8. Water Resources, Forests, and Their Related Issues in Japan
Hideji MAITA1
1. INTRODUCTION
Water management in Japan has traditionally focused on manipulating the country’s
abundant supplies of freshwater to meet the needs of users. This “supply management”
approach has resulted in the building of large dams and water conveyance systems.
Increasing development costs, capital shortages, government fiscal restraints,
diminishing sources of water supply, polluted water, and a growing concern for the
environment in particular have forced water managers and planners to rethink
traditional approaches to management and to experiment with new ones.
New approaches to water management are necessary to respond to concerns about
a return to a more natural hydrologic regime to restore ecological and geomorphic
functions to rivers. Therefore, the planners cannot depend largely upon dams, as in
previous traditional approaches. However, since dams play important roles in water
resource provision, flood control, and hydropower, it is difficult to establish alternative
water management approaches. Thus, forests are increasingly expected to assume a role
in water resources and flood control.
This paper describes the present status of water resources and forests in Japan and
some characteristics of the forest influences on water resources based on the results of
forest-watershed experiments in Japan.
2. PRESENT STATUS OF WATER RESOURCES
1) Precipitation
Precipitation is the original water resource. It falls directly into river channels and
bodies of standing water and also onto vegetation and the land surface beneath it. Some
of this water evaporates or is transpired to the atmosphere, but much of what falls on the
land is transferred over and though the soil to contribute to river flow or downwards to
contribute to groundwater.
Although the average annual precipitation in Japan from 1966 to 1995 was 1710
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Institute of Agricultural and Forest Engineering, University of Tsukuba
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mm, twice as much as the world average of 970 mm, the per capita precipitation is not
substantial, in fact it is somewhat inadequate because of the high-density population
(Fig. 1). The annual amount of precipitation and its seasonal patterns are very different
in each region because the Japanese archipelago stretches south to north and is divided
into the Pacific Ocean side to the east of the central mountain ranges and the Japan Sea
side to the west. Much of the precipitation in most of the archipelago falls in the rainy
season caused by monsoons (June and July) and typhoons (July to October), although
precipitation on the Japan Sea side in the north is characterized by heavy winter
snowfalls.
Fig. 1 Annual precipitation, per capita annual precipitation, and annual water
resources in Japan and other countries (from Ministry of Land, Infrastructure and
Transport, 2001).
Annual water resources in Japan are the same as the upper limit of water resources, or equal to the
net annual precipitation obtained by subtracting the annual evapotranspiration from the annual
precipitation. Annual water resources in other countries are the same as the annual internal
renewable water resources (from World Resources Institute, 2001).
Figure 2 illustrates the changes of annual precipitation from 1897 to 2000. The
variation of annual precipitation is substantial in Japan and has become particularly
significant since about 1970. The trend of precipitation has decreased by approximately
7 percent since 1897.
2) Upper Limit of Water Resources
The upper limit of water resources is equal to the net precipitation obtained by
subtracting the evapotranspiration from the precipitation, since we cannot directly
utilize the portion of precipitation that returns to the atmosphere as a water resource.
Water resource reports in Japan (Ministry of Land, Infrastructure and Transport, 2001)
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indicate that the total volume of the average annual precipitation in the Japanese islands
(average annual precipitation×area of Japanese islands) is 650 billion cubic meters per
year (billion m3/y) and that of the annual precipitation from the islands is 230 (billion
m3/y). Therefore, the specific net annual precipitation, the upper limit of annual water
resources, is a volume of 420 (billion m3/y). This value is 280 (billion m3/y) for drought
years that may occur once a decade.
Fig. 2 Changes of annual precipitation from 1897 to 2000 (from Water Resources
Department, Ministry of Land, Infrastructure and Transport, 2001).
The annual precipitation in this figure is the average value of 46 sites.
A) Annual precipitation. B) Five-year moving average. C) Mean precipitation. D) Trend.
3) Water Use
The total amount of freshwater withdrawals during 1998 was estimated to be 88.7
(billion m3/y) for agriculture, industrial, and domestic water use (Fig. 3). Of this amount,
agriculture water use was 58.6 (billion m3/y), accounting for 66 percent; the domestic
water use was 16.4 (billion m3/y), accounting for 18.5 percent; and the industrial water
use was 13.7 (billion m3/y), accounting for 15.5 percent. The withdrawals by the source
for this amount were estimated to be 77.7 (billion m3/y), or 87.6 percent, for surface
water, and 11 (billion m3/y), or 12.4 percent, for ground water.
Agriculture water use is mainly categorized into three groups, irrigation water use
for paddy fields and for crop fields and water use for livestock. The greatest volume of
water used for each category of agriculture water use in 1998 was irrigation water use
for paddy fields. This was estimated to be 55.4 (billion m3/y), accounting for 95 percent
of agriculture water use. The other categories are much smaller compared with this;
irrigation water use for crop fields was estimated to be 2.8 (billion m3/y) and water use
for livestock was 0.5 (billion m3/y). Water use in Japan can be characterized from the
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standpoint of quantity by the fact that more than 60 percent of the total amount of
withdrawals is utilized for irrigation to paddy fields.
Fig. 3 Changes of freshwater withdrawals for agriculture, industrial, and
domestic water use from 1975 to 1998 (from Water Resources Department,
Ministry of Land, Infrastructure and Transport, 2001).
Domestic water use includes water for normal household purposes, such as
drinking, food preparation, washing clothes and dishes, flushing toilets, watering lawns
and gardens, and washing cars. It also includes water for hotels, restaurants, office
buildings, and other commercial and public facilities. Since public supply is the
dominant source for domestic water use, public supply of domestic water can be used to
represent the domestic water use. Therefore, an effective public supply that excludes
water losses in deliveries to users and groups can also represent effective water use.
Changes in domestic water use and per capita water use represented by an effective
water use base (Fig. 4) indicate that domestic water use has increased from 8.8 (billion
m3/y) in 1975 to 14.4 (billion m3/y) in 1998. However, the yearly growth rate in
domestic water use has rapidly decreased from 2.7 percent prior to 1989 to 1.0 percent
after 1990. Per capita water use has remained stable at approximately 320 litters per day
(l/d) since 1990, although it increased from 247 (l/d) in 1975 to 308 (l/d) in1989. Thus,
it should be noted that the growth rate decline in domestic water use and per capita
water use began in 1990. This trend could be influenced by the general public having a
recently enhanced awareness about water resources and conservation issues, as well as
the fact that the Japanese economy has declined since the 1990s.
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Fig. 4 Changes of domestic water use and per capita water use from 1975 to 1998
(from Water Resources Department, Ministry of Land, Infrastructure and
Transport, 2001).
The domestic water use and per capita water use in this figure represent the effective water use base.
Fig. 5 Changes of industrial water use from 1965 to 1999 (from Water Resources
Department, Ministry of Land, Infrastructure and Transport, 2001).
The values of freshwater withdrawals are marginally smaller than the values for industrial water use
in Fig. 3 because water use of small factories (less than 29 employees) is omitted.
Industrial water use includes water for such purposes as processing, washing, and
cooling in product manufacturing facilities. The total amount of industrial water use was
estimated to be 56.4 (billion m3/y) during 1998, and 42.7 (billion m3/y) of this amount
was accounted for as recycled water and 13.7 (billion m3/y) as freshwater withdrawals.
This water use is characterized by recycled water; the ratio of recycled water to the total
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industrial use in 1998 reached 76 percent. The total industrial water use from the mid
1960s to mid 1970s rapidly increased 2.5 times in this period, but subsequently leveled
off for the most part up to the present, as shown in Fig. 5. The rapid increase between
the mid 1960s and mid 1970s was supported by an increase (4.5 times) in recycled
water during this period. Though withdrawals increased slightly to 15.8 (billion m3/y) in
1973, they subsequently decreased and have recently remained stable at about 12.5
(billion m3/y). The major industries that use recycled water are steel, chemical, and
allied product industries. The ratio of recycled water to industrial water use reaches 80
to 90 percent in these industries, whereas the ratio in paper and allied product industries,
plastic product industries, and textile industries is 20 to 40 percent.
3. PRESENT STATUS OF FORESTS AND FORESTRY
1) Decline of Forestry and its Problems
Forests cover about 25 million hectares or 67 percent of Japan’s national land area.
Planted forests consisting of coniferous trees, such as Japanese cedar (Cryptomeria
japonica) and Japanese cypress (Chamaecyparis obtuse), make up 10 million hectares,
accounting for 40 percent of the entire forested area, and natural forests and others make
up 15 million hectares, accounting for 60 percent. Plantations that were planted in the
1960s and the 1970s to enhance the wood supply capability will reach the age for final
cutting in the near future (Fig. 6).
Fig. 6 Age distribution of a planted forest in Japan in 1995 (from Japan Forest
Investigation Committee, 2000)
Forest age class: 1) 1-5 years, 2) 6-10 years, 3) 11-15 years, 4) 16-20 years ….
Japan may have recently entered an age of having the most abundant forests in the past
few centuries, from the standpoint of forest quantity. However, forests have recently
developed many problems in terms of forestry and forest quality; the profitability of
forestry has been declining sharply and forest owners have lost their interest in forestry
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due to low and stagnating wood prices and the increase in management costs since the
1980s. As a result, tending and thinning to promote normal growth for planted forests
have not been practiced as the profitability in forestry has declined. This negatively
influences not only timber production but also public benefits of forests, such as soil
and water conservation, recreational use, and environmental protection.
2) New Demands for Forests and Forestry
The public interest toward forests has become even more diversified and specific in
recent years and includes growing expectations for conservation of the natural
environment, recreation, and the mitigation of global warming, as well as concerns
about water resources and flood control. Therefore, the Forest Agency has recently
shifted the forest policy of Japan to fulfillment of a multi-functional role for forests. The
management of national forests has also been shifted to social-benefit-oriented forest
management. The area of forests managed primarily for social objectives, which include
water resources, soil conservation, recreational use, and environmental protection, is
being enlarged from 50 percent to 80 percent of the total national forest area, based on
laws related to the reform of the national forest that were enacted in 1998 (Fig. 7).
Fig. 7 Reform of the National Forests Zoning System for more social-benefitoriented forest management (from Forest Agency, Ministry of Agriculture,
Forestry and Fisheries, 2001)
This change strongly influences Japan’s forests and forestry since national forests cover
approximately 20 percent of Japan’s land area and about 30 percent of the entire forest
area. In addition, a large portion of these forests are located in the backbone mountain
ranges or upstream water reservoir areas, including pristine natural forests. Management
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that protects forests and meets public demands is more important than ever before. The
protection forests, which are expected to promote soil and water conservation,
recreational use, and environmental protection, cover about 8.6 million hectares,
accounting for 34 percent of the Japan’s overall forest area in 1997. Water resource
conservation forests comprise 72 percent of all protection forests. Thus, the forest
function for water resource conservation predominates in the multi-functional role of
forests. However, management for the protection of forests has not always been
practiced appropriately, although protection of the forest system of Japan has a long
history, reaching back to 1897. Forests, particularly water resource conservation forests,
should be appropriately managed and protected based on scientific knowledge to meet
the public’s expectations.
4. FOREST INFLUENCES ON THE AMOUNT AND TIMING OF STREAM
FLOW
1) Increasing Water Yields
Japan’s studies on the relationship between the forest and stream flows at small
watersheds revealed that the annual stream flow increased after clear cutting and
decreased following forest regrowth (Nakano 1976). These increase responses of the
annual stream flow after clear cutting have been observed without exception in similar
studies throughout the world (Satterlund and Adams, 1992). Nakano (1976) reported
that the volumes of annual increase after clear cutting are 190 to 270 mm in Japan.
These responses depend significantly on the deforested area. Greater responses tend to
occur where clear cutting is practiced, lesser responses occur where partial cutting
systems remove a small portion of the cover (Fig. 8). This suggests that increased water
yield from forests could be attained primarily by reducing transpiration and interception
losses by timber harvesting, thinning, and similar means.
2) Changes in Storm Flow
A reasonably good understanding has been reached regarding the relationship
between forest practices and storm flow. Japan’s forest-watershed experiments
conducted at small watersheds indicated that storm-flow volume and peak flows
increased following clear cutting, the storm-flow volume by 1.5 to 2 times and peak
flows by 1.05 to 1.9 times (Nakano, 1976). However, the storm-flow volume did not
always increase after removal of the forest. The storm-flow volume increased after clear
cutting with increasing rainfall when the rainfall exceeded 200 mm at the TatsunokuchiKitadani watershed, in an area with scarce precipitation. In contrast, it decreased when
the rainfall was less than 100 mm (Fig. 9). This suggests that the storm-flow volume for
a smaller rainfall event depends significantly on the soil moisture deficiency before the
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event.
Fig. 8 Percent of increase or decrease in forest cover versus change in annual
stream flow (from Satterlund and Adams, 1992).
Deforestation typically increases stream flow, whereas reforestation decreases it.
Fig. 9 Changes of direct runoff by clear cutting at Tatsunokuchi-Kitadani
watershed (Nakano, 1976).
○: Very wet soil moisture just before a flood. ●: Very deficient soil moisture just
before a flood.
The above results for water yields and storm flow obtained from experiments in
smaller watersheds suggest that vegetation changes in any forest could cause changes in
the route of inflow to the stream channel or changes in evapotranspiration losses, or
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both. These in turn could cause changes in the stream-flow regimen. Thus, potential
effects of forest management exist; however, our present understanding of this issue,
particularly for larger watersheds, is insufficient to derive specific cause-and-effect
relationships.
5. CONCLUSION
Recent water demands in Japan have remained stable, or at least have not increased
since the 1990s. At the same time, public awareness of the environment has forced
water managers and planners to rethink traditional approaches to water management in
favor of new ones that are necessary to respond to concerns about a return to a more
natural hydrologic regime to restore ecological and geomorphic functions to rivers. As a
result, expectations of forest functions such as soil and water conservation and
environmental protection have grown. In fact, forest treatment has the potential to
control the amount and timing of stream flow by modifying the water route from its
place of impact on the watershed to the stream channel, thereby modifying the
distribution and amount of evapotranspiration losses. Appropriate forest and watershed
management practices are useful methods to support new approaches to water
management that are in harmony with the environment. However, the potential for
modifying the amount and timing of water yields through modification of the watershed
by forest treatment varies greatly from one watershed to another and even within
different parts of the same watershed. Although forest hydrologic research has made
considerable progress, few research results have been validated over a wide range of
conditions. We must conduct more research into developing forest and watershed
management techniques to maintain or improve water resources and put appropriate
forest and watershed management for water resources into practice.
6. ACKNOWLEDGMENTS
I would like to thank Dr. Masayoshi Satoh for his useful information about water
resources in Japan and also thank Drs. Haruyuki Mochida and Kazuto Shiga for their
suggestion about the literature on forest and forestry in Japan.
7. REFERENCES
Forest Agency, Ministry of Agriculture, Forestry and Fisheries (2001): Annual Report of
Trends of Forestry, Japan Forestry Association, 382pp.
Japan Forestry Investigation Committee (2002): Forest and Forestry; Data Book 2002,
127pp.
Hideaki Nakano (1976): Forest Hydrology, Kyoritsu Shuppan, 228pp.
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Donald Sattlerlund and Paul W.Adams (1992): Wildland Watershed Management, John
Wiley & Sons, 436pp.
Water Resources Department, Ministry of Land, Infrastructure and Transport (2001):
Water Resources in Japan 2001, Printing Bureau, Ministry of Finance, 328pp.
Water Resources Institute (2001): World Recourses 2000-2001, 389pp.
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