Annual rainfall is increasing here: Experts

Dear expert panel,

I refer to the 12 Jan 2012 Straits Times report “Annual rainfall is increasing here: Experts” [1].

It was reported that rainfall increased by 15mm a year on average between 1968 and 2008. The table below was constructed using government rainfall data [2]. It shows that by shifting the years by one or two, a completely different picture emerges. For example between 1967and 2009, rainfall decreased by 23.5mm a year on average.

Period Annual rainfall increase / decrease (mm / year)
1967-2009 -23.5
1967-2008 -14.3
1967-2007 -0.6
1968-2009 -3.8
1968-2008 6.2
1968-2007 20.7
1969-2009 -9.2
1969-2008 0.9
1969-2007 15.7

Regressing annual rainfall on year between 1960 and 2011 gives a p-value of 0.4, an R square value of 0.014 and a coefficient of 3.4. Thus, rainfall might be increasing by 3.4mm per year but there is 40% chance that this is all fluke. Thus, annual rainfall data doesn’t provide a lot of confidence to your claim that annual rainfall is increasing.

It was also reported that the number of days when there were at least 70 mm of rain per hour went up from 5 in 1980 to 13 in 2010. If that contributed to Orchard Road floods in 2010, why were there no Orchard Road floods in 2008 when the number went up to 14 (Figure 1)? If increase from 5 to 13 in 2010 is cause for concern, shouldn’t increase from 5 to 14 in 2008 be cause for more concern? Yet there were no Orchard Road floods in 2008.

Figure 1: Number of days when at least 70mm rain fell per hour [3A]

Referring to Figure 1 again, 1993 is the year with the highest number of days with at least 70 mm of rain per hour. If number of days with at least 70 mm of rain per hour is a parameter that is linked to Orchard Road flooding, shouldn’t 1993 have experienced even more serious Orchard Road flooding?

The ‘evidence’ provided by the expert panel is weak and doesn’t support your view that weather patterns have changed and that higher rainfall intensity contributed to flooding.

Next I refer to your Jan 2012 flood report [3].

Page 18, paragraph 3.2.2 of your report says there is strong year-to-year variability in maximum rainfall intensity and that the amplitude of that variability increased considerably over the last thirty years. But if high rainfall intensity or amplitude is a cause for concern, how come there is no flooding in 1995, 1999, 2007, 2008 and 2009 which have higher amplitudes compared to 2010?

Figure 2: Rainfall intensity [3B]

Considering Figure 1 and Figure 2 together, we find that 2008 experienced more frequent heavy rains and more intense rains than 2010. Yet, there were no Orchard Road flooding in 2008. In just two years, something has changed dramatically to cause Orchard Road to flood despite lower rainfall intensity and lower frequency of heavy rains. Therefore, the culprit for the 2010 Orchard Road flooding is unlikely to be higher rainfall since rainfall intensity was lower and frequency of heavy rains was also lower. We have to look elsewhere to explain Orchard Road floods. For a start, you might want to examine any major underground works or canal diversions in the vicinity of Orchard Road between 2008 and 2010.

Page 21, paragraph 3.3.1 of your report says that 7 stations in southwest and northeast Singapore show statistically significant uptrend in hourly rainfall total. Question is, how much of the rain falling on those seven stations went into Stamford Canal? If not much, why should it matter?

Page 22, Figure 3-7 shows stations registering statistically significant increase in rainfall intensity of at least 70 mm per hour. These stations are located in areas like Seletar, Lim Chu Kang, Boon Lay, Pandan Gardens and Sentosa. How is rainfall in these areas going to affect Orchard Road and Stamford Canal?

[1] Straits Times, 12 Jan 2012, Annual rainfall is increasing here: Experts


[3] Report on Key Conclusions and Recommendations of the Expert Panel on Drainage Design and Flood Protection Measures, January 2012,

  • [3A]Section 3: Rainfall Analysis, page 22, figure 3-6
  • [3B] Section 3: Rainfall Analysis, page 19, figure 3-2
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