at the 
" 
.RS-1 
of 2.7 meters/second to a maximum of 43 
meters/second between 8:40 a.m. to 9:40 a.m. On the 
same day, the ERS-1 SAR image over the area was 
acquired at 10:28 a.m. and consequently imaged 
active lahars. These appear with bright tones on the 
image because the surface of active lahar is very 
rough (Figure 4). In the upper Bucao and Santo 
Tomas-Marella rivers, a comparison with the ancient 
lahars shows that the active lahars have slightly 
higher backscatter values (80-120 dB for the July 
image and 75-150 dB for the August image). 
Textural aspects reflected in the tresse 
patterns are good indicators of the active lahars. 
Considering morphological aspects, the two images 
show changes in the traces of thalwegs. In addition, 
the August image shows a fan-like structure with a 
small lake in the upstream portion of the Bucao River 
formed by the deposition of pyroclastic materials. 
As a result of the blockage by pyroclastic 
materials of the tributaries of the upstream portions of 
the Santo Tomas-Marella rivers and Mapanuepe 
rivers, a lake (called Mapanuepe) was formed 
immediately west of the tailings pond of the Benguet 
Dizon open-pit mine of in San Marcelino, Zambales. 
This lake which appears in cyan on the lower portion 
of the composite covers an estimated area of 787 
hectares based on the digital image classification of 
an April 1993 Thematic Mapper image. The digital 
image analysis of a January 1992 SPOT multispectral 
image showed that the lake covered 566 hectares 
only. A possible breaching of this lake poses a 
continuing hazard to the population centers and vast 
agricultural tracts downstream. 
7.2 Visual interpretation of multitemporal 
color composites for the western portion. 
The color composites were generated by 
displaying the July 9 image in blue and green and the 
August 13 image in red (Figure 5, west of crater). 
The area shown is approximately 50 kilometers by 60 
kilometers. The composites show the crater of Mount 
Pinatubo after the June 1991 eruptions reduced by at 
least 150 meters from its pre-eruption elevation of 
1,750 meters above mean sea level. The high slopes 
surrounding the crater show in shades of white the 
volcanic ash that was deposited in the 1991 eruption. 
The middle and lower portions of the Bucao and Sto. 
Tomas Rivers which drain the volcano on the west 
side show in bright red the active lahars that were 
imaged by the August 13 scene. The cyan areas 
Immediately north of the mouth of the Bucao River 
on the northwest portion of the image show the 
municipality of Botolan in Zambales with the effects 
443 
of widespread flooding over the town and its 
surrounding agricultural fields caused by the filling 
up with sediments of the Bucao River and causing the 
river to overflow its banks. 
The pertinent changes due to lahars are very 
visible in the valleys only. The distinct red colors in 
the lower valleys indicate surfaces which changed 
from dark tones (dry smooth lahars) to bright tones 
(active tumultuous lahars). This may be an indication 
that more recent lahar were flowing over ancient 
rough surfaces. The light red colors in the upper 
valleys indicate active lahars which have rougher 
surface than ancient lahars. Thus, all red colors in 
the valleys indicate active lahars. The blue color of 
the fan-like structure upstream of the Bucao River is 
possibly caused by the predominance of fine lahar 
over coarse materials. The Mapanuepe Lake located 
on the southeast part of the composite has a cyan 
appearance. 
7.3 Visual interpretation of multitemporal 
color composites for the eastern portion. 
The first set of multitemporal color 
composites was generated by displaying the July 25, 
August 29 and October 3 images in various 
combinations of blue, green and red. The second set 
of color composites displayed the July image in blue 
and green and the October image in red. The second 
set was used in the interpretation work. 
The composite for the eastern portion 
includes five major drainage systems, namely, the 
O'Donnell, Sacobia-Bamban, Abacan, Pasig-Potrero 
and Porac-Gumain river. The bright red features 
comprising lahar flow are very evident in the 
O'Donnell River which flows northward to the towns 
of Tarlac and Capas in Tarlac province. The central 
part of the eastern image shows the Sacobia-Bamban 
River which abuts the northern boundary of the 
former United States Clark Air Force Base and 
Mabalacat town in Pampanga. A lake formed by the 
blocking of a tributary of the Sacobia-Bamban River 
northwest of the former military base appears black 
indicating the smooth calm water surface at the time 
of imaging. Along the Porac-Gumain River, the 
bright red areas show the extent of damage of lahar in 
the western and southern vicinities of the Philippine 
Air Force Base located in Floridablanca, Pampanga. 
The dark-red areas on the southeast portion of the 
image show the aquaculture-rich towns of Pampanga 
and Bulacan which have been affected by lahar 
drained through the Pasig-Potrero and Porac-Gumain 
rivers (Figure 5, east of crater). 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996