Return period refers to the probability of extreme weather events like heavy rain or floods occurring, based on historical data. For example, a 100-year rainfall event has a 1% chance of happening in any given year, but two such events could occur in consecutive years. Various government agencies in the Philippines monitor rainfall and flooding risks. They produce hazard maps showing areas at risk of flooding for different return periods, using data from weather radars, satellites, and rainfall observations. These maps are publicly available online from sources like DOST-NOAH and NAMRIA to help with disaster prevention and mitigation efforts.

1. HYDROMETEOROL
OGICAL HAZARD
MAPS

2. RETURN PERIOD,
also referred to as recurrence interval,
is the probability of an event such as
heavy rain, intense typhoon of floods
to happen. It is a statistical
measurement mainly based on
historical data that gives the estimated
time interval between similar extreme
events.

3. FOR EXAMPLE,
the return period of a heavy rainfall event is 100
years, this can be expressed as an event with a
probability of happening equivalent to 1/100 or
1%. This does not mean that the next similar
extreme event will happen 100 years after,
instead, it means that in a given year, there is a
1% chance that the event will happen.
Consequently, 100-year floods can happen in 2
consecutive years. In analysis of extreme
hydrometeorological events, “return periods” are
usually used to infer the severity of an event.

4. RAINFALL IN THE PHILIPPINES
Rainfall is the most important climatic
element in the Philippines. Rainfall
distribution throughout the country
varies from one region to another,
depending upon the direction of the
moisture-bearing winds and the
location of the mountain systems.

5. THE MODIFIED CORONAS CLIMATE CLASSIFICATION
(FIG. 1) DESCRIBES THE MONTHLY RAINFALL
VARIATIONS IN DIFFERENT PARTS OF THE
COUNTRY.

7. RAINFALL OBSERVATION
Aside from rain gauge monitoring in different parts
of the Philippines, PAGASA publishes regularly
updated color-coded satellite images (Fig. 2) from
the Multi-Functional Transport Satellite or MTSAT
(as of Dec 2015, but will soon shift to a newer
satellite called Himawari-8) to visualize rain
distribution across the country. Color code is as
follows: Red – heavy rain; Yellow – light to
moderate rain; White – clouds; Blue – clear sky.

8. In addition to the satellite observation, PAGASA
operates 10 weather radar stations (as of 2015)
all over the Philippines. Fig. 3 shows the
locations of these weather radar systems.
Weather radar can detect hydrometeors (e.g. rain,
clouds) by transmitting electromagnetic radiation
to the atmosphere then analyze the returning
“echoes” reflected by weather elements present in
the sky. Figure 4 shows a sample radar image
over Bataan also using a color coding scheme to
visualize rainfall in the region.

12. THE NATIONWIDE OPERATIONAL
ASSESSMENT OF HAZARDS OR NOAH
is a program under the
Department of Science and
Technology (DOST) with the
mission to undertake disaster
science research and
development, advance the use of
cutting edge technology and
recommend innovative information
services in government’s disaster
prevention and mitigation efforts.
DOST-NOAH can be accessed
online at http://noah.dost.gov.ph.

13. THE FOLLOWING FIGURES ARE COMPARISON
OF FLOOD HAZARD MAPS WITH DIFFERENT
RETURN PERIODS FOR MARIKINA IN NCR
(FIG. 9) AND CAGAYAN DE ORO (FIG. 10).

14. Another local website,
http://www.nababaha.com, a
non-profit project by the
members of the Volcano-
Techtonics Laboratory of the
National Institute of Geological
Sciences of University of the
Philippines, publishes a list of
flood hazard maps of different
regions
(http://www.nababaha.com/list.h
tm), with the same dataset as
DOST-NOA

15. National Mapping and Resource
Information Authority (NAMRIA) is
an agency of the Philippine
government under the Department of
Environment and Natural Resources
(DENR) responsible for providing the
public with mapmaking services and
acting as the central mapping
agency, depository, and distribution
facility of natural resources data in
the form of maps, charts, texts, and
statistics. On its website’s download
page,
http://www.namria.gov.ph/download.php,
hazard/susceptibility maps for storm
surge, rain-induced landslide and flood
are freely available.
On the left: Figure 11:
Flood/Flashflood susceptibility map (left
image)

16. The Mines and Geosciences Bureau (MGB), a government agency also
under the Department of Environment and Natural Resources (DENR), is
responsible for the conservation, management, development and proper
use of the country’s mineral resources including those in reservations and
lands of public domains. MGB maintains a geohazard visualization portal
accessible at this URL http:// gdis.denr.gov.ph/mgbgoogle/.