Criteria for Irrigation Scheduling

Meaning; Irrigation scheduling means deciding when and how much water to apply to a crop field.
It’s about supplying water at the right time, in the right quantity, and by the right method, so that crops get the needed water without waste.

Concept; The concept of irrigation scheduling is based on matching crop water requirements with available water supply. The main goal is to maximize crop yield and quality while conserving water, energy, and labor.

It involves considering:

• When to irrigate? (based on soil moisture status, crop growth stage, or climatic conditions)
• How much to irrigate? (amount of water required to refill the soil root zone to field capacity)

i) Soil Water Depletion Approach

Concept:

• Irrigation is scheduled when the soil water content falls below a certain depletion level of Available Soil Moisture (ASM).
• Crops can use only a fraction of ASM before stress occurs.

Key Points:

• Readily Available Water (RAW): Portion of soil water that plants can use without stress (≈ 50% of ASM in most crops).
• Depletion Levels:
  • Shallow-rooted crops (vegetables, potato) → irrigate at 30–40% depletion.
  • Most field crops (wheat, maize, groundnut) → 50% depletion.
  • Deep-rooted/drought-tolerant crops (sorghum, pulses) → 60–70% depletion.

Measurement Methods:

• Gravimetric soil sampling
• Tensiometer (effective up to 1 atm suction)
• Gypsum blocks (electrical resistance)
• Neutron probe (very accurate, costly)
• Advantage: Scientific & crop-specific
• Limitation: Requires instruments, labor-intensive

ii) Plant Indices Approach

Concept:

• Crop itself is used as an indicator of water stress.
• Irrigation scheduled based on physiological or morphological changes in the plant.

Major Indices:

• Leaf Water Potential (LWP): Measured with a pressure chamber (“pressure bomb”). More negative LWP = higher stress.
• Leaf Temperature / Canopy Temperature: Measured with infrared thermometer. Stressed plants are hotter due to reduced transpiration.
• Stomatal Resistance / Conductance: Measured with a porometer. High resistance = water stress.
• Crop Water Stress Index (CWSI): Based on canopy-air temperature difference.
• Visual Symptoms: Leaf rolling in rice, wilting in cotton, leaf angle changes in maize.
• Advantage: Direct measure of crop stress, more precise.
• Limitation: Costly, research-level, less practical for farmers.

iii) Climatic Parameters Approach

Concept: Irrigation scheduled according to evapotranspiration (ET) demand of atmosphere, which depends on climatic variables.

Key Climatic Parameters:

• Temperature → Higher temp → Higher ET.
• Relative Humidity (RH) → Low RH → Higher ET.
• Wind Velocity → Increases ET by removing vapor from leaf surface.
• Solar Radiation / Sunshine hours → Directly increases ET.
• Rainfall → Effective rainfall reduces irrigation need.

Methods Used: Pan Evaporation Method: Formula: ETc =Kc × ETo

1. • ETo = reference ET (pan evaporation)
   • Kc = crop coefficient

• IW/CPE Ratio Method (very popular in India): Irrigate when ratio of irrigation depth (IW) to cumulative pan evaporation (CPE) reaches a set value (e.g., 0.8, 1.0).
• FAO Penman–Monteith Method: Most scientific & globally accepted. Considers radiation, wind, humidity, and temperature.

• Advantage: Scientific, reflects atmospheric demand.
• Limitation: Needs weather data & calculations.

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