El Nino Damage Assessment in Central Luzon using
Multitemporal Remotely-Sensed Images
The El Nino Phenomenon
El Nino is a large scale oceanographic phenomenon that develops in the pacific Ocean, and which is associated with extreme climatic variability. It is the invasion from time to time of warm surface waters from the western equatorial Pacific Basin to the eastern equatorial region and along the coasts of Peru and Ecuador. This condition can prevail for more than a year causing anomalous atmospheric circulation in the tropical Pacific; thus, resulting to heavy rains or strong winds in some areas and drought in other. It can adversely affect the economy in both the local and global scale.
In the Philippines, the effect of El Nino has always been associated with droughts. But the expected second and third order impacts include a number of environmental, social and economic effects. Examples of environmental effects include degradatin of soil which could lead to desert-like conditions if persistent effect on water quality like salt-water intrusion, high forest/grass/bush fire risk and domestic water supply shortage.
Based on the records of the Philippine Atmospheric. Geophysical, Astronomical Services Administration (PAGASA), the pattern of El Nino recurrence in the country is at least every three to five years from 1968-1987. During this period there were five drought events recorded (1968-1969; 1972-1973; 1976-1977; 1982-1983 and 1986-1987).
The frequency of recurrence intensified starting late 1980s, with the earlier interval of three to five years trimmed down to just tow years and one year of the following decade. For instance, it took a two-year respite before El Nino recurred in 1989 following
the 1986-1987 drought event, the last warm episode to happen within a two-decade period from 1969 to 1989.
The worst El Nino event to hit the country in terms of areas affected was during the 1982-1983 period but the longest and worst (in terms of damages) El Nino event happened in 1990-1995. The drought, which started in late 1989, actually took off in 1990 as a separate episode which later baffled climatologists because of its protracted lifespan.
The said event lasted up to 1995, lying dormant for a shore while in 1993 when the country experienced a record-breaking number of tropical cyclonesentering the Philippine area of responsibility. That year, there were a total of 33 tropical cyclones, way above the normal, which is 20 per annum. But to note, this was this was the first time that an El Nino related drought was prolonged. The 1990-1995 warm episode had some similarities in terms of duration to an El Nino event that occurred in 1935.
Some climatologists think that the 1997-1998 warm episode would likely to follow the 1982-1983 drought events in terms of intensity and areas covered because of the high positive anomaly of the sea surface temperature.
Methodology
Two multitemporal remotely-sensed images (MOS -1 and ADEOS) were acquired to assess the impact of El Nino on the agricultural lands of San Fernando, Pampanga. The images were taken during the wet and dry seasons, respectively, of the years when El Nino affected these areas.
Based on a 1:50,000-scale map, the images were rectified using the ER Mapper software. To generate a map showing the agricultural and non-agricultural lands of the study area for 1990 and 1997, the images were classified using the unsupervised classification technique. Then to show the irrigated and non-irrigated areas of each image, both images were classified using the supervised technique. To determine the relative changes in land use between 1990 and 1997, the images were overlaid. The accuracy of the information generated was checked through ground truthing.
