8.1 Irrigation System Types, Evapotranspiration & Scheduling

Irrigation System Types, Evapotranspiration & Scheduling

Effective irrigation management is paramount for optimizing crop yields, conserving water resources, and minimizing environmental impacts. This requires a deep understanding of irrigation delivery systems, how crops use water, and when to apply that water to prevent drought stress without over-irrigating.

Irrigation System Types

Choosing the right irrigation system depends on factors like topography, soil type, water availability, crop type, and capital investment capabilities. The three primary categories are surface (flood), sprinkler (center pivot), and micro-irrigation (drip).

Flood (Surface) Irrigation

Flood or surface irrigation is the oldest and most widely used method globally. It involves water flowing over the soil surface by gravity.

• Characteristics: Includes furrow, border, and basin irrigation.
• Efficiency: Generally the least efficient (40-60%) due to deep percolation, runoff, and evaporation losses.
• Suitability: Best suited for heavy soils (clays, clay loams) with low infiltration rates and relatively flat topography. It is common for crops like rice, alfalfa, and some row crops.
• Management: Requires careful leveling (often laser leveling) and management of flow rates and set times to improve uniformity and reduce runoff.

Center Pivot (Sprinkler) Irrigation

Center pivot systems are self-propelled sprinkler systems that rotate around a central pivot point.

• Characteristics: Highly automated, can cover large areas, and can apply fertilizers and chemicals (fertigation/chemigation) uniformly.
• Efficiency: Moderately high efficiency (70-85%). Low-Energy Precision Application (LEPA) or drop nozzles can further improve efficiency by reducing wind drift and evaporation.
• Suitability: Adaptable to rolling topography and a wide range of soil types, especially sandy soils with high infiltration rates where flood irrigation is impractical. Used for a vast array of agronomic crops (corn, soybeans, wheat).
• Management: Application rates must be matched to soil infiltration rates to prevent runoff, especially near the outer edge of the pivot where the system travels fastest.

Drip (Micro) Irrigation

Drip irrigation delivers water slowly and precisely directly to the plant root zone via emitters or driplines.

• Characteristics: Can be surface or subsurface (SDI). Applies water frequently at low volumes.
• Efficiency: The most efficient system (90-95%+) because it minimizes evaporation, runoff, and deep percolation.
• Suitability: Ideal for high-value crops (vegetables, orchards, vineyards) and areas with severe water scarcity or challenging topography.
• Management: Requires significant capital investment and meticulous maintenance to prevent emitter clogging from biological growth, chemical precipitation, or physical particulates. Filtration is mandatory.

Evapotranspiration (ET) and Crop Water Use

Evapotranspiration (ET) is the fundamental driver of crop water requirements. It consists of two processes occurring simultaneously: evaporation of water from the soil surface and transpiration of water through plant stomata.

Understanding ETc, ETo, and Kc

Crop water use is calculated using the following relationship: ETc = ETo × Kc

1. Reference Evapotranspiration (ETo or ETr): The ET rate of a reference crop (typically well-watered grass or alfalfa) under specific weather conditions. It represents the atmospheric demand for water, driven by solar radiation, temperature, wind speed, and relative humidity. ETo data is usually obtained from local weather station networks (e.g., CIMIS, CoAgMet).
2. Crop Coefficient (Kc): A factor that relates the water use of a specific crop to the reference crop. Kc values change throughout the growing season based on the crop's development stage.
   • Initial stage: Kc is low (mostly soil evaporation).
   • Canopy development: Kc increases rapidly.
   • Mid-season (full cover): Kc is at its peak (highest water demand).
   • Late season (senescence): Kc declines.
3. Crop Evapotranspiration (ETc): The actual water requirement of the specific crop at its current growth stage.

ET Calculations in Practice

If the daily ETo is 0.25 inches and the corn crop is in the silking stage with a Kc of 1.20, the ETc is:

• ETc = 0.25 inches * 1.20 = 0.30 inches/day. The irrigation manager must replace this 0.30 inches, factoring in the irrigation system's efficiency and any effective rainfall, to maintain optimal soil moisture.

Irrigation Scheduling and MAD

Irrigation scheduling answers the questions: When to irrigate? and How much to apply? It relies on tracking soil moisture status, often using a "checkbook" method where inputs (irrigation, rain) are balanced against outputs (ETc).

Soil Moisture Concepts

• Field Capacity (FC): The amount of water held in the soil after excess gravitational water has drained away (typically 24-48 hours after a soaking rain or irrigation).
• Permanent Wilting Point (PWP): The soil moisture level at which plants can no longer extract water and will wilt irreversibly.
• Plant Available Water (PAW) or Available Water Holding Capacity (AWHC): The water held between FC and PWP. PAW varies significantly by soil texture (e.g., sand holds less PAW than silt loam).

Management Allowable Depletion (MAD)

Allowing the soil to dry out to PWP would severely damage the crop. Therefore, irrigation is triggered long before PWP is reached. This trigger point is based on Management Allowable Depletion (MAD).

MAD is the maximum percentage (or amount) of PAW that can be depleted from the root zone before crop stress or yield reduction occurs.

• MAD is often expressed as a percentage (e.g., 50%).
• If a soil has 4.0 inches of PAW in the root zone and the MAD is 50%, irrigation should be triggered when 2.0 inches of water have been depleted.

Factors Influencing MAD:

1. Crop Type and Growth Stage: Drought-sensitive crops (like most vegetables) or critical growth stages (like corn silking or soybean pod fill) require a smaller MAD (e.g., 30-40%) to ensure no stress. More tolerant crops or vegetative stages can handle a larger MAD (e.g., 50-60%).
2. Soil Texture: Sandy soils have low PAW, so the allowable depletion is small in total inches, requiring frequent, light irrigations. Finer soils hold more water, allowing for larger, less frequent irrigations.
3. Irrigation System Capacity: If a system (like a center pivot) takes several days to complete a circle, the manager must start irrigating before the MAD is reached in the driest part of the field to prevent stress before the system arrives.

By accurately calculating ETc and managing soil moisture within the MAD limits, growers can optimize yield while maximizing water use efficiency.