Metropolitan areas in the Philippines are sinking mainly due to extraction of groundwater, a new University of the Philippines (UP) study that did maps and measurements showed.

Land subsidence, defined as the “gradual settling or sudden sinking of the Earth’s surface,” is caused by human activities such as extraction of water, oil, and mineral resources from the ground.

The study conducted by a group from UP Resilience Institute (UPRI) and National Institute of Geological Sciences led by geologist Mahar Lagmay, identified subsidence in cities, towns, and villages in metropolitan areas namely, Greater Manila Area, Metro Cebu, Metro Davao, Metro Iloilo, and Legazpi City.

As cities sink, sea level rises

Sea level rise is caused by climate change. Because of increasing temperatures, ice sheets and glaciers melt and seawater expands, contributing to the increase in water level of our oceans.

That, and the sinking of lands due to excessive groundwater use — a consequence of development — threaten vulnerable countries, especially its coastal communities.

“Due to high subsidence rates, the decrease in ground elevation creates a basin conducive to flooding, worsening the inundation depth,” the study read.

“In addition, sea-level rise due to climate change further aggravates the flooding problem in coastal communities.”

The Philippine Climate Change Assessment report in 2016 said mean sea level in Manila has been increasing at 1.33 millimeters per year since the 1900s. 

The country’s National Adaptation Plan (NAP) has looked into the link between excessive groundwater extraction and increasing sea level, saying that “these changes have the potential to inundate vast areas and affect millions of people, emphasizing the urgent need to address the risks associated with rising sea levels.”

One way to adapt, the plan recommended, is to manage water use effectively.

Researchers correlated subsidence with land use and development.

In industrial complexes, economic zones, technoparks, residential areas, and agricultural fields, water is extracted from the ground to make sure that operations in these places run smoothly. 

Understandably, where there are more people, the consumption of water is higher.

That’s why researchers are recommending that the government should improve how it monitors groundwater extraction. According to the British Geological Survey, more than half of the Philippines’ potable water supply comes from groundwater.

“The growing population, urbanization, and poor urban planning in Philippine metropolitan cities present a big problem in managing the environment,” the study said. 

“One such problem is the monitoring of groundwater extraction, which, if not done properly, can lead to unabated extraction and consequent subsidence.”

Meanwhile, they found no significant link between rainfall and flooding after an analysis of rainfall data taken from synoptic stations in Metro Manila from 2000 to 2020.

There had been a slight increase in the average monthly rainfall volume, from 1.289 millimeters to 1.334 millimeters within the two-decade period.

The study used radar data from the European Space Agency’s Sentinel-1, starting 2014, for the Greater Manila Area, Metro Cebu, and Metro Iloilo. Researchers used later data for Legazpi City and Metro Davao.

They used Interferometric Synthetic Aperture Radar (InSAR), a technique to monitor ground movement up to millimeter-scale deformation at a national scale and over a long period of time.

Adapting our cities

As cities grow and, consequently it seems, the ground sinks, adjustments should be made.

“[E]very time you have a change, a subdivision is built, another roadway is built, you have to rethink, you know, another big flood happens, maybe another another flood mitigation scheme [that] you have to adapt,” said Guillermo Tabios in an episode of Rappler’s Be The Good. 

To prevent flooding, cities around the world are rethinking how they can absorb water. The trend now is attempting to combine gray infrastructure (such as dams, seawalls, pipes) with green infrastructure (such as forests, wetlands, mangroves). 

“It’s not just about flood control, we’re also thinking of housing, livelihood,” design anthropologist Pamela Cajilig said in a mix of Filipino and English during the same interview. 

Nature-based flood management would include, said Cajilig, restoration of wetlands and watersheds. At a smaller scale, say at a community or household level, Cajilig said having urban gardens could work. 

By 2040, NAP predicts that around 160 square kilometers of land in the Philippines will be inundated, and “may disproportionately impact coastal areas in Luzon, such as Bulacan and Pampanga, where there is land subsidence due to groundwater activity.”

This means 85,000 residential homes flooded, exacting damage that could reach P7 billion per year. And poverty plays a big role in this, as most of these homes are in informal settlements and coastal communities.

Shared global problems

These issues — land subsidence, sea level rise, and extreme flooding — are not unique to the Philippines. 

During the media launch of the Asia Pacific Ministerial Conference on Disaster Risk Reduction on August 22, Defense chief Gilberto Teodoro Jr. emphasized the opportunity to share lessons, technology, and knowledge on disaster risk reduction with Asia-Pacific nations. 

The dialogue would also open access to funds, which Environment Secretary Toni Yulo-Loyzaga said, could cover green and gray infrastructure. 

“[T]hat covers both infrastructure as well as livelihoods, knowledge [on] how to evacuate, how to use early warning systems,” said Loyzaga on August 22 in a mix of Filipino and English.

The environment chief also emphasized the use of space technology to avert hazards turning into disasters, something that the group of researchers from UPRI recommended in their paper. 

The group said the data presented in the study could be used as a baseline. “Authorities can use this information to make informed decisions about groundwater extraction rates, land-use planning, and mitigation measures to prevent excessive land subsidence,” according to the paper.

– Rappler.com

Subsidence rates are converted from millimeters to centimeters in this story.