A Study of Rainfall Variations in the Philippines: 1950-1996

Bonifacio Pajuelas


The long-period rainfall variations in the Philippines are studied using unfiltered and filtered Rainfall Anomaly Index (RAI). To have RAI’s that are representative for each group, zones of quasi-homogeneous climate were constructed based on highly correlated stations (r > 0.75), narrow standard deviation, and period of maximum rainfall using the 1950-1996 monthly rainfall total.

Variance analyses of the RAI’s suggest that unfiltered samples do not significantly differ from the normal distribution except for the western part (climate type 1) that have significant positive skewness and peakedness. The RAI’s contain a significant amount of non-random elements and a significant negative change in mean is reflected over the central Visayas and Mindanao (climate type 3). Filtered RAI’s that are not significantly different from the normal distribution (at least for c2 test) indicated significant trend over areas with high-variable rainfall (i.e., climate types 1, 2, 4 & 5).

In general, long-period rainfall may have changed over the period of study. The 10-year filtered RAI’s have the possibility of falling rate over climate types 1, 2 & 5, but increasing rate over climate type 4. These trends are indicated towards the rainfall-sensitive months (i.e., February through May) during El Niño or La Niña events. Falling rate is also significant from October through January over climate type 4. Longer periods (30-year filtered RAI’s) have significant negative trend for climate types 2 &4, but positive trend for climate type 5. These trends also occurred during February through May.


Long-period rainfall variability; Rainfall variations; RAI

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