A crop or a plant requires water for its growth. 

The crop yield from an agricultural land depends upon a number of factors. 

Perhaps the most important factor is that the crops should get adequate water at the various stages of the growth. 

If the rainfall is adequate to meet the total water requirements of the crop and it occurs at the time when the crops require it, then irrigation is not required. 

However, such ideal conditions are rare, especially in a tropical country like India. 

The factors which necessitate irrigation can be summarized as follows:

1. Inadequate rainfall : 

When the rainfall at the place is inadequate to meet the water requirements of the crops, irrigation is necessary. 

This is generally essential for arid and semi-arid regions. 

In such a case, water is stored and diverted from the rivers, where it is available in abundance, to the arid region for irrigation. 

For example, the water from the river Sutlej is conveyed to the Thar desert of Rajasthan through the Indira Gandhi Canal.

2. Non-Uniform rainfall :

The rainfall in a particular area may not be uniform over the entire crop period. 

During the early periods of the crop, rains may be there, but no rain water available at the end of crop period.

3. Increasing the yield of crops : 

The yield of crops per hectare is substantially increased by irrigation as the supply of water is properly controlled. 

Although the rainfall may be adequate for ordinary crops, it may not cater to the needs of high-yielding varieties which require more water. 

In such a case, the yield can be substantially increased by irrigation.

4. Growing a number of crops : 

The rainfall in a region may be sufficient to grow only one crop in a year but it may be inadequate when a number of crops (2 or 3) are grown in the same year.

Irrigation is necessary if a number of crops are grown during the same year in different crop periods. 

5. Growing perennial crops : 

Sugarcane is an important perennial crop which remains in the field practically throughout the year. 

As the rainfall is not regular, irrigation is invariably needed for sugarcane.

6. Growing superior crops : 

Irrigation is necessary when superior crops are grown. Some inferior crops, such as millets, require less water. For such crops the rainfall may be sufficient and irrigation may not be required. 

But for growing superior crops and cash crops such as oil seeds, cotton, fruits, vegetables, etc. which require large quantities of water, irrigation is a necessity.

7. Insurance against drought: 

The crops in a region may not require irrigation during a rainfall year, but in the case of drought, it may be necessary. Irrigation is necessary to provide insurance of crops against drought and famine.

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