How DOST-PAGASA’s weather guide dam operations in between wet and dry seasons

Ambuklao Dam in Benguet is among the six major dams in the Philippines monitored by DOST-PAGASA. (Photo from the DOST-PAGASA’s Hydro-Meteorology Division)

In the Philippines, where typhoons and monsoon rains abound, storing this large volume of water is a crucial resource that benefits local farmers and millions of households in the rural and urban areas.

And as heavy downpours from the effects of the Southwest Monsoon rains during its peak, the Department of Science and Technology–Philippine Atmospheric, Geophysical and Astronomical Services Administration (DOST-PAGASA) watches over the country’s weather situation not only to provide timely rainfall bulletins but also to give timely updates on the dams’ water levels.

The Weather Bureau transforms these weather forecasts, rain-gauge readings, and water level data into critical information for dam operators. This information is vital as they make one of the most significant decisions during a storm: when to open the dam gates.

The DOST-PAGASA monitors six major dams across the country: Angat Dam in Bulacan; Binga Dam and Ambuklao Dam in Benguet; San Roque Dam in Pangasinan; Pantabangan Dam in Nueva Ecija; and Magat Dam, which serves areas in Isabela and Ifugao.

But opening a dam’s gates is not a decision made simply because rain is falling heavily, as a DOST-PAGASA expert explained.

Behind every release is a continuing process of monitoring, forecasting, computation, coordination, and public warning—one that can begin well before a reservoir reaches a critical level. The goal is not simply to release water, but to manage it carefully and give communities downstream enough time to prepare.

Turning rainfall into a critical dam decision

Ms. Rosalie Pagulayan, Assistant Weather Services Chief at the DOST-PAGASA’s Hydro-Meteorology Division and Chief of its Flood Forecasting and Warning Section, pointed out that rainfall is one of the most important pieces of information in this equation.

“Number one precursor ng pagbaha and even pagpuno ng atin [pong] mga dams ay yung ulan,” Pagulayan explained.

(The number one precursor to flooding and even to the filling of our dams is rainfall.)

Rainfall over a watershed determines how much water may eventually flow into a reservoir.

However, not all rain produces the same amount of water at the same rate. Different parts of a watershed can experience different rainfall intensities and volumes, which dam operators must take into account when estimating how much water goes into the reservoir.

“So, yan po yung magiging basis ng dams to compute the inflow. Ano yung volume and the resultant dam elevation?” she said.

(So, that will serve as the basis for the dams to compute the inflow. What is the volume and the resultant dam elevation?)

Pagulayan said that the process becomes a race against time when heavy and continuous rainfall begins to push water levels upward.

She emphasized that DOST-PAGASA does not simply provide one forecast and step away. During critical weather conditions, the country’s weather bureau continuously feeds dam operators with updated weather and rainfall forecasts and hydrometeorological to read: information, particularly the downstream water level of the dam.

“Hindi po talaga kami tumitigil ng pagbigay ng forecast rainfall. Kasi po, very crucial yun sa ating po mga dams. So, continuous yun,” she said.

(We never stop providing rainfall forecasts because this information is crucial to our dams. It is a continuous process.)

At the same time, DOST-PAGASA monitors what is happening along river systems, including downstream conditions of dams. This information becomes especially important when a dam may need to release water.

“Kung halimbawa po, talagang malakas ang pag-ulan at kailangan mag-release,” Pagulayan said. “Isa po rin na kinoconsider nila ay ano ba yung estado naman dito sa downstream.”

(For example, if the rainfall is really heavy and a release is necessary. One of the factors they also consider is the downstream conditions.)

This means the decision is not based solely on what is happening inside a reservoir.

Coordinating warnings before water is released

What is happening across the wider river basin also matters.

Pagulayan described the dam operation as a collaborative undertaking. The DOST-PAGASA provides meteorological conditions across the watershed, forecast rainfall, and information from monitoring facilities along river systems, while dam operators use these alongside their own operational data and forecasting tools.

“It’s really a collaborative undertaking,” she said. “Hindi po pwedeng mag-exist lang mag-isa ang DOST-PAGASA or the dam operators pagdating sa dam operation. It’s always a concerted effort.”

(It’s really a collaborative undertaking. DOST-PAGASA or the dam operators cannot operate on its own when it comes to dam operations and it’s always a concerted effort.)

The collaboration becomes more structured when conditions indicate that water releases could affect communities downstream.

The DOST-PAGASA can activate the Flood Forecasting and Warning System for Dam Operation (FFWSDO).

Pagulayan explained that dams have identified target areas that may be heavily affected by water releases. Before conditions reach the point where a release becomes imminent, DOST-PAGASA begins the warning process.

“Habang hindi pa po sila, or talagang doon na papunta, nag-a-activate na tayo noon,” she said. “So we activate the Flood Precaution Phase within the target areas of the dam. This will ensure that both DOST-PAGASA and the dam operators are ready to undertake all the activities pertaining to dam operation in the event that the opening of the spillway gates is really imminent and has been decided.”

(Before conditions reach that point, or as they are approaching that point, we activate the system,” she said. “So, we issue what is called the Flood Precaution Phase.)

This early warning is critical because water released from a dam does not disappear. It moves downstream, potentially adding to the volume already flowing through rivers and affecting communities located along the river system.

Technology plays an important role in keeping this information flowing.

The DOST-PAGASA uses rainfall and water-level sensors to automatically collect data, particularly in the downstream areas of the dams and share this information to the dam operators.

Pagulayan said that these instruments need regular checking and maintenance to ensure that they continue providing reliable information when it is most needed.

“Kaya po, lahat po ng sensors natin, rainfall, water level, talagang kailangan po ay merong periodic na pag-check, merong maintenance,” Pagulayan said.

(That is why all our sensors, including rainfall and water-level sensors, need to undergo regular checks and maintenance.)

These automated measurements help reduce the need for personnel to physically travel to remote areas to measure rainfall and water levels. The information is instead transmitted to monitoring and operations facilities, helping personnel respond more quickly to changing conditions.

But even with technology and forecasting, dams have physical limits.

Pagulayan explained that dams are designed to withstand a particular level and volume of water. Going beyond their designed limits can compromise the safety and structural stability of the reservoir.

“Hindi siya talaga pwedeng mag-store ng water beyond its maximum design flood,” she explained.

(It is not something that can go on indefinitely; it also has a limit based on its design.)

The stakes become even higher when water enters a reservoir at an exceptionally rapid rate.

Recalling an extreme rainfall event in the Pangasinan province, Pagulayan described a situation in which the inflow was so strong that the dam had to open its gates widely to release water.

“Kailangan i-open yung gates ng malalaking opening para po lumabas yung tubig kasi ang bilis ng inflow niya,” she said.

(The gates need to be opened wide to release the water because the inflow is coming in so rapidly.)

That moment captures the essence of dam operations during extreme weather: the water coming in must be closely watched because there is only so much time to respond.

It takes a network to keep communities safe

For communities downstream, however, the opening of dam gates should not be viewed as an isolated event. It is part of a larger chain of science-based monitoring and communication—from rainfall observations in the watershed, to forecasts, inflow estimates, river monitoring, dam operations, and ultimately warnings delivered to local authorities and communities.

Pagulayan noted that the dissemination of the DOST-PAGASA’s flood warning information and dam operator’s dam discharge warning information at the local level is structured so that it will directly reach the concerned Local Government Units from the provincial, municipal levels down to the barangay levels.

Both DOST-PAGASA and dam operators provide their respective warning information directly to the LGUs where they operate. The same information is also provided to the National Disaster Risk Reduction and Management Council (NDRRMC) head office.

“Although, kung titignan po natin how we operate, it’s really a collaborative undertaking. Hindi po puwedeng mag-exist nang mag-isa ang DOST-PAGASA pagdating sa dam operation, at hindi rin po puwedeng mag-stand alone ang ating mga dam operators. It’s always a concerted effort,” Pagulayan emphasized.

(Although, if we look at how we operate, it’s really a collaborative undertaking. DOST-PAGASA cannot operate on its own when it comes to dam operations, and our dam operators cannot do it alone either. It’s always a concerted effort.)

Meanwhile, Pagulayan emphasized that the warnings issued by DOST-PAGASA are intended primarily for the early actions of the community, ensuring their safety from hydro-meteorological hazards. She added that any questions or concerns regarding the warnings are welcome to be raised with the agency for clarification, urging the public to continue listening to and following the weather agency’s advisories.

DOST-PAGASA is one of the attached agencies of the Department of Science and Technology (DOST) aimed at providing science-based, innovative, and inclusive solutions across four strategic pillars: human well-being, wealth creation, wealth protection, and sustainability. These pillars embody the mantra OneDOST4U: Solutions and Opportunities for All.