Western Sustainable Agriculture Research & Education
(SARE) Farm Internship Curriculum and Handbook

Irrigation

Learning Objectives
The learner will:

• Understand reasons for irrigation
• Learn to recognize the effects of water stress
• Learn methods of determining the proper frequency of irrigation
• Examine various system designs and delivery methods and investigate specific advantages and disadvantages.

Why Irrigate?

• Maintain and moderate best temperature for plant life
• Gives structure and support to plant – water molecules in tissue
• Unlocks biological and chemical processes in the soil that support plant growth
• Plants use water to form oxygen and carbohydrates
• Irrigation protects crops from frost

Effects of Water Stress

• Water stressed plants have lower immunity to pests and disease
• Decreased yield. Plants are particularly sensitive at these stages
• Flowering stage
• Fruit/yield set
• Seedling
• Fruit ripening

Terms and Definitions

• GPM (gallons per minute) – A measurement of flow (volume of water at given source in one minute)
• PSI (pounds per square inch) – A measurement of water pressure (the force that water exerts on a given area)
• (e.g. Water coming out of a pipe can be expressed both in terms of rate of flow and the force applied to that flow, as in 35 GPM @ 50 PSI.)
• Velocity – Rate at which water moves through a closed system. As velocity increases, pressure decreases. Velocity should be 5ft/sec or less. (Use table to determine.)
• Evaporation – Loss of water from soil to atmosphere
• Transpiration – Loss of water from plant to atmosphere
• Evapotranspiration (ET) – Evaporation plus transpiration
• Evapotranspiration rate (ETo) – Measurement of ET in inches/day
• Hygroscopic Water – Water held too tightly in soil to be available to plants
• Capillary Water – Water that is held in pore spaces of soil; available to plants
• Gravitational Water – Water draining from soil; not available to plants
• Capillary Action – Movement of water in soil from wet to dry areas
• Percolation - Movement of gravitational water down through soil
• Permanent Wilting Point – Boundary between capillary water and hygroscopic water. Plant begins to sustain damage and will die if water is not applied.
• Field Capacity – Boundary between gravitational water and capillary water (upper limit for soil moisture available to plant)
• Available Water – Amount of water available to plants

Frequency of Irrigation

• Soil Test Method
• Manual test – Soil is felt and observed at the root zone of the plant. Water is applied when soil is at 50%-75% (depends on crop specifics). Charts are available as a guide to this method, but judgment is largely based on site-specific experience.
• Mechanical test – Tensiometer

• Soil Budget Method
• Calculate site ETo
• Get from local extension office
• Measure time for one inch of water to evaporate from a pan.
• Replace water as it evaporates from field capacity using measured amounts from an irrigation system.
• Example/Exercise: If local ETo is .4in/day, how much water needs to be applied in a week?
• Formula is PR = (96.3xGPM)/(SxL)
• PR = precipitation rate measured in in/hr
• 96.3 = constant
• GPM = gallon per minute water flow in measured area
• S = in-line spacing of sprinklers or emitters in feet
• L = lateral spacing of sprinklers or emitters in feet

Factors Affecting Irrigation Frequency

• Climate
• As temperature increases, ETo increases
• As wind increases, ETo increases
• As humidity increases, ETo decreases
• As precipitation increases, ETo decreases

• Soil Type (see charts)
• Coarse – sand – drains quickly, increase frequency of irrigation
• Medium – loam – drains moderately
• Fine – clay – drains slowly, decrease frequency of irrigation

• Slope (see charts)
• The steeper the slope, the less water the soil can absorb before run-off.

• Crop Specifics
• Water loving or drought tolerant
• Germinating direct seeded crops
• Dry down during ripening
• Perennials

Irrigation Systems
Design Considerations

• Determine Source
• Pond or other open source (gravity feed or pump)
• Well

• Determine GPM and PSI at delivery point
• Flow test with bucket and stopwatch
• PSI test with gauge
• Determine pipe size using tables
• Determine delivery method based on GPM, PSI, and field requirements

• Drip or Micro
• For row crops, typically T-tape coming from 1/2"-1" manifold, with filters and pressure regulators.
• Advantages
• Good for limited water source
• Efficient direct delivery of water to root zone
• Disadvantages
• Maintenance on filters
• Limited life
  
  
• Overhead Sprinkler
• Buried PVC - must be designed correctly and buried deep enough to avoid cultivation
• Aluminum Hand Line - 20'-30' pieces are moved as needed. Have impact heads. Connect to fittings at top of field or row.
• Advantages
• More ambient cooling
• Longer life
• Disadvantages
• Less efficient
• Can promote mold/disease
• Takes higher flow

• Flood
• Sheet of water over established vegetation. Best for pasture or cover crop.

Assessment/Review

• What are the symptoms and effects of water stress in plants?
• What are some factors affecting irrigation frequency?
• Name several types of irrigation systems and discuss advantages and disadvantages of each.

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