Hydrological Characteristics in Chao Phraya River, Thailand○Shunji KOTSUKI
Kenji TANAKA(Graduate School of Engineering, Kyoto Univ., Japan)(Disaster Prevention Research Institute, Kyoto Univ., Japan)
kotsuki.water@gmail.com

1. Background

3. Study area: Chao Phraya River Basin

2. Scientific questions

Intermittent heavy rain from June 2011 led to massive
flooding in the Chao Phraya River basin, Thailand. The WorldBank estimates that the total economic damage and losses fromthe floods stands at 1.425 trillion Baht (45.7 billion US$). Thesedamages have affected not only Thailand but also industries inother countries such as Japan, as has been widely reported in thepress. After the massive flooding in 2011, The Thai government
intends to revise the master plan (prepared in 1999) for
management of the entire river basin.
The Japan International Cooperation Agency (JICA) launched aflood‐control project following the massive flooding. The authors
join the project through the Integrated Study on Hydro‐
Meteorological Prediction and Adaptation to Climate Change in
Thailand (IMPAC‐T) project.

①

Catchment Area
Total: 162, 800 km2, upper C.2: 110,000 km2

Natural Flood (NF)
Increase in rainfall
brings increase in runoff.
(2011, 2006, 1980, 1995)

What produces
the difference ?

Non Natural Flood (NoNF)
Reservoir Volumes
Bhumibol Dam: 13.5 km3, Sirikit Dam: 9.5 km3

Increase in rainfall does not
result increase in runoff.

Annual rainfall and runoff (excluding dam effects)(1999, 2008, 2001, 2010, 1988)

Flood and inundation records② Can we capture indications of floods before September?
Through hydrological modeling and observations③ How have runoff characteristics been changed ?
Land cover change (deforestation), strengthened rainfall, etc…

Keyword: Floods, Chao Phraya River, Numerical simulation

4. Results

Rainfall observation
rainydrydry

Recorded river discharge at Nakhon Sawan (C.2)

Nakhon
Sawan

Seasonal rainfall (mm/month)
Rainfall during rainy season
account for about 90 %of annual rainfall
In the basin.

The Ping, Wang, Yom, and Nan Rivers
flow down from the northern mountain areaand merge at Nakhon Sawan (C.2).
Floods have been caused in the years whenriver discharge at Nakhon Sawan
is beyond 3500 m3per second.

Topography and river networks

IMPAC‐T

GAME‐T

Geographical Characteristics

ave. Rainfall (mm/yr)

Stations of observation (RID & TMD)

There are more rain in
northeast mountainthan south lower part
via Asia Monsoon.

Rainfall‐observing stations is not substantial
in north east mountain although
it have much rainfall in the area.
Adding stations in the area is desire in future.

Long‐term Trends

Hydrological ModelingTime series of NDVI at double
cropping area (Thailand)

Both annual rainfall and runoff show
increase trends slightly in this 30 years.

Observed runoff(mm/month)

Average annual runoff ratio is 17. 2% , which means
more than 80% of rainfall is lost throughevapotranspiration in the basin.

Simulation agrees
with observation.

Rainy years have large runoff ratios
as shown in 1980, 1995, 1996, 2006, and 2011.
Because runoff ratio is very low, small increase in rainfall
can result large increase in runoff.

Trends of annual Rain, Roff, and Runoff Ratio

Significant increase trend in runoff ratios
have not been captured in this 30 years,
It suggests that land cover changes such as deforestation
and urbanization do not have much impact, currently.

Floods have beencaused when
TWS exceed limit.

Simulated runoff(mm/month)evap

1.Increase ofbase runoff

canopyPPrec

TWS limit

2. Saturated soil
prevents rainfall
infiltrations.

Results

Prec

Schematic illustration

Simulated total water storage (TWS)

W1

Edc1

W2

Edc2

W3

ED

Evap
discontinuation
D1
Surface
D2 Root
D3

Q12
Q23
Q3

Numerical modeling

Vertical water balance

discRoff

Horizontal water transport

5. Discussion
Rainfall – Runoff & TWS

Rainfall ‐ Runoff
Natural Flood year: 2011・+43% of average rainfall brought+125% of average runoff,
which accounts for about +29 km3.
・Bhumibol (13.5 km3) and Sirikit (9.5 km3)
Dams could not prevent floods.
1106

Natural Flood years: 1980, 1996, 2006
・+10% rainfall brought +50‐75% runoff

9680

Determination coefficient(a‐1) 0.6481: between JJA rain and ASO runoff(a‐1) 0.5276: between ASO rain and ASO runoff(a‐3) 0.7260: between JJASO ran and ASO runoff(b‐1) 0.505: between JJA rain and peak TWS(b‐2) 0.393: between ASO rain and peak TWS(b‐3) 0.765: between JJASO rain and peak TWS

Comparison between rainfall and observed runoff (excluding dam effect)

1. Rainfall in meddle rainy season (JJA) is
more dominant than that in late rainy season.

1. No dam’s operation may preventthe massive flooding in 2011 perfectly.

Comparison of annual rainfall and runoff (with and without dam effect)□: rainfall－: observed runoff

2. Small increase in rainfall can results
large increase in runoff because oflow runoff ratio in the basin.

－: runoff excluding dam effect

JJA: June July August, ASO: August September OctoberComparison between rainfall and simulated total water storage (TWS)

2. TWS exceeded its limit in Natural Flood years,
because they had much rainfall in JJA.
Saturated soil increases not only base runoff,
but surface runoff via preventing infiltrations.

6. Conclusion
Summary
1. +43% of average rainfall in 2011 brought +125% of average runoff, which accounts for about +29
km3. No dam’s operation may prevent the massive flooding perfectly because increased runoff
exceeded the capacity of Bhumibol Dam (13.5 km3) and Sirikit Dams (9.5 km3) .
2. Small increase in rainfall can result large increase in runoff because of low runoff ratio.
3. Rainfall in meddle rainy season (JJA) is more dominant than that in late rainy season. TWS
exceeded its limit in Natural Flood years, because they had much rainfall in JJA. Saturated soil
increases not only base runoff, but surface runoff via preventing infiltrations.

Acknowledgements
The authors wish to thank RID(Royal Irrigation Department of Thailand), EGAT(Electricity
Generating Authority of Thailand), and TMD(Thai Meteorological Department) for providing theirvaluable data sets. This work has been supported by a research project of the JICA entitled“Integrated Study on Hydro‐Meteorological Prediction and Adaptation to Climate change inThailand (IMPAC‐T)” and the Japan Society for the Promotion of Science (JSPS).