Threshold Precipitation for Defining Meteorological Droughts
The onset of meteorological drought is marked by the 3/4th departure from normal
precipitation (Thurow and Taylor, 1999). Thus while delineating the spatial
pattern of meteorological droughts 3/4th departure from normal precipitation is
critical to decide the threshold precipitation value. According to World Bank
Report (2010), average precipitation in depth in Pakistan in 2009 was 494 mm;
thus 3/4th of precipitation departure according to this value is 370.5 mm. Mean
annual rainfall figure calculated from the data acquired from PMD for the time
period 1985-2012 was 376.78mm and 3/4th departure calculated from this data is
282.585 mm. Another research conducted by Faisal and Ghaffar (2012) concludes
that the mean area weighted rainfall of Pakistan for a 50 year time period, i.e.
1961-2010, is 297.6 mm. The 3/4th departure from annual average rainfall value in
this case is 223.2 mm. Keeping in view the varied climatic conditions of Pakistan,
meteorological droughts have been defined in this paper taking 282.585mm (11.12
inches) as the threshold precipitation value, since it has been calculated using the
reliable data collected by PMD.