Gov. Vicente Magsaysay said he has ordered the evacuation of at least 100 families, mostly Aetas, from Barangays Villar, Ugik and Morasa.
The Philippine Institute of Volcanology and Seismology (Phivolcs) said residents of other barangays here, including the town center, may also have to evacuate.
Magsaysay, Phivolcs director Raymundo Punongbayan and Senior Superintendent Reynaldo Berroya, Central Luzon police director, flew by helicopter over the crater lake and observed that it was only about 4.5 meters from the lowest rim, also known as the Maraunot notch since it leads to the Maraunot River.
Berroya said they had a dialogue with the villagers to explain to them the danger they are facing.
Jaime Sincioco, resident volcanologist at Clark Field, said the Department of Public Works and Highways is set to carry out Phivolcs’ recommendation to artificially breach the Maraunot notch to allow a "controlled and systematic flow" of the rising lake water via the Maraunot and Bucao-Balin Baquero rivers.
Magsaysay urged the DPWH to start work in two days.
Phivolcs, in a situationer report yesterday, said, "If the lake water overtops and headward erosion occurs, a breach depth of 10 to 20 meters on the notch may release lake water volumes between 28 and 55 million cubic meters."
In a worst-case scenario, Phivolcs said the released water may "bulk up or incorporate more sediments as it flows downslope, to form lahars."
"We certainly don’t want to alarm people to the point of panic, but we also don’t want them to be complacent either," Phivolcs said.
Sincioco said the DPWH will airlift workers and "bobcat" excavators to the Maraunot notch.
"The workers would first have to shovel and pressure-hose some ashfalls about 1.5 meters thick before the excavators could remove the next level of pyroclastic materials called ‘brecia’ down to the level of the crater lake," Sincioco said.
Phivolcs said, "Plans must be made for a precautionary evacuation of the whole valley, including Botolan town proper. Such plans need not be only on paper but actually tested."
The institute said "an evacuation is greatly simplified if the dam is breached intentionally at a known date and time."
"In general, ground that is 10 meters above the valley floor should be safe enough, but exceptions may be near the volcano like Villar, Morasa and Ogik, and perhaps, areas near Malumboy," it added.
Aetas abandoned these villages following the 1991 eruption of Mt. Pinatubo, but some of them have reportedly returned there.
Sincioco said the regional disaster coordinating council should operate watchpoints not only at the Maraunot notch, but also along the Maraunot and other rivers.
"Those manning these watchpoints must be equipped with efficient radio communications since there are no cellular phone signals there," he said.
Berroya said the National Disaster Coordinating Council will provide funds for the artificial breaching of the crater lake.