Journal of Japan Society of Civil Engineers, Ser.
B1 (HydraulicEngineering),
Vol.
67, No.
4, I_85-I_90,
2011.
Annual Journalof HydraulicEngineering,
JSCE,
Vol.55,2011, February

CLIMATE CHANGE IMPACT ON RIVER FLOW
OF THE TONE RIVER BASIN, JAPANSunmin KIM 1, Yasuto TACHIKAWA2, Eiichi NAKAKITA3,
Kazuaki YOROZU4 and Michiharu SHIIBA5
1Member of JSCE, Dr. Eng., Lecturer, Dept. of Civil and Earth Resources Eng., Kyoto University(Nishikyo-ku, Kyoto 615-8540, Japan) E-mail:sunmin@hywr.kuciv.kyoto-u.ac.jp
2Member of JSCE, Dr. Eng., Associate Professor, Dept. of Civil and Earth Resources Eng., Kyoto University
3Member of JSCE, Dr. Eng., Professor, DPRI, Kyoto University (Gokasho, Uji, Kyoto 611-0011, Japan)
4Member of JSCE, Dr. Eng., Assistant Professor, Dept. of Civil and Earth Resources Eng., Kyoto University
5Member of JSCE, Dr. Eng., Professor, Dept. of Civil and Earth Resources Eng., Kyoto University

Future river flow changes in the Tone River basin, Japan, were investigated using a fine resolution
distributed hydrologic model and the MRI-AM20km projection output, which is simulated under the A1B
climate change scenario. Multiple dam reservoir operations and current water usage information werealso considered for realistic river flow simulations. The future water usage and reservoir operation was
assumed to be the same as under current conditions. Analysis results show that the river flow change
downstream of the basin seems to be insignificant compared to the area upstream of the basin; however,more frequent water shortage days are expected in the future, especially at the Tone-Ozeki station.
Revised dam reservoir operation rules should be prepared unless the current water demand changes
properly in the future.
Key Words: Climate change, river discharge, future projection, the Tone River basin

1. INTRODUCTION
Climate change is expected to strongly affect thehydrologic cycle in the coming decades1). In a
warmer world, water cycle would be accelerated,
shifting seasonal patterns and increasing extreme
events in many regions2,3). Climate change impacts
on the hydrologic cycle should be a main concern if
we want a sustainable society in the future.
Assessing water resources in the future is difficult
due to various uncertain factors related to nature andsociety4). Magnitude of the climate change impact
depends on the river basin characteristics, and thus
there have been a number of regionalized climatechange impact analyses so far5,6). Especially inJapan, which has severe seasonal variations ofprecipitation and high population density, futurehydrologic impact analysis should be carried out
with much consideration for the complicatedgeographical shapes of basins and the sophisticatedwater control and usage information available7,8).
Under the nationwide climate change research
project of Japan, Kakushin219), a series ofhydrologic impact analyses has been performed
using GCM outputs. Takino et al.10) investigatedclimate change impacts on river discharge regimes
in all Japanese river basins using the output of asuper high resolution atmospheric model (hereafter,
MRI-AM20km). According to their investigation,
annual maximum discharge seems to haveincreased, especially in the Hokkaido, Northern
Touhoku, and Chugoku areas. And the monthly
discharge patterns in snow-dominant regions are
expected to change, though the change patterns
differ region to region. Kim et al.11) also
investigated the precipitation changes across Japan
using the MRI-AM20km output, and they showedthat we can expect a slight increase in annual
precipitation and more apparent increases insummer precipitation at the end of this century.
However, because of increased evapo-transpiration
amounts, they detected decreased fresh water
resources, especially in the Kanto and Tohoku areas.
Focusing on the Tone River basin, Takara et al.12)
estimated the water resources changes that will
occur in the future with the Standardized
Precipitation Index (SPI), and they also found that
there will be a greater possibility of drought in the
future than the possibility of a flood hazard.

I_85

In this study, a very fine resolution distributedhydrologic model was utilized to estimate future
river flow changes in the Tone River basin byutilizing the future projection output from the
MRI-AM20km. Flow control of seven multipurpose dams and water usage data about the river
basin were also considered to arrive at a realistic
simulation of the river flow as it was tested in ourprevious studies13,14).
In addition, we analyzed the hydrologic impact
on the Tone River basin under the A1B climatechange scenario for the near future (2015-2039) andthe future (2075-2099), looking at three aspects.
First, the flow duration curves were investigated at
the following check points in the subject basin:
Murakami, Yakatahara, Yattajima, and Tone-Ozeki(see Fig. 1). Second, we examined the average
discharge and minimum discharge, taking into
consideration current water usage, in an attempt todetermine any possible water shortage problems in
the future. Finally, we compared hourly peak anddaily peak discharges from the future projection tothat of the current climate scenario to figure out
possible changes in flood hazards in the subject
basin.
The organization of this paper is outlined below.
Section 2 describes the atmospheric input data andthe hydrologic model setup. Section 3 provides
simulation results of the hydrologic model andanalyzes the changes of future river flow, focusing
on its seasonal variations. Section 4 describes thechanges in the flood peak of the check points in thebasin. Finally, the last section summarizes theanalysis results.

2. DATA AND METHODOLOGY(1) Input Data
Very fine resolution of atmospheric output datafrom the MRI-AM20km, which was developed in
the Meteorological Research Institute of Japan, wasutilized in this study. The atmospheric model uses
the HadISST1 dataset15) of observed monthly mean
climatologic sea surface temperatures (SST) for aboundary condition of the current climate simulation(1979-2003). The SST for the future projection
simulation (2015-2039 for the near future and2075-2099 for the future term) was utilized from theensemble mean of GCM simulation output under the
A1B emission scenario16). Refer to Kitho et al.9),
Mizuta et al.17) and Kitoh and Kusunoki18) for details
on the MRI-AM20km.
Utilizing the MRI-AM20km output in the
distributed hydrologic model is the same method
employed in our previous studies13,14). The output
variable, prcsl (daily rainfall amounts that reach thesoil layer) and sn2sl (daily snowmelt amounts in thesoil layer) are downscaled into data recording thehourly precipitation output (precipi). The evpsl(evaporation from the soil layer, daily) and trnsl(transpiration from the soil root zone, daily) values
were utilized in daily resolution. In this study, there
was no bias correction or downscaling on theatmospheric data.
(2) Runoff Simulation
A distributed hydrologic model was built on an
object-oriented hydrologic modeling system,
OHyMoS19). For the hill slope runoff simulation
using 250m resolution DEM, we utilized thekinematic wave equation, which incorporates astage-discharge relationship for sub-surface andsurface flow20). Channel flow simulation was also
conducted by solving the kinematic wave equationand the model produce hourly based hydrograph ateach river segment.
The hydrologic model was calibrated manuallyfor the 16 sub-basins in the Tone River basin using
the observed input and output data. There are fiveparameters to be optimized in the model: roughness
coefficient n, soil depths ds and dc, and hydraulicconductivities ka and kc. The model parameters
have set for each sub-basin was calibrated andvalidated using the observation data of 1994-1998.
Details on the model structure and the calibration
procedure are found in Kim et al.13,14).
The hydrologic model system was previously
tested for the evaluation of the MRI-AM20km
controlled simulation output. Kim et al.10) showedthat the simulated river discharges match quite wellwith the historic river discharge, especially for alarge basin area (>5,000 km2) even without any bias
correction or downscaling on the atmospheric model
output. Considering this characteristic of the
MRI-AM20km output, this study focused on an
analysis of the Yattajima (5133.6 km2) andTone-Ozeki (6058.8 km2) stations, and the resultsfrom the Murakami (1249 km2) and Yakatahara(1677.5 km2) stations are given to provide additional
information for checking the functionality of dam
operation. See Fig. 1 for the location of the checkpoints.

Fig. 1 Basin map of the Tone River basin.

I_86

(3) Reservoir Operation and Water Usage
There are seven multipurpose dam reservoirs in theTone River basin, and the reservoir operation modelin this study was designed to reproduce the current
release pattern of each dam reservoir. Thispattern-reproducing method provides very efficientalgorithm for the long-term simulation of a reservoir
operation13,14).
We also included water usage information in the
distributed hydrologic model. We surveyed annual
water usage data (mainly for living and agricultural
usage) of 5 main points in the basin from the Dam
Management Office of the Tone River basin, andutilized this information for the model simulation.
The water demand at the check points is given as
figures in the next section. We may consider futurechanges in water usage pattern and demanding
amount. However, future water demand was assume
to be the same as the current one in this study.
Based on this condition, it was able to check the
impact of climate change on current water usage.

Fig. 2(a) Flow duration curves at the Murakami station.

3. SEASONAL PATTERN CHANGES(1) Analysis with Flow Duration Curves
To understand the seasonal pattern changes at thecheck points of the basin, flow duration curves(FDC) were produced for present, near future, andfuture terms. The FDC for each year was preparedfirst, and representative FDC was generated byaveraging the FDC over 25 years. In other words, n
day river flow of the representative FDC is theaverage of n day river flow of 25 years.
The representative FDCs for check points are
given in Fig. 2. Because there is no dam reservoir
and no flow control upstream of the Murakami
station (see Fig. 2(a)), the FDCs for that pointshows only natural river flow behaviors. There is no
significant change in the near future; however, a
noticeable increase is there in the low-flow seasonof the future term. The Murakami station is locatedin the western upper part of the Tone River basin,
and the expected shift in snowmelt discharge in the
future will create a different shape of FDC.
The other three points - Yakatahara (Fig. 2(b)),
Yattajima (Fig. 2(c)), and Tone-Ozeki (Fig.
2(d)) - are all located downstream of several dam
reservoirs, and the simulation results provide twodifferent river flow patterns: without and with dam
reservoir controls. When we first checked thenatural river flow pattern (no dam reservoir
operation), all three points showed similar behaviors
with different magnitudes, which are an expected
increase of low flow in the future term. The upper
region, especially the snowmelt of this region, ishighly affected by the increased temperatures at theend of this century. Because of the shifting
snowmelt in the upper region (e.g. Yakatahara St.),

Fig. 2(b) Flow duration curves at the Yakatahara station.

Fig. 2(c) Flow duration curves at the Yattajima station.

Fig. 2(d) Flow duration curves at the Tone-Ozeki station.

I_87this area will be a subject to experiencing changed
river flow regimes in the future.
Currently, there are five dam reservoirs upstreamof the Yakatahara station, and one more dam is thereat the upper basin of the Tone-Ozeki station. Using
the same test utilized in the Yagisawa dam
reservoir9), we tested the reservoir outflow
regulations in those six dam reservoirs andsimulated a controlled river flow in this study.
Focusing on the Yakatahara station, which is locatedat the right downstream of five dam reservoirs, thereservoir operation effects clearly appeared in the
FDCs (see Fig. 2(b)). Due to the reservoir
operations, the high flow amounts decreased, and it
stabilized with increased river flow in the low-flowseason. However, the reservoir operation effects
decrease as the check point is being away from thereservoirs.

Fig. 3(a) Annual river flow at the Murakami station withthe minimum discharge (daily) of each day.

(2) Considering the Water Demand
One of the main reasons to control the river flow
with a dam reservoir operation is to provide a safe
water supply for those who live downstream. To
figure out whether the future river flow will be able
to meet the current water demand under the current
reservoir operation rules, the minimum riverdischarge of each day was compared to the amountof water demand.
Fig. 3 shows the average river discharge (to see
the seasonal pattern of each term) and the minimum
discharge (to check the water supply safety) for the
check points. Different from the averaging method
used for making the representative flow duration
curves (sorting first in maximum order andaveraging n day’s order discharge in each term), theaveraged river discharge in Fig. 3 is a 25-year
averaged value of each day from January 1st to
December 31th. Using the same method, theminimum discharge is the minimum daily discharge
amount of each day within the 25-year period ofeach term.
At the Murakami station (see Fig. 3(a)), where
there is no dam reservoir upstream, we noticed a bitof a shift in river flow in the future term during theearly spring season. And the minimum discharge ofthe future term shows a very low flow in the spring
season (from approximately day 100 to 130, whichcorresponds to the middle of April and the middle ofMay). On the other hand, at the Yakatahara station(see Fig. 3(b)), which is located downstream of five
dam reservoirs, the minimum discharge during all
three terms shows a very similar pattern due to thereservoir operation effects.
The shifted and flattened river flow in the futuresnowmelt season was also very noticeable at theYakatahara station before applying the reservoir
operation, and the minimum discharge of the future

I_88

Fig. 3(b) Anl. river flow (daily) at the Yakatahara St.

Fig. 3(c) Anl. river flow (daily) at the Yattajima St.

Fig. 3(d) Anl. river flow at the Tone-Ozeki St.
Minimum flow (daily) of near future and future termshows more frequent failure of the water supply atthis station.

term was not able to satisfy the water demand inmany times (it is not shown in the figure). However,
river flow regulated by the reservoir operationshows a safe water supply at the Yakatahara station.
While the Yattajima station also is expected tomeet the demand for water, the Tone-Ozeki station(see Fig. 3(d)) will have a critical failure to meetdemands for water supply in both the near futureand future, especially in June. The Tone-Ozeki
station is the main fresh water source formetropolitan Tokyo. At present, about 75% of all
water and 88% of the metropolitan domestic watersupply come from the Tone River and its tributaries.
On average, 6.5 million m3/day (around 75 m3/sec)of water is sent to Tokyo through the Musashi
Canal, which starts at the Tone-Ozeki station21).
According to the simulation results, continuous
water supply through the Musashi Canal can besustainable in future. However, agricultural water
demand in the late spring shows critical failure in
the near future and future. It seems the current river
flow source in the late spring, which comes from thesnowmelt of the upstream, will be shifted to theearly spring in the future, and thus it cause limited
water resources in some drought years.
The reservoir operation rules adopted in thisstudy follow the current reservoir operation rules
not only for the present term but also for the futureterms. Thus there is room for improvement to meet
the water demand at the Tone-Ozeki station.
Revised dam reservoir operation rules should be
prepared unless the current water demand changes
properly in the future.

Fig. 4(a) Daily peak discharge in near future and futurecompared with the present peak discharge at the
Murakami St.

Fig. 4(b) Hourly peak (left) and daily peak (right)
discharge at the Yakatahara St.

4. FLOOD PEAK CHANGES
Finally, hourly peak and daily peak discharges in
the near future and future term were compared tothe one in the current climate scenario, to figure out
any possible changes in flood hazards in the Tone
River basin. The hydrologic model produced hourly
based hydrograph, and it was up-scaled into daily
discharge to get daily peak discharge amount. In
each 25-year term, the annual maximum hourly anddaily peak discharges were collected and sorted in
maximum order. The scatter-gram in Fig. 4 shows
the 25 pairs of the ordered maximum hourly (anddaily) peak discharges with the present term as thex-axis and the near-future term as the y-axis.
One is able to see that the future peak discharges
are not bigger than those of the present. The peakdischarge expected in the future is a similar amountand it is even a smaller amount in some cases(especially at the Murakami St. and Yakatahara St.).
Kim et al.11) investigated precipitation changes
across Japan using the output of the MRI-AM20km,

Fig. 4(c) Daily peak discharge at the Tone-Ozek St.

and they showed that maximum precipitation
amounts in the future will vary regionally.
According to their results, the Chugoku and Shikoku
region will apparently experience an increase ofdaily and hourly maximum rainfall in the near future
and the future terms. However, Kanto and someparts of the Tohoku region will have decreased
maximums in both the near future and future terms.
Analysis of the Tone River basin for this study
using the river discharges also showed mildly
decreased or slightly changed peak river flows.
It should be noted that the MRI-AM20km is oneof general circulation models and the resolution may

I_89not yet be precise enough to simulate extreme
events on a complicated geographic shape. We
should keep in mind that the GCM uses 20km
spatially averaged topographic data, which is very
rough, to represent complex mountainous areas ofJapan, and it adopts several parameterization
schemes to generate convective rain-cells. Dynamic
downscaled future projection output using a
non-hydrostatic regional model might be necessary
to analyze such a flood event in small basins ofJapan.

5. CONCLUDING REMARKSThis study estimates the hydrologic impacts on
the Tone River basin undergoing climate changeusing a very fine-resolution distributed hydrologic
model. The MRI-AM20km projection output for thenear future and the future terms was adopted toprovide the input data into the hydrologic model.
River flow controls provided by multiple dam
reservoirs and water usage information were also
considered by checking main gauging points in thesubject basin.
First, an investigation of the flow duration curves
found that a noticeable increase is anticipated in the
low flow season in the future. Downstream of thedam reservoirs, the reservoir operation effects
clearly appeared in the flow duration curves; highflow amounts decreased and low flow amounts
stabilized. However, the reservoir operation effects
decrease as the check point is being away from thereservoirs.
Secondly, minimum discharges of each term
were examined taking into consideration current
water usage information. While the other stations
showed that a stable supply will meet the demand
for water, the Tone-Ozeki station showed a critical
failure of water supply in both the near future andfuture term, especially in June. Revised reservoir
operation rules should be considered unless current
water demand changes properly in the future.
Finally, hourly peak and daily peak discharges
from the future projection were compared to the one
in the current climate scenario. The flood peakdischarge of future was not significantly different
from the peak in the present climate scenario or
showing even decrease amount in some places.
Further investigation with other GCM outputs may
be necessary to adopt it properly into the extreme
event analysis in a complicated topographic region.
ACKNOWLEDGEMENTS
This work was done under the framework of the"Integrated assessment of climate change impacts onwatersheds in a disaster environment (PI: Prof.
Nakakita at DPRI, Kyoto Univ.)" supported by the
KAKUSHIN Program of MEXT.

REFERENCES

1) Milly, P.C.D., Dunne, K.A. and Vecchia, A.V.: Global pattern oftrends in streamflow and water availability in a changing climate,
Nature, Vol. 438, pp. 347-350, 2005.
2) Betts, R.A., Boucher, O., Collins, M., Cox, P.M., Falloon, P.D.,
Gedney, N., Hemming, D.L., Huntingford, C., Jones, C.D., Sexton,
D.M.H. and Webb, M.J.: Projected increase in continental runoff
due to plant responses to increasing carbon dioxide, Nature, Vol.
448, pp. 1037-1041, 2007.
3) Oki T. and Kanae, S.: Global hydrological cycles and world water
resources, Science, Vol. 313, pp. 1068-1072, 2006.
4) Vorosmarty, C.J., Green, P., Salisbury, J. and Lammers, R.B.: Global
water resources: Vulnerability from climate change and population
growth, Science, Vol.289, pp. 284-288, 2000.
5) Fowler, H.J., Kilsby, C.G. and O'Connell, P.E.: Modeling the impactsof climatic change and variability on the reliability, resilience, andvulnerability of a water resource system, Water Resour. Res., 39,1222, doi:10.1029/2002WR001778, 2003.
6) Kim, B.S., Kim, H.S., Seoh, B.H. and Kim, N.W.: Impact of climatechange on water resources in Yongdam Dam Basin, Korea,
Stochastic Environmental Research and Risk Assessment, Vol. 21,
No. 4, pp. 355-373, DOI: 10.1007/s00477-006-0070-5, 2007.
7) Islam, M.S., Aramaki, T. and Hanaki, K.: GCM-based analysis ofwater availability along the Tone River and Tokyo Metropolitan
Area under climatic changes, Regional Environmental Change,
Vol. 7, No. 1, pp. 15-26, DOI: 10.1007/s10113-007-0022-8, 2007.
8) Fujihara, Y., Ode, M., Kojiri, T., Tomosugi, K. and Irie, H.: Effects
of global warming on the water resources of the Tone River basin,
Anl. Journal of Hydraulic Eng., JSCE, Vol. 50, pp. 367-372, 2006
(Japanese with English abstract).
9) Kitoh, A., Ose, T., Kurihara, K., Kusunoki, S., Sugi M. and
KAKUSHIN Team-3 Modeling Group: Projection of changes infuture weather extremes using super-high- resolution global andregional atmospheric models in the KAKUSHIN Program: Resultsof preliminary experiments, Hydrological Research Letters, Vol. 3,
pp.49-53, 2009.
10) Takino, S., Tachikawa, Y., Shiiba, M., Yamaguchi, C. and Yorozu,
K.: Estimation of changes of river flow regimes in Japanese river
basins under climate change, Anl. Journal of Hydraulic Eng., JSCE,
Vol. 54, pp. 475-480, 2010 (Japanese with English abstract).
11) Kim, S., Nakakita, E., Tachikawa, Y. and Takara, K.: Precipitation
changes in Japan under the a1b climate change scenario, Anl.
Journal of Hydraulic Eng., JSCE, Vol. 54, pp, 127-132, 2010.
12) Takara, K., Kim, S., Tachikawa, Y., and Nakakita, E.: Assessing
Climate Change Impact on Water Resources in the Tone River
Basin, Japan, Using Super-High-Resolution Atmospheric Model
Output, Journal of Disaster Research, Vol.4, No.1, pp. 12-23, 2009.
13) Kim, S., Tachikawa, Y., Nakakita, E., and Takara, K.:
Reconsideration of Reservoir Operations under Climate Change:
Case Study with Yagisawa Dam, Japan, Anl. Journal of HydraulicEng., JSCE, Vol. 53, pp. 597-611, 2009.
14) Kim, S., Tachikawa, Y., Nakakita, E. and Takara, K.: Hydrologic
Evaluation on the AGCM20 Output Using Observed River
Discharge Data, Hydrological Research Letters, Vol. 4, pp.35-39,
2010.
15) Rayner, N.A., Parker, D.E., Horton, E.B., Folland, C.K., Alexander,
L.V., Rowell, D.P., Kent, E.C., and Kaplan, A.: Global analyses ofsea surface temperature, sea ice, and night marine air temperaturesince the late nineteenth century, Journal of Geophysical Research,
108, No. D14, 4407, doi:10.1029/2002JD002670, 2003.
16) Nakicenovic, N., Alcamo, J., Davis,G., deVries, B., Fenhann, J.,
Gaffin, S., Gregory, K., Grübler, A. et al.: Special Report on
Emissions Scenarios, Working Group III, IPCC, Cambridge
University Press, Cambridge, 2000.
17) Mizuta, R., Oouchi, K., Yoshimura, H., Noda, A., Katayama, K.,
Yukimoto, S., Hosaka, M., Kusunoki, S., Kawai, H. and Nakagawa,M.: 20-km-mesh global climate simulations using JMA-GSM
model -Mean climate states-, J. of the Meteorological Soc. ofJapan., Vol. 84, pp. 165–185, 2006.
18) Kitoh, A. and Kusunoki, S.: East Asian summer monsoon
simulation by a 20-km mesh AGCM, Climate Dynamics, DOI
10.1007/s00382-007-0285-2, 2007.
19) Takasao, T., Shiiba, M. and Ichikawa, Y.: A runoff simulation withstructural hydrological modeling system, JHHE, Vol. 14, No. 2, pp.
47-55, 1996.
20) Tachikawa, Y., Nagatani, G. and Takara, K.: Development ofstage-discharge relationship equation incorporating saturated-unsaturated flow mechanism, Anl. Journal of Hydraulic Eng.,
JSCE, Vol. 48, pp. 7-12, 2004 (Japanese with English abstract).
21) Ibbitt, R., Takara, K., Desa, M.N.M. and Pawitan, H. (Eds.),
Catalogue of Rivers for Southeast Asia and the Pacific – Volume
IV, The UNESCO-IHP Regional Steering Committee for Southeast
Asia and the Pacific, UNESCO-IHP Publication, pp. 95-109, 2002.

I_90(Received September 30, 2010)