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Drought: Drought in the Barren River Area
Drought is a result of the variable nature of climate in the Barren
River Area. However, climatic variation is best understood in
reference to what is normal. Under normal climatic conditions, the
Barren River Area receives about 50 inches of precipitation over a
year (Figure 4-1), with eastern counties averaging slightly higher
amounts than western counties.
Figure 4-1. Mean annual precipitation for Kentucky.
Proximity to moisture from the Gulf of Mexico combined with seasonal
atmospheric circulation patterns results in a fairly uniform
distribution of precipitation throughout the year (Figure 4-2). October
is typically the driest month, but it still averages more than three
inches of precipitation.
Figure 4-2. Average monthly precipitation, based on 1961-1990 normals for Bowling Green, Glasgow, Mammoth Cave National Park, and Scottsville.
The weather station located at the Bowling Green Warren County
Regional Airport has the longest continuous record of precipitation in
the Barren River Area. Figure 4-3 shows the distribution of annual
precipitation totals. Bowling Green received more than 45 inches of
precipitation in 70 percent of the years but received less than 40
inches in about one of every seven years.
Figure 4-3. Annual precipitation totals, Bowling Green, 1888-2000.
The Palmer Drought Severity Index (PDSI) is widely employed as an
indicator of drought. Precipitation and temperature data, along with
information about soil properties, contribute to the index (Hayes,
1999). The PDSI is calculated on a weekly basis by factoring in
precipitation and temperature data from the current week to update the
previous week's index. Because the index is calculated for climate
divisions, it does not necessarily describe conditions at any
particular place within the division. The index is scaled to zero,
with positive values indicating wet conditions and negative numbers
indicating dry conditions (Table 4-1). A severe drought, defined by a
PDSI of -3, becomes extreme if the index drops to -4.
Table 4-1. Interpretation of Palmer Drought Severity Index.
| PDSI |
Description |
| 4.00 and above |
Extreme moist spell |
| 3.00 to 3.99 |
Very moist spell |
| 2.00 to 2.99 |
Unusual moist spell |
| 1.00 to 1.99 |
Moist spell |
| 0.50 to 0.99 |
Incipient moist spell |
| 0.49 to -0.49 |
Near normal |
| -0.50 to -0.99 |
Incipient drought |
| -1.00 to -1.99 |
Mild drought |
| -2.00 to -2.99 |
Moderate drought |
| -3.00 to -3.99 |
Severe drought |
| -4.00 and below |
Extreme drought |
A monthly time series of the PDSI for Kentucky's Central Climate
Division is shown in Figure 4-4. Eight of the ten counties in the Barren
River Area are located in this division, making it the single best
indicator of drought history. The Barren River Area experienced severe
drought conditions on 12 separate occasions over this past century when
the PDSI dropped from being positive to a value less than -3. That
translates to a drought recurring about every eight to nine years.
However, droughts were more frequent and intense during the first half
of the century when severe levels were reached in 1901, 1904-05,
1908-09, 1913-15, 1930-31, 1936, 1939-42, and 1943-44. Since the
drought of 1952-55, only three droughts, occurring1963-64, 1988, and
1999-2000, have reached the severe level. According to the PDSI, the
recent drought of 1999-2000 was the longest and most intense since the
1950s.
Figure 4-4. Drought history of Kentucky's Central Climate Division.
While there remains much debate over projections for global warming,
some climate models suggest that the southeastern United States will
experience more intense droughts in the future, even if average annual
precipitation increases (National Assessment Synthesis Team, 2000).
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