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When water flows under gravitational pull through the artificial canal towards the agricultural land, it is known as Flow irrigation. In this system, the head of the canal should always be at a higher elevation than the land to be irrigated. The Flow irrigation can be of two types, Inundation irrigation and Perennial irrigation.
1. Inundation irrigation
In Inundation irrigation, the canals are excavated from the bank of the inundation river (i.e .the river which overflows in the rainy season but nearly dried up in summer and winter).
In this case, water flows to the agricultural land in rainy season only. There is no regulator at the head of the canal to control the flow of water.[Fig.(a)]
The bed level of the canal is fixed at such level that the water can flow through the canal only when the water level of the river rises above the canal bed. [Fig.(b)]Again, the flow of water through the canal stops automatically when the water level of the river falls below the canal bed.[Fig.(c)]
So, this system of irrigation depends completely on the water level of the river.
As there is no regulator at the head of the canal, over-irrigation is possible resulting in damaging the crops.
2. Perennial irrigation
In Perennial irrigation, either a weir or a barrage is constructed across the perennial river(i.e., the river which flows throughout the year in its full capacity) to raise the water level on the upstream side or a dam is constructed to form a storage reservoir. Then the main canal is constructed on either or both the banks of the river. The regulator is constructed at the head of the canal to control the flow of water through the canal towards the agricultural land. This system is reliable as water is available throughout the year. The perennial system of irrigation may be of the following types:
(a) Direct irrigation system
In this system, a weir is generally constructed across a perennial river to raise the water level on the upstream side up to a certain limit so that the water can flow through the canal. Here, the water level on the upstream side will remain at a constant height and the excess water flows over the weir. Sometimes, a barrage is constructed, in place of a weir, to regulate the water level on the upstream side. The hydraulic structure which is constructed in a direct irrigation system is known as Diversion Head Works.[Fig.(1.2)] (b) Storage irrigation system
In this system, a dam is constructed across a river valley to form a storage reservoir.[Fig.(1.3)]The main canals may be taken from both sides of the dam. The flow of water through the canal is controlled by head regulator. This storage reservoir is also known as a multipurpose reservoir as it serves the following purposes:
(b) Water supply
(c) Hydro-electric power generation
(e) Flood control
Book – Irrigation Engineering, Writer – N N Basak