NUS researchers study Singapore’s rainfall and runoff to predict urban floods

NUS researchers study Singapore's rainfall and runoff to predict urban floods

As climate risks intensify, NUS researchers are developing innovative solutions to strengthen resilience across natural and urban environments. This two-part feature explores sustainability research on coastal protection and flood prediction.

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In this series, NUS News explores how NUS is accelerating sustainability research and education in response to climate change challenges, and harnessing the knowledge and creativity of our people to pave the way to a greener future for all.

Singapore is often considered safe from major natural disasters, but climate change is quietly changing that.

In fact, Singapore experiences around 200 days of thunderstorms each year. Over the past few decades, the Meteorological Service Singapore has observed a steady increase in both annual rainfall and intense downpours.

When rain falls faster than the ground can absorb it, saturation excess and infiltration excess can occur, causing the excess water to become surface runoff (also called overland flow).

In a low-lying city like Singapore, sudden and intense rainfall can quickly overwhelm the ground and drainage systems, resulting in flash floods. Understanding how rainfall generates runoff is therefore critical, particularly as hydrologists still do not fully understand these processes in tropical environments due to the limited field data. With climate change expected to intensify extreme rainfall events, improving flood prediction is increasingly important for safeguarding Singapore.

At the NUS College of Design and Engineering, Professor Simone Fatichi from the Department of Civil and Environmental Engineering is working on the first-of-its-kind research in a tropical city, to track what happens from the moment rain hits the ground.

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Using the campus as a living laboratory, Prof Fatichi and his team have been studying an 8.6 ha land area stretching behind NUS Central Library to NUS Business School in order to track what happens when it rains - how much water seeps into the soil, how much of it turns into runoff, and how it flows through the drainage system.

Using meteorological stations, soil moisture sensors, groundwater wells, and pressure transducers and cameras to measure water flow in drains, the team records data every five minutes to capture the rapid changes in infiltration, soil moisture and runoff caused by Singapore's short but very intense rainfall.

After 22 months of research, the team found that saturated hydraulic conductivity - a measure of how easily water can move through soil - is generally very high in parks and forests, allowing water to be absorbed quickly. However, during major rainfall events, soils in many areas become nearly fully saturated, producing hydrological responses that are almost as intense and rapid as those from impervious areas.

To illustrate, imagine a lake just a few tens of centimetres below the ground, which leads to large volume of water released as flash floods. This effect can be mitigated by highly vegetated areas which deplete soil water storage by up to a few meters below the ground during dry periods, thereby buffering the runoff response during intense rainfall events.

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"Generally, if we can make better predictions of the catchment response to these extreme rainfall events and convective storms, we can alert the public to avoid certain areas. We can even come up with models which allows simulation of floods all around Singapore ─ to pinpoint and forecast flood risk hotspots," explained Prof Fatichi.

Staying one step ahead

This research goes beyond simply understanding rainfall-runoff response as it has the potential to transform how Singapore prepares for floods by improving its model capabilities.

With more accurate predictions, authorities can identify flood-prone areas earlier, enabling better planning and more targeted deployment of resources to mitigate impact. The public can also be alerted in advance, allowing them to take necessary precautions.

This is the second story in a two-part feature on NUS' sustainability research on coastal protection and flood prediction. Read about the first part here.