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.
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.
- 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
- 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
- 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
Irrigation Systems Design Considerations
- Determine Source
- Pond or other open source (gravity feed or pump)
- 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.
- Good for limited water source
- Efficient direct delivery of water to root zone
- 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
- More ambient cooling
- Longer life
- 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.
- 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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