2010 AIT-KU JOINT SYMPOSIUM ON HUMAN SECURITY ENGINEERING
Bangkok, Thailand, November 25-26, 2010

Projection of River Discharge in Thailand
under Climate Change and its Impact
on Water Resources
Yasuto TACHIKAWA1, P. B. HUNUKUMBURA2, Kazuaki YOROZU3,
and Somkiat APIPATTANAVIS4
1Associate Professor, Department of Civil and Earth Resources Engineering, Kyoto University(C1-116, Kyoto University Katsura Campus, Nishikyo-ku, Kyoto 615-8540, Japan)
E-mail: tachikawa@hywr.kuciv.kyoto-u.ac.jp
2Research Associate, Department of Civil and Earth Resources Engineering, Kyoto University(C1-116, Kyoto University Katsura Campus, Nishikyo-ku, Kyoto 615-8540, Japan)
E-mail: hunu@hywr.kuciv.kyoto-u.ac.jp
3Assistant Professor, Department of Civil and Earth Resources Engineering, Kyoto University(C1-116, Kyoto University Katsura Campus, Nishikyo-ku, Kyoto 615-8540, Japan)
E-mail: yorozu@hywr.kuciv.kyoto-u.ac.jp
4Researcher, Office of Research and Development, Royal Irrigation Department
200 Tivanon Road, Pakkred, Nonthaburi 11120, Thailand
E-mail:skavis@yahoo.com

The impact of climate change on river flow in Thailand is analyzed by feeding future climate projection
data into a distributed rainfall-runoff model. The projection data used consists of daily hydrologic data
downscaled by hourly precipitation for the present climate (1979-2003), the near future climate(2015-2039), and the future climate (2075-2099), which were simulated by a 20km spatial resolution
general circulation model (MRI-AM20km) developed by the Meteorological Research Institute, Japan
Meteorological Agency. It is found that a change of river flow appears at the tributaries of the Chao Phraya
River. The change pattern differs according to location, and it is expected to decrease of water resources at
the Pasak River basin. Thus, we develop a water resources assessment model at the Pasak River basin,
which includes a distributed hydrologic model, a dam reservoir water storage prediction model, and a plant
growth model for rice production assessment.
Key Words : Thailand, Chao Phraya River, Pasak River, water resources, climate change, river flow,
rice production

the Intergovernmental Panel on Climate Change, 4thassessment report (WG1, WG2; 2007) describes
increase of global average surface temperatures, andpotential increase of frequency of heavy rainfall, andso on based on long term observations. The report
also shows the projections of climate change ac-

1. INTRODUCTION
Global warming will give us a serious impact on
our life. Frequencies and magnitudes of floods andsedimentation disasters are predicted to increase dueto the change of precipitation extremes. The IPCC,
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cording to several greenhouse gas emission scenarios
and the impacts of climate change on water-related
disasters and water resources.
To cope with water-related disasters induced byclimate change, both mitigation measures and adaptation measures are essential. For adaptation measures, prediction of future water resources is a key
issue. In this paper, a distributed hydrologic model of
the Chao Phraya River basin is developed. Then,future river discharge is projected using the latest
GCM output to detect the hotspots of river dischargechange. It is found a change of flow characteristics at
the tributaries of the Chao Phraya River, and water
resource at the Pasak River basin is decreased. Thus,
we develop a water resources assessment model at
the Pasak River basin, which includes a distributedhydrologic model, a dam reservoir water storage
prediction model, and a plant growth model for riceproduction assessment.

2. DISTRIBUTED HYDROLOGIC MODEL(1) Watershed model
The topography of the catchment is modeled using
the eight direction method which assumes the flow
direction one-dimensionally to the steepest gradientdirection illustrated in Fig. 1. Each slope element determined by the flow direction is represented by a rectangle formed by the two adjacent nodes of grid cells.
The watershed model is developed using a digital elevation model, DEM, included in HydroSHED, which
cover the globe with about 100m spatial resolution.
Figure 2 shows the delineated catchments of the Chao
Phraya River basin (160,400km2), Thailand using theDEM. The area, length, and gradient of each rectangular slope element used for runoff and channel routing
are calculated according to the watershed model.

Fig. 2 Chao Phraya River basin in Thailand derived from DEM.

(2) Flow model
The kinematic wave model is applied to all rectangular slope elements to route the water to downstream according to the derived watershed model.
The continuity equation for each rectangular slope
element is:∂A ∂Q+= q (t )
∂t ∂x

(1)

where t is time; x is distance; A is cross-sectional
area; Q is discharge; and q(t) is the lateral inflow perunit length of slope or channel given as runoff generation provided by MRI-AM20km. The Manning
type relation of the discharge and the cross-sectional
area:

i ⎛1⎞
Q = αA , α = 0 ⎜ ⎟
n ⎝B⎠
m

m −1

, m = 5/3

(2)

is combined with the continuity equation to route thewater, where i0 is slope; n is roughness coefficient;and B is width of the flow. The slope i0 is determinedaccording to the watershed model. The model parameters of the flow model are B and n. The value of
B is determined using the regression relationship
B=aSc , where S is the catchment area, and a and c isconstant parameters. The value of n is determined to
0.03m-1/3s when the size of the catchment is larger

Fig. 1 Schematic drawing of a catchment modelling using
DEMs.

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than 250km2 and 11.0m-1/3s when smaller than
250km2. These values were tuned to reproduce theobserved data at two different catchments and applied to all basins.
The flow model was applied to all catchments and75 years runoff simulations were conducted. The
simulated river discharge data of daily maximum anddaily mean were stored for each day with about 1km
spatial resolution.
Fig. 3 Hydrologic projection data provided by MRI-AM20km
used for river discharge simulation.

3. GCM DATA USED FOR RIVER FLOWPROJECTION
The projection data used here is simulated by thegeneral circulation model (MRI-AM20km) developed by the Meteorological Research Institute inJapan. MRI-AM20km realizes 1920×960 of gridcells of about 20 km spatial resolutions (Kitoh et al.,2009). The products of MRI-AM20km consists ofvarious atmospheric and hydrologic variables of thepresent climate experiment (1979-2003), the near
future climate experiment (2015-2039), and the future climate experiment (2075-2099), which were
simulated under the SRES A1B scenario.
The river discharge for the Chao Phraya Riverbasins is predicted by feeding the future climate
projection data into the 1km-spatial resolution distributed hydrologic model. The hydrologic projection variables related to river discharge is shown in
Figure 3. The inputted data to the distributed hydrologic model is daily surface runoff generation anddaily sub-surface runoff generation data, which are
simulated by the land-surface process model embedded in the MRI-AM20km.
The time-scale of daily runoff generation data is
insufficient to reproduce the hourly flood peak discharge, thus it was downscaled using the time-seriesof hourly precipitation data of MRI-AM20km to add
the same hourly distribution pattern into the daily
surface runoff generation data. We confirmed thatriver discharge simulation with the timely downscaled surface runoff generation and daily subsurface
runoff generation data successfully reproduced almost similar river discharge simulated by using
hourly precipitation, daily snowmelt, daily evaporation and daily transpiraion (Takino et al., 2010).
Thus, the timely downscaled surface runoff generation data and daily subsurface runoff generation data
were used as inputted data to the distributed hydrologic model.

Fig. 4 Change of the difference of the mean of the annual average
daily temperature in near future and future climate with respect
to the present climate.

4. CHANGE OF BASIC METEOROLOGI-CAL PARAMETERS
According to the MRI-AM20km projection data
for the near future climate experiment, the surface
temperature found to increase roughly by 0.5 to 1
degree from the present temperature whereas thetemperature increased by about 2.5 to 3 degrees in the
future climate. Figure 4 shows a spatial pattern of thechange of the difference of the mean of the annual
average daily temperature in the near future and thefuture climate experiment with respect to the present
climate experiment.
Figure 5 shows a spatial pattern of the change ofthe percentage difference of the mean annual rainfall
in the near future and the future climate experimentwith respect to the present climate experiment. The
percentage difference PD is obtained as

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Fig. 7 Change of the percentage difference of the mean annual
daily potential evapotranspiration in the near future and thefuture climate experiment with respect to the present climate.

Fig. 5 Change of the percentage difference of the mean annual
precipitation in the near future and the future climate experimentwith respect to the present climate experiment.

Figure 6 shows a spatial pattern of the change ofthe percentage difference of the annual maximum
hourly rainfall in the near future and the future climate experiment with respect to the present climate
experiment. It is observed that a clear increasing
trend of the annual maximum hourly rainfall overThailand for both time periods of the climate experiments. Notably, in the future climate experiment,
about 20% increase of the annual maximum hourly
rainfall in many parts of the region.
Figure 7 shows a spatial pattern of the change ofthe percentage difference of the mean annual daily
potential evapotranspiration in the near future and thefuture climate experiment with respect to the present
climate experiment. It is noticed that the potential
evappotranspiration increase by 4% to 6% in the near
future climate experiment and 4% to 10% in the future climate experiment.

Fig. 6 Change of the percentage difference of the mean annual
maximum hourly rainfall in the near future and the future climate
experiment with respect to the present climate experiment.

PD = 100 ×r f − rp(3)

rp

5. IMPACT OF CLIMATE CHANGE ON
RIVER FLOW REGIME

where rf is the annual mean precipitation in the near
future or the future climate experiment and rp is theannual mean precipitation in the present climate experiment. 1% to 5% decreases of annual precipitation
is observed in the central region of Thailand and 1%
to 10% of increase in annual precipitation is observedin the southern region in the near future climate experiment. In the future climate conditions, it is expected to have 1% to 10% of increase in annual precipitation in the north mountainous region and 5% to
10% increase in the northeast region.

Runoff simulations of 75 years for the present
climate experiment (1979-2003), the near futureclimate experiment (2015-2039), and the future climate experiment (2075-2099) were conducted.
Runoff simulation data of hourly maximum and dailymean are stored for each day with about 1km spatial
resolution. The simulated discharge data were analyzed to discuss the change of the flood risk andwater resources.

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Fig. 8 Change of the ratio of the mean of the annual maximum
hourly discharge in the future climate experiment with respect tothe present climate experiment.

Fig. 10 Change of the ratio of the mean October river discharge
in the future climate experiment with respect to the present
climate experiment.

(1) Change of flood
Annual maximum hourly discharge data wascompiled and the statistical characteristics were analyzed. Figure 8 shows the change ratio of the mean of
the annual maximum hourly discharge for the present
climate experiment and the near future climate experiment. Generally, the annual maximum dischargeof the main stream of the Chao Phraya River does notchange, however the one of the tributaries changes
location to location.
Figure 9 shows the change ratio of the 10-year
return period river discharge in the future climate
experiment with respect to the future climate experiment using the Gumbel distribution. The spatial
pattern is similar to the change of the mean of theannual maximum river discharge. It is important torecognize that the change of the discharge would
appear especially at the tributaries.
(2) Change of water resources
Figure 10 shows the change ratio of the mean
October discharge in the future climate experimentwith respect to the present climate experiment.
Generally, the monthly discharge on October decreases in the middle and lower part of the Chao
Phraya River basin.

Fig. 9 Change of the ratio of the 10-years return period riverdischarge in the future climate experiment with respect to thepresent climate experiment.
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Fig. 11 The location of the Pasak Dam reservoir which is locatedat the Pasak River, a tributary of the Chao Phraya River.

Fig. 14 Detail topographic modeling using HydroSHED for a
distributed hydrologic model. Left shows the entire Chao Phraya
River basin and Right shows the Pasak River basin.

Figure 12 shows the last 10% daily flow duration
curve for each 25 years climate experiment at thelower reach of the Pasak River shown in Figure 11 in
the red circle. The figure shows the decrease tendency of the low discharge at the area. Figure 13
shows the projected monthly discharge at the lower
reach of the Pasak River. The figure shows the clear
pattern of the decrease of the October discharge.

Fig. 12 Change of the ratio of the mean October river discharge
in the future climate experiment with respect to the present
climate experiment.

6. WATER RESOURSES ASSESSMENT
MODEL AT THE PASAK RIVER BASIN
Through the river discharge projection research of
the entire Chao Phraya River, it was revealed that the
Pasak River basin would be one of the hotspot basins
for the river discharge change. Thus, we focus thecatchment and develop a water resources assessment
model to analyze the change of hydrologic cycle,
water resources, and its impact on rice production(Tachikawa and Yorozu, 2009). Figure 14 shows thedetail topographic modeling using HydroSHED. Left
figure shows the entire Chao Phraya River basin andright figure shows the Pasak River basin. The topography data is used for a detail distributed hydrologic model.

Fig. 13 Change of the monthly discharge at the lower reach of
the Pasak River.

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7. SUMARRY
The impact of climate change on river flow regimes in the Chao Phraya River basin was analyzed.
A possible change of water resources are indicated in
the near future climate experiment and it becomes
clearer in the future climate experiment. We foundthe Pasak River basin would be one of hotspots ofriver discharge change. Thus, the following researches are in progress at the Pasak River basin:zzFig. 15 Irrigated area extended over the lower part of the Pasak
dam reservoir.

zzz

Development of a detailed distributed hydrologic model at the Pasak River basin,
Development of the Pasak Dam storage prediction model,
In-situ observation of crop production cycle atthe irrigation area of the Pasak dam reservoir,
Development of crop production model, andHydro-meteorological observation at the study
area.

ACKNOWLEDGMENT: The hydrologic projection data used for the river flow simulations wasprovided by the Meteorological Research Institute,Japan. The hydrologic observation data at the Pasak
River basin was provided by the Royal Irrigation
Department, Thailand.

REFERENCES
IPCC WG I : Climate Change 2007 : The Physical Science Basis,
Contribution of Working Group I to the Fourth Assessment
Report of the Intergovernmental Panel on Climate Change(IPCC), Cambridge Univ. Press, Cambridge, UK, 2007.
IPCC WG II : Climate Change 2007 : Impacts, Adaptation and
Vulnerability, Contribution of Working Group II to the
Fourth Assessment Report of the Intergovernmental Panel on
Climate Change (IPCC), Cambridge Univ. Press, Cambridge,
UK, 2007.
Kitoh, A., Ose, T., Kurihara, K., Kusunoki, S., Sugi, M. and
KAKUSHIN Team-3 Modeling Group : Projection ofchanges in future weather extremes using super-high-resolution global and regional atmospheric models
in the KAKUSHIN Program: Results of preliminary experiments, Hydrological Research Letters, Vol. 3, pp. 49-53,
2009.
Takino, S., Tachikawa, Y., Shiiba, M., Yamaguchi, C., andYorozu, K. : Estimation of changes of river flow regimes inJapanese river basins under climate change, Annual Journalof Hydraulic Engineering, JSCE, 54, pp. 475-480, 2009 (inJapanese) .
Tachikawa, Y. and Yorozu, K.: Impact Analysis of Climate
Change on Water Resources in Japan and Thailand using a
GCM Hydrologic Projections and a Distributed Rainfall-Runoff Model, Proc. of 2009 AIT-KU Joint Symposiumon Human Security Engineering, Bangkok, Thailand, November 19-20, pp. 149-156, 2009.

Fig. 16 A hydro-meteorological observation instrument installedat the Rama VI office, RID.

Figure 15 shows the irrigated area at the lower partof the Pasak Dam, where water is taken from thelower reach of the Pasak River for paddy fields extended over the lower part of the Pasak River basin.
To predict a potential rice crop yield for the irrigatedarea, a development of a crop production model
combined with a distributed hydrologic model and adam storage prediction model is in progress. We haveinstalled a hydro-meteorological observation instrument at the Rama VI office, RID to develop acrop yield model (Fig. 16).

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