4 Obs_Sylhet_short
3
obs SPI
2 1 0 -1 -2 M6 1987
1983
1989
M3 1981
1979
1977
1975
1973
1971
1969
1967
1965
1963
M1 1961
-4
1985
-3
Fig. 7.1.5. (a). SPI calculated using Observed data in different short month-length at Station Sylhet. 4 Obs_Sylhet_long
3
obs SPI
2 1 0 -1 -2 -3 1989
1987
M24 1985
1983
M12 1981
1979
1977
1975
1973
1971
1969
1967
1965
1963
1961
M9 -4
Fig. 7.1.5. (b). SPI calculated using Observed data in different long month-length at Station Sylhet. In the Sylhet station according to Observed SPI calculation there were moderate to severe droughts (SPI ≤-1) in the year of 1962, 1970, 1972, 1973, 1978, 1979, 1981, 1982, 1983 and 1988, 1989. And Extreme drought (SPI ≤-2) years were detected in the year 1962, 1979, 1981, 1986. Overall, detection of drought in different stations, different month lengths are not exactly same. This may happen due to many reasons such as chosen stations are well separated, though Bangladesh is small country, there is a difference in geographical and environmental from region to region. So, drought detection may vary from station to station. Moreover, the magnitude of drought also varies between different month-lengths. It observed that short month length i.e. M1, M3 can detect more peak then the long month-length i.e. M12, M24 etc. Due to station point information used in calculation of SPI, the exact same drought years are not possible to detect by the data values. Therefore, regional average of neighbour stations may provides better performance of using the SPI for drought analysis. Also the statistical analysis is needed for understanding that how much reliability is there to use RegCM outputs for drought diagnosis. 7.1.2 Observed Regional Average Data To observe the drought condition of whole Country, 27 station data utilized into four regions for detail study which are describe below. (a) Central Region