Effects of deficit irrigation on yield and WUE of some crops under semi-arid conditions (Bekaa Valley of Lebanon)

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Effects of deficit irrigation on yield and WUE
of some crops under semi-arid conditions(Bekaa
Valley of Lebanon)

• K. Karaa, F. Karam, N. Tarabey

4th WASAMED Workshop on Water Use Efficiency and
Water Productivity Amman, Sep 1st - Oct 4th
2005
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ACTION PLAN

• Determine water use, yield, and WUE in four
  annual crops with contrasting response to deficit
  irrigation (DI)
• Maize (1998-1999)
• Soybean (2000-2001)
• Cotton (2001-2002)
• Sunflower (2003-2004).
• Determine the relationships between yield and
  biomass, in one hand, and evapotranspiration in
  the other.

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• Maize, a determinate species with a limited
  capacity to adjust grain yield in response to
  water availability (Karam et al, 2000 2003)
• Soybean, an indeterminate species with a high
  capacity to compensate the effects of early water
  stresses (Karam et al., 2005)
• Cotton, an indeterminate species with a larger
  capacity to adjust the number of dehiscent bolls
  under stressful conditions (Karam et al., in
  press, Agric. Water manag.)
• Sunflower, a determinate species with a single
  inflorescence and an aptitude to tolerate
  moderate water stresses.

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YIELD RESPONSE TO WATER

• Yields increase with water availability in the
  root zone, until a saturation level, above which
  there is little effect.
• Yield response curve of specific crops depends on
  weather conditions and soil type as well as
  agricultural inputs.

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Soil Water Retention Capacity
Field Capacity ()
Management Allowed Deficit ()
Permanent Wilting Point ()
Available Water FC - PWP
MAD 40-50 AW
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Soil Water Vs. Depth
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Water relations in a warmer world
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Changes in VPD with temperature
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Solutions

• Develop new irrigation scheduling approaches, not
  necessarily based on full crop water requirement,
  but ones designed to ensure the optimal use of
  allocated water Partial irrigation

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Deficit Irrigation

• DI or RDI is one way of maximizing water use
  efficiency (WUE) for higher yields per unit of
  irrigation water applied.
• The crop is exposed to a certain level of water
  stress either during a particular growth period
  or throughout the whole growing season, without
  significant reduction in yields.

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Objectives

• To increase WUE of a crop by eliminating
  irrigations that have little impact on yield.
• The resulting yield reduction may be small
  compared with the benefits gained through
  diverting the saves water to irrigate other crops.

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Water Use Efficiency

• WUEg,b (kg/m3) Yield or biomass (kg/m2)/ ET
  (m3/m2)
• (1 kg m-3 1 g m-2 mm-1).
• Subscripts g, and b indicate grain yield and
  biomass

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Water Use Efficiency

• For maize, soybean, and sunflower WUE was
  calculated as the ratio of yield (Y) and biomass
  (B) at dry bases to the amount of crop ET (Y/ET)
  and (B/ET).
• For cotton, WUE was calculated as dry lint yield
  to the amount of ET.

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Working hypothesis

• The relationship between yield and ET is an
  appropriate framework to investigate the pattern
  of DI.
• Linear models were fitted to the data
• Y a1 (ET) b1
• B a2 (ET) b2
• (WUE Y ET-1 WUE B ET-1)

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Advantages

• Simplest and more often used to describe the
  relationship between Y and B, and ET.
• Have important implications for WUE, either at
  grain or seed basis, or biomass basis.
• Depending on whether the slope is constant or
  variable, and whether the intercept is zero or
  negative, the expected relationship between Y and
  B and ET can be outlined (WUE Y ET-1)
• Departure from linearity can be tested through
  regression of log Y on log ET, or log B on log ET.

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Disadvantage

• Can produce misleading results when the
  y-intercept differs from zero. At this point,
  polynomial regressions are preferred.

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CROP WATER REQUIREMENTS

• Defined as the amount of water, applied at
  appropriate periods of time, to control the soil
  moisture deficit, caused by the potential
  evapotranspiration of the crop.

• An estimation of the crop water requirements can
  be made using different methods.
• Hourly
• Daily
• Seasonally

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CROP WATER REQUIREMENTS

• Two general methods were used for CWR
• Indirect Method The Climatic-Water Balance
• Empirical formulas
• Direct Method The Soil-Water Balance
• Rye-grass Lysimeter
• Crop Drainage Lysimeter
• Crop Weighing Lysimeter

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The Climatic-Water Balance FAO Penman-Montheith
Equation
ETc ETo Kc
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Daily Time Course of Potential Evapotranspiration
at Tal Amara
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CONCEPT OF THE SOIL-WATER BALANCE

• Dynamic-oriented Process
• Where t2t1 is the time interval over which
  measurements are made, zo is the soil surface and
  z is the depth to the lowest point of measurement
  and ? is the volumetric soil water content.

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Weighing Lysimeter (ETcrop)

• ET measurements (Hourly and Daily)
• Location (middle of the Exp. field)
• Area (4 ? 4 m²)
• Watered at 30 of SWD
• Linked to a weight indicator
• Weight loss recorded (4 times/hr 94
  readings/day)

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Drainage Lysimeters (ETcrop and ETref)

• ET measurements (3-to-4 day interval)
• Location (middle of the Exp. field)
• Area (2 ? 2 m²)
• Watered at 30 of SWD using digital water markers
• ET I D ?Q
• (?Q 0 when irrigation are frequent)

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Irrigation treatments
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Daily and Seasonal ET of Soybean by the weighing
lysimeter
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Daily Crop Coefficients of Soybean
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Daily and Seasonal ET of Cotton by the Drainage
lysimeter
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Maize (1998 - 1999)
(data points are means of five quadrates of 1m2
each per treatment)
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Soybean (2000-2001)
(data points are means of five quadrates of 1m2
each per treatment)
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Cotton (2001-2002)
(data points are means of five quadrates of 1m2
each per treatment)
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Sunflower (2003-2004)
(data points are means of five quadrates of 1m2
each per treatment)
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Concluding remarks

• Improvement of water use efficiency of the
  cultivated crops requires information on water
  stock in the root zone
• Irrigation timing and volume requires information
  on the sensitivity of the different growth stages
  to DI
• Creation of a national database for the irrigated
  crops in Lebanon with emphasis to agro-climatic
  zoning.