Annual Journal of Hydraulic Engineering, JSCE, Vol.53, 2009, February
Annual Journal of Hydraulic Engineering, JSCE, Vol.53, 2009, February

RECONSIDERATION OF RESERVOIROPERATIONS UNDER CLIMATE CHANGE:CASE STUDY WITH YAGISAWA DAM, JAPANSunmin KIM 1, Yasuto TACHIKAWA2 , Eiichi NAKAKITA3 and Kaoru TAKARA4
1Member of JSCE, Dr. Eng., Assistant Professor, DPRI, Kyoto University(Gokasho, Uji 611-0011, Japan) E-mail:sunmin@flood.dpri.kyoto-u.ac.jp
2Member of JSCE, Dr. Eng., Associate Professor, Dept. of Urban and Env. Eng.,
Kyoto University (Nishikyo-ku, Kyoto 615-8540, Japan)
3Member of JSCE, Dr. Eng., Professor, DPRI, Kyoto University (Gokasho, Uji 611-0011, Japan)
4Fellow of JSCE, Dr. Eng., Professor, DPRI, Kyoto University (Gokasho, Uji 611-0011, Japan)

To investigate the adaptability of current dam reservoir operation rules under climate change condition,
a dam in the upper part of Tokyo, Japan was modeled and simulated. For the dam inflow data under thepresent and future climate condition, hydrologic output of a general circulation model was converted into
discharge data through a distributed hydrologic model. A simple type of dam model was prepared toreproduce (1) historical water levels and (2) historical dam outflows. Simulation results show that there
will be very limited inflow in May and June due to decreased and shifted snowmelt inflow in the future.
Under the changed climate condition, the dam outflow from January to March should be reduced andstored in the reservoir, or the current water level regulations should be revised.
Key Words: Climate change, dam operation, water resources, snow-dominated regions

1. INTRODUCTION
Climate change is now an unequivocal truth, and
it is expected to strongly affect the hydrologic cyclein the coming decades1,2). Even though climatechange would accelerate water cycles and therefore
freshwater resources may be less limited in the next
century, risk of water related problems will be still
remaining in the future due to changes in seasonalpatterns and increased extreme events3,4).
Water supply condition, especially for freshwater
resource in the future is difficult to assess owing tovarious uncertain and changing factors of nature. If
any considering area is in snow-dominated regions,seasonal variation of the water resources underclimate change condition becomes more apparent.
In a warmer world, there will be less snowfall in
winter and the melting of winter snow occurs earlier
in spring, therefore much of surface runoff will beshifted in the earlier season5).
Under this uncertain future hydrologic condition,
Japan, where flood has been more considerable
extreme events, may not be an exception for thewater resources problem. Water supply condition in
Japan is not stable even now due to its severe
seasonal variation and high population density6). So

far, this water related problems have been skillfully
handled with many reservoirs and multi-purpose
dams. Every reservoir has own optimized operation
rules to maximize its function under given water
problems. However, this current reservoir operation
rules may not work properly under the changedhydrologic cycle in the future.
To evaluate current dam reservoir operation rules
under the changed climate conditions, one of dam
reservoirs in Japan was modeled and simulated.
Future climate condition was set by the output of avery high resolution atmospheric model that wasdeveloped by the Japan Meteorological Agency(JMA) and the Meteorological Research Institute(MRI), Japan7). The subject area for this study is theYagisawa Dam basin (167.6km2), which is locatedat the upper part of the Tone River. The Tone Riveris the main water source to the metropolitan Tokyo,
Japan8), and the upstream of the basin is in snowdominated regions.
The organization of this paper is as follow:
Section 2 illustrates data and models used in thisstudy. Section 3 and 4 provides the simulation
results of hydrologic model and the virtual dam
operation results and discuss with the results.
Finally, Section 5 summarizes this study.

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Table 1. Annual mean amount of AGCM20 output(20 years of average for each term)

2. DATA AND MODELS(1) Very High Resolution Atmospheric Model,
AGCM20 and Hydrologic Output Data
JMA and MRI have developed a prototype of thenext generation of global atmospheric model, which
produces 20-km spatial and 1-hour time resolution
output, for the use of both climate simulations andweather predictions9). The global atmospheric model(hereafter, AGCM20) performs at a triangulartruncation 959 with the linear Gaussian grid
(TL959) in the horizontal, which has 1920×960 ofgrid cells of about 20 km size, and with 60 levels inthe vertical. Refer to Mizuta et al.7) and Kitoh andKusunoki9) for more details on the model.
The hydrologic data used in this study are thetest-running output of the AGCM20 provided by the
MRI as of 2008. The model provides two separate
outputs for present climate (1979~1998) and futureclimate (2075~ 2094). The model adopts observed
monthly mean climatologic sea surface temperature(SST) as boundary condition of the controlled
simulation. For the projection simulation, future
SST was estimated from the previous GCM outputs
simulated under the A1B scenario.
The AGCM20 output data used for this study aremainly related to hydrologic variables. Five variables
as shown in Table 1 were used as input data of a
distributed hydrologic model and it was converted
into discharge data. The prcsl indicates daily rainfall
amounts reached to the soil layer, and sn2sl is daily
snowmelt amount into the soil layer. For moreaccurate runoff simulations, the daily prcsl andsn2sl data were downscaled in hourly resolution
using the time series of the precipi (rainfall +snowfall, hourly) variables, and the evpsl(evaporation from the soil layer, daily) and trnsl(transpiration from the soil root zone, daily) values
were used as it is.
(2) Yagisawa Dam and Its Modeling
Yagisawa dam is located at the upper part of theTone River basin and it has 167.6 km2 ofcatchments area (see Fig. 1). The dam was built in1967 for flood control, hydropower generation, anddrinking water supply10). Observed daily waterlevel, inflow and outflow of ten years (1994~2003)
were prepared from the dam database10) of the
Ministry of Land, Infrastructure and Transport(MLIT), Japan and presented in Fig. 2 and 3. Dayby-day averaged values (bold blue and red lines)
show the annual trends of the inflow and outflow.
Average annual inflow and outflow are 16.788m3/s
and 16.921m3/s, respectively. It has 204.3 million m3
of total reservoir volume and 175.8 million m3 ofeffective volume. The normal high water level(NHWL) is 850.0m (mean sea level), and the lowest
water level (LWL) is 796.5m (see Fig. 4).

Output of
AGCM20precipi
prcsl1sn2sl2
evpsl3trnsl4
Net-Precipi.
1+2-3-4

Annual mean of the Yagisawa dam basin
Present(unit: mm)
1907.27(1,775.96)
804.76(1,335.58)
880.06(269.25)110.71(277.55)
237.43(240.28)
1336.68(1,087.00)

Future(unit: mm)
1907.82(1,851.03)
1005.03(1,510.99)
677.26(167.55)
150.81(329.92)273.25(275.84)1258.21(1,072.78)

Changes(unit: %)+ 0.03(+ 4.23)+ 24.89(+ 13.13)- 23.04(- 37.77)+ 36.22(+ 18.87)+ 15.09(+ 14.80)- 5.87(- 1.31)* ( ) is the values of the Tone River basin

Fig. 1 Location of the upper Tone River basin (outlet:
Kurihashi station, 8,772 km2) and the Yagisawa dambasin (167.6 km2). Red diamond marks (right) are thecenter of the AGCM20 output grid cells.

Fig. 2 Observed inflow (blue) and outflow (red) of eachyear (1994~2003) and day-by-day averaged values of
inflow (bold blue) and outflow (bold red).

Fig. 3 Observed water level of each year (1994~2003)
and day-by-day averaged values of W.L. (bold blue)- 116 -

The main water source into the dam reservoir issnowmelt inflows from April to May. The reservoir
prepares some storage space in the early spring forthis snowmelt inflow, and provides some amount ofwater to the downstream. The water level reaches to
the NHWL (850.0m) in the middle of May andmaintains this level until July (concretely, from 21st
May to 1st August). Another big release from thedam is in August and September to prepare flood
inflow. During this release, water level should behigher than 830.0m until 26th August, 827.8m until 1st
September, and 816.8m until 25th September. Afterthe end of September, the dam holds inflow little bylittle and water level rises to the NHWL.
The observed W.L. (Fig. 3) generally followed
the prescribed regulations, but there were also manytimes of irregular operations. Because the amounts
and arrival times of dam inflows will be different
every year and hard to expect, a certain level ofirregular operation by operator’s intuition might beunavoidable. However, it is not easy to consider thiskind of irregularity into a dam model, and if the dam
model works on the perfect inflow information, thesimulation is unrealistic.
In this study, a simple type of dam model was
prepared to check the aptitude of the current dam
reservoir operation rules under the changed climate
condition. Objectives of the dam modeling in thisstudy are to reproduce (1) historical water levels and(2) historical outflows. The historical outflows andwater levels are day-by-day averaged values of
10-years observation, which are shown in Fig. 2 and3, respectively. More details of the dam model
developed in this study are described in Section 4.
(3) Hydrologic Model and its Calibration
For the simulation of dam inflow using the
AGCM20 output data, a distributed hydrologic model
was composed for the Yagisawa dam basin, using
an object-oriented hydrologic modeling system,
OHyMoS11). The kinematic wave model for surface
and subsurface flow simulation12) was utilized in thesystem to simulate rainfall-runoff procedure, andriver channel routing is also considered by solving
the kinematic wave equation. This type of modelcomposition was successfully applied on the Yodo
River basin by Sayama et al.13) to evaluate theimpact of climate change on flood disasters in the
future, including dam reservoir operation models forthe flood control. However, the dam models that are
introducing in this study is different to the dam
model of Sayama et al. 13)
Snowmelt functions are not included in thehydrologic model of this study, and the model
directly utilizes the snowmelt amount (sn2sl) data
among the output of AGCM20. Evapo-transpiration
losses are considered in the model with the
AGCM20 output, evpsl and trnsl.

Fig. 4 H-V (water level - reservoir volume) relationshipand the specific W.L. of the Yagisawa Dam.

Fig. 5 Calibration results of the Kinematic wave model.
The calibration was fulfilled with summer season(July~October) observation data of 1994~1998.

The hydrologic model was calibrated manuallyusing the observed data of rainfall and daily inflow
into the Yagisawa dam within the summer season(July ~ October) of five years (1994 ~ 1998). The
model produces hourly discharge data, and it was
up-scaled to daily and compared with the daily
observed inflow data. The rainfall data was preparedusing the AMeDAS data, and a monthly uniform
evaporation amount was estimated from theobserved data14). There are five parameters to be
optimized in the model and they are roughness
coefficient n (= 0.03 m3/s), soil depths ds (= 0.40 m),
dc (= 0.21 m), hydraulic conductivities ka (= 0.108m/s) and kc (=0.012 m/s), which determine thevelocity of saturated and unsaturated subsurface
flow, respectively. As shown in Fig. 5, the calibrated
model shows good model performances on both low
flows and peak flows.
(4) Methodology
Procedures of this study to evaluate the current
dam operation rules are as follows. Firstly, rainfallrunoff simulations were conducted with thecalibrated hydrologic model, using the AGCM20
output for the present (1979~ 1998) and future(2075~2094) climate condition, and dam inflow
data were prepared. Secondly, dam models werecomposed (1) to reproduce the historical waterlevels and (2) to reproduce the historical dam
outflows. Thirdly, these two dam models simulated- 117 -

water level and outflow with the present and future
inflow. Lastly, the simulation results were analyzed
to investigate adaptability of current dam reservoir
operation rules under climate change condition in
the future.

3. RUNOFF SIMULATIONS WITH
THE AGCM20 OUTPUT DATA
(1) Dam Inflow Simulations
Rainfall-runoff simulation to calculate Yagisawa
dam inflows was fulfilled using the output of theAGCM20, and the results are presented in Fig. 6. As
it was expected, the melting of winter snow in the
future occurs earlier, and much of runoff is shiftedand flattened in the earlier season. The pattern of thesummer season discharge does not show noticeablechanges in the future. The day-by-day averaged
discharges for the present and the future provide
apparent distinction of those two terms.
(2) Water Balance Checking
Average annual inflow from the rainfall-runoff
simulations under present climate condition is 7.172
m3/s (6.988 m3/s under future climate condition),and it is equivalent to 1,349.6 mm of annual
precipitation (area of the Yagisawa dam basin is167.6 km2). However, the annual average inflow of
the observation, 16.788 m3/s is equivalent to 3,158.9mm of annual precipitation. Because there was a big
difference of water volume between the observed
and simulated inflow, it was not able to carry out
proper dam model operations.
According to the AMeDAS observed data on thedam basin (Nikko-41166: 2,127.9 mm, Nas-41011:
1,865.4 mm, Fujiwara-42046: 1,777.7mm), annual
precipitation amount is not sufficient to match withthe annual dam inflow. However, it is also difficult
to sure whether the AMeDAS has successfully
observed much snowfall in a highly mountainous
area. Therefore, the observed inflow was set as thebase water volume, and the simulated inflow weremodified to match its water amount to the observed
inflow. The simulated inflows both for the present
term and future term were revised by multiplying afactor 2.340, which is a ratio of the observed annual
mean inflow, 16.788 m3/s and the simulated annual
mean, 7.172 m3/s.
The revised inflows are presented in Fig. 6 bothfor the present (blue line) and for the future (redline). The discrepancy of the average inflow of thepresent (bold blue line) to the observed inflow (boldblack line) in Fig. 6 is because of inherent error in
the input precipitation. Monthly precipitation of theAGCM20 output also has some discrepancies to theobserved monthly precipitation.

Fig. 6 Simulated inflow to the Yagisawa Dam using the
AGCM20 output for the present term (1979~1998,
blue) and for the future term (2075~2094, red). Muchof snowmelt of present will be decreased and shiftedto the earlier season because of the temperature
increasing in the future. *Simulated inflow was
modified to match the water balance with theobserved inflow by multiplying 2.340 (=16.788m3/s /7.172m3/s).

Fig. 7 Simulated (for the given water level) outflow fromthe Yagisawa Dam. Because of the shifted snowmelt
season, there will be decreased water resources in
May and June.

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Fig. 8 Simulated (for the given water level) water levelof Yagisawa Dam. Observed water level, which is
the target of the simulation, is presented with boldblack line. There are many water shortages during
May and June in the future compared to the present.

4. SIMULATION OF DAM OPERATIONWITH THE GIVEN CONDITIONS(1) Simulations with the Water Level Regulations
The first dam model developed in this study isfor reproducing the historical water levels (averaged
values) shown in Fig. 3. This 10-year of averagewater level is the target water level in the first dam
model. The simulation rules to operate the dam
under this purpose are as follow: (1) if water level ishigher than the target W.L., release the extra water
to have the same W.L. to the target W.L., (2) if
W.L. is lower than the target W.L., outflow is 0.0
m3/s until the W.L. reaches to the target W.L., (3)dam outflow can not exceed 60.0 m3/s, and theexceeding outflow is carried over to the next day,and (4) the relationship of the dam W.L. and thereservoir water volume is following the H-V
relationship given in Fig. 4.
The simulated dam outflow and water level withthe given water level regulations are presented inFig. 7 and 8. The variant simulation results of year
by year were summarized with the day-by-day
averaged values. The observed outflow and waterlevel are also presented for the reference to thesimulation results analysis.
From this dam simulation results, it is able to see
the apparent results of the shifted snowmelt season
in the future. From January to the end of March,dam outflows of the future will be larger than thepresent one (see Fig. 7) because of abundant inflows
in this season (Fig. 6). However, due to decreased
snowmelt inflow, there will be very limited outflow
in April and May (Fig. 7). If it is necessary to keep
the water level higher than the simulation result andto release some amount of water from April to June,
the target water level in March should be higher
than 835.0 m in the future, and dam release fromJanuary to March should be smaller than thesimulated one.
Dam simulation results with the present term
data also shows some amount of discrepancy in theoutflow pattern compared to the observations: overrelease in April and water shortage in May and June.
This is mainly caused the inherent discrepancy inthe inflow of the present term to the observed inflow(see Fig. 6), and this is originally comes from the
AGCM20 output data error as explained in theprevious section. After August, both dam simulation
results for the present and future climate condition
show successful match to the given water level andoutflow as well.
(2) Simulations with the Outflow Regulations
The second dam model developed in this study isto reproduce the given dam outflow, which is theFig. 9 Simulated (for the given outflow) water level ofYagisawa Dam. While the results of the present showrather similar pattern to the observed water level, theresults of the future show many extreme conditions:water abundant in April and water shortage in thesummer season (July~ September).

Fig. 10 Simulated (for the given outflow) outflow of theYagisawaDam. Both simulation results successfully
keep the given condition (designed outflow) except
excesses in April and shortage in summer of future.

averaged historical outflow shown in Fig. 2 with thebold blue line. This 10-year of average outflow is
the target outflow in the second dam model. The
regulation rules to reproduce the given outflow are
as follow: (1) if W.L. is between the minimum W.L.
(816.8 m) and the surcharge W.L. (854.5 m), store
the inflow and release the target outflow, (2) if W.L.
reaches to the surcharge W.L., release the target
outflow and inflow as well, (3) if W.L. is lower thanthe minimum W.L., outflow is 0.0 m3/s until the
W.L. is getting higher than the minimum W.L., and(4) the relationship of the W.L. and the reservoir
volume is following the H-V relationship given in
Fig. 4.
Dam simulation results using the given outflow
regulations are presented in Fig. 9 and 10. Bothsimulation results for the present and the future arerather successfully following the designed outflow(see Fig. 10). Except several times of exceedingcases in April and shortage problem in summer
season of the future, the day-by-day averaged- 119 -

outflow of the present (bold blue line) and future(bold red) are almost matched with the designed
outflow (bold black).
The simulated water level with the future term
data show very different pattern to the historicobservation, and many extreme conditions are also
found in the simulation results. There are high waterlevels in April, but soon after the level falls down
drastically due to the increased outflow in May andJune (Fig. 10). After all, many years of water levelin the future goes down to the minimum water level(816.8 m) in the summer season (July~September).
The inflow of May becomes very low in the futurecomparing with the current inflow (Fig. 6), andtherefore the water volume (water level) will besignificantly decreased (fell down) if there is big
release as the current condition. If it is necessary tokeep the current dam release pattern for somereason, such as downstream water demand andwater intake in a certain season, the averaged waterlevel presented with the bold red line in Fig. 9 canbe proposed as a new water level regulation in the
future. However, reservoir water insufficiency in thesummer season of the future should be carefully
considered.
The simulated water level of the present also
shows some difference, however, it comes from theoriginal difference between the present term inflow
and the observed inflow, especially in May and June(see Fig. 6).

5. CONCLUING REMARKSThe Yagisawa dam in the Tone River basin wasmodeled and simulated to investigate adaptability ofcurrent dam operation rules under climate change.
To prepare the dam inflow of the present term
(1979~2008) and the future term (2075~2094), thehydrologic output of AGCM20 were converted into
discharge data using a distributed hydrologic model.
Through the dam model simulation to reproduce
historical water levels, it was found that there will
be very limited outflow in April and May, and it
will be difficult to keep the current water level
regulation in the future due to decreased and shifted
snowmelt inflow. If it is necessary to release someamount of water from April to June in the future,
dam outflow from January to March should be
reduced and stored in the reservoir. The second dam
model simulations to reproduce the current dam
release pattern show that it is able to realize thepresent outflow pattern in the future. In this case,however, the water level regulations should be
revised, and the shortage of the reservoir water insummer season should be carefully considered.
The dam models used in this study are notfollowing the realistic operation rules, and thesewere developed only to evaluate current reservoir
operations under climate change condition. Moreinformation on dam operations and water demand/
usage from the reservoir should be defined for morereasonable dam modeling. Further researches are on
development of realistic dam operation models andproposing revised dam operation rules for the proper
water resources control in the future.
ACKNOWLEDGEMENTS
This work is 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
KAKUSHIN Program of MEXT.
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