Irrigation is defined as the artificial or manmade application of water to soil or land for farming purposes. In other words, irrigation is the artificial application of water to the soil to supplement insufficient rainfall. Irrigation is mainly practised in areas where there is insufficient rainfall.
Water is the most important determinant in the growth of crops. The choice of irrigation system depends on:
- Availability of water: For irrigation to operate, water must be available in sufficient quantity. Water should therefore come through rivers, streams and boreholes.
- Slope of the land: The choice of irrigation system depends on the shape and slope of the land. While surface irrigation requires sloping land, sprinkler irrigation requires a flat land.
- Soil type: The choice of any irrigation system depends on the type of soil in the area. Surface irrigation may be effective in a clayey type of soil to aid water runoff.
- Type of crops: The choice of any irrigation system also depends on the type of crops to be grown. While cereals and leguminous crops will require sprinkler irrigation, tree crops may require surface irrigation.
- Size of farmland: Irrigation system depends on the size of farmland. Irrigation becomes effective and efficient if the farm land is large.
AIMS OF IRRIGATION
- To add water to the soil in order to supply moisture for plant growth.
- To ensure the survival of crops during droughts.
- To cool the soil and the atmosphere, thereby making the environment more favourable for plant growth.
- To make early planting possible.
IMPORTANCE OR EFFECTS OF IRRIGATION ON CROP PRODUCTION
The effects of irrigation on crop production
- Irrigation softens the soil for easy tillage operations.
- It provides moisture in the soil for root absorption.
- It reduces the amount of salts accumulated in the top soil, which could be harmful to crop plants.
- It also cools the soil by reducing the soil temperature during the dry season when the soil temperature is very high. Plant growth is thus improved.
- Irrigation enables the crop to be well established even when rain is erratic.
- It increases crop productivity or crop yield.
- Irrigation provides humid environment for the breeding of pest and pathogens of crops.
- It helps in the improvement of microbial decomposition of organic manure to release nutrients for crop plant.
- Irrigation dissolves nutrients for root absorption.
- It assists in facilitating crop production all year round.
- Irrigation encourages the spread of weed seeds.
TYPES OF IRRIGATION
Irrigation systems or types can be grouped into three main classes: Surface irrigation, sub-surface irrigation and overhead irrigation.
a) Surface Irrigation
In this system, water from rivers, dams or streams flows along the surface of the land to farmland. This can be in form of channels, flooding, contour ditch, furrow, basin, border dyke, border and ditch. The most important condition for surface irrigation to be efficient is for the land to have a gentle slope towards the direction of the farmland.
- It removes excess water caused by heavy rainfall especially in heavy soils.
- It is easier to construct.
- Irrigation is cheap to establish and maintain.
- It prevents accumulation of alkalis.
- It increases the activities of microbes in top soil.
- It helps in the collection and disposal of surface run-off.
- It cannot be practised where the land is hilly.
- It cannot be successful in sandy soil.
- The volume of water, especially in flooding, may be difficult to control.
- Some crops may not tolerate heavy flooding.
Factors to be considered in choosing surface irrigation method
- The topography or sloppy land.
- The soil type.
- The type of crop to be grown.
- The size of the farmland.
b) Sub-surface Irrigation
In this system, water is applied below the soil surface by maintaining an artificial water level at certain depth. This, however, depends on the type of crop and the nature of the soil. In all cases, perforated pipes are used to deliver water within the soil, and this water gets to the roots of crops through capillary action.
- By irrigation, it is possible to maintain water at optimum depth for crop need.
- In addition, water use efficiency is high.
- It also ensures low evaporation losses from soils.
- It does not create obstruction or obstacles when carrying out cultural practices.
- Water which has a high salt content cannot be used.
- It requires some level of expertise or technical know-how.
- It is a bit expensive.
Factors to be considered in choosing sub-surface irrigation method
- The permeability of substratum.
- The level of water table.
- Soil texture or capillarity.
- Soil structure.
c) Overhead Irrigation
In overhead irrigation system, water is supplied to the farmland above the surface of the soil.
Problems associated with overhead irrigation
- Wind distorts sprinkler patterns and causes uneven distribution of water.
- The spray force can damage ripening soft fruits.
- A stable water supply is required for economic use of the equipment.
- The water must be clean and free of sand, debris and large amounts of dissolved salts.
- The initial investment is usually high.
- Power requirements for pumping water is high.
- Requires technical know-how.
- Can spread diseases.
- Lateral pipes hinder movement of farm machines.
- High cost of maintenance/repairs/spare parts not readily available.
Overhead irrigation exists in two forms: sprinkler irrigation and drip irrigation.
I) Sprinkler Irrigation: In this system, water is sprayed from the air and allowed to fall on the ground surface somehow as rainfall, through nozzles under high pressure.
Pumping machine which operates under high pressure is needed to pump water into irrigation pipes which are spaced to ensure even distribution of water.
Advantages of Sprinkler Irrigation
- The amount of water supplied is regulated.
- It economises the use of water.
- It is suitable for high arid lands with high evapo-transpiration rate.
- Soluble fertilizers and herbicides can be applied through the irrigation water.
- It can be used for all crops except tree crops like cocoa.
Disadvantages of Sprinkler Irrigation
- Sprinkler irrigation is costly to operate.
- High wind velocity may prevent distribution of water.
- A stable water supply is required.
- It requires high power to produce the high pressure needed to pump water into pipes.
- There is lack of sufficient technical know-how on sprinkler irrigation operation.
II) Drip Irrigation: Drip or trickle irrigation is a new method of irrigation where water is discharged through nozzles called emitters or drippers at selected spacing to deliver water to the soil surface near the base of the plant. With this system, loss of water through deep percolation, run-off and evaporation is greatly reduced.
Advantages of Drip Irrigation
- It is economical in the use of water.
- It reduces salt concentration in the root zone.
- Fertilizers can be applied through this system.
- It operates with slower line pressure.
- It is the most appropriate system needed in arid and semi-arid areas.
Disadvantages of Drip Irrigation
- It is very expensive to set up and maintain.
- Water cannot be distributed in a sloppy farmland evenly.
- Water with high salt content cannot be used.
PROBLEMS ASSOCIATED WITH IRRIGATION
There are many problems associated with irrigation and these are:
- Inadequate water supply for irrigation.
- Irrigation equipment is expensive.
- Pests from dry surroundings may invade and damage crops in irrigated land.
- Disease-causing organisms increase with increasing humidity.
- Vectors of diseases like snails and tsetse flies breed freely in irrigated areas.
- Irrigation equipment is expensive to maintain.
- Lack of adequate technical know-how on irrigation on the establishment and operations of irrigation scheme.
- Excessive dissolution of salt in irrigated water prevents proper growth of crops.
- It may lead to pollution, if it contains dangerous chemicals.
- It disturbs the free movement of farm machinery.
How to reduce disease build-up in irrigated farm
Reduction of disease build-up in an irrigated farm may be achieved through:
- Use of disease-resistant varieties.
- Crop rotation.
- Use of appropriate chemicals.
- Use of clean tools.
- Use of clean or healthy planting materials.
- Destruction of diseased crops to prevent its spread.
- Avoidance of excessive irrigation.
- Weed control.
Methods of Irrigating a small Nursery
Small nursery can be irrigated through:
Manual sprinkling e.g. hand, brooms and brush.
- Use of watering can.
- Drip irrigation.
- Channel irrigation.
- Sprinkler irrigation.
Sources of water for irrigation include rivers, lakes, streams, ponds, dams, bereholes, wells and reservoirs.