Rainfall or precipitation data was collected from 18 monitoring stations (Figure 4.1) maintained by Indian Meteorological Department (IMD) and Directorate of Economics and Statistics – Karnataka between 1901 to 2015.  115 years average rainfall data was considered to understand the rainfall dynamics – spatio temporal variability, etc.  in Greater Bangalore (Figure 4.2).

Figure 4.1: Rain gauge stations

Spatial Analysis of rainfall indicates that the western portion of Bangalore receives higher rainfall than the east. Monthly analysis (Figure 4.3, Figure 4.4) of rainfall indicated that rainfall in Bangalore is spread across 7 months i.e., 86.9% of rainfall occurs between the months of May to November, September being the highest with average rainfall of 156 mm.
Annual rainfall pattern (Figure 4.5) indicates higher variability in rainfall with respect to mean. The trend line and the moving averages indicate increasing rainfall

Figure 4.3: Spatial rainfall distribution pattern (all units in mm)
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Figure 4.4: Monthly rainfall variations

Figure 4.5: Annual rainfall variations (hyetograph)

Figure 4.6: Rainfall characteristics - frequency, dependability (%) and return period (years)

Statistical analysis (Figure 4.6, Table 4.1) of annual rainfall indicates that average annual rainfall in Bangalore is 787 mm with 75% dependability and return period of 5 years. The coefficient of variation is 0.23 which indicates that there would be 23% variability in rainfall between consecutive years (http://dimtecrisk.ufs.ac.za/nc/precipitation.html). Rainfall distribution as per the IMD classification (http://www.imd.gov.in) of rainfall conditions nearly 67.7% of the time at least normal rainfall can be observed in Bangalore, 20.7% drought and 11.7% deficient rainfall conditions. The return period of normal rainfall is 3.5 years.

Table 4.1: Rainfall characteristics