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The Effects of Climate Change on Water Resources
So KAZAMA1 and Taikan OKI2
1

Graduate School of Environmental Studies, Tohoku University
6-6-20, Aoba, Sendai, Miyagi 980-8579, Japane-mail: kazama@kaigan.civil.tohoku.ac.jp
2
Institute of Industrial Science, The University of Tokyo

Abstract
This paper introduces examples of influences of global warming on water resources, and means ofadaptation. Problems regarding water resources widely and complexly interact with each other. Therefore current technologies for coping with these influences and making integrated assessments include
much uncertainty and are difficult to perform. Many researchers have done their best, however, to develop numerical simulation techniques which can evaluate each interaction and the integrated influences.
Numerical simulations with GCM data are elucidating problems on floods and droughts. They also help
us understand the impact of fresh water resource loss and human activities in coastal regions due to sea
level rise. Although no models of water quality under the influence of global warming have been established yet, there is some qualitative understanding of the influence on water quality. Even though research and discussions for adaptation of water resources have already started, no predictions of socialand climatic changes have been made yet. Therefore it is necessary to develop numerical models
expressing the relationship between water resources and social conditions.
Key words: coastal water resources, droughts, floods, snow, water environment

1. Introduction
Global warming has already impacted various
fields including ecology, agriculture, and infectious
diseases, which are strongly related to water. Freshwater influences many phenomena, both actively andpassively, and both directly and indirectly. It would be
too difficult to get an intuitive grasp of the complex

organic links.
Figure 1 shows the links between the simple processes of global warming influence and water resource
problems. It is difficult for us to grasp even these influences quantitatively because it is necessary toobtain not only the function of each process but also
the function of the feedback.
Numerical models are utilized to comprehend theTemperature increase
Strong stratification
Highly intense rainfall

Ice and snow reduction

Increase in sedimentationand nutrients

Erratic flooding

Largeevapotranspiration

Sea level rise

Decreased water quality
Boom in infectious
diseases

Decreased baseflow
base flow

Saline intrusion

Poorer water resources

Fig. 1 Influence propagation from temperature increase to water resources.
Global Environmental Research
10(2)/2006: 201-206
printed in Japan©2006 AIRIES202

S. KAZAMA and T. OKI

complex link in an integrated manner. Many numerical models cannot perfectly express real world conditions but can provide an objective quantitative evaluation easier than by the intuitive feel (Tosaka, 2003).
Another reason numerical models are useful is thecontribution they make to decision-making and policy
determination by enabling future prediction undervarious scenarios.
Many water resource evaluation models rely on
climatic models. In other words, effects on water resources are discussed using evaluation models andaltering boundary conditions, which are given by ageneral circulation model (GCM) expressing futurewater and energy movements in the ocean and atmosphere. GCMs simulate climatic conditions startingfrom bold assumptions called the scenario, which affects simulation results. The scenario itself is a future
supposition in policy or social environments and includes a large degree of uncertainty. We cannot predict
future scenarios reliably. Therefore this model is too
abstruse to elucidate the interaction between water
resources and social environments. Current evaluations on water resources under global warming appear
simple although they involve great efforts to forecastchange in water resources.

2. Risk of Floods and Droughts
GCM data are used in forecasting floods anddroughts in the future because they result from precipitation. Many researchers have mentioned precipitation patterns in connection with global warming. For
example, there will be more incidents of severe rainfall in Japan (Water resources committee in Science
Council of Japan, 2005) such as tropic shower. Thismeans an increase in extreme rainfall and extension of
non-rainfall period though annual precipitation remains the same. The reason is the escalation of sea
surface temperatures results in stronger and longerlasting typhoons and larger convection clouds. Thesetypes of rainfall produce severe floods and droughts.
Japan has successfully prevented flooding bybuilding dams and dikes on large rivers since WorldWar II. However, medium and small rivers managed
by rural governments are still vulnerable to extreme
rainfall due to poor maintenance. Heavy rainfall generated from cumulonimbus clouds over a small area
damages medium and small rivers.
Some urban districts have underground diversion
channels to protect areas with high economic activity.
Nowadays there is talk of limiting budgets for thetypes of infrastructure used as countermeasures. The
Japanese government’s master plan for integrated
flood protection recommends percolation systems,
retention ponds, and allowances for inundation, which
are widely used countermeasures.
Drought problems are more serious. Althoughdams and reservoirs have been able to keep the watersupply stable thus far, extreme rainfall patterns will

make the control difficult under global warming.
Stable runoff is provided predominantly by groundwater storage in the mountainous forests. The name“green dam” has sometimes been applied to the forest
function of recharge and late runoff processes. Most ofhighly intense rainfall due to global warming reduces
the green dam effect because the rainfall flows downstream without infiltration and the groundwater levelrecedes. Also higher temperatures diminish snow storage. Therefore the base flow of the river will decrease
and lose stable water supply will be lost. Drought
problems heighten regional temperatures generatingheat islands and accelerate the lack of water to suppress the temperature.
Some people say that drought problems will besolved by the future depopulation, which will reducewater demand. However, new water resources will beneeded for various forms of water utilization such asenvironmental water for ecosystems or watering torelieve the heat island effect. Global warming andchanges in the social environment may continue toimpact on water supply and demand. Dams and reservoirs are the most effective adaptation for providing
water resources stability and are making the situation
tolerable. Facilities finding wider use recently like
infiltration masses or a house tanks are the main waysof adapting in Japan. Different physical countermeasures involving constructing facilities have been promoted as have institutional countermeasures, which
include, for example, land development prohibitions
and amendments in water right rules. Many rural governments have taken these actions innovatively andenacted fundamental laws concerning water resources.
Numerical simulation is adequate for grasping thequantitative effectiveness of adaptations. Figure 2
shows the distribution of rainfall, soil moisture, riverdischarge, and runoff in the 2004 flood in the
Hokuriku district, Japan (Okazawa et al., 2005). It iseasy to grasp the relative intensity occurring over awide area and to detect weak areas. Also it can express
the distribution of water resources using GCM data
for the future projections. These are excellent points
providing criteria for objective evaluation of postglobal-warming environments.
It is reported that population explosions in India
and African countries will encourage more food production, which will make water resources scarcer on
the world market. Japan depends on overseas foodproduction and will import more water indirectly.
Figure 3 explains the importation of virtual water toJapan. We can see that Japan imports water from
Middle Eastern and African countries, which havefewer water resources (Oki et al., 2003). If Japan uses
more water and boosts the water price, international
criticism will be incurred. Water resource problems
are a potential foreign trade concern and impact uponvarious commodities in the world, not only crops but
also services. The saying that water can be a strategic
material is not hyperbole.

The Effects of Climate Change on Water Resources

Fig. 2203

Distribution of rainfall (upper left), runoff (upper
right), soil moisture (bottom left), and discharge(bottom right) during a typhoon in Japan.

Fig. 3 Importation of virtual water to Japan (100,000,000m3/year).

3. Snow Resources
Snow is one of the things most notably influenced
by global warming. Snowfall changes to rainfall.
Stored snow melts quickly afterward. Snow plays animportant role in maintenance of water resources by
providing long constant recharge and storage volume.
The Japan Sea side of our country is famous as having
among the heaviest snowfalls in the world. In particular, the Tohoku District has less rainfall in spring,
therefore water from snowmelt contributes to irrigation water for large paddy areas. The amount of snow
in the Tohoku District is about 7 km3, estimated byremote sensing technology, and is equal to quadrupleof household water for the Tokyo metropolitan district(Kazama & Sawamoto, 1996). Reduced snow will
influence people’s life in the Tohoku District.
Numerical simulation of snow variation can be accomplished with precipitation and temperature data as
input, and a satellite image can be used for calibrationof the model parameter. The simulation gives accurate
variations as shown in Fig. 4. The difference in theamount of snow between light and heavy snow years
reaches more than 30% in the simulation. Also RCM(Regional Circular Model) also shows a decline in theamount of snow in the Tohoku and Hokuriku Districts(Inoue & Yokoyama, 2003). Moreover, the long slow
recharge rate of snowmelt is good for ground water
storage. These results foretell of a formidable task to
manage water resources.

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S. KAZAMA and T. OKI

Fig. 4 Snow water equivalent distributions in Japan (mm).
Left: heavy snow year, Feb., 2000; Right: little snow year, Feb., 1990.

depend on snowmelt resources, for example, theMekong and Ganges Rivers in the tropical monsoon
region, which provide base flows from snowmelt
water in the dry season. In addition, the large areas
covered by snow influence global climate conditions
and water resources in snow-free regions directly andindirectly (Saito et al., 2004).

4. Sea Level Rise
Fig. 5 Snow variation with different LAIs.

Adaptations for snow decrease need to be massivein scale on account of the huge amounts involved.
However, if snowmelt season can be expanded, adaptations on some level will possible without the massive structure. For example, one successful measure
could be snow dams using dynamite discharges todeposit snow in valley bottoms, where the snow canlast for a long time. Vegetation can also be used fordelaying snowmelt. Izumi et al. have investigatedsnow variation in forests of different vegetation using
energy balance and wind turbulence models (Izumi
et al., 2006). This calculation involves the effects ofinterception, wind speed profile, and radiation. It is
known that deciduous trees have less snow accumulation but longer deposit periods than evergreen trees asshown in Fig. 5. This suggests that vegetation change
can be used as an adaptation to global warming
although these changes may be unallowable withrespect to ecological conditions.
Not only Japan, but other large areas around world

Sea level rise due to global warming has been detected at many points and is affecting coastal water
resources. Under conditions of higher sea levels, tidal
surges and tsunamis cause damage directly and salt
pollution indirectly. Also a gradual sea level rise
would allow saline water to intrude along the rivers
and spread areas of salt pollution through lower regions such as deltas or aggraded valley plains which
are frequently occupied by grain belts or major urban
areas.
Not only surface water but also groundwater resources are influenced because many cities in thelower areas will be demanding increased fresh
groundwater as their populations increase. Lower
groundwater levels also result in saline intrusion, too.
Ranjan et al. estimated fresh groundwater reduction incoastal zones due to global warming using GCM dataand sea level rise data and detected larger areas of
reduction in northern Australia and eastern Africa asshown in Fig. 6 (Ranjan et al., 2007). These kinds ofmodels involve much uncertainty regarding futuresocial environments but give us an idea of the strong
factors influencing the results. Ranjan’s results

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Fig. 6 Reduction in coastal fresh groundwater resources over next 100 years.

mentioned that population and land-use change were
the strongest factors influencing groundwater resources and indicated that control of those factors is
one means of adapting to groundwater reduction due
to the global warming.
People residing on islands are at the mercy of fresh
groundwater resources. Many small islands also havethe same and more serious problems because the area
recharges on the island shrinks when the sea level
rises. This is another water-stress factor in the hydrologic environment on small islands.

5. Water Quality
Water quality problems impact on acquisition ofwater resources. Water treatment plants are an adaptation to this problem under global warming, but thismeans that poorer quality gives rise to higher costs.
Higher water temperatures decrease DO (Dissolved
Oxygen) and inhibit river self-purification. Sometimes
high temperatures in cool zones cause the reverse tohappen because bacteria work more actively.
Heavy rainfall is more important than high temperatures in global warming with regard to water quality because highly intense rainfalls produces a large
amount of sedimentation and nutrients, which growexponentially as runoff discharge increases. The sedimentation and nutrients are deposited in the reservoirs
and lakes, resulting in eutrophication and increased
algae, which give the water a moldy odor and prevent
flocculation and filtration. Recently it has been foundthat pre-chlorine treatment as a countermeasure tothese problems produces carcinogens. Also depositionof sediment decreases reservoir capacity and increases
environmental degradation. Shallow water levels are

sensitive to temperature change and extremely highwater temperatures occur sometimes and kill almost
all of the aqueous life.
Snowmelt also affects water quality because slope
failure and landslides due to snowmelt generate huge
sedimentation yields. Figure 7 (b) indicates the distribution of 10 years probability of landslides due tosnowmelt in the Tohoku region. This probability wasestimated using the multi-logit function with variablesof geology, slope gradient and hydraulic gradient. The
function parameters were evaluated with actual records of slope failures (Kawagoe et al., 2006). A highprobability indicates a large sedimentation yield.
Reduction in the amount of snowmelt due to global
warming may decrease the sedimentation yield andresult in better water quality but, on the other hand,raindrops may accelerate surface erosion and gully
and finally lead to slope failure and sedimentation.
Figure 7 (a) shows 10-year probabilities of landslides
due to rainfall in the same way as Fig. 7(b) for rainfall.
Comparing both figures, rainfall produces a bigger
sedimentation yield than snowfall. Global warming
has a higher potential to cause loss of reservoir function and result in a poorer water quality environment.

6. Conclusions
Water resources problems under conditions ofglobal warming were the main focus of our research
results. The accurate forecasting of effects on water
resources in the future depends on good meteorological and climatic forecasting but foremost on determining social scenarios. Changes in population and landuse dynamically affect hydrologic cycles and wateruse directly, and substance cycles and human activities

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Fig. 7 Slope hazard probability in rain and snow.
(a) 10 years with extreme rainfall, (b) 10 years with heavy snow.

indirectly. Policies also influence human activities andwater resources, but models have not been established
for those influences yet. With regard to future water,
more uncertainty exists in social forecasting than inhydrological and climatic models. It is clear that
accurate predictions of social actions in the future aremost important to predicting future water resources(Uesugi, 2003). It seems that policies and social
environments in the future cannot be forecasted even a
couple of years in advance.
Many researchers are doing their best right now to
get an understanding of future water problems andadaptations to global warming.
References
Inoue, S. and K. Yokoyama（2003）Estimation of snowfall depth,
maximum snow depth and snow pack conditions in Japan byusing five global warming predicted data. Journal of Agricultural Meteorology, 59: 227-236.
Izumi, H., S. Kazama and M. Sawamoto (2006) Influence ofthe forest for evaluation of snow water equivalent. Annual
Journal of Hydraulic Engineering, 50: 421-426. (in Japanese)
Kawagoe, S., S. Kazama and M. Sawamoto (2006) Slopehazard probability involving return period of extreme
precipitation. Annual Journal of Hydraulic Engineering, 50:

619-624. (in Japanese)
Kazama, S. and M. Sawamoto (1996) Estimation of the snow
depth distribution and snow water resources in wide area.
Proc. Int. Conf., Water Resources & Environment Research,
659-666．
Okazawa, T., N. Hanasaki, K. Yoshimura, T. Oki and S. Kanae(2005) Development of river flow forecasting using GPV of
JMA. Proc. 2005 Annual Conference, JSHWR, 148-149. (inJapanese)Oki, T., M. Sato, A. Kawamura, M. Miyake, S. Kanae and K.
Musiake (2003) Virtual water trade to Japan and in the world.
Virtual water trade, Proceedings of the international expert
meeting on virtual water trade, Value of Water Research
Report Series, 12: 221-235.
Ranjan, P. S., S. Kazama and M. Sawamoto (2007) Climatechange and fresh groundwater resources in coastal regions.
Global Environmental Change.
Saito, K., T. Yasunari and J. Cohen (2004) Changes in the
sub-decadal covariability between Northern Hemispheresnow cover and the general circulation of the atmosphere.
International Journal of Climatology, 24: 33-44.
Tosaka, H. (2003) Hydrological cycle modeling for subsurface.
Global Environmental Research (J), 8: 37-48.
Uesugi, T. (2003) Integrated evaluation of water environment.
Global Environmental Research (J), 8: 81-88．
Water resources committee in Science Council of Japan (2005)
Preparation for flood and draught, Committee report. (inJapanese)(Received 13 July 2006, Accepted 26 September 2006)