Rainfall-Runoff-Inundation (RRI) model
Comparable to IFAS, the other key technology ofICHARM is the Rainfall-Runoff-Inundation (RRI)
model developed by Dr. Takahiro Sayama, currently
an Associate Professor at Kyoto University, during
his tenure in ICHARM. This model simultaneously
analyzes the hydrological processes from rainfall torunoff to flood inundation in a river basin, eliminating
two-stage simulation employed in conventional flood
analysis models in which a river channel runoff model
first calculates the flow rate and then a flood inundation
model uses it as the boundary condition for furthercalculation.
An event that had a great impact during the developmentof this model is the 2011 Thailand Flood along the Chao
Phraya River. At that time, Japanese diplomatic missions
and companies operating in Thailand tried to figure out
the entire scale of the flood based on limited information.
However, since satellite-based information was limited
in its observation frequency, the temporal change of theflood was difficult to predict. Therefore, despite the stricttime constraints, ICHARM hurried the development of
the RRI model and developed a 1-2-km grid model for
the Chao Phraya River basin. The flood movement wassequentially analyzed as emergency simulation, and theresults were published in cooperation with the Ministry ofLand, Infrastructure, Transport and Tourism (Figure 1).
The development of the RRI model and provision ofemergency information were highly appreciated, andFigure 1 Results of emergency simulation for the Chao Phraya River(Left: (a) RRI, Right: (b) Satellite observation)

the developer, Dr. Takahiro Sayama, was awarded by
the Minister of Education, Culture, Sports, Scienceand Technology in the science and technology field in2013 (Young Scientist Award), the Ministry of Land,Infrastructure, Transport and Tourism in 2013 (15thInfrastructure Technology Development Award), and theJapan Society of Civil Engineers in 2014 (Best PaperAward).

Figure 2 RRI model

The unique features of this model include: (1) flood
simulation in lowlands by using a 2D diffusive wave
approximation model; (2) simple expression ofhydrological processes such as vertical infiltration
flow in plains and lateral infiltration flow in mountains,
evapotranspiration, and impacts of dams; and (3) highspeed and reliable numerical computation incorporating
the Step Runge-Kutta Method, which automaticallyvaries time steps and parallel algorithms using Open MP.
This model, as well as IFAS, can use global data andsatellite-based rainfall data (Figure 2).
The real-time flood forecasting system incorporating
the RRI model was installed in the Royal Irrigation
Department (RID) in Thailand in 2013, as part of JICA’s
flood management project in the Chao Phraya River after
the 2011 floods.
The Indus-IFAS developed under a UNESCO Pakistan
project also uses the RRI model for flood simulation overthe lower Indus River. Thus, the Indus-IFAS becomes acoupling model taking advantages of the two powerfulcomponents: IFAS for high-speed simulation of runoff
in upstream mountains and RRI for flood inundation
simulation in downstream rivers.
RRI has been used in flash flood analysis in the Kabul
River and in flood inundation analysis of the Chikusa
River in Hyogo Prefecture as a verification tool for flood
events. ICHARM also undertakes research to improveaccuracy for forecasting the temporal development of aflood from rainfall to inundation by feeding the RRI modelwith ensemble information based on numerical weather
forecasts targeting domestic river basins.
To further expand and disseminate the RRI model,ICHARM started the development of GUI in 2014 foreasy input and output, and released it with an execution

program on its website in 2016 (http://www.icharm.pwri.
go.jp/research/rri/rri_top.html). The RRI-GUI is also
used to learn the operation of flood simulation models intraining held as part of the ADB project in Myanmar andalso as part of the JAXA-SAFE project (2016) for theKalu Ganga River basin in Sri Lanka.
ICHARM has also focused its support on training for theRRI model. The IFAS training by JICA in 2016 included
the RRI training. Also, the training for Arab countries
includes introduction and practice of the RRI model. With
development of GUI, the base of RRI users is expected
to expand further. In addition, to study adaptation
measures against climate change and analyze thephenomena of extreme external force exceeding thedesign scale, the needs for flood simulation using the RRI
model are expected to increase, including applications tosmall and medium-sized rivers in Japan. As mentioned
earlier, the RRI model uses approximation of the diffusive
wave to perform simulations efficiently; thus it is suitablefor the analysis of low, flat land, but has limitations in therepresentation of complicated flood hydraulic phenomena
such as those around building structures. Further studies
are needed for the model to produce more accurate
simulation results by reflecting detailed cross sections ofrivers in simulation and considering combinations withsewerage/drainage networks and pump drainages in city
areas. The model should also be verified for applicability
to hydraulic phenomena not only in the Asian monsoon
area but also in arid regions. RRI users are encouraged
to use the model at their disposal, which should motivate
further improvement in the future. ICHARM is fully
open to active discussions with a wide variety of users
to enhance the capacity of the RRI model. Currently
ICHARM is also working on coupling the RRI model with
WEB-DHM (water and energy budget-based distributedhydrological model) to include initial conditions of soil
moisture in simulation.

Figure 3 Inundation frequency analysis in Chao Phraya river basin for 25 years between present and future climate by using MRI-AGCM 3.2S
Left: Present(1979-2003), Center: Future(2075-2099), Right: Change (F–P)