Fact Sheet - Historic Droughts in Kentucky
Drought is essentially a prolonged and abnormal moisture deficiency as defined by Walter C. Palmer (Palmer, 1965). In addition to defining a drought episode, he developed a quantification system referred to as the Palmer Drought Severity Index, or the PDSI. The PDSI is a numerical index used to better define when wet or dry spells begin or end. To see and learn more about how the system was developed go to the explanation of the Palmer Drought Severity Index.
The PDSI consists of eleven numerical categories, ranging from an extreme moist spell (=> +4) to an extreme drought (=< -4) situation. For this look at droughts in Kentucky, the PDSI was used to determine the drought intensity, when it began, and when it ended by using the moderate drought category as a threshold. A mean PDSI value was calculated for each year from 1895-1998 for each of the four climatologica lregions in Kentucky. To identify the major droughts over all of Kentucky, the sum of PDSI values for the extreme and severe years were totaled for each region then divided by the number of regions in Kentucky, to get an average PDSI value for the drought year. The top five drought years in Kentucky are the results of three significant drought episodes. These episodes in Kentucky were as follows:1930-1931, 1953-1954, and 1941.
The graph of the Bluegrass Region from 1895-1998 concentrates on droughts that ended the month of May in the moderate category and how they progressed through June, July, and August. Out of the thirteen moderate droughts in May, nine or 69% continued to be moderate or drier over the next three months. In June, nine or 69% of the thirteen moderate droughts in May continued to be moderate or drier. In July, eight droughts remained in the moderate or drier category. By August, nine of the May moderate droughts were continuing in the moderate or drier pattern. It seems when May ends in the moderate drought category it continues or becomes drier in June, July, and August.
The conclusions drawn from the graph above can help to analyze current PDSI values from the Bluegrass region. This graph shows the PDSI values for each month in 1999 in the form of a line graph. The Bluegrass region began 1999 in the mild drought category and reached the moderate category in April. The month of May ended even further in the moderate category. The information from the previous graph shows 69% of the droughts that ended in the moderate category in May, continued to be moderate or drier. An expectation would be for the drought in 1999 to continue through the summer. As expected, the Bluegrass region ended June of 1999 in the moderate category as well.
In the first graph, nine droughts continued to be moderate or drier out of the original thirteen. Of the nine droughts that persisted in June, eight of those continued to be moderate or worse in July. The year of 1900 managed to make the mild drought category in July, only to go back into the moderate category in August. A statistical conclusion could be drawn stating that the drought conditions in 1999 will continue through August at the same intensity or drier.
The current values for the Eastern region in 1999 are similar to the numbers in the Bluegrass region for the same year. The Eastern region reached the mild drought category in February. May ended in the moderate drought category and continued throughout June and July with the same conditions. August will most likely sustain the dry trend over the Eastern region.
The Central region remained in the mild drought category from January to June 1999. In July the PDSI value dropped into the moderate drought category. A climatological outlook would have August remaining dry in the Central region.
United States Department of Commerce. Weather Bureau. (1965). Meteorological Drought. (Research Paper 45). Washington, D.C.