EXPLAINER: When and how will Angat Dam return to normal levels?

The water level at Angat Dam rose to 160.10 meters on July 3, slightly higher than its critical level of 160 meters.

However, it is still below its minimum operating level of 180 m and far from its normal high water level of 210 meters above sea level.

But when and how will the water level in Angat Dam return to normal?

In recent days, a Low Pressure Area (LPA) that became Tropical Depression Henry brought rain to several parts of the country, particularly Luzon.

This reportedly helped increase the water level in Angat Dam, but it still won't be enough to return to normal levels.

Rainfall level

"Kung June 30, doon siya nakaranas ng ulan, tapos kahapon may pag-ulan. Kung ngayon walang ulan, maaaring bahagyang huminto ‘yung pag-angat ng elevation ng Angat. Tapos kung hindi pa rin uulan sa ibang araw, ay bababa uli ‘yan," said Edgar dela Cruz, a hydrologist at PAGASA.

(If June 30, we experienced rain, then yesterday there was rain. If there is no rain today, the elevation of the water level at Angat may stop slightly. Then if it still doesn't rain for another day, it will go down again.)

Kaya kailangan natin ng continuous rain kahit na mga light to moderate rains. Pero tuloy-tuloy, makikita natin ‘yung dahan-dahan siyang papanhik ‘yung elevation," he added.

(That's why we need continuous rain even if it's light to moderate rains. But continuously, we will see the elevation slowly rise.)

The amount of rainfall needed to enter the watershed is between 900 mm to 1,000 mm so that the normal water level is maintained.

Ondoy level

In comparison, Typhoon Ondoy dropped more than 400 millimeters of rain in just one day in 2009.

This means that the total amount of rainfall needed in the watershed is greater than this.

However, according to PAGASA, it should not pour all at once because it could cause severe flooding.

"Natatandaan n’yo ‘yung Bagyong Ondoy? Mga 400 plus millimeters of rain, binuhos sa isang araw. Kita mo naman na naging delubyo sa Metro Manila nu’ng gano’ng karaming ulan," said Dela Cruz.

(Do you remember Typhoon Ondoy? It dumped 400 plus millimeters of rain in one day. You can see how much rain caused a deluge in Metro Manila.)

"So kung yung 900, ibubuhos mo sa short period of time, talagang magkakaroon tayo ng flooding talaga. Kaya kailangan ‘yung forecast rainfall na kailangan 900 millimeters of rain, kailangan nakai-spread out dyan sa within one month. 3x ba? Kung 400, dapat 1200 instead of 900?" he added.

(So if you pour 900 in a short period of time, we will definitely have flooding. That's why the forecast rainfall that needs 900 millimeters of rain needs to be spread out within one month. Is it 3x? If 400, should it be 1200 instead of 900?)

Typhoon season

Angat Dam usually returns to normal levels around the end of July until the month of August.

"Sa trend, ganu’n talaga ‘yung recovery niya. Puwede namang mabago ‘yung trend na ‘yan. Pero sa trending talaga, ‘yung pag-ahon ng elevation ng Angat, pumapalo sa last week of July, papasok ng August. Kasi d’yan pumapasok ‘yung mga bagyo," the hydrologist said.

(In the trend, the water level recovery is exactly like that. That trend can be changed. But in terms of trending, the rising elevation of Angat, would be in the last week of July, or at the first few days of August. Because that's when the storms enter the country.)

PAGASA clarified that they do not expect it to immediately return to normal high water levels come September.

What is needed is to have enough water by the end of the year.

"Ang gusto lang namin, dahan-dahan siyang umakyat na pagdating ng end of the year, nasa normal high water level siya, by December or January. Para pagpasok ng summer next year, sufficient ‘yung tubig natin, hindi tayo mamuong problema sa tubig pagdating ng summertime," Dela Cruz said.

(All we want is for the water level to slowly rise so that by the end of the year, it will be at the normal high level, by December or January. So that by the start of summer next year, we will have sufficient water, we will not have a water problem when summertime arrives.)

Crucial water supply

It is important that Angat Dam has sufficient water levels because the majority of Metro Manila draws its water supply from here.

When the water level at Angat Dam falls below normal operating levels, the release of water from it for irrigation is reduced or temporarily stopped, which may cause service disruptions in some areas.

"Talagang d’yan na po lahat nanggagaling ang tubig ng Metro Manila. Ayaw ko lang kung itong sa Montalban, yung Wawa Dam, kung... Kasi imagine-in mo, hindi lang basta Metro Manila, greater Metro Manila na. Outside na sa [Metro Manila], katulad ng Bulacan, kumukuha na riyan," Dela Cruz said.

(That's really where all the water for Metro Manila comes from. I just don't about Montalban, the Wawa Dam, if... Just imagine, it's not just Metro Manila, it's greater Metro Manila. Outside of [Metro Manila], like Bulacan, it's getting water from there.) — BAP, GMA News