Scheduling of Irrigation

Irrigation Scheduling is the process of determining when to irrigate and how much water to apply, based on the crop’s water needs. It ensures that water is applied in an optimal way, enhancing crop growth while minimizing water wastage.

Approaches for Scheduling Irrigation

Irrigation scheduling decisions depend on various factors like crop type, soil properties, climatic conditions, and water availability. The general approaches for scheduling irrigation can be grouped into several categories:

1. Soil Moisture Regime Approach

This method uses soil moisture content to determine irrigation schedules. The soil moisture regime is based on measuring the moisture available in the soil and applying irrigation when moisture levels fall below a critical threshold. Various techniques within this approach include:

1.1. Soil Water Content Approach

• Objective: Irrigation is based on measuring the soil’s water content at various depths in the root zone.
• Techniques: Gravimetric method, where the soil sample is weighed before and after drying to measure water content.
• Drawback: This method may not always be practical for large fields due to labor-intensive nature and variability of moisture content.

1.2. Depletion of Available Soil Moisture (DASM)

• Objective: Irrigation is scheduled when a certain percentage of available soil moisture is depleted.
• How It Works: The amount of water remaining in the soil is calculated, and irrigation is done when it falls below a certain threshold, to avoid yield reduction.

1.3. Soil Water Tension or Soil Moisture Potential

• Objective: This method uses the soil’s water tension to assess moisture levels.
• Tools: Tensiometers, irrometers, and electrical resistance blocks measure soil water potential at different depths.
• Best Use: This approach is more suitable for orchards, vegetables, and coarse-textured soils.

1.4. Feel and Appearance Method

• Objective: Farmers with experience assess soil moisture based on the feel and appearance of the soil.
• How It Works: Soil samples are taken and tested based on their texture and appearance when squeezed or handled (i.e., whether they form a ball, crumble, or remain loose).
• Advantage: Simple, but requires significant experience and judgment.

2. Climatological Approach

This method uses climatic data, such as Evapotranspiration (ET) or pan evaporation, to schedule irrigation.

2.1. Potential Evapotranspiration (PET) Measurement

• Objective: Irrigation is scheduled by estimating the amount of water lost from the soil through evapotranspiration.
• Techniques: PET can be estimated using empirical formulas, lysimetric methods, or energy balance methods.
• How It Works: PET represents the total water that would evaporate and transpire if water were freely available.

2.2. Cumulative Pan Evaporation (CPE)

• Objective: This approach uses open pan evaporimeters to measure the cumulative evaporation and schedule irrigation accordingly.
• How It Works: Crop water use is related to pan evaporation. The cumulative pan evaporation is calculated, and irrigation is scheduled based on this value.
  • Example: Wheat may require 75–100 mm CPE, while sugarcane may require 75 mm CPE.

2.3. Irrigation Water to Cumulative Pan Evaporation (IW/CPE) Ratio

• Objective: This ratio is used to schedule irrigation based on the amount of water applied and the pan evaporation value.
• How It Works: An irrigation amount is applied equivalent to the amount of water lost in evaporation. For example, if 10 cm of pan evaporation occurs, and 5 cm of irrigation is applied, the IW/CPE ratio would be 0.5.
• Benefit: Simple and practical for scheduling irrigation in regions with adequate water.

3. Plant Indicator Approaches

These approaches rely on direct or indirect physical measurements of plant water status to schedule irrigation.

3.1. Visual Plant Symptoms

• Objective: Irrigation is scheduled based on observable symptoms of water stress in plants, such as wilting, leaf curling, or discoloration.
• Common Symptoms:
  • Drooping or curling leaves
  • Reduced leaf size or leaf angle
  • Wilting or rolling of leaves, especially in crops like maize or beans.
• Limitations: This method can be imprecise as it depends on visual judgment and may not be applicable to all crops.

3.2. Plant Water Content or Water Potential

• Objective: This approach measures the relative water content (RWC) and leaf water potential, which gives an indication of plant hydration.
• How It Works: RWC is calculated as the ratio of the difference between the fresh and dry weight of the leaf to the turgid weight of the leaf.
  • Formula: RWC = (Fresh Weight – Dry Weight) / (Turgid Weight – Dry Weight).
• Benefit: Can provide a precise indicator of water stress when monitored regularly.

3.3. Canopy Temperature and Plant Temperature

• Objective: The difference in temperature between the plant canopy and the surrounding air temperature is used as an indicator of water stress.
• How It Works: A significant difference between canopy and air temperature suggests that the plant is under stress due to insufficient water.
• Tools: Infrared thermometers are often used to measure canopy temperature.

3.4. Critical Crop Growth Stages

• Objective: Irrigation is scheduled during specific critical growth stages when crops are most sensitive to water stress.
• Examples:
  • Rice: Initial tillering, flowering.
  • Wheat: Crown root initiation, tillering, jointing, flowering, dough stages.
  • Maize: Early vegetative stage, tasseling, and silking stage.
• Benefit: Ensures the crop gets adequate water during the most sensitive growth periods.

3.5. Indicator Plants

• Objective: Certain plants, like maize or sunflower, can serve as “indicator plants” for scheduling irrigation.
• How It Works: When these plants show signs of water stress (e.g., wilting), it is a cue to irrigate other crops in the field.

4. Hybrid Approaches

Sometimes, a combination of the methods mentioned above is used for more accurate irrigation scheduling, especially when water availability is uncertain or variable.

• Example: Using both soil moisture content and PET or CPE to determine irrigation requirements.

Key Factors Influencing Irrigation Scheduling

1. Crop Type: Different crops have varying water needs based on their root structure, leaf area, and growth stage.
2. Soil Type: The moisture retention capacity of soil affects how often irrigation should be applied.
3. Climate: Climatic conditions, particularly temperature and humidity, play a key role in evapotranspiration.
4. Water Availability: The availability of irrigation water determines how efficiently it can be used to maximize crop production.
5. Growth Stage of the Crop: Crops require different amounts of water at different growth stages, particularly during critical stages like flowering, fruiting, and grain development.