Introduction of Dryland Science
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Development of economic and sustainable
agricultural technology in dryland
Contents
1
What is dryland ?
2
What are the causes of desertification ?
3
How to combat desertification using technology ?
Division of Afforestration and Land Conservation,
Arid Land Research Center, Tottori University
Mitsuhiro INOUE
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What is dryland ?
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What is dryland ?
Characteristic
Little precipitation
High potential
evapotranspiration
Distribution map of average annual precipitation
Distribution of Drylands
Distribution map of average annual potential evapotranspiration
（UNEP, 1991）
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Distribution map of drylands in the world
Rain shadow deserts
Takla Makan desert
Subtropical desert
Sahara desert
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Terrestrial equator
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Atacama desert
Cold coastal desert
Hyper-arid
Arid
Humid
Semi-arid
Cold climates
Dry sub-humid
（UNEP, 1991）
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potential evapotranspiration (mm)
Precipitation (mm)
Dryland and humid region
Shabul (Iran)
Dryland
Potential evapotranspiration
Precipitation
Humid region
Tottori (Japan)
Precipitation
Potential
evapotranspiration
year
Precipitation (P) in arid region is far less than
potential evapotranspiration (PET) on average.
Potential evapotranspiration (PET) has been defined as the volume of water per unit
area of field evaporated and transpired by a dense stand of actively growing short
grass that is well endowed with water.
(Matsumoto)
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Classification of Drylands using the Aridity Index
Hyper-arid region
Arid region
P / PET ＜ 0.05
0.05 ＜ P / PET ＜ 0.2
Average annual precipitation is less than 200mm （Rain season in Winter）
Average annual precipitation is less than 300mm （Rain season in Summer）
Semi-arid region
0.2 ＜ P / PET ＜ 0.5
Average annual precipitation is less than 500mm （Rain season in Winter）
Average annual precipitation is less than 800mm （Rain season in Summer）
Dry sub-humid region
0.5 ＜ P / PET ＜ 0.65
Aridity Index (AI) is defined as dryness ratio, which is ratio of P and PET.
P：Average annual precipitation，PET：Average annual potential evapotranspiration
UNEP (United Nations Environmental Program)
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Coefficient of variation (CV) and
Maximum value of precipitation
Annual precipitation (mm)
place
time
annual
maximum
precipitation intensity
Chicama (Peru)
1925
Aozou (Sahara)
1934 Mar.
4 mm
394 mm/yr
30 mm
370 mm/3d
Swakopmund (Namibia) 1934
15 mm
Sharja (UEA)
107 mm
1957
Tamanrasset (Sahara) 1950 Sep 27 mm
50 mm/yr
74mm/50min
44mm/3hr
Bisra (Algeria)
1969 Sep. 148 mm
210mm/2d
El Djem (Tunisia)
1969 Sep
275 mm
310mm/3d
Lima (Peru)
1925
46 mm
1524mm/yr
About 10 times more precipitation in
3 days than annual precipitation, and
more precipitation in 3 hours than
annual precipitation were recorded.
Coefficient of Variation (CV) of
annual precipitation (%)
Coefficient of variation (CV) is standard deviation of
precipitation divided by average annual precipitation.
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Northern circle
Terrestrial equator
Southern circle
CV of annual
precipitation
Distribution map of Coefficient of Variation of annual precipitation
CV of annual precipitation in dryland is more than 30%.
CV of annual precipitation in dryland is significantly large.
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Aridity is defined as P/PET
Hyper-arid region
P / PET ＜ 0.05
Arid region
0.05 ＜ P / PET ＜ 0.2
Sahara desert
Anuual P < 200mm (Winter rainy season)
Annual P < 300mm (Summer rainy season)
Semi-arid region
0.2 ＜ P / PET ＜ 0.5
Anuual P < 500mm (Winter rainy season)
Annual P < 800mm (Summer rainy season)
Dry sub-humid region
0.5 ＜ P / PET ＜ 0.65
P：Average annual precipitation
PET：Average annual potential evapotranspiration
Classification of drylands by United Nations Environmental Program (UNEP)
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Drylands Area
Region
Area
（Mha）
Cold
1765.0
Humid
5100.4
Dry sub-humid 1294.7
Semi-arid
2305.3
Arid
1569.2
Hyper-arid
Total
978.1
13012.7
Drylands
Dryland ＝Dry sub-humid＋Semi-arid＋Arid＋Hyper-arid
47.2%
Susceptible dryland ＝Dry sub-humid＋Semi-arid＋Arid
39.7 %
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What is dryland ?
1
2
3
4
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Average precipitation (P) in arid region is less than
average potential evapotranspiration (PET).
UNEP (United Nations Environmental Program)
classifies drylands using the aridity index, into four
types; (1) hyper-arid (2) arid, (3) semi-arid and (4) dry
sub-humid.
For example, aridity index is less than 0.05 in hyperarid region, and aridity index is between 0.05 and 0.2.
is arid region.
Drylands are related not only to less precipitation, but
also to large coefficient of variation of annual
precipitation.
Susceptible dryland covers 39.7%, approximately
40% of the Earth’s land area.
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What are the causes of
desertification ?
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dry land region with very little
precipitation, and vegetation
What is desert ?
？
What is desertification ?
（Photo: INOUE ）
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Sahel
Desertification
Map of desertification
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Definitions of desertification/land degradation
Definition of desertification is significantly different among organizations
(GLASOD, UNCOD, CCD, UNCED). The terms ‘desertification’ and ‘land
degradation’ have been used with various meanings, for which internationally
accepted definitions have evolved over time.
The United Nations Conference on Desertification (UNCOD) in 1977 defined
desertification as ‘the diminution or destruction of the biological potential of land
in arid, semi-arid and sub-humid areas mainly due to human activities’.
The Global Assessment on Soil Degradation (GLASOD) in 1990 stated that
‘desertification/land degradation, in the context of assessment, is land
degradation in arid, semi-arid and dry sub-humid areas resulting mainly from
human activities’.
At the United Nations Conference on Environment and Development (UNCED) in
1992 and in the convention to combat desertification (CCD) in1994, it was stated
that ‘desertification is land degradation in arid, semi-arid and dry sub-humid areas
resulting from various factors, including climate variations and human activities’.
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Global status of desertification
Land area affected
by desertification
Desertification region in
agriculturally used drylands
Population affected
by desertification
North America
1.1B
約10億
South America
Europe
Africa
3.6Bha
Australasia
One-forth of total land
（14.96 billion ha）
One-six of population
（6.6 billion)
Asia
According to UNEP assessment, about 70% of drylands used worldwide for
agriculture are already degraded. Desertification/land degradation now affects
almost 30% of the total land area of the world and one-sixth of the world’s
population. About 22% of Asia’s drylands and 15% of its other land area is
affected by desertification/land degradation. Developing countries are the most
affected, but developed countries also face problems with desertification/land
degradation.
( UNEP, 1991)
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Desertification region in agriculturally used drylands
Rangelands
（1.223 billion ha）
73% (3.3 billion ha)
4.56 billion ha
88%
Rainfed cropland
47% (216 million ha)
（242 million ha）
0.46 billion ha
9%
Irrigated land
30 % （43 million ha） （103 million ha）
0.15 billion ha
3%
Total cultivated drylands
（1568 million ha）
70% (3.592 billion ha)
5.16 billion ha
Desertification area
Non-desertification area
Causes of Desertification/Land Degradation
Natural causes
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１３％
Global climate change
Long-term drought
Desertification
Reduced precipitation
Aridification
Human-induced factors
８７％
Soil Erosion
Over-grazing
Water Erosion
Over-deforestation
Wind Erosion
Over-exploitation
Agriculture
Over-cultivation
Over-irrigation
Waterlogging
Salt accumulation
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Natural causes
Precipitation (mm)
100-years trend
of reduced
precipitation and
aridification
Year
Long-term drought
100-years variation of annual precipitation in Senegal
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Human-induced Desertification is land degradation in drylands
resulting primarily from human activities
factors
Over-grazing
soil erosion
Over-deforestation
water erosion
Over-exploitation
water erosion
Agriculture
Over-cultivation
water erosion
Over-irrigation
salt accumulation
Over-irrigation
waterlogging
Current worldwide desertification is taking place much faster than historically
observed and usually arises from the demands of increased populations that settle on
the land in order to grow crops and graze animals.
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Humaninduced
factors
Main causes of soil degradation are over-grazing,
over-deforestation,
over-exploitation
and
agriculture. Agriculture activities include overirrigation and over-cultivation.
Main causes of soil degradation by region in susceptible drylands (Mha)
Region
Over-grazing
Drylland area
Over-exploitation
Over-deforestation
others
Agriculture
Total dedraded
Africa
Asia
Australasia
Europe
North America
South America
Total
UNEP 1992
Over-grazing is the most important cause of degradation in the dryland areas,
especially, in Africa and Asia. Agricultural activities represent the most important
cause of dryland degradation in North America, and also important in Asia and
Africa. We will focus on over-grazing and agriculture in next discussion as the
causes of soil degradation in susceptible drylands.
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Over-grazing
wind and water erosion
Overgrazing has been long regarded as a goat and a sheep for many
dryland degradation problems
The loss of perennial grasses has been widespread from grasslands,
savannas, and open woodland, often with replacement by unpalatable
shrubs, and heavy grazing pressures continue to reduce ground cover,
especially during droughts, laying landscapes bare to wind and water
erosion.
Wind erosion and water erosion are the carrying away or displacement
of soil particles by wind and water, respectively.
(UNEP 1997, 46)
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Agriculture
Evaporation
waterlogging
Over-irrigation
Irrigation Canal
Rising water table
water leaking from canal
Waterlogging caused by rise in the
water table due to canal irrigation.
Over-cultivation
soil erosion
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Soil erosion
0 ton
0.4 %
Forest land
Soil erosion (t/ha)
Sheet erosion
0 ton
Grass land
1.9 %
Ratio of outflow
to rainfall (%)
78 ton
Farm land
26.0 %
Rill erosion
Bare land
146 ton
50.4 %
Gully erosion
Soil erosion and ratio of outflow to rainfall under
different vegetation cover in dryland
Erosion types
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Soil erosion
Land degradation
Gully erosion makes a large ravine and breaks
structures for life.
Once it occurs, it is
difficult to force back.
Gully erosion in Loess Plateau in China
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Agriculture
Over-cutting
soil erosion
fuel wood
Over-irrigation
salt accumulation
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salt accumulation
High evaporation rates in drylands contribute to the surface accumulation of
salts that would otherwise be leached out of the system in wetter
environments. Therefore, salinization refers to the surface or near-surface
accumulation of salts, mainly chlorides (Cl), sulphates (SO4) and carbonates
(CO2) of sodium (Na), calcium (Ca) and magnesium (Mg). Salt accumulation
reduces soil pore space and its ability to hold soil air and nutrients. High salt
concentrations are toxic to many plants especially during the seedling stage.
Plants can suffer from salt burn and salt stress, causing an inability to take up
the moisture necessary for growth. The salinization problem also embraces
alkalization, the excess accumulation of sodium, which rises in association
with the dissolved salt load of irrigation water.
Over-irrigation
Evaporation
capillary rise
water table rise
Water table
Water table
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salt accumulation
Sodification is a physical consequence of salinization. Sodification
occurs when saline water in a soil, often from irrigation in drylands, is
concentrated by evapotranspiration, leaving sodium ions (Na) dominant
in the soil solution as calcium (Ca) and magnesium (Mg) components
tend to precipitate as carbonates. The sodium ions (Na) tend to be
adsorbed by aggregates of very fine clay particles, which consequently
become broken down. The result is a structureless soil which is
unfavourable for root development, almost impermeable to water and
highly erodible, particularly by piping.
Definition of salt affected soil
Salt affected soil
ECe
ESP
ｐH
Saline soil
4 < EC
ESP<15
pH<8.5
Sodic soil
EC < 4
15<ESP
8.5<pH
Sodic-saline soil
４ < EC
15<ESP
8.5<pH
ECe: Electrical conductivity of saturated extraction (dS/m)
ESP:Exchangeable Sodium Percentage (%)
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Distribution of salt-affected soils in the world
Sodic
Saline soil is
a big problem
in America
and northern
parts of Africa.
Saline
Sodic soil is a serious problem
in Australia, southern part of
South America and middle of
Africa.
Some 24% area of irrigated land in the world was affected by salinization.
Agriculture in more than 100 countries is affected by sodic and saline soil.
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What are the causes of desertification ?
• Desertification is land degradation in arid, semi- arid and
dry sub-humid areas resulting from various factors,
including climate variations and human activities.
• Human-induced factors include over-grazing, overdeforestation,
over-exploitation,
and
inadequate
agricultural management.
• Inadequate agricultural management includes overirrigation and over-cultivation.
• Current worldwide desertification is taking place much
faster than historically observed and usually arises from
the demands of growing population.
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How to combat desertification
using Technology ?
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Pastoral farming
Over-grazing
In pastoral farming, over-grazing
is a main cause of desertification
/ land degradation.
Approximately four sheep are
necessary for a person to live
in herding society.
Soil erosion
6 sheep / ha
Over grazing
→ bare soil
2 sheep / ha
4 sheep / ha
As population increases, the
number of sheep needed
increases, resulting in an
expansion of bare land.
How to combat over-grazing?
Rotational grazing technique
To halt the expansion of bare land and to
keep vegetation, it is necessary to maintain
suitable livestock numbers with intensive
grazing management.
Intensive grazing management is the
practice of using rotational grazing
technique and careful, usually daily,
management to get optimal production.
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Land reclamation
Soil erosion
Over-deforestation
Over-exploitation
Over-deforestation and over-exploitation caused by population growth is serious problem
How to combat over-deforestation?
Forest recovery by reforestation
The term reforestation can also refer to afforestation, the process of restoring and
recreating areas of woodlands or forest that once existed but were deforested or
otherwise removed or destroyed at some point in the past. The resulting forest
can provide both ecosystem and resource benefits and will have the potential to
become a major carbon sink.
Domestic house making
and cooking materials
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Lack of fuelwood
Over-deforestation
Over-deforestation caused by population growth is a serious problem
Low
combustion
efficiency
High
combustion
efficiency
How to combat over-deforestation?
Saving domestic
fuel wood
Saving fuel wood using improved furnace
Using natural gas instead of
fuel wood for cooking
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Rainfed farmland
water erosion
Over-cultivation
Over-cultivation caused by population growth is serious problem
Silt in Yellow river (6-7kg/m3)
How to combat water erosion?
Contour reforestation in steep sloping land
Plant cover is a major factor influencing the erodibility
of dryland soils by both wind and water
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Control of water erosion
How to combat water erosion
using engineering methods?
Terrace for saving water
level terraces
level terraces in winter
Sloping terraces
Leveling land by engineering methods is an effective way of increasing water
percolation into the soil and thus decreasing, even eliminating water erosion on
slopes and in gullies. Numerous studies have shown its effectiveness. Many
forms of terraces have been used, including level terraces, sloping terraces and
level terraces alternating with slopes, consisting of a terrace ridge, level terrace
area and natural slope, being used on land with slope of 10 to 15 degree.
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Control of water erosion
Level narrow belts
How to combat water erosion
using engineering methods?
Planting trees and grasses in
level narrow belts and fishscale pits.
Fish-scale pits
Fish-scale pits
Slopes formed inside gullies also need controlling in order to reduce runoff to the
bottom. This has been done by planting trees and grasses in fish-scale pits, level
furrows, level narrow belts, and the level terrace alternating with slopes made by
engineering methods.
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Control of water erosion
How to combat water erosion using engineering methods?
Check dam to store water and to make land
To block runoff and
induced sediments
Check dam
to store water
from runoff
Controlling the gully bottom is the last, most important step. For this, engineering
measures are more powerful, especially constructing dams to block runoff and the
induced sediments. For different purposes, there are different dams such as check
dams for consolidation of the eroded base and therefore preventing further erosion,
detention dams or silt arresters for arresting flooding sediment in the gully bottom to
make land, small reservoirs to protect water conservancy projects, and to store
water from runoff for other uses such as irrigation.
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Over-irrigation
Irrigated farmland
Lack of groundwater
In irrigated farmland, over-irrigation caused by population growth is important problem
Flood irrigation
Furrow irrigation
Waterlogging
Salt accumulation
Center-pivot irrigation
Sprinkler irrigation
Water Harvesting
How to combat over-irrigation?
Traditional agricultural
Saving water
Drip irrigation
To combine the modern and traditional agricultural technologies.
Efficient Use of water in cropping systems
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To combine the modern and traditional agricultural technologies.
Vegetative measures are very important for
controlling soil erosion and for other interests.
Alternative land-use systems recently
established are called tree-cum-crop and
tree-cum-forage, versions of agroforestry
systems. In these systems, food or forage
crops are grown in alleys formed by
hedgerows of trees, generally fruit trees or
shrubs.
Agroforestry
Agroforestry can serve as an effective
means of controlling wind erosion by
reducing wind speed near the soil surface,
thereby improving crop microclimate.
Multi-cropping systems include “intercropping” systems, in which rows of one crop
are alternated with those of another; “relay cropping” systems, in which an earlyseeded crop is later inter-sown with a second, later-maturing crop; and “alleycropping” or “agroforestry” system, in which crop species are grown between
woody or tree species.
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Significance of vegetation in soil degradation studies
1. Vegetation is a major pathway for the loss of water from drylands,
through evapotranspiration.
2. Plant cover is a major factor influencing the erodibility of dryland
soils by both wind and water.
3. Vegetation is key source of soil nutrients and therefore changes
in vegetation cover can significantly contribute to the chemical
deterioration of soils.
4. Natural vegetation supports pastoralism which is a major land use
in dryland areas.
5. Vegetation is an important source of domestic fuel in many
developing world drylands.
6. Vegetation communities and plant cover densities are susceptible
to changes caused by natural drought and human factors.
7. Changes in vegetation communities may be an important
indicator of pending changes within the soil system.
(UNEP 1997, 50)
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Exercise
1) Please describe the characteristics of drylands.
2) Please list the causes of desertification in drylands.
3) Please describe the sustainable agricultural technology that can mitigate
desertification.
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