Irrigation Scheduling

Definition; Irrigation scheduling = deciding when to irrigate and how much water to apply to meet crop water needs efficiently, ensuring optimum yield with minimum water use.

Criteria Used in Scheduling

• Soil water depletion level (e.g., 50% of available water).
• ET demand (Pan evaporation or ETc estimation).
• Crop water requirement at critical stages.
• Climatic demand (high temp. → more frequent irrigation).

Objectives of Irrigation Scheduling

• Avoid crop stress (over- or under-irrigation).
• Improve Water Use Efficiency (WUE).
• Optimize yield with minimum water.
• Minimize deep percolation, runoff, and salinity hazards.

Modern Concepts

• Deficit irrigation: Apply water below full requirement → saves water, minimal yield loss.
• Supplemental irrigation: In rainfed farming → irrigation given during dry spells.
• Conjunctive use: Combining canal + groundwater for reliability.
• Precision irrigation: Drip/sprinkler with automated scheduling using sensors.

Approaches of Irrigation Scheduling

Irrigation scheduling = deciding when and how much water to apply to crops. Different approaches are used depending on soil, crop, climate, and management level.

i) Soil-Based Approach

• Irrigation is scheduled based on soil moisture status.
• Methods:
  • Gravimetric soil sampling
  • Tensiometers, gypsum blocks, neutron probe
• Criteria: Irrigate when soil moisture is depleted by 40–60% of Available Water Holding Capacity (AWHC).
• Advantage: Scientific, crop-specific.
• Limitation: Needs instruments & labor.

ii) Climatic / Evapotranspiration-Based Approach

• Based on atmospheric demand (ET, temperature, sunshine, wind, RH).
• Methods:
• Pan Evaporation Method: ETc = Kc × ETo
  • IW/CPE ratio method → widely used in India.
  • Advantage: Directly reflects climatic variation.
  • Limitation: Requires data & calculations.

iii) Crop-Based Approach

• Irrigation scheduled according to critical crop growth stages.
• Example: Wheat → CRI (21 DAS), booting, flowering. Rice → Panicle initiation, flowering
• Advantage: Very practical, easy for farmers.
• Limitation: May not optimize water fully in water-scarce areas.

iv) Plant-Based Approach

• Relies on plant stress indicators to decide irrigation timing.
• Indicators:
  • Leaf water potential
  • Leaf temperature (infrared thermometer)
  • Stomatal resistance
  • Crop Water Stress Index (CWSI)
• Advantage: Direct crop response measurement.
• Limitation: Research-level, not suitable for routine farming.

v) Fixed Interval / Calendar Approach

• Irrigation applied at regular intervals (e.g., every 7 or 10 days).
• Advantage: Simple, no instruments needed.
• Limitation: Ignores soil, crop, and climate variations → inefficient use of water.

vi) Modern Approaches

• Deficit Irrigation: Supplying less than full requirement, maximizing WUE.
• Supplemental Irrigation: In rainfed areas, irrigation applied only during dry spells.
• Conjunctive Use: Combining surface + groundwater for reliability.

vii) Sensor-based & Precision Irrigation: Automated scheduling using soil moisture sensors, remote sensing, or drip/sprinkler with ET models.

Summary Table (Exam-Ready)