mild-moderate to severe.
The lack of congruency between the SPI-1 and
PMDI, especially at the drought's onset, is largely
related to the computational and theoretical characteristics of the Palmer Drought Index. The latter
responds slowly to precipitation departures since it
was designed to integrate antecedent weather conditions. Instead, it responds to deteriorating soil moisture conditions, making it more of an agricultural
drought index (and perhaps even a hydrologic indicator) than a meteorological one (Wilhite, 1983). Thus,
the severe precipitation deficits of December 1998
were not, by and large, well captured by the PMDI.
The failure to capture the onset of the drought in the
fall 1998 and its continuation into the winter and
spring of 1999 is related to the fact that all precipitation is treated as rainfall in the computation of the
index (Hayes et al., 1999), even though snow and
freezing rain are the more likely forms of winter precipitation in Vermont. The slow response of the PMDI
closely resembles the character of the SPI-9 in Divisions 1 and 2. The height of the drought was observed
in August 1999. Division 3 differs in that it was the
only division in which the summer of 1998 was not
marked by precipitation surpluses. Thus, drier
antecedent conditions may help to explain why the
PMDI was able to indicate the presence of drought
from as early as August 1998 and for most of the period of interest.
In order to circumvent the long-term nature of the
Palmer Drought Severity Index, the Crop Moisture
Index (CMI) was derived by Palmer (1968). The CMI
examines the relationship between subnormal precipitation levels, soil moisture supplies and evapotran-
spiration demand (Walsh, 1987). It is essentially a
measure of evapotranspiration anomalies. The CMI
differs from the PMDI and SPI in that its interpretation depends on whether the index values are increasing or decreasing. Across all three divisions, moist or
favorable topsoil conditions existed from September
instead of August in Division 1 and in week 11
(March) instead of the AprilfMay time frame in Division 2. In Division 3, however, the mild to moderate
drought conditions of September to December 1998
were not borne out in the weekly values, although
coincidence with mild to moderate drought conditions
was observed in March. In all three divisions, the
rapid response of the upper soil layers to the precipitation inputs of Hurricane Dennis and Tropical Storm
Floyd was striking at both the weekly and monthly
time scales.
These results highlight a number of difficulties
involved in using each drought index. The SPI-1 on
average performed better than the PMDI in terms of
detecting the onset of dry conditions and the severity
of conditions in April and August 1999. The slow
response of the monthly PMDI in Divisions 1 and 2 is
problematic for drought monitoring and assessment.
However, by computing the SPI-1 and monthly PMDI
at the divisional level, neither index takes into
account either the spatial or temporal distributions of
the precipitation received. This lack of spatial congruency may partly stem from the fact that climatic divi-
sions across the U.S. are primarily defined by
"drainage basins or major crops" (Guttman and
Quayle, 1996:300) and may not always be climatically
homogeneous. Thus, the high magnitude precipitation
events of March, May, July, and September 1999
skewed the monthly index values towards anomalously moist conditions. Given these fluctuations between
drought and moist conditions, neither drought index
is truly able to account for the cumulative drying of
the landscape as a hydrologic system and should be
combined with other parameters (e.g., soil moisture,
surface, and subsurface flow) if the true scope of
drought severity is to be quantified. In this respect,
the weekly CMI values provided an added dimension
of the temporal distribution of the precipitation.
DISCUSSION
1998 through March 1999 (week 9). From then
Hydrometeorological Characteristics of Drought
in Vermont
onwards, the topsoil was abnormally dry in Division 2
to the extent that yields were jeopardized by the end
of August (week 35). Division 3 displayed similar
From the foregoing description as well as from
analyses of recent droughts (particularly the 19941995 event), the following characteristics of drought
in Vermont emerge. Several types of drought were
observed suggesting that a multi-faceted definition of
characteristics with several weeks in March and May
returning to favorably moist conditions in response to
the precipitation inputs. In Division 1, surface mois-
ture levels were only threatened in August 1999
(week 35).
moving beyond the corresponding monthly data. For
drought needs to be considered. The deficits in precipitation reflect one component of meteorologic drought,
drought conditions were observed in week 17 (April)
the system to meet the demand. The excessive drying
Weekly PMDI values are particularly useful in
such that inadequate moisture is being supplied to
example in Divisions 1 and 2, mild to moderate
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of the atmosphere resulting from the combination of
high temperatures and low humidities are indicative
of atmospheric drought. As the combined influence of
inadequate moisture and an enhanced drying capacity
of the air impacts the vegetation and upper soil profile, agricultural drought is observed. Finally, stream-
flow drought exists when streams, lakes, and
reservoirs are at low levels and sectors such as
hydropower that depend on them are affected.
Drought analyses can therefore be performed either
on the stochastic nature of these individual components or by examining the interaction among them as
one hydrologic system. The tight coupling of the these
individual components of the Vermont landscape suggests that it should be considered as one hydrologic
system at the macro level, with spatial variability at
deficits only became marked in the mid to late summer 1999. Superimposed on this picture is the observation that even within severely-impacted regions,
drought impacts tended to be localized as a result of
differences in soils, geology, land cover, and elevation.
Both the recent drought and the widespread event
in 1995 have been typified by a number of record-set-
ting maximum temperatures, which serve to contribute to the moisture and thermal stresses on plant
life. In 1995, 15 new records were set during the
months of June and July, when the drought was most
intense. Four years later, nine new records were set
for the period May 31 through July. It is interesting to
note that many of the previous record maxima had
also been set during drought years.
tation. Rather, low precipitation months are often
An important causal link setting off a drought is
the presence of persistent high pressure systems over
the region. The source areas for these pressure systems fluctuate and may represent a northwest expansion of the Bermuda high or a southeast expansion of
interspersed with those of above or near normal precipitation, often with a skewed temporal distribution.
The onset of drought during the cooler season is likely
to be observed only in the precipitation signal, since
ground water and stream levels are at their natural
annual minima and declines in surface soil moisture
Given that high pressure systems are associated with
subsidence, this acts as a convective cap, which also
prevents moist, tropical air from invading the region.
This, in turn, results in an absence of precipitationproducing disturbances or humid air masses. More
the micro level.
The onset of meteorologic drought in Vermont is
not marked by successive months of low or no precipi-
a high pressure over the Hudson Bay in Canada.
may be difficult to detect under snow cover or in
sunshine therefore reaches the earth's surface,
frozen ground. At first the precipitation deficits may
not be noticeable since water consumption tends to be
low at this time (National Weather Service, 1999; personal communication), but they become more so during early spring (March/April). As these shortfalls
continue into the summer, the severity of the impacts
increases. The influence of the precipitation deficits
on the hydrology of the region may either be: (a) overshdowed by spring snow melt (a positive or buffering
effect); or (b) exacerbated by the fact that water levels
tend to be naturally low at this time of the year, since
recharge occurs during the fall.
The severity/intensity of the precipitation shortfalls is related to the existing ground cover or land
use, prevailing temperature and humidity characteristics of the atmosphere, vegetation phenology, plant-
increasing the likelihood of record-setting temperatures being attained.
Related to the influence of anticyclonic circulation
is a phenomenon known as the "ring of fire." This
relates to the fact that along the edges of the high
pressure regions where the subsidence is weaker,
instability of the atmosphere leads to the formation of
precipitation-producing systems. The movement of
these systems along the northern and northeastern
portions of the state may help to account for the fact
that deficits in these locales were not as severe as the
western and southern portions of the state.
Although every drought is unique in terms of its
spatial and other characteristics, the 1994 to 1995
and 1998 to 1999 droughts shared a number of
marked similarities that have implications for the
understanding of the drought cycle. Both events
began in the preceding fall and intensified over the
ing/growing cycles, and the water recharge cycle.
When the deficits coincide with planting dates or critical times during the growth cycle, crop growth may
be stunted or lost.
The inequality of precipitation receipt is translated
course of the subsequent summer. In both cases, the
streamflow and ground water levels were severely
impacted, and in the case of the 1994 to 1995 event
water quality issues became important. These components of the hydrologic cycle are not as adversely
affected when the drought onset occurs in the spring
or summer before terminating in the fall. Another
similarity between the two events was the manner in
which the drought ended or was reversed. In October
1995, Hurricane Opal — followed by a series of strong
cold fronts — brought sufficient precipitation to
into variations in the spatial extent, severity, and
magnitude of the drought. Deficits were first observed
in the southeastern portions of the state, followed by
the western sections (that fall in the rain shadow of
the Adirondack mountains and tend to be drier than
the rest of the state). Impacts, especially agricultural
losses were most severe in these sectors, and much
less so in the northeastern portions of the state where
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Dupigny-Giroux
Vermont to end the statewide drought with the
Division 3) where the atmospheric dynamics and
wettest October on record. In September 1999, the
land-surface interactions differ from the other parts of
arrival of Hurricane Dennis and Tropical Storm Floyd
the state. In addition, the monthly time scale has
(within a two-week span) together with a variety of
cold fronts, contributed substantially to recharging
the streamfiow and ground water levels. The drought
of 1998 to 1999 eventually ended with the receipt of
above normal precipitation in the late winter/early
been shown to be too coarse to capture the temporal
variability of the precipitation received, and as such a
weekly time step is suggested.
Finally, the SPI has been shown to perform better
than the PMDI in terms of capturing precipitation
departures, although it is recommended that any such
precipitation-based indices be complemented with
measurements of surface conditions, ground water
levels and crop indices specific to the agricultural
interests of the state.
spring 2000.
SUMMARIZING REMARKS
The onset of a drought during the cooler season of
the year creates a series of cascading effects on agriculture, water supplies, and other important socioeconomic sectors, that may not always be apparent until
ACKNOWLEDGMENTS
The author would like to thank the following individuals for
their assistance in providing data and technical assistance: Craig
the subsequent spring/summer. Both the onset and
Altemose, University of Vermont Extension Service; Charles McGill
and Roger Hill, National Weather Service/Burlington International
continued existence of the 1998 to 1999 drought were
interrupted by a number of high magnitude, shortduration events that skewed the temporal distribution of precipitation, making monthly totals (and
hence derived statistics or indices) questionable.
Airport; James Shanley, Jon Denner, and Chandlee Keirstead,
USGS- New Hampshire and Vermont; the Vermont Department of
Agriculture, Food, and Markets; the Vermont Farm Service Agency;
Thomas Heddinghaus, NOAAJNCEP; Mark Svoboda, National
Drought Mitigation Center; and Robert Shedd, Northeast River
These two factors together tend to delay the perceived
Forecast Center. Helpful suggestions were provided by two external
reviewers.
urgency of the threat of drought until the summer.
However, given the financial and economic expendi-
tures that were made in the drought's aftermath, it
becomes imperative that adequate monitoring and
mitigative strategies be implemented to improve
future resource allocation. The increased awareness
of this fact is borne out by the over 700 wells that
Alley, W. M., 1984. The Palmer Drought Severity Index: Lirmtations and Assumptions. Journal of Climate and Applied Meteo-
were drilled in December 1999 in the drought's aftermath (Bazilchuk, 1999). It is interesting to note that
Bazilchuk, Nancy, 1999. Well Levels Inch Back to Normal:
LITERATURE CITED
rology 23:1100-1109.
Drought's Affects Keep Drillers Busy. Burlington Free Press,
in Lamoille County (which was not as severely
December 21, pp.1B, 6B.
impacted by the 1998 to 1999 drought as it has been
in previous dry events), the overall impact was lessened because residents had used funds from the ECP
Program following the drought of the 1980s, to drill
wells (Linda Langdell, 1999; personal communica-
Beran, M. A. and J. A. Rodier, 1985. Hydrological Aspects of
Drought. A Contribution to the International Hydrological Pro-
gramme, World Meteorological Organization, Studies and
Reports on Hydrology 39, Paris. France.
Buckley, J. Taylor, 1999. Fish Feeling the Heat: Warm Weather,
Lack of Rain Takes Its Toll on Trout. Burlington Free Press,
tion).
August 8, p.8C.
Comstock, Jeff, 1999. Nitrate Thxicity Alert. Agriview 63(18);1,4.
Duke, Abbey, 1999. Sunset Lake Runs Dry: Residents Form Associ-
Given the multifaceted nature of drought in Vermont, an interdisciplinary approach that considers
ation to Combat Problems. Burlington Free Press, September 3,
the cumulative drying of the environment (soils, flora,
p. 4B.
fauna, and hydrology) in response to precipitation
shortfalls should be developed. The details of this
Dupigny-Giroux, Lesley-Ann, 2001. Towards Characterizing and
Planning for Drought in Vermont — Part II: Policy Implications.
pooled approach is discussed in a subsequent paper in
this volume (Dupigny-Giroux, 2001).
Drought planning efforts should be focused at the
Journal of the American Water Resources Association 37(3):527531.
Ebert, Charles H.V., 1997. Disasters — Violence of Nature and
Threats by Man (Third Edition). Kendall/Hunt Publishing Company, City?, Iowa.
county level or smaller to eliminate potential biases
or generalizations that currently exist by using
Golec, Matt, 2000. Crop Losses Put Bite on Feed Supplies: 150
statewide or climate division data. The use of county
Farmers Expected to Apply for Aid. Burlington Free Press, Jan-
data will also be at an appropriate spatial scale to
capture observed latitudinal and longitudinal varia-
uary 14, pp. 1A, 4A.
Guttman, Nathaniel B., 1998. Comparing the Palmer Drought
Index and the Standardized Precipitation Index. Journal of the
tions in precipitation inputs. Special attention should
be paid to the southeastern counties (which comprise
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American Water Resources Association 34(1):113-121.
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JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
'lbwards Characterizing and Planning for Drought in Vermont — Part I: A Climatological Perspective
Guttman, Nathaniel B., 1999. Accepting the Standardized Precipitation Index: A Calculation Algorithm. Journal of the American
Water Resources Association 35(2):311-322.
Guttman, Nathaniel B. and Robert G. Quayle, 1996. A Historical
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