Irrigation Systems – SS2 Horticulture and Crop Production Lesson Note (Second Term)

Irrigation is the artificial application of water to the soil to help crops grow when there is not enough rain.

Behavioural objectives

By the end of the lesson, students should be able to:

1. define irrigation and state why it is needed;
2. describe drip, sprinkler and centre pivot irrigation;
3. state the advantages of modern irrigation systems;
4. list ways of conserving water on an irrigated farm;
5. state the main challenges of modern irrigation.

What is irrigation?

Irrigation is the artificial supply of water to the soil so that crops can grow when rainfall is too little.

• Idea: it works like “artificial rain”. The farmer, not the cloud, decides when the crop is watered.
• Delivery: water is brought from its source straight to the plant’s roots.
• Importance of water: plants can go a short time without food, but they cannot survive long without water.
• Main benefit: irrigation protects farmers against drought and dry spells.
• Dry-season farming: it lets farmers grow vegetables such as tomato and pepper outside the rainy season.

What are the sources of water for irrigation?

Irrigation water comes from surface sources and underground sources.

• River or stream: water is pumped or channelled from the river to the farm.
• Borehole: a deep drilled hole that brings up underground water, usually with a pump.
• Well: a dug hole that reaches underground water, drawn by hand or by pump.

What are the traditional methods of irrigation?

Traditional methods use simple tools and a lot of human labour.

• Bucket or watering can: water is carried and poured on each plant by hand.
• Channels: simple ditches dug in the soil carry water from the source to the crop rows.

What are the modern methods of irrigation?

There are three modern methods of irrigation: drip, sprinkler and centre pivot.

1. What is drip (trickle) irrigation?

Drip irrigation is a method in which water drips slowly through small holes in thin plastic pipes onto the base of each plant.

• Equipment: a network of thin plastic pipes with tiny holes (emitters) placed at the base of each plant.
• How it works: water drips drop by drop directly onto the roots.
• Advantage: very little water is wasted, and less water reaches the spaces between plants where weeds grow.

2. What is sprinkler irrigation?

Sprinkler irrigation is a method in which water is pumped through pipes and sprayed over the crop through nozzles.

• Equipment: a pump, pipes and rotating nozzles that spray water into the air.
• How it works: the water falls like natural rain over a wide area.
• Advantage: it suits sandy soil, where water sinks in too fast for methods that flood the land.

3. What is centre pivot irrigation?

Centre pivot irrigation is a method in which a long sprinkler pipe on wheels moves in a circle around a central water source.

• Equipment: a long pipe fitted with sprinklers and supported on wheeled towers.
• How it works: the pipe turns slowly around the centre and waters a circular area.
• Advantage: one machine can water a very large farm with little human effort.

How do the three modern methods compare?

What are the advantages of modern irrigation systems?

Modern irrigation systems give the farmer control over water, fertiliser and labour.

• Control: the farmer decides how much water the plant gets, so there is no “drowning” and no “thirst”.
• Fertigation: liquid fertiliser is mixed into the irrigation water, so the plant gets “food” and “drink” together.
• Night watering: systems can be set to run at night when it is cool, which reduces evaporation.
• Saves labour: pipes and machines replace carrying water by hand.
• Drought protection: crops still get water when rain fails.

How can water be conserved on an irrigated farm?

Water conservation means using irrigation water wisely so that none is wasted.

1. Scheduling: do not water between about 12:00 pm and 3:00 pm, because much of the water evaporates. Water in the early morning or late evening.
2. Mulching: cover the soil around irrigated plants with dry grass. This layer keeps the soil damp for longer.
3. Maintenance: check pipes regularly. Even a small leak in a plastic pipe wastes a lot of water over time.
4. Sensors: soil moisture sensors are placed in the ground and tell the pump to stop when the soil is wet enough.

What are the challenges of modern irrigation?

There are three main challenges of modern irrigation: cost, power supply and salt build-up.

• Cost: pumps, pipes and sprinklers are expensive for a student or a small farmer.
• Power: most pumps need petrol, diesel or electricity. Without fuel or light, the plants stay thirsty.
• Salt build-up: if water containing salts is used for many years, the salts stay in the soil and can make the land harmful to crops.

Evaluation

1. Define irrigation.
2. Describe how drip irrigation works and state one advantage of it.
3. Why is sprinkler irrigation suitable for sandy soil?
4. What is fertigation?
5. State three challenges of modern irrigation.

Assignment

1. List the four ways of conserving water on an irrigated farm, explain any two of them, and state why watering in the early morning or late evening saves water.

Teacher’s classroom tip

• Ask: “You have one bucket of water for ten tomato plants. Do you pour it all on them at once, or use a small cup at the base of each plant?”
• Explain that the “cup” method is the basic idea behind drip irrigation.