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Home > Master Publication List > Western Sustainable Agriculture Research & Education (SARE) Farm Internship Curriculum and Handbook > Irrigation

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

Tom and Maud Powell and Michael Moss, Sustainable Farmers, Jackson County, OR.
Technical advisor: Tim Franklin, Jacksonville, OR.
Curriculum advisor: Peter O'Connell, Jacksonville, OR.
Web advisor: National Center for Appropriate Technology, Butte, MT.

Published 2007
Updated 2010


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 - (A system in which sprinklers come directly from a buried PVC via a vertical riser)
    • Must be designed correctly and buried deep enough to avoid cultivation
      • Advantages
        • More ambient cooling
        • Longer life
      • Disadvantages
        • Less efficient
        • Can promote mold/disease
        • Takes higher flow

  • Aluminum Hand Line (Pipe and sprinklers are all above ground and are supplied with water from a riser valve coming off of the main system at the top of the row or field)
    • Requires less buried PVC.
    • 20' to 30' pieces are moved as needed. Have impact heads. Connect to fittings at top of field or row.

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



  • 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.



  • Rain Bird Drip/Standard Irrigation Design Manual
  • Turf Irrigation Manual


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This page was last updated on: August 25, 2014