●Research Trends and Results

Evaluating the impact of climate change on thenatural flow regime of rivers
KIKUMORI Yoshito, Senior Researcher, Climate Change Research Team
KATO Takuma, Researcher, Climate Change Research TeamYOSHITANI Junichi, Research Coordinator for Watershed Management
River Department(Key words) Climate change, climate

model, drought

1. Introduction
Climate change will greatly change future rainfall
patterns in Japan. It is feared that declining snow
accumulation and earlier spring thaw than in the past
may increase the risk of droughts. The NILIM has, aspart of the Innovative Program of Climate Change for the21st Century, obtained climate model based predictions
as information which contributes to evaluating impact onwater resources and studying the application ofcountermeasures, translated these into hydrologicalinformation for regions throughout Japan, and analyzed
change of the flow regimes of rivers flowing into dams
distributed in climate zones throughout Japan in thefuture (end of 21st century) and near future (30 yearsfrom now). This was accompanied by the presentation ofdifferences in the change of flow regimes revealed by theresults of four different climate models, in order to beable to resolve the uncertainty of climate models.
2. Climate models used
The climate models used to calculate future river
flow regimes are the four models shown in Table 1
considering the fact that they output values throughout
the year and that they satisfy the spatial resolution (about20km) necessary to calculate a river flow regime.

ClimateTable 1. Climate

Models UsedEmission
scenario

Calculation period
Near
PresentFuture

model

Reform Stage 2:
GCM20
Reform Stage 1:
GCM20
Coexistence:
GCM20
MeteorologicalAgency RCM20

using a four-level tank model, and the discharge causedby the spring thaw was handled as the quantity flowing
into the top level tank.
The results of the analysis show that in the near futureand future, tendencies common to the four climate
models will appear: large annual fluctuation as the flowrate falls during years of light rain and rises in years ofheavy rain, accompanied by a remarkable fall of thesummer flow rate beginning in May at dams in Hokkaido,
Tohoku, Kanto, and Hokuriku, where the spring thaw
runoff is remarkable. Because similar trends appeared in
all four climate models, it is hypothesized that it seemsto be quite certain that these trends will appear in thefuture. Figure 1 shows an example of calculation results(inflow into a dam on the Japan Sea coast side ofHokkaido).
3Flow rate (m /s)

A1B

1979-1998

2015-2039

2075-2099

A1B

1979-2003

2015-2039

2075-2099

A1B

1979-2003－2075-2099

A2

1981-2000

2031-2050

2081-2100

3. Estimating the river flow regime
The flow regimes of the flow into reservoirs in thefuture and the near future were calculated for 15 dams
which are the furthest upstream dams in their respective
rivers throughout Japan. The rainfall values used for thecalculation are climate prediction model values with theirbiases corrected by AMeDAS observed values. And therainfall and snow were again distinguished based on
ground surface temperature, then corrected so they werein harmony with the observed accumulated snow depthvalue. The quantity flowing into the dams was estimatedPresent
Near future
FutureSpeeding up spring
thaw peak
Fall of flow ratefrom May to July
Speeding up startof spring thawJan.

FutureAverage daily flow rate (Reform: Stage 1)

Feb. MarchAprilMayJune

JulyAug.

Sept.

Oct.

Nov.

Dec.

Figure 1. Estimated River Flow Regime (Dam on theJapan Sea Coast of Hokkaido)
4. Future endeavors
In the future, examples of proposed application
measures will be presented by analyzing differences
between the change of regional flow regime trends andby assessing drought risk including predicted change ofwater use.
[Reference]
Ministry of Education, Culture, Sports, Science andTechnology: The Innovative Program of Climate
Change for the 21st Century, Prediction of the Changeinfuture
weather
extremes
using
SuperHigh-Resolution Atmospheric Models, Reporton Research Achievements of 2011, March 2012.