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As one can see in Figure 4, a drought's characteristics change with time and at longer timescales droughts become less frequent but their duration increases. The time series of 3-month SPI (Figure 4(a)) shows that the station experienced frequent moderate, severe and extreme droughts. Analysis of the relative frequency of occurrence (Table 2) of the annual minimum monthly SPI, during rainy seasons as mentioned earlier, shows that May is the month during which the 3-month SPI most frequently takes the annual minimum value (13.7%), and it is followed by July (8.22%). The annual minimum SPI values for the 6-, 9-, 12-, and 24-month time series for the period of analysis were mainly observed in June, May, August and June, respectively (Table 2).

Table 2

Relative frequency of occurrence (%) of annual minimum monthly SPI for various timescales using SPI at Bahir Dar station

MonthSPI 3SPI 6SPI 9SPI 12SPI 24
January 9.59 7.35 4.84 6.67 7.50 
February 0.00 8.82 6.45 5.00 7.50 
March 10.96 10.29 9.68 3.33 7.50 
April 9.59 7.35 6.45 6.67 5.00 
May 13.70 10.29 12.90 8.33 7.50 
June 6.85 11.76 9.68 13.33 12.50 
July 8.22 8.82 9.68 10.00 10.00 
August 6.85 10.29 11.29 13.33 10.00 
September 6.85 5.88 6.45 10.00 10.00 
October 6.85 5.88 8.06 6.67 7.50 
November 6.85 5.88 6.45 8.33 7.50 
December 13.70 7.35 8.06 8.33 7.50 
MonthSPI 3SPI 6SPI 9SPI 12SPI 24
January 9.59 7.35 4.84 6.67 7.50 
February 0.00 8.82 6.45 5.00 7.50 
March 10.96 10.29 9.68 3.33 7.50 
April 9.59 7.35 6.45 6.67 5.00 
May 13.70 10.29 12.90 8.33 7.50 
June 6.85 11.76 9.68 13.33 12.50 
July 8.22 8.82 9.68 10.00 10.00 
August 6.85 10.29 11.29 13.33 10.00 
September 6.85 5.88 6.45 10.00 10.00 
October 6.85 5.88 8.06 6.67 7.50 
November 6.85 5.88 6.45 8.33 7.50 
December 13.70 7.35 8.06 8.33 7.50 

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