Understanding Rainfall Return Periods

By Rich Ybanez

When talking about typhoons and tropical storms which have hit the Philippines in the past, like Ondoy, Sendong, Milenyo, and Reming, one of the common phrases we hear from news outlets and scientists is “that storm was a 50-year rainfall” or “malakas yung ulan noon kasi100-year rainfall yun”.

But what exactly does this phrase mean, and more importantly, imply?

First of all, this is what is called as the rainfall return frequency or period. There are 5-, 10-, 25-, 50-, and 100-year rainfall return periods. Generally, the higher the return period, the larger the accumulated rainfall will be, but it will also be less likely to occur. Unfortunately, because of the unpredictability of nature, there’s more to understand than this. We must understand first, how these numbers come to be.

It is already common knowledge that every year, around 20 typhoons will enter the Philippine Area of Responsibility (PAR), and around 9 will make landfall over Luzon, Visayas, or Mindanao. The others will just pass over the Philippine Sea then move on to Taiwan or China. This number of 20 and 9 was determined by observation of these typhoons over several decades. It is a statistical average, so sometimes we get 19 or 21 typhoons, and sometimes more than 9 will make landfall.

Accumulation of Typhoon Occurrences in and near the PAR from 1972-2001. Joint Typhoon Warning Center

Accumulation of Typhoon Occurrences in and near the PAR from 1972-2001. Joint Typhoon Warning Center

The same way that these typhoons are observed and a statistical estimate produced for forecasting the number of typhoons a year, rainfall rate and duration data for every day, every year, over several years are collated, correlated, and analysed. One thing we get would be the dry and wet seasons for the Philippines. We know that, in general, the dry months are from December to May, and the wet season is from June to November; with the rainiest month being August (after two years of Habagat in August, everyone probably knows this by now).

Average precipitation in Quezon City based on a 12 year record. WeatherBase.com

Average precipitation in Quezon City based on a 12 year record. WeatherBase.com

Again, this may not prove true every year. Certain conditions may change the rainiest month in 2014 to July or September, but on average, the rainiest month is still August. It is not 100% sure for every year, but this product of statistics aids in forecasting and disaster preparation.

Finally, we come to the rainfall return period.

Something else that was observed from rainfall per month over several years, besides identifying the rainiest month, is that there are certain years that receive very high rainfall. For example, according to data from the Philippine Atmospheric, Geophysical, & Astronomical Services Administration (PAGASA) analysed by the National Hydraulic Research Center (NHRC) in the University of the Philippines Diliman, Tropical Storm Ondoy was a 180-year rainfall event.

Rainfall return period computed from maximum rainfall over 6 and 12 hours with data from TS Ondoy. Science Garden Station (PAGASA) and NHRC

Rainfall return period computed from maximum rainfall over 6 and 12 hours with data from TS Ondoy.
Science Garden Station (PAGASA) and NHRC

From this graph, we can see that the 2-year return period is at 120mm over 6 hours. This means every 2 years, we can expect this amount of rain to fall in this area. For the 10-year return period, 225mm of rain can fall over a 6-hour period. This is much more rainfall, but occurs only around every 10 years. Return periods are usually only determined up to the 100-year frequency (nearly 350mm in this graph), so Ondoy had to be projected using existing data as well as the fresh data from Ondoy itself.

Again, as with the number of typhoons per year, and the rainiest month of the year, the rainfall return period is determined by statistical means and is not a 100% certainty. Just because we encountered nearly 250mm of rain this year, does not mean we won’t experience it again in another 10 years. It just means that observing the past decades, it probably won’t. But it may, and it may happen in 9, 8, or even 5 years.

According to the NHRC, Ondoy was a 180-year rainfall return period. But only three years later in 2012, we experienced the rains of Habagat which competed with the rainfall intensity of Ondoy, catching everyone off guard. We were told in 2009 that Ondoy was a strong but rare tropical storm and should not occur again in at least another 100 years.

It is important to understand that Habagat 2012, and 2013 as well, were caused by weather anomalies and sheer bad luck. Habagat 2012 was caused by Typhoon Haikui in southeast China being rendered nearly stagnant for several days by high pressure systems in central China. This year, Tropical Storm Maring was nearly stagnant southeast of Taiwan and northeast of Batanes, causing a pull in the southwest monsoon. These rains were caused not by the usual passage of a tropical storm through the Philippines, but weather systems being in the wrong place, at the wrong time.

Rainfall return periods are produced by observing the past, and while we can say with some confidence that these numbers are accurate, nature is always unpredictable. Some experts say that with climate change, rainfall return periods should be adjusted. 10-year rains can become 5-year, and 100-year become 75-year rainfall return periods. Regardless of what the statistics result in, one thing we can be sure in taking from history is that very strong rainfall can occur in the Philippines, and that we should always be prepared for it. We should always be prepared for strong rains, and should never be complacent in anticipating the next year’s rainfall season.

Rich Ybanez is a faculty member at the National Institute of Geological Sciences – Volcano-Tectonics Laboratory, University of the Philippines Diliman. 

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