Dryland Agriculture, Irrigation, and Salinity

1
Dryland Agriculture, Irrigation, and Salinity
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Dryland Agriculture

• Applies to semi-arid areas, see temp./rainfall
  limits (Tivy, 1992)
• Characterized by variable/inconsistent rainfall
  short growing season may restrict
  number and kinds of crops grown

3
Relationship between Precipitation, Temperature,
and Climate
Semi-Arid
Humid
Arid
Tivy, 1992
4
Dryland AgricultureRemedies and Adaptations

• Choice and timing of planting of adaptable crops
• Irrigation (much required balance of water
  supply and demand over time?)
• Moisture conservation practices (mulching, summer
  fallow to reduce weed transpiration, etc.)

5
Examples fragile agroecosystems in which crops
produced and timing have been worked out over
many years

• Midwest US --- corn, wheat (adapted crops)
  irrigation
• Mediterranean region --- cereals, adapted trees,
  grazing
• Tropical Africa --- crop selection sorghum,
  millet, cassava

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Desertification

• expansion of arid lands into semi-arid
  agricultural areas
• Can be a consequence of mismanaged dryland
  agroecosystems
• Caused when drought interacts with mismanagement
  practices

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Desertification on World Basis
Sahel Region
Tivy, 1992
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Desertification Caused when drought interacts
with mismanagement

• Overgrazing
• Overcultivation
• Deforestation

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Desertification Caused when drought interacts
with mismanagement

• Overgrazing

Carrying capacity (good year)
Carrying capacity (poor year)
Animal Population
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Desertification Caused when drought interacts
with mismanagement

• Overgrazing

Problem occurs when poor year follows good year
Carrying capacity (good year)
Carrying capacity (poor year)
Animal Population
Limited vegetation depleted if animal population
over carrying capacity
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Desertification Caused when drought interacts
with mismanagement

• Overgrazing
• Overcultivation, reduction of fallow and
  regeneration periods (needs time for groundwater
  supplies to regenerate)
• Deforestation for firewood, etc. erosion

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Desertification Effects

• Adverse effects on water supplies, wells, etc.
• Adverse impact on hydrological cycle

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Desertification Impact on Hydrological Cycle
Drought Less vegetation
Erosion
Exposed soil Baked by sun Sealed by rain
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Desertification Impact on Hydrological Cycle
Drought Less vegetation
Erosion
Exposed soil Baked by sun Sealed by rain
Less percolation to groundwater (slower recharge)
More evaporation Increased runoff
15
Remediation Depends on how far process has
gone slow re-establishment of native vegetation
is critical
Drought Less vegetation
Erosion
Exposed soil Baked by sun Sealed by rain
Less percolation to groundwater (slower recharge)
More evaporation Increased runoff
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Arid Agriculture and Irrigation

• Arid deserts occupy 1/6 of world land area
• No agriculture without irrigation

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Irrigation Sources

• Surface water limited in arid regions (flow
  from canals, etc.)
• Ground water caution that withdrawal is not
  faster than recharge

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Irrigation Types(see text pp. 314-317 for many
examples)

• Furrow (ditches, etc.)
• Sprinkler (overhead irrigation, center pivot)
• Drip (trickle, microirrigation)

Also used to provide frost protection May create
environment for plant diseases
Most efficient in water delivery Most expensive
system to set up
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Overhead Irrigation for Citrus Freeze Protection
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Irrigation Types

• Furrow
• Sprinkler
• Drip (trickle, microirrigation)

Most efficient in water delivery Most expensive
system to set up
Main water line
Outlets to individual plants
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Main Lines from Drip Irrigation System in Coleus
Experiment
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Efficiency of Water Use

• Need to optimize amount of water for each crop
• Conserve limited water supplies
• Save (if water costs )
• Limit salinity problems
• Avoid ecological disasters

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Ecological DisastersAral Sea, Kazakhstan
Tivy, 1992
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Ecological DisastersAral Sea, Kazakhstan
Tivy, 1992
Once large enough to be seen on world map !
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Ecological Disasters Aral Sea(see Hammond, 1990)

• Kazakhstan formerly part of USSR
• Semi-arid grain-growing area
• Push to increase ag production in USSR
• Heavy irrigation withdrawals from 1960-87
• Lost 40 of its area
• Lost 60 of its volume
• Lost almost all fishing industry

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Ecological Disasters Aral Sea
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Salinization

• salt buildup in soil
• Some basic definitions
• Salinity high salts
• Sodicity high sodium Na
• Alkalinity high pH

Note These three problems can occur at the same
time in the same soil !
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Soil Salinity

• Several types of salts important in agriculture
  (see text, pp. 320-322)
• Measured by electrical conductivity (EC)
• Salt tolerance varies among crops (as EC
  increases, crop choices become more limited)

Especially Na Cl- Mg2
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Salinity Sources

• Saline soils
• Fertilizer salts
• Irrigation

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Sources of Salts from Irrigation

• Minerals, salts in groundwater (natural)
• Minerals, salts in groundwater (from fertilizer
  salts)
• Salt water intrusion

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Sources of Salts from Irrigation

• Minerals, salts in groundwater (natural)
• Minerals, salts in groundwater (from fertilizer
  salts)
• Salt water intrusion

Usually confined to coastal areas
Ocean
Crop
Groundwater
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Fertilizer Salts in Soil and Groundwater

• Common problem in arid climates
• Fertilizer salts can accumulate in groundwater
• Groundwater may already contain natural mineral
  salts
• When water is pumped, salts added to crop and
  soil
• Gradual accumulation of salts over long periods
  of time in arid climates
• Lack of sufficient rainfall to leach salts out

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Irrigation Effects on Soil Salinity

• Furrow irrigation highest accumulation of salts

Salts accumulate as water evaporates
Furrow
Watch out for whitish crust at soil surface
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Irrigation Effects on Soil Salinity

• Irrigation water with salts

Soil
In arid climate, leaching limited by low
rainfall, so salts remain and accumulate in soil
Normally, salts leach out
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Irrigation Effects on Soil SalinityHigh Water
Table
Plant roots remain in water with salts
Na
Na
Na
Na
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Salinity and Alkalinity

• Review definitions
• Salinity high salts
• Sodicity high sodium Na
• Alkalinity high pH

Note These three problems can occur at the same
time in the same soil !
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Salinity and Alkalinity

• Important in loss of agricultural land on global
  level !!

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Salinity and Alkalinity

• Added Na Na replaces Ca

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Salinity and Alkalinity

• Added Na Na replaces Ca
• Multiple problems result
• 1. Less Ca available to plant
• 2. Na salt adverse effect on plant
• 3. pH increases (alkalinity)
• 4. Ca2 leaches out

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Salinity and Alkalinity

• Added Na Na replaces Ca
• Multiple problems result
• 1. Less Ca available to plant
• 2. Na salt adverse effect on plant
• 3. pH increases (alkalinity)
• 4. Ca2 leaches out

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Salinity and Alkalinity

• Added Na Na replaces Ca

pH increases (alkalinity)
Affects availability of P and micronutrients (Fe,
Zn, Mn,..)
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Salinity and Alkalinity

• Added Na Na replaces Ca
• Multiple problems result
• 1. Less Ca available to plant
• 2. Na salt adverse effect on plant
• 3. pH increases (alkalinity)
• 4. Ca2 leaches out

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Salinity and Alkalinity

• Added Na Na replaces Ca

Ca leaches out
Ca Important in maintaining structure of clay
fractions
Dispersion of clay and breakdown of soil structure
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Salinity and Alkalinity

• Added Na Na replaces Ca

Less leaching and removal of salts
Ca leaches out
Dispersion of clay and breakdown of soil structure
Less drainage
Less permeability (reduced pore space)
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Remediation Salinity and Sodicity

• Must treat sodicity first --- sodic soils
  poorly drained, so difficult to flush out salts
• Remediate sodicity
• Remediate salinity

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Remediation Sodicity

• Expensive
• More difficult if soil structure is destroyed
• Add CaSO4 (gypsum) acid reaction and adds Ca
  too
• Must treat sodicity first, then treat for
  salinity too

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Remediation Salinity

• Can be costly
• Lower water table
• Flush with clean water

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Minimizing Soil Salinity

• Good quality water for irrigation
• Irrigate at correct time and amount to leach
  salts
• Adequate drainage to prevent rise of water table

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References

• Text, Ch. 15, desertification in Ch. 16, pp.
  333-334.
• Hammond, A.L. 1990. World resources, 1990-91.
  Oxford University Press, New York. Ch. 1.
• Tivy, 1992. Chs. 12,13.