3.10 Drought 



Drought has many different meanings and definitions vary widely. 

 For example, it may be termed an agricultural drought when precipitation is 

 inadequate to maintain the moisture content of the soil at optimum levels 

 thereby affecting agricultural production. To others, droughts are character- 

 ized by more severe precipitation deficits that persist to the degree that 

 streamflow and ground water and thereby man's water supply are affected. A 

 satisfactory definition for the Long Island Sound Region is that of Thomas 

 ( 38) "drought is a meterologic phenomenon that occurs during a period when 

 precipitation is less than the long-term average and when this deficiency is 

 great enough and continues long enough to hurt mankind." 



The area may be affected by deficient precipitation for short 

 periods in any year or season. A table prepared for the region gives the 

 distribution of the total number of occurrences of consecutive days with less 

 than 0.1 inch of precipitation for durations of from 1 to 19 days and 20 or 

 more days. The total number of occurrences of 5 or more consecutive dry days 

 (less than 0.1 inch of precipitation) is uniform throughout the area. The 

 number of occurrences of 20 or more consecutive dry days however shows con- 

 siderable areal variation, ranging from 20 to 30 along the perimeter of the 

 Sound to ]k to 26 in interior parts of southern Connecticut. The chance of 

 an extended dry spell appears to be 15 to 50 percent greater along the coast 

 than in the hilly interior areas. 



A widely used measure of drought severity and duration is the 

 Palmer Drought Index (24) . This index is based on the accumulated weighted 

 difference between actual precipitation and that required for the particular 

 time and place. Figure 6 is a plot of the monthly Palmer Drought Index for 

 the period 1929-72 and can be used to determine the length and severity of 

 both wet and dry periods in the region. 



Figure 6 shows a total of 21 periods of drought lasting 3 or more 

 months. Droughts of severe or extreme intensity lasting a month or more occurred 

 7 times encompassing a period of 53 months. The first prolonged drought in 

 the period of record began in July 1929 and continued through August 1932; the 

 most extreme conditions were reached in September and October of 1930. The 

 longest and severest drought affecting the entire northeastern United States, 

 began in March I962 and continued through I967. This drought is classed as 

 severe or worse from August 1964 through February I967. 



In addition to affecting plant life these lengthy droughts resulted 

 in reduced streamflow and lowered ground-water levels. The greatest impact 

 was felt during the growing season when both agriculture and water supply must 

 be satisfied by precipitation plus the water stored in reservoirs and the 

 ground. The effect of the 1960's drought on streamflow and ground-water 

 levels has been summarized by Barksdale and others (J_) and Cohen and others 

 (6). Figure 7 depicts the streamflow and antecedent ground-water conditions 

 in 1965 when the hydrologic effects of the drought were most severe. 



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