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Irrigation—Principles & Practices

Unit 1.5 | Part 1 – 207

Lecture 1: Irrigation—Concepts & Terminology

Pre-Assessment Questions

1. Why is water important for growing crops?


2. How is water volume commonly measured in agricultural systems?


3. How does irrigation water cycle through an agricultural system?


4. How does water stress negatively affect crop development and yield?

A. The Role of Irrigation Water in Agriculture Systems

1. Sustains soil biological and chemical activity and mineralization during dry periods: In
   seasonally dry areas, irrigation water artificially extends the time period in which soil
   biological activity and nutrient release are elevated, creating more optimal growing
   conditions for cultivated crops


2. Promotes soil solution and nutrient uptake: Irrigation water becomes the medium into
   which soil nutrients are dissolved (soil solution) and through which nutrients are made
   available for plant uptake


3. Provides carbohydrate building block: 6 CO 2 + 6 H 2 O —> C 6 H 12 O 6 + 6 O 2 : Through the
   process of photosynthesis, water molecules taken up by plants are broken down and their
   constituent atoms rearranged to form new molecules: carbohydrates and oxygen


4. Provides plant structure/support: Water molecules contained within the water-conducting
   vascular bundles and other tissues of plants serve to provide physical support for the plant
   itself


5. Promotes the maintenance of optimal temperatures within the plant: The loss of water
   through the process of evapotranspiration (defined below) liberates heat from the plant,
   thereby regulating plant temperature


6. Protects crops from frost damage: Irrigation water is commonly used to lower the freezing
   temperature in orchard systems during threats of damaging frost


7. Reduces plant stress: By reducing stress on the plant, proper irrigation improves plants’
   resistance to pest and disease damage and improves crop quality (see E, below)

B. Water Cycling in Agricultural Systems

1. Definition of terms (see also Appendix 1, Water Cycling Terms)

a) Transpiration: Water transfer to the air through plant tissues.

b) Evaporation: The loss of water to the atmosphere from soil and plant surfaces

c) Evapotranspiration: The loss of water to the atmosphere by the combined processes of
evaporation and transpiration (see more at Supplement 1, Evapotranspiration (ET) and
the Factors that Affect ET Rates)

d) Capillary action: The movement of water through very small pores in the soil from
wetter areas to drier areas. Water may move vertically and horizontally.

e) Infiltration: The process by which water on the ground surface enters the soil

f) Percolation: The gravitational process of water moving downward and through the soil
horizons

Lecture 1: Irrigation—Concepts & Terminology