Water Resources and Rice Paddy Cultivation in the Asian Monsoon Region - Dr. Shigetaka Taniyama
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INTERNATIONAL WATER CONFERENCE
Hanoi, Vietnam,
October 14-16, 2002
Water Resources and Rice Paddy Cultivation in the
Asian Monsoon Region
By:
Dr. Shigetaka TaniyamaWater Resources and Rice Paddy Cultivation in the Asian Monsoon Region
Dr. Shigetaka Taniyama1
OUTLINE
1. Overview
2. A Survey of the Asian Monsoon Region
• Rainfall
• Rice Paddy Cultivation and Water Resources
3. The Rice Paddy in the Asian Monsoon Region
• Irrigation and Crop Production
• Characteristics of Rice Paddies in the Asian Monsoon Region
• Efficiency of Water Use in Rice Paddies
4. Irrigation in the 21st Century
• Projected Water Demand in the 21st Century
• Considerations Relating to Rice Paddies in the Asian Monsoon Region
5. A Case Study of Water Redistribution in Japan
6. Conclusions
7. References
8. Acknowledgements
ABSTRACT
The farmers of the Asian monsoon region have traditionally practiced forms of rice
cultivation uniquely adapted to the climate of the region that is characterized by annual precipitation
far in excess of the global average of 800 mm and clearly delineated wet and dry seasons. In the
Asian monsoon region, water is used dynamically as part of a larger cycle embracing the whole
catchment basin from its upper to lower reaches. This is markedly different from the pattern of
water utilization observed in dryland irrigation farming which uses water to the last drop or in static
water- delivery paddy farming in arid and semiarid regions. Rice is a staple food crop in much of
Asia and other continents. Yet, rice paddy cultivation has been criticized as a wasteful consumer of
water. This paper counters this criticism of rice paddy cultivation as a wasteful consumer of water
by pointing out how that agriculture skillfully harnesses the Asian monsoon region's dynamic
hydrologic features. It goes on to identify factors that will need to be taken into consideration, such
as appropriate water transfer to other sectors of society during water shortages, in order to facilitate
sustainable development of paddy irrigation in the coming era of water shortages.
1
Advisor, Japanese Association of Rural Sewerage and Vice President, International Commission on Irrigation and
Drainage (ICID)
11. Overview
The farmers of the Asian monsoon region have traditionally practiced forms of rice
cultivation uniquely adapted to the climate of the region. The Asian monsoon region is
characterized by more than 1,000 mm of precipitation per year, far in excess of the global average
of 800 mm – even precipitation of 1,500 to 2,000 mm is not rare — and clearly delineated wet and
dry seasons. One common type of farming in the Asian monsoon region is rice paddy irrigation in
which water is used as part of a larger cycle embracing the whole catchment basin from its upper to
lower reaches in a dynamic fashion that varies depending on day-by-day fluctuations in
precipitation. This is markedly different from the pattern of water utilization observed in dryland
irrigation farming which uses water to the last drop or the static fashion of water delivery that is
characteristic of paddy farming in arid and semiarid regions.
The 21st century, it is predicted, could witness water shortages on a regional or even global
scale as the population explosion fuels additional demands for food and fiber production while rural
people flock increasingly to the cities. Efficient water use and appropriate redistribution of water
resources between various sectors of society will thus be essential in the 21st century. Rice is a staple
food crop in much of Asia and other continents. Yet, rice paddy cultivation in the Asian monsoon
region has been criticized in some quarters as a wasteful consumer of water.
This paper addresses the above criticism by describing the temporal and spatial distribution
of available water resources in the Asian monsoon region and outlining how paddy cultivation has
skillfully harnessed those dynamic hydrologic features. It is argued that paddy irrigation in the
Asian monsoon region is based on a completely different approach to water use from that found in
arid and semiarid regions and in dryland agriculture. Moreover, this form of paddy irrigation in the
Asian monsoon region makes a significant contribution to the catchment basin's hydrologic cycle
and local biodiversity. Factors are then identified that will need to be taken into account in order to
facilitate sustainable development of paddy irrigation in an era of water shortages. Finally, a case
study is presented of voluntary temporary transfer of water supply from irrigation use to domestic
use at the Aichi waterworks in Japan during periods of water shortfalls.
2. A Survey of the Asian Monsoon Region
• Rainfall
Asia with 23.5% of the Earth's land area is the world's largest continent and is home to
60.7% of humankind. Asia can be divided broadly into the Asian monsoon region which gets some
of the heaviest rainfall on the planet and an Asian arid region occupying the remainder of the
continent. These two regions differ greatly in lifestyles and modes of agricultural production
because they differ so much in climate, particularly the intimate association between the availability
of water resources and human activity. The term monsoon, which is derived from the Arabic word
mausim (season), refers to a seasonal prevailing wind that switches direction in a semiannual cycle.
2For the purposes of this paper, the Asian monsoon region is defined as that region of Asia receiving
more than 1,000 mm of rain a year: East Asia including southeast China, Japan, South Korea and
Taiwan, Southeast Asia including the Philippines, Indochina, Indonesia and Myanmar, and that part
of South Asia consisting of India east of the Deccan Plateau, Sri Lanka and Bangladesh.
The Asian monsoon region is characterized by abundant precipitation carried by seasonal
winds. Annual precipitation exceeds 2,000 mm in some areas, and the rains can be extremely heavy.
This monsoon region has distinct wet and dry seasons. As Table 1 shows, even national capitals,
which are typically located in low-lying areas where less precipitation falls than in the mountains,
record over 70% of their annual precipitation in the six months of the wet season or at least receive
over 1,000 mm of rain during that period.
Table 1. Precipitation in National Capitols in the Asian Monsoon Region.
Country City Annual Rainfall Percentage Duration of
Rainfall during Wet Wet Season Wet Season
(mm) Season Rainfall
(A) (mm) (B/A)
(B)
Japan Tokyo 1,466 981 66.9 May-Oct.
South Korea Seoul 1,343 1,126 83.8 Apr.-Sep.
Taiwan Taipei 2,452 1,665 67.9 Apr.-Sep.
Philippines Manila 1,714 1,487 86.8 May-Oct.
Indonesia Jakarta 1,903 1,382 72.6 May-Dec.
Vietnam Hanoi 1,704 1,429 83.9 May-Oct.
Thailand Bangkok 1,529 1,336 87.4 May-Oct.
Malaysia Kuala Lumpur 2,390 1,113 46.6 Mar.-Oct.
Myanmar Yangon 2,261 2,102 93.0 May-Oct.
Bangladesh Dacca 2,143 1,844 86.0 Apr.-Sep.
Sri Lanka Colombo 3,305 1,676 50.7 Mar.-Sep.
China Shanghai 1,937 1,414 73.0 May-Oct.
Calculated values based on 1971-2000 average precipitation
Source: Rika Nenpyo (Chronological Scientific Tables) 20021
• Rice Paddy Cultivation and Water Resources
There are those who believe that farmers in the Asian monsoon region opted for water-
greedy rice paddy cultivation precisely because the region gets so much rain, most of it concentrated
in the wet season. That perception is a mistaken notion rooted in a superficial knowledge of the
region's hydrological and topographical features and indeed of soil science and crop physiology.
To begin with, this region, encompassing as it does the Circum-Pacific and Himalayan
mobile belts, is steep in the lay of its land and the gradient of its rivers. Such topography leads to the
3problem of how to avoid damage from flooding and soil erosion caused by excessive rainfall while
effectively diverting river water on its sometimes brief journey to the ocean. Rice paddies enclosed
by levees within which water can collect function as dams in their own right where large quantities
of water can be temporarily stored. As such they are beneficial in both preventing flooding and
ensuring adequate water supply. The ponded water is also an effective barrier against the rank
growth of weeds that flourish in the steamy climate. Moreover, leaching of soil moisture washes
away saline
and toxic components (e.g., hydrogen sulfide and organic acids), thereby preventing them from
accumulating in the root zone and causing damage to rice plants. Finally, for a portion of the year
the soil is covered with water (i.e., in a reduced condition) and for a portion of the year the soil is dry
(i.e., in an oxidized condition), resulting in a fluctuation of microorganisms within the soil such that
a beneficial microbial balance and soil fertility are preserved. Damage from continuous rice
cropping is in this way averted, which is why rice has been successfully grown for centuries as the
sole crop on the same land in many areas in the Asian monsoon region.
Thus, paddy rice cultivation is the inevitable choice in the search for a sustainable form of
agriculture best suited to the Asian monsoon region. Asia grows 91.4% of the world's rice, with the
Asian monsoon region accounting for the lion's share of that figure. With a few exceptions such as
Thailand, most of that rice is consumed locally2. Consequently, as shown in Figure 1, Asians
depend on rice on the average for about 32% of their caloric food intake. In Myanmar, Bangladesh,
and Vietnam rice provides over 70% of their caloric intake. Furthermore, rice is capable of keeping
many
mouths fed since it produces such a high yield per unit area. As shown in Table 2, the countries of
the Asian monsoon region where rice is the staple food have a high population density, ranging
from twice the European average in Malaysia to 27 times in Bangladesh.
Figure 1. Rice as a Percentage of Total Caloric Intake: World and Selected Regions (1998)2
35
30
Caloric intake ,%
25
20
15
10
5
0
ia
lia
pe
a
ld
ica
A
ric
or
As
US
ra
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Af
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W
Eu
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Region/Country
4Table 2. Population Density in the Asian Monsoon Countries.
Total Area Farmland Population Population Population
(million ha) (million ha) (millions) per Total per Farmland
(A) (B) (C) Area (pop./ha)
(pop./ha) (C/B)
(C/A)
Europe* 2,195 293 703 0.32 2.40
Asia* 2,744 531 3,507 1.28 6.60
Japan 37.70 4.90 126.50 3.36 25.82
South Korea 9.90 1.90 46.50 4.70 24.47
Taiwan 3.60 0.99 21.47 5.96 21.69
Philippines 30.00 9.90 74.50 2.48 7.53
Indonesia 190.40 30.90 209.30 1.10 6.77
Vietnam 33.10 7.20 78.70 2.38 10.93
Thailand 51.30 20.30 60.90 1.19 3.00
Malaysia 32.90 7.60 21.80 0.66 2.87
Myanmar 67.60 10.10 45.00 0.67 4.46
Bangladesh 14.40 8.20 126.90 8.81 15.48
Sri Lanka 6.50 1.80 18.80 2..89 10.44
China 959.60 135.30 1,226.80 1.28 9.07
Source: ICID, Annual Report (2001-2002)3.
*Figures for Europe represent totals for the 28 European countries and for Asia, total of 27 Asian
countries affiliated with the ICID.
The Asian monsoon region has an abundant stock (supply) of water resources due to the large
quantity of rainfall, but, as illustrated in Table 3, some countries' stocks are not that great when
viewed on a per capita basis. Japan in particular with its dense population has a lower level of water
stock per capita than the European average. On the other hand, in terms of exploiting water
resources, the Asian monsoon region taps a lower percentage of its stock because of natural
conditions — rainfall is concentrated in a short period, plus the gradient of the landscape and its
rivers is steep — and because of inadequately developed water infrastructure. Per capita available
water is thus far lower than the average for Europe and the developed world.
5Table 3. Per Capita Water Stocks and Annual Water Withdrawals.
Water Stocks Annual Water Withdrawals
Total Per Capita Total Percentage Per Capita
(km3) (1000km3) (km3) of Stocks (m3/capita)
Utilized
World 40,673 7.69 3,296 8 660
Asia (excluding the 10,485 3.37 1531 15 526
former USSR)
Japan 335 2.65 89 26 706
South Korea 63 1.45 11 17 298
Philippines 323 5.18 29 9 693
Indonesia 2,530 14.02 17 1 96
Vietnam 376 5.6 5 1 81
Laos 270 66.32 1 0 228
Cambodia 88 10.68 1 0 69
Thailand 110 1.97 32 18 599
Malaysia 456 26.3 9.42 2 765
Myanmar 1,082 25.96 3.96 0 103
Bangladesh 1,357 11.74 22 1 211
Sri Lanka 43 2.51 6 15 503
China 2,800 2.47 460 16 462
Europe 2,321 4.66 359 15 726
North and Central 6,945 16.26 697 10 1,692
America
Source: Figures for water stocks are from Kazuto Nakazawa, Mizu Shigen no Kagaku (The Science of Water
Resources, 1996)4.
The values given are generally out of date. Those for South Korea are from 1976, for
Malaysia and the Philippines 1975, and for China 1980. All other figures are from 1987 except
those for Japan, which have been corrected in light of the latest data.
In highly industrialized Europe, about 54% of the water resources are used for industry and
33% for agriculture. In most of the countries of the Asian monsoon region, by contrast, over 70% of
managed water resources are destined for agricultural use. On the other hand, as can be seen from
Table 4, the Asian countries with the exception of Japan divert less water for domestic use both in
6absolute terms and as a percentage of the total because their water infrastructure is less developed.
These countries are struggling with problems of hygiene such as rampant water-borne microbial
diseases, and in many areas people are unable to live in sanitary, comfortable conditions.
Table 4. Per Capita Water Withdrawals and Domestic Water Consumption.
Annual Water Percentage Annual Daily
Withdrawals Used for Domestic Water Consumption
per Capita Domestic Consumption (liters) US$
(m3) Consumption (m3)
Japan 706 17 120 329 32,350
Indonesia 96 13 12 32 640
Vietnam 81 13 11 30 350
Laos 228 8 18 49 320
Cambodia 69 5 4 11 260
Thailand 599 4 24 66 2,160
Myanmar 103 7 7 19 ---
Bangladesh 211 3 6 16 350
Sri Lanka 503 2 10 27 810
China 462 6 28 77 750
UK 507 20 101 277 21,240
France 606 16 97 266 24,210
Spain 682 12 82 225 14,100
Sources: Figures for per capita GNP are from Sekai Kokusei Zue (World Nations Databook) 2000-
20015.
Other figures are Kazuto Nakazawa, Mizu Shigen no Kagaku (The Science of Water Resources)
(Asakura Shoten)4. Data for South Korea and Malaysia have been excluded since they are out of
date.
Figure 2 shows the striking correlation between per capita GNP and domestic water
consumption in the Asian monsoon region and the major countries of the world, daily water
consumption rising with increasing GNP. Domestic water consumption in the countries of the
Asian monsoon region is expected to increase dramatically as economies grow, the population
burgeons, and people flock to the cities.
7GNP and Dom estic W ater Consum ption
100000
GNP(US$ per pers
10000
1000
100
0 50 100 150 200 250 300 350
W ater Consum ption(liters per person)
Figure 2. GNP and Domestic Water Consumption
3. The Rice Paddy in the Asian Monsoon Region
• Irrigation and Crop Production
Rice production around the world can be classified into several categories. In addition to
irrigated paddy rice, there are the "rain-fed lowland rice" grown in the alluvial regions of Thailand
and Bangladesh, which depends on rainwater alone; "flood-prone rice," which is cultivated in
floodwaters in the river deltas and lowland areas of Cambodia and Bangladesh; and the "upland
rice" of Brazil and Africa.2. While both rain-fed lowland rice and flood-prone rice are grown in the
Asian monsoon region, irrigated paddy cultivation has, thanks to its ability to enhance productivity
dramatically, spread the most widely and is constantly being improved in the process. It does not
merely improve productivity on previously cultivated land through irrigation during the wet season,
it also allows crops to be planted in dry season where that was impossible before and opens up new
land to agricultural production. Rice yields soar in areas where irrigation has been adopted, thus
sustaining a larger population.
Rice farming --- -Irrigated paddy cultivation -------Humid regions (e.g., Asian monsoon regions)
-----Arid regions (e.g., US and Australia)
- -Rain-fed lowland rice
- -Flood-prone rice
- -Upland rice
At the beginning of the 20th century the world had only 50 million ha of irrigated farmland.
By the end of the century that figure had skyrocketed to 270 million ha, with most of the increase
attributable to Asia. Growth in irrigated agriculture has been especially dramatic since 1950.
According to the Annual Report of the International Commission on Irrigation and Drainage
(ICID)3, 69% of the world's irrigated land in 1999 was in Asia, where, as shown in Table 4, irrigated
8farmland accounted for 33.7% of the total stock of agricultural land (this figure is the average for the
27 Asian countries affiliated with the ICID). That percentage, far higher than on any other continent,
attests to the crucial role of irrigation in Asia. Agriculture in the Asian monsoon region consists
primarily of paddy rice cultivation and agricultural water is used almost exclusively for paddy rice.
Figure 3 plots the relationship between crop yield per unit area and percentage of irrigated
farmland in the countries of the Asian monsoon region. While not all the gains can be ascribed to
irrigation, in addition to its direct benefits irrigation has also prompted the adoption of new, better
strains of rice and led to use of fertilizer, thereby ultimately boosting crop production.
Table 4. Irrigated Farmland by Continents.
Total Area Farmland Irrigated Land Irrigation Percentage of
(million ha) (million ha) (million ha) Rate (B/A) Global
(A) (B) Irrigated
Land
Asia (27) 2,744 531 178.9 33.73 69.0
Africa (24) 1,820 157 10.9 6.94 4.2
Oceania (2) 803 56 2.8 4.82 1.1
The Americas 3,771 370 39.6 10.69 15.3
(16)
Europe (28) 2,194 293 27 9.2 10.4
ICID-affiliated 11,333 1,407 259 18.40 100
Countries (97)
World 13,386 1,511 271 17.93
The figures in brackets indicate number of countries.
Source: ICID, Annual Report (2001-2002).3
8000
7000
Crop Yield (kg/ha)
6000
5000
4000
3000
2000
1000
0
0 20 40 60 80
Irrigation Rate (%)
Figure 3. Relations between Irrigation Rate and Crop Yield per Unit Area in Different Countries
(Irrigation rate is defined as the percentage of irrigated farmlands out of total farmlands).
9• Characteristics of Rice Paddies in the Asian Monsoon Region
The rice paddies of the Asian monsoon region evolved in humid climatic conditions with
abundance of rainfall. The mechanism of water use is completely different from that in dryland
irrigated farming. In a dry field, crops are supplied with the quantity of water they physiologically
require (the amount of evapotranspiration) and leaching if the root zone is salt-affected. Any other
water use is considered wasted or not beneficial to crop production, and the idea is to reduce water
losses to a minimum. In a rice paddy of the Asian monsoon region, by contrast, rain water is pooled
on the soil surface, and enough water is retained in the paddy to replenish the amount by which the
water level has dropped over the course of the day. Therefore, when calculating how much water is
necessary, one must take into account more than simply the quantity which the crop physiologically
requires (the amount of evapotranspiration). One must also factor in how much water percolates
vertically into the soil to recharge the groundwater and how much trickles back into drainage
channels and paddies downstream through lateral percolation.
Some experts on dryland agriculture regard such water not consumed by crops as wasted
and cite it as evidence of the low water efficiency of paddy farming. Other experts point out that
irrigation water not consumed by crops in a particular irrigated subunit oftentimes is reused in
another downstream subunit in the irrigated basin and the overall basin irrigation consumptive use
coefficient (irrigation performance) is high compared to that in the upstream irrigation subunit6. The
Asian monsoon region is blessed with an abundance of water during the wet season, and rainfall
can fluctuate wildly over a short span. Thus, it is essential to consider how to make the most
effective use of these water resources within the context of the catchment basin as a whole — as
part of a broad-based hydrologic cycle — not just in terms of the water balance in a single paddy.
Moreover, moisture that penetrates the soil plays an important role in maintaining the paddy's
sustainability.
Figure 4. Paddy Irrigation and Estimated Water Circulation
10(in ten thousands of cubic meters; 1,000 ha over a 120-day period)
An illustrative example is given in Figure 4, which shows the annual water balance for
1,000 ha of rice paddy in Japan's Tone River basin as monitored over a 120-day irrigation period. In
this illustration, the ineffective rainwater is 14.2% (400 m3) of the total of intake (1,920 m3) and
rainfall (900 m3).
The following is a summary of the benefits of rice paddy irrigation in the Asian monsoon
region in addition to the direct consumption of irrigation water by the crops:
1. Paddy irrigation alleviates runoff of rainwater, the amount of which fluctuates dramatically in
the Asian monsoon climate, by temporarily pooling precipitation that has fallen on the paddy
within the levees.
2. Moisture that penetrates through the soil helps to recharge groundwater, which serves as a
stable water source for a multitude of purposes.
3. As the ponded water seeps into the underground it leaches out of the root zone saline
components and other substances harmful to crop growth. That is why rice paddies in the
Asian monsoon region do not suffer salt and other damages.
4. The paddy is left dry during the off-season, then submerged in water during the irrigation
season. That constant seasonal repetition puts the soil through a cycle switching between
oxidative and reductive conditions, a process that promotes proliferation of beneficial soil
microbes to crops and prevents damage from continuous cropping.
5. The ponded water makes the soil anaerobic, and water quality is improved through
denitrification.
6. Paddies where water is pooled serve as a kind of shallow wetland, providing a habitat for
aquatic plants and animals and thus helps maintain a diverse ecosystem.
• Efficiency of Water Use in Rice Paddies
Professor Tony Allan of the University of London terms the water absorbed by crops via
the soil as "virtual water" and estimates that it takes 1,000 tons of water to raise a ton of wheat7. But
according to Professor Mushiake8, 3,200 m3 of water are required per ton of wheat production,
assuming a yield of 2.6 tons per hectare and a daily irrigation depth of 3 mm for duration of 270
days. There is quite a discrepancy between these two estimates. For comparison, dryland winter
wheat grown in the Texas High Plains in the USA for 18 years averaged 1.13 tons/ha (with about
235 mm/yr effective rainfall) and irrigated wheat, 2.68 tons/ha9. While such calculations on applied
water to paddy rice yields in Asian monsoon region are virtually meaningless, let us for the sake of
comparison apply the same formula as for wheat. According to the example in Figure 4, 1,920 m3
of withdrawn water plus 900 m3 of rainwater has, on superficial examination, been applied for a
total of 2,820 m3 to 1,000 ha of rice paddy. Assuming a yield of 6 tons/ha, that gives a figure of
4,700 tons of water per ton of rice. But if we assume that approximately half the water that seeps
back into the river is reused downstream, the number works out to 2,850 tons — not much different
11from irrigated wheat. Although no general conclusions are possible on beneficial water reuse
because conditions vary depending on the topographical location of the paddy, the nature of the
waterways, and land use downstream, the argument that water seepage all goes to waste does not
stand up to close scrutiny.
4. Irrigation in the 21st Century
• Projected Water Demand in the 21st Century
Over the course of the 20th century the world's population increased 3.8 times from 1.6
billion to six billion, while the area of irrigated land rose 6.6 times. Crop production tripled in the
last fifty years of that century alone. By the end of the 20th century 17% of the world's farmland,
much of it in Asia, produced 40% of the world's crops. Population growth is expected to slow down
in the coming 21st century, but the number of people is expected to still reach the 7.5-billion mark
by 2025. Food production will need to be boosted in order to feed all those mouths, and it is
predicted that increased intake of animal protein will drive up crop demand even further. Given the
fact that extensive expansion of farmland is no longer a feasible way to increase yields, the sole
option is to bolster food production per unit area. That can only be accomplished by increasing the
amount of irrigated land, which in turn, requires some additional water resources. Forecasts also call
for dramatic increases in industrial and domestic water use as living standards rise in the developing
world. Domestic water consumption in particular is expected to skyrocket with the ongoing
concentration of population in the cities.
As shown in Table 5, the World Water Vision unveiled by the WWC in 200010 predicts that
by 2025 water demand will climb 10%, most of that increase being attributable to domestic
consumption, while the amount of water used in irrigation will rise 6%. The latter figure assumes
that, while irrigated land area will increase 20%, gains will be made in irrigation efficiency. The
International Commission on Irrigation and Drainage (ICID) for its part has put together A Vision
for Food and Rural Development in collaboration with such bodies as the World Bank and the
national committees of the ICID11. They predict that, even on an optimistic assumption of crop
yields and irrigation efficiency, 15-20% more irrigation water will be needed over the next 25 years.
At any rate increased water demand is unavoidable, though exact forecasts differ considerably.
Table 5. Outlook by the World Water Vision Committee10.
User Water Use (km3) Percentage
1995 2025 Increase
Agriculture 2,500 2,650 6
Industry 750 800 7
Domestic 350 500 43
Reservoirs 200 220 10
Total 3,800 4,200 10
Note: Agriculture is taken as equivalent to irrigation.
12• Considerations Relating to Rice Paddies in the Asian Monsoon Region
In the face of increased pressure on water resources, some hydrologists criticize irrigated
agriculture for wasting too much water, and irrigation engineers are doing their best to develop and
disseminate more efficient structural and nonstructural technologies. But such efforts are generally
tailored to irrigated agriculture in the more arid regions where annual precipitation is low or rainfall
is not heavy. Ensuring the sustainable development of paddy rice agriculture in the Asian monsoon
region in the coming age of water shortages will require coming to grips with challenges quite
different from those facing dryland farming.
First, the Asian monsoon region has an abundance of water resources. One might even go
so far as to contend that paring back on annual water withdrawals during the rainy season would be
largely pointless. The important issue is how to adjust supplies between users when shortages occur
due to large swings in the amount of precipitation. Consider a scenario involving building a dam in
order to develop new water resources. Usually the size of the reservoir is determined based on past
hydrological data assuming a dry year once every decade. Now, one might jump to the conclusion
that water supply cutbacks will be needed only once every ten years, but that assumption does not
hold in monsoon regions that receive large amounts of irregular rainfall. If the level of the water
behind the dam drops to less than half, there is no guarantee that rain will subsequently resume
falling on the pattern of a typical year. Therefore, one will have to invoke Phase 1 of your water
conservation plan. Should the dry spell continue, Phase 2 contingency plans will need to be
implemented. In the real world this type of thing happens a good deal more often than once a
decade. Hence there is not much point in boosting efficiency of water use in wet years when rainfall
is plentiful. A more germane consideration is how to share water among users when reserves are
low.
Second, since paddies in the Asian monsoon region consume only some of the water
withdrawn, it is important to improve recycling so that seepage from paddies can be reused
downstream. Needless to say, the quality of effluent water will have to be maintained in good
condition for reuse.
Third, in order to adjust distribution and foster recycling as described above, it is not
enough to leave irrigation and sewage treatment facilities exclusively to public management. A
powerful organization of water users is required to manage things collectively with input from local
residents. In the case of irrigation water, so-called Land Improvement Districts (LID) as in Japan
fulfill that role. However, these organizational bonds have weakened in recent years as aging
farmers consist the majority of members of LID and city people take up residence in farming
communities due to urbanization. In addition, LID remain undeveloped in rainfall-dependent
regions where paddy irrigation has failed to evolve.
Fourth, there is the question of recycling the 90% of waste water from domestic use that
ends up being discarded. In the developing countries waste water is disposed untreated or
minimally treated, and if it is to be recycled it needs to be treated to a permissible level. Presently the
13developing world uses only minuscule amounts of water for domestic purposes, none of which is
recycled, but demand is expected to pick up in the future as urbanization increases. If that waste
water can be treated to a certain usable level, it could become a valuable water resource for
agriculture and other users. Given the fact that stream flow declines during dry season, the quality of
effluent water will need to be maintained in good condition for recycling.
Let us examine how much domestic waste water can be recycled. Assuming a farm village
with a population density of eighty persons per hectare (non-DID density) that generates 330 liters
of domestic waste water per person per day, 240 m3 of sewage would be produced for every 1,000
ha. If all the waste water were reused in rice paddies, that would be equivalent to 2.4 mm of water
per day. In other words, some 10% of irrigation needs could be met from domestic waste water that
is too large an amount to ignore under water shortfalls. Thus, promoting sewage treatment of
domestic waste water is important not just from the viewpoint of hygiene but from that of making
effective use of water resources as well.
5. A Case Study of Water Redistribution in Japan
Next we examine an example of conversion from agricultural and industrial water use to
domestic water use involving the Aichi waterworks located in Japan's Kiso River basin12.
Completed in 1961, the Aichi waterworks is a multipurpose water development that carries water
for irrigation and domestic and industrial purposes from the Kiso River system to the area stretching
from the plain east of Nagoya to the Chita Peninsula. The Makio Dam was constructed upstream to
serve as the water reservoir. Water is withdrawn at Kaneyama on the middle stretch of the river
system and channeled along a waterway to the tip of the Chita Peninsula, a distance of some 110
km. In 1994 Japan suffered serious water shortages beginning in the early spring due to low rainfall
throughout the country. In the Kiso River basin, which supplies the Aichi waterworks, users began
cutting back on water use on June 9th. However, the dry spell continued with no substantial rains,
and the level of water behind the Makio Dam fell rapidly. This situation led to a progressive beefing
up of restrictions on water withdrawals. Supply of water for irrigation use was cut back a maximum
of 65%, that for domestic use 35%, and that for industrial use 65%. Finally, substantial rains started
to fall on September 16th, putting an end to the immediate crisis, and from October 1st the
restrictions were gradually relaxed until they were lifted completely on November 14th. Throughout
the period in question supply of water for agricultural and industrial use was cut back by about
double the percentage as that for domestic use12.
We turn next to withdrawals by type of use as shown in Table 6. In terms of breakdown of
water rights, agricultural use accounted for a 70.5% stake before restrictions were imposed but only
65.7% of supply after the restriction was lifted, while industrial use experienced a similar decline
from 21.0% to 19.6%. Domestic use, by contrast, increased from 8.5% to 14.7% after the
restrictions. This clearly represents a shift in utilization of a developed water source away from
agricultural and industrial use to domestic use.
14Table 6. Fluctuations in Withdrawals during the Water Shortage in Kiso River Basin12.
Water Rights Withdrawals after Restrictions Imposed
Type of Use
Volume Percentage Level of Volume Percentage Relative
3 3
(m /sec) of Total Cutbacks (m /sec• of Total Change
Volume (%) Volume (B/A)
(A) (B)
Domestic 2.594 8.5 35 1.686 14.7 1.73
Agricultural 21.514 70.5 65 7.530 65.7 0.93
(Irrigation)
Industrial 6.411 21.0 65 2.244 19.6 0.93
Total 30.519 100.0 11.460 100.0
As mentioned in Section 4, such water transfers from agricultural and industrial use to
domestic use are not restricted to years of serious drought. Table 7 lists cases of water supply
cutbacks over the last 29 years at the Makio Dam, which supplies the Aichi waterworks12. Some
type of cutback has been implemented in 19 of the last 29 years, in each case resulting in a water
transfer from agricultural and industrial to domestic users.
Table 7. Water Supply Cutbacks over the Past 29 Years12.
Year Number of Level of Cutbacks in Percent
Days Agricultural Domestic Industrial
Cutbacks
Imposed
1973 109 30 20 30
1977 189 20 10 20
1978 55 15 10 20
1979 10 10 5 10
1982 34 40 20 40
1983 52 20 10 20
1984 246 30 15 30
1986 148 40 20 40
1987 191 37 17 37
1990 32 20 10 20
1992 51 20 10 20
1993 25 20 15 20
1994 166 65 35 65
1995 210 44 22 44
1996 29 20 20 20
1997 7 10 5 10
1999 9 10 5 10
2000 78 50 25 50
2001 144 40 20 40
15The problem that arises here is how to determine the level of cutbacks. In the case of the
Kiso River basin, a deliberation committee was initially set up to decide by how much to restrict
water supply to various users. This committee consisted of the affected users including the LID in
the case of agricultural use, and municipalities and the prefectural bureau of industry in the case of
domestic and industrial use. Then, if the drought grows more severe and the need arises for
adjustments throughout the catchments basin, an emergency committee is convened to regulate
water users throughout the river system and decide on drought measures. This committee consists
of representatives from the affected prefectures and the local branches of the Ministry of Land,
Infrastructure and Transport which manages the country's rivers, the Ministry of Agriculture,
Forestry and Fisheries which has jurisdiction over agricultural water use, and the Ministry of
Economy, Trade and Industry which has jurisdiction over industrial water use.
In this case, it is important that water users decide among themselves what water
conservation measures are to be implemented, for they are the ones who will suffer the economic
consequences. One school of thought argues that the process of reallocation should be left to the
market and the relevant rights traded for money. But in monsoon regions the price of water is
speculative, and it is inconceivable that this approach would result in its being pegged at the right
level13. A prolonged dry spell would send the price of water through the roof, yet a single torrential
rainfall might solve everything overnight. After all, unlike arid and semiarid regions, the monsoon
climate is fickle and completely unpredictable even over the short term.
Figure 5 is an example of transition of cutbacks in 1994 to that of normal year. The graph
shows a change of the ratio of water reserve of 1994 and cutbacks ratio during water saving
operation period.
Dams in Japan are designed with capacity that can bear a drought that occurs once a decade.
The amount of the storage capacity is calculated on the assumption that the catchments area should
have the designated rainfall. But it cannot be known in advance during water discharging operation
period if the catchments area will suffer a heavier shortage of rainfall than that in designated year.
Therefore:
1. Cutbacks start whenever the ratio of water reserve to that of normal year is 50% or lower,
and
2. Cutbacks ratio change according to the S/C.
Temporary water transfers from agricultural and industrial use to domestic use during
severe water shortage period are operated in a voluntary way under such an unpredictable situation.
16W ater Reserves and W ater Supply Cutbacks in Drought
Year (1994)
80
70
60
50 W ater reserves
percent
Agricutural
40
Dom estic
30
Indutrial
20
(1) Water reserves
10
(%):
0
(Storage/Capacity) ×
ly uly ~10
3
ne ~7
ne ~14
Ju ne ~16
~J 4
~N 16
l 5
Au ugu 3
st 1
t. t. 0
ne ~10
pt 31
.1
25 7~2
1
gu st2
O c O 1~3
Ju uly
Ju e 1
17 1~
~A 1~
Se 22~
Ju J y 5
Ju 11
Ju 14
Ju 7
ov
100
ne 1
14 1
.
n
Ju
c
(2)Agricultural,
D ti
Dates
Figure 5. Relationship between Water Reserves and Water Supply Cutbacks in a Typical Year
(1994)12.
Also note that, since water supply cutbacks in the Tone River basin are decided publicly
without direct input from users, those cutbacks are all set at the same percentage without any
variation. This fact underscores the importance of users making decisions for them.
6. Conclusions
Rice is the ideal crop for the Asian monsoon region with its dense pattern of habitation.
Sustainable ways must be found of expanding agricultural production for the population that is
expected to grow even larger in the 21st century. Moreover, urbanization and economic
development is expected to result in a massive increase in domestic water consumption. For these
reasons large amounts of water will be required. Paddy irrigation, which occupies an important
place in the Asian monsoon region, is practiced within the context of the overall hydrologic cycle
and uses water in quite different fashion from dryland farming. In the 21st century the rice paddy's
unique features will need to be harnessed in the quest to make effective use of water resources and
achieve sustainable development.
17ACKNOWLEDGEMENT
I express my gratitude to Mr. Kazumi Yamaoka of the Japanese Institute of Irrigation, and
Drainage (JIID), Mr. Fukuda of the Second Engineering Division of the Water Resources
Development Public Corporation, Japan and Prof.. Emeritus Kenneth K. Tanji of the University of
California and Japan for their valuable opinions during the writing of this paper.
REFERENCES
1. Rika Nanny (Chronological Scientific Tables) 2002
2. Shigetaka Taniyama, Keynote Addresses. 1st Asian Regional Conference, 2001.
3. ICID, Annual Report, 2001-2002.
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1996.
5. Sekai Kokusei Zue (World Nations Databook) 2001-2002, 2002.
6. K.H. Solomon & B. Davidoff, relating unit and subunit irrigation performance, Transactions,
ASAE 42:115-122, 1999.
7. Tony Allan, "Moving Water to Satisfy Uneven Global Needs: 'Trading' Water as an Alternative to
Engineering It." ICID Journal 1998.
8. Katsumi Mushiake, Nihon o Chushin to Shita Kasosui no Yushutsunyu (Import and Export of
Virtual Water in Japan). The Japan Society of Hydrology and Water Resources, 2002.
9. J.T. Musick & K.B. Porter, Chapter 20, Wheat, IN Irrigation of Agricultural Crops, B.A. Stewart
& D.R. Nielsen, co-editors. Agronomy 30, American Society of Agronomy, 1990.
10. William J. Cosgrove & Frank R. Rijsberman, World Water Vision. 2000.
11. ICID, Water for Food and Rural Development, 2000.
12. Water Resources Development Public Corporation, Heisei 6 Nen Kassui Hokokusho (Report on
the Water Shortage of 1994). 1995.
13. The Japanese Institute of Irrigation and Drainage (JIID), Suido no Chi (Wisdom based on "Land
and Water"). 2002.
Miscellaneous
Per Pinstrup-Andersen, Rajul Pandya-Lorch and Mark W. Rosegrant, World Food Prospects:
Critical Issues for the Early Twenty-first Century. International Food Policy Research Institute,
1999.
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