Irrigation is generally defined as applying water to the soil to supply moisture essential for plant growth.

Objectives of irrigation

Irrigation is essential for crop production. Following are the objectives of irrigation-

• To supply water for the nutrition and growth of a plant.
• To leach or wash out injurious salt from the soil.
• To remove the stress (moisture deficit) condition of plants.
• To release nutrients in soil solution for absorption by plants.
• To keep the activity of soil microbes at the optimum level and to hasten the decomposition of organic matter in the soil.
• To maintain the temperature and humidity of the soil.
• To control harmful soil insects.
• To create water stagnation for controlling dryland weeds.
• To create conditions suitable for puddling.
• Irrigation is necessary to reclaim fallow land and sometimes brings desirable conditions for tillage.

Importance of irrigation

Plants must be supplied with water as required for successful crop production. Irrigation is necessary for crop production due to following reasons:

1. Uncertainty of monsoon rainfall: Monsoon wind cause 80 percent rainfall in Bangladesh. But the monsoon rain is very uncertain. There is no like schedule that exactly when the rain starts and the duration of the season. In fact, these are variable every year. So irrigation is very important to supply water to plants when needed, i.e., at the right time and in the right amount.

2. Uneven distribution of rainfall: Rainfall is uneven in all the places of Bangladesh. Irrigation is essential for crop production in an area having low and uncertain rainfall.

3. Winter rainfall: Rainfall is rare in the winter season in our country. Rabi crops need much water for their growth and development. Therefore, irrigation is very important for the cultivation of Rabi crops. The cultivation of boro rice is impossible without irrigation.

4. Cultivation of high yielding crops: The high yielding crops are being cultivated in our country, which requires sufficient water for their growth and development. Irrigation has a key role in the successful cultivation of high-yielding crops.

5. Difference in the water-holding capacity of soil: Different types of soil in our country have a difference in water holding capacity. In such cases, irrigation must be provided depending on the soil texture.

Sources of Irrigation

Rainfall: Rainfall is the primary source of water for irrigation. But it is not available throughout the year in all arable areas, and appropriate quantities are required by each crop variety at the different stages of its growth. Rainfall reserved in other water bodies is used as irrigation water resources for an indefinite time. Following are the sources of irrigation water:

Surface water: Surface water is provided by the flowing water of the river from still waters of tanks, ponds, lakes, pools, or artificial reservoirs such as dams, barrages, and diversionary bunds with different storage capacities. Water from these sources is carried to the field by lifting them with the help of water-lifting devices used for irrigation.

Groundwater: Water is reserved underground enriched by percolation of the surface water through the porous state of the earth’s crust land; this type of water is known as groundwater. The underground water is tapped by digging or drilling wells. Wells may be shallow (9- 11 m) or deep (25-30 m or more); the waterfront wells are lifted to be used for irrigation.