SOIL-WATER RELATIONSHIPS:

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Unit

• SOIL-WATER RELATIONSHIPS
• MICROIRRIGATION

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Learning Objectives
1. Define infiltration, application rate, and
wetted pattern.
2. Describe the relationships between soil
texture, infiltration rates, and wetted pattern.
3. Analyze a specific site for inputs into design
and management of a microirrigation system.
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Terms
Application rate Capillary flow Emitter
Infiltration Tensiometer Wetted pattern
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What are infiltration, application rate and
wetted pattern?
Irrigation success is founded on many basic
principles.
- Infiltration is the process by which water
moves into the soil.
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Infiltration, Rates, Patterns
- Application rate is the amount of water that is
applied expressed as a volume over time.
-Some examples would include 10 gallons per hour
or 5 liters per hour.
- Wetted pattern is the shape that is formed
below the soil surface of the water that has
infiltrated into the soil.
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How do infiltration rates and application rates
affect wetted pattern?
Water moves through the soil in many ways.
- The soil has properties that determine the
movement and storage of water, which affects the
availability of the water to the plants.
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How do rates/ affect patterns

• Water moves through the pores (spaces) in the
  soil.
• The water moves laterally and vertically up and
  down caused by the cohesive forces of water.

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How do rates/ affect patterns

• Soils with large pore spaces have less of a
  capillary flow, upward and lateral flow caused by
  molecular forces in some liquids.

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How does rates/ affect patterns

• The soil will have a unique wetted pattern
  dependent upon its texture.
• Sandy soils have larger pore spaces so the
  wetted pattern will be more vertical since the
  gravitational forces will affect most of the
  water.

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How does rates/ affect patterns
- Clay soils will have a more lateral wetted
pattern due to the stronger cohesive forces and
the smaller pore spaces. - Loam soils will have
a more rounded wetted pattern since both lateral
and gravitational forces will act in relatively
equal proportions.
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How does rates/ affect patterns

• The water is applied using an emitter, a device
  that meters the amount of water discharged.
• If the water is applied at a high rate soils tend
  to have a wider wetted pattern than those soils
  that are irrigated with a low application rate.

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How can you analyze a specific site for the
management of amicroirrigation system with
various inputs?
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Microirrigation
The movement of water through the soil in a
microirrigation system needs to be monitored.
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Microirrigation
- A tensiometer is a device that can measure
water tension and can describe wetted patterns.
- Other methods of monitoring include
spot-checking the soil and determining if the
water is reaching the root zone.
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Terms Relating to microirrigation
-Infiltration is the process whereby water moves
into the soil.
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Terms relating to micrirrigation
-Wetted pattern is the shape produced by
infiltrated water into the soil.
-Application rates are the amounts of water that
are applied to the soil in gal/hr or liters/hr.
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WETTED PATTERN AT A GIVENAPPLICATION RATE
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Soil Moisture Curves
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Available Soil Moisture
is the difference between the amount of water in
the soil at field capacity and the amount of
water in the soil at the permanent wilting point.
-Field capacity (FC), permanent wilting point
(WP), and available soil moisture (AM) vary by
soil texture. See the table of moisture holding
capacities of different soils.
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Moisture Holding Capacities of Soils
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Water movement in Various soils

Soil Type
Radius Wetted Area (ft)
Coarse sand
0.51.5
1.03.0
Fine sand
Loam
3.04.5
Heavy clay
4.06.0