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CATCHY CATCHMENTS: ESTIMATING OUTFLOW OF THE IPONAN WATERSHED IN
MINDANAO ISLAND IN THE PHILIPPINES USING A SYSTEM DYNAMICS MODEL
Julius Sempio, Jerimae Acosta, Joana Decilos, Leigh Lunas
Department of Geodetic Engineering, University of the Philippines, Diliman
EXTENDED ABSTRACT
The Philippine Islands has directly witnessed the adverse potentials of climate change in
the form of storms. The nation’s falling prey to nature’s wrath first reached international
headlines in 2009 when Typhoon Ketsana (“Ondoy”) caused extreme flooding in the
Metropolitan Manila area (CNN, 2009). Two years later, Tropical Storm Washi (“Sendong”)
ravaged in the Mindanao area (Samenow, 2011); and a year later, Typhoon Bopha (“”Pablo”)
brought further devastation to the same island (BBC, 2012). And, perhaps the most memorable
of them all, Typhoon Haiyan (“Yolanda”) “wiped out or damaged practically everything in its path
as it swept ashore… with seven-metre storm surges destroying around 90 percent of the city of
Tacloban in Leyte province” (Al Jazeera, 2014).
Majority of the devastation brought about by the abovementioned storms were due to
severe floods in communities located along rivers. In particular, floods caused by the first three
storms mentioned were due to unusually high precipitation values pouring into watersheds
(estimated to around 50 mm per hour of rainfall for each of the three storms), resulting to great
volumes of water rampaging through receiving rivers and crashing into these low-lying
communities (Olan, 2014) (Malig, 2011) (Panela, 2012). Having the ability to predetermine
watershed outflow due to increasing precipitation levels can therefore help in assessing the
vulnerability of Philippine settlements located along river systems, and this can be done through
reliable models aimed at predicting floods during severe storm events (Ali, Khan, Alsam, &
Khan, 2011). In turn, these models can warrant the determination of best adaptation strategies
to be taken before and during a high precipitation event.
This paper aims to present the formulation and development of a computerized model
of the Iponan Watershed in Mindanao Island that will estimate water outflow during different
precipitation events in a system dynamics simulation software. The resulting model’s output will
then be compared to officially published outflow values that made use of another simulation
software (in this case, HEC-HMS) to assess its applicability and accuracy.
Significance
Knowing the salient workings of a hydrologic model can be a key aspect in designing
software that is suited to conditions that are needed to be addressed. HEC-HMS is a hydrology
modelling software developed by the United States Army Corps of Engineers’ Hydrologic
Engineering Center (HEC), designed to simulate the complete hydrologic processes of dendritic
watershed systems (USACE, n.a.). It is free to be used by anyone – but its source code is
closed, disallowing potential modification. The HEC does, nonetheless, provide a technical
reference manual that explains how the software works, thus allowing other developers to
create their own models fashioned after HEC-HMS but are adapted to local settings.
Hydrologic models do not have to be constructed “from the ground up” if there are
readily available modelling tools that can provide users enough capability to simplify the
development process. For the purposes of this project, the AnyLogic simulation software is to be
used in creating the hydrologic model of the Iponan watershed. Said software’s object-oriented
approach in model development, built-in visual user interface, and capability to provide tools in
system dynamics modelling (AnyLogic, n.a.) can become advantages in performing the
abovementioned tasks.
The project aims to develop a system dynamics model of the outflow of the Iponan
watershed in AnyLogic and compare the resulting output with official data from the Climate
Change Commission’s Project Climate Twin Phoenix. Thus the steps and considerations taken
in the latter’s development of the HEC-HMS-based watershed outflow model (such as usage of
SCS curve number values for surface runoff computation) will be simulated in the AnyLogic
model. Other functions that are available in HEC-HMS will be set aside.
Related literature
The rising average global temperatures can mean increased potential for extreme
weather events, and “flood caused by storm events is a major concern in many regions of the
world”, warranting the necessity of reliable flood models to predict severe storm situations (Ali,
Khan, Alsam, & Khan, 2011). Extreme weather events are not only stronger, but also erratic –
Mindanao island in the Philippines, traditionally considered safe from storms with a “one storm
every ten years” rate according to the US Navy and Air Force Joint Typhoon Warning Center
(JTWC), was devastated by successive storms for two years (Panela, 2012), and storm
occurrence may be even become a “new normal” for the island (GMA News, 2013). Whether the
shift in storm patterns is due to climate change is being challenged, on the other hand, when the
Manila Observatory noted that in its historic records Mindanao was struck by an average of a
storm per year from 1883 to 1900 (Suarez, 2012).
The magnitude of damage that tropical storms and typhoons brought to a country
supposedly used to such weather caused the Philippine government to initiate climate-related
studies and projects as a means to answer future challenges imposed by climate change. An
example of such a project is the Nationwide Operational Assessment of Hazards (NOAH) – a
consortium of Philippine and international organizations –
launched “to undertake disaster
science research and development, advance the use of cutting edge technologies and
recommend innovative information services in government’s disaster prevention and mitigation
efforts”, with the vision of achieving “a disaster-free Philippines where communities are
empowered through open access to accurate, reliable and timely hazard and risk information”
(Project NOAH, n.a.). And the Climate Change Commission of the Philippines (CCC) launched
Project Climate Twin Phoenix, aimed at assessing “the flood hazards of the cities of Cagayan
de Oro (CdO) and Iligan in Mindanao, Philippines and surrounding areas to meteorological and
meteorologically-induced hazards due to climate change” – areas greatly affected by Tropical
Storm Washi in 2011 (UP TCAGP, 2013).
Increased precipitation due to storm events is not the only culprit in causing severe
floods. The hydrologic characteristics of a watershed also contribute to flood occurrence, as
determined by “complex spatio-temporal hydrological processes that are in turn related to
numerous meteorological, surface and subsurface characteristics”, with land use and its
transformation with time “among the most critical factors influencing various components of the
hydrologic budget such as evaporation, surface runoff, infiltration, and groundwater recharge”.
In view of this, land use is considered “a key input in various applications such as water
resources management problems, flood prediction analyses, assessing of soil degradation and
nutrient loss, and biodiversity conservation studies” (Öztürk, Copty, & Saysel, 2013).
Fig.1. The four major river basins studied in the CCC Twin Phoenix project (UP TCAGP, 2013), with the
Iponan watershed marked by a red box
There have been studies on modelling the otherwise complex hydrologic processes of
watersheds especially during rainfall events. A modelling software known as the Soil and Water
Assessment Tool (SWAT) was used to “simulate water, nitrogen, phosphorus, and sediment
dynamics under multiple land-use and climate change scenarios at the watershed scale” for the
Teshio River in northern Hokkaido (Fan & Shibata, 2015), while HEC-HMS was used “to
quantify the impacts of potential land use change on the storm-runoff generation in the Lai
Nullah Basin” in Islamabad, Pakistan (Ali, Khan, Alsam, & Khan, 2011). As mentioned earlier,
the latter was used in developing a model for the Iponan watershed (UP TCAGP, 2013).
Fig.2. The Iponan watershed as modelled in HEC-HMS for the Twin Phoenix project (UP TCAGP, 2013)
Study area
The Iponan watershed is situated in northern Mindanao, and its water outlet is situated
within Cagayan de Oro City – one of two cities severely affected by Tropical Storm Washi (the
other being Iligan City). Its topography was determined via LIDAR data, and its soil
characteristics obtained by assessing data from the Philippine Department of Agriculture’s
Bureau of Soil and Water Management. Land cover was classified and determined using
processed Landsat 8 satellite imagery, and curve number values for rainfall excess runoff
calculations determined using the classified images.
The resulting outputs of the
aforementioned activities became inputs for the HEC-HMS model, with additional precipitation
values supplied by the Philippine Atmospheric Geophysical and Astronomical Services
Administration (PAGASA) through the agency’s climate model (UP TCAGP, 2013).
Because the setup of the Iponan HEC-HMS model involves the usage of stock and flow
elements, it can be inferred that it is a case of system dynamics modelling – of which the
AnyLogic software is capable of performing.
Fig.3. Classified land cover map of the Iponan watershed (UP TCAGP, 2013)
Fig.4. Soil texture map of the Iponan watershed (UP TCAGP, 2013)
Methodology
AnyLogic is a Java-based software that is capable of modelling process-centric
(discrete) events, system dynamics and agent-based activities. The said software is primarily
inclined to development of models for efficiency purposes especially in business processes
(AnyLogic, n.a.), but there have been researches on using the software for applications other
than commerce.
AnyLogic was used to validate a mathematical model on pedestrian evacuation, and the
effects of panic impacts, in a Chinese train platform. The study’s authors noted that conducting
a real evacuation trial “is oftentimes unaffordable as it is extremely expensive and may cause
severe injury to participants”, and so simulation models “as an alternative have been used to
overcome the aforementioned issues in recent years” (Li, Chen, Wang, & Feng, 2014). The
software was also used for evaluating supply chain energy consumption for carbon footprint
reduction of operational processes via a combined discrete-event and system dynamics
approach, claiming that progressive companies that make conscious efforts for reducing
environmental impact of their operations “obtain a competitive advantage towards customers in
addition to making a contribution to a more sustainable future” (Jain, Lindskog, Andersson, &
Johansson, 2013).
This paper will present the development of a functional AnyLogic system dynamics
model of the Iponan watershed that utilizes stock and flow operations. In theory, initial values for
rainfall precipitation, formulated CN and soil values and other hydrologic considerations will be
read through either a spreadsheet file or a database. The flow elements will contain HEC-HMSinspired formulations for the transfer of rainfall excesses from one subwatershed (represented
by stock elements) to another, with the last stock element containing the final outflow that will be
compared with published catchment values from the Twin Phoenix project.
The paper will also discuss the development of a modelling framework that would
provide a template of stock and flow elements that can be reused in the modelling of other
watersheds, provided formula normalization can be achieved so that the system will simply
require from the user the entry of raw data.
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