Blaine Hanson LAWR 21130 Feb 8, 1997 Slides

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Salinity Control under Saline Shallow Ground
Water Conditions of the San Joaquin Valley,
California
Blaine Hanson Department of Land, Air and Water
ResourcesUniversity of California,
Davisbrhanson_at_ucdavis.edu
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Co-participants

• Don May Farm Advisor (retired), University of
  California Cooperative Extension
• Jirka imunek Professor, Department of
  Environmental Sciences, University of California,
  Riverside
• Jan Hopmans Professor, Department of Land, Air
  and Water Resources, University of California,
  Davis

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Situation ? Source of soil
salinity saline, shallow ground water along the
west side of the valley ? Subsurface
drainage systems cannot be used for water
table and salinity control ? No drainage water
disposal method exists ? Improved
irrigation practices, drainage water reuse, and
increased shallow ground use by crops are the
only options available
San Joaquin Valley
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Salinity control furrow, border (flood), and
sprinkle irrigation
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Soil salinity for furrow, border (flood), and
sprinkle irrigation under saline shallow
groundwater conditions
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Salinity control (furrow, sprinkle, and flood
(border) irrigation apply pre-plant irrigations
for periodic reclamation leaching in the late
winter/early spring

• Low salinity irrigation water (0.3 to 0.5 dS m-1)
  except during periods of drought
• Soil salinity increases during the crop season
  due to upward flow of saline ground water
• Objectives of salinity control
• Maintain spring soil salinity at acceptable
  levels
• Return fall soil salinity levels to spring levels
• Replenish fall soil moisture depletion
• Leaching requirement apply about 25 mm of water
  per 0.30 m of soil for leaching after
  replenishing the soil moisture depletion

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Salinity control drip irrigation
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Experiments in commercial fields on subsurface
drip irrigation of processing tomatoes under
saline shallow ground water conditions

• Field conditions
• Drip irrigation systems 16 to 32 ha drip line
  depth 20 to 30 cm
• Salinity (EC) of irrigation water 0.3 to 1.1 dS
  m-1 salinity (EC) of ground water 4 to 16 dS
  m-1
• Water table depth 0.45 to 2 m
• Results
• Average yields of subsurface drip irrigation was
  90.7 Mg ha-1 compared to 75.9 Mg ha-1 for
  sprinkle irrigation
• Profit increased by 1,195 ha-1 US
• Yield increased as the amount of applied water
  increased
• Yield unaffected by water table depth and soil
  salinity
• Little water table response to drip irrigation
• Drip irrigation now commonly used in the
  salt-affected soils of the San Joaquin Valley

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ECi 0.3 dS m-1 ECgw 8-11 dS m-1 GW depth 2 m
Ece (dS m-1)
ECe (dS m-1)
Salt distributions for subsurface drip
irrigation under saline shallow ground water
conditions
ECi 0.3 dS m-1 ECgw 5-7 dS m-1 GW depth 0.6
1 m
ECi 1.1 dS m-1 ECgw 9-16 dS m-1 GW depth
0.6 1 m
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Factors affecting root zone soil salinity with
drip irrigation under saline shallow groundwater
conditions

• Depth to shallow groundwater
• Salinity of shallow groundwater
• Irrigation water salinity
• Amount of applied water
• Location of drip line relative to plant row
• Soil type

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Surface drip irrigation saline irrigation water
no shallow ground water
No leaching under drip irrigation
ECe (dS m-1)
Soil Salinity (ECe in dS m-1)
Leaching under drip irrigation
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Effect of amount of applied water on volume of
low salt soil under subsurface drip irrigation
and saline shallow ground water conditions
ECe (dS m-1)
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Subsurface drip irrigation
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Soil water content
Root density
Plant row
Plant row
Drip line
Drip line
Drip line
Drip line
Drip line
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Controlling soil salinity under drip irrigation

• Highly concentrated leached zone near drip lines
  due to localized wetting patterns if sufficient
  irrigation water is applied
• Zone of highly concentrated leaching increases as
  the amount of applied water , and thus the
  leaching fraction, increases
• Salinity of leached zone depends on the
  irrigation water salinity
• Salinity beyond the leached zone reflects upward
  flow of saline ground water
• Pre-plant irrigation with sprinkle irrigation may
  be required for stand establishment for
  subsurface drip irrigation systems
• Effect of drip line placement on root zone
  leaching
• Drip line and plant row locations coincide
  concentrated leaching in the root zone
• Drip line and plant row locations offset may
  have salinity control problems

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Effect of amount of applied water on tomato yield
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Factors affecting increased yield with increased
leaching fraction

• Larger leaching fractions increase the volume of
  low salt soil near drip lines
• Larger leaching fractions decrease the soil
  salinity near drip lines for a given irrigation
  water salinity
• Larger leaching fractions increase the soil
  moisture near drip lines

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What is the leaching fraction under drip
irrigation?

• Soil salinity measurements soil water content,
  soil salinity, and root density vary spatially
  around drip lines
• Water balance approach for estimating the
  leaching fraction
• Commonly used for estimating leaching fractions
• Leaching fraction 100 x (AW ET) AW where
  AW applied water and ET crop
  evapotranspiration
• Field-wide average leaching fraction
• Irrigation event or seasonal leaching fraction

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Water balance leaching fractions under drip
irrigation
Localized leaching around drip line
ECe (dS m-1)
Drip line
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Estimating the leaching fraction under drip
irrigation using the HYDRUS-2D computer
simulation model

• Depth of drip lines 20 cm
• Water table depths of 50 and 100 cm
• Irrigation water salinities of 0.3, 1, and 2 dS
  m-1
• Ground water salinities of 8 and 10 dS m-1
• Applied water amounts of 60, 80, 100, and 115
  percent of the estimated crop evapotranspiration
• Irrigation frequencies of 2 per week and daily

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Computer simulations (HYDRUS-2D) water balance
leaching fraction and actual leaching fraction
for drip irrigation

• Notes
• ? Applying an amount of water equal to the ET
  does not result in an
• irrigation efficiency 100 for drip
  irrigation, as is commonly assumed
• ? High irrigation efficiency under drip
  irrigation occurs only for
• severe deficit irrigation conditions
• ? This behavior is due to the wetting pattern
  around drip lines
• and can not be avoided

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HYDRUS-2D simulations of reclamation over time
ECi 0.3 dS m-1, water table depth 1 m,
applied water 100 of ET
ECsw (dS m-1)
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Effect of amount of applied water of volume of
low salt soil under subsurface drip irrigation
computer simulation study using the HYDRUS-2D
computer simulation model
ECsw (dS m-1)
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HYDRUS-2D simulations on the effect of irrigation
water salinity on soil salinity near the drip
line
ECsw (dS m-1)
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Recommended salinity control practices for
subsurface drip irrigation of processing tomatoes
under saline, shallow groundwater conditions

• Sufficient localized leaching must occur near
  drip lines to maintain profitable yields
• Seasonal water applications should be about equal
  to the seasonal evapotranspiration (ET) to
  provide sufficient localized leaching and to
  reduce water table response to irrigation
• Use relatively low salt irrigation water (ECi
  equal to or less than about 1 dS m-1)
• Irrigation frequency daily to 2 to 3 times per
  week.
• Periodic leaching of salt accumulated above
  buried drip lines may be necessary with
  sprinklers for stand establishment
• Drip system should be designed for high
  uniformity (DUgt90 along drip line)
• Periodic system maintenance must be performed to
  prevent clogging of drip lines, which will
  reduce the localized leaching

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Conclusions based on research and field experience

• Drip irrigation has the potential to be highly
  profitable in the salt-affected soils of the
  valley
• Soil salinity control can be achieved through
  proper system design, management, and maintenance
• Little water table response to properly managed
  drip irrigation
• Small applications per irrigation
• Distributed more or less uniformly over time
• Sufficient natural drainage
• Experience indicates that subsurface drainage
  systems may not be needed for drip-irrigated
  fields

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The End