Gravity or flow irrigation is the type of irrigation in which water is available at a higher level as to enable supply to the land by gravity flow. In flow irrigation water is supplied to the fields though the canals off taking from head works. Gravity flow irrigation is cheaper compared to lift irrigation. The gravity irrigation is further classified as under.
Non â€'Perennial Irrigation:
Also called restricted irrigation. Canal supply is generally made available in non â€' monsoon period from the storage in small dams as in Kandi areas which inadequate to feed all the year round, and / or canal water is not required during monsoon due adequate rainfall in the command area.
Tanks on local streams form a significant source of irrigation especially in the peninsula area in the States of Karnataka, Maharashtra and Tamil Nadu. Tank irrigation belongs to category of storage irrigation. Tanks are small sized reservoirs formed by small earthen embankments to store runoff for irrigation. The site is selected within a watershed protected by vegetation and containing minimum of cultivated land so as to ensure minimum rate of sedimentation which lowers its storage capacity. Adequate soil conservation measures are essentially adopted to ensured quantity and quality of water inflow into the tank. The essential components of irrigation that are tank embankment, surplus or escape weir, and outlet sluice. A suitable breaching section also sometimes provided to ensure that the tank embankment is not overtopped in the event excessive discharge from the catchment. The breaching section is a low level embankment of certain length designed to have a localized breach to escape excessive inflow.
Irrigation tanks are classified, as under, according to the nature of supply of water:
1. System tanks: The system tanks get assured supply from nearby rivers or canal system at as such they may not have their own catchment.
2. Non â€'system tanks: Also called â€˜isolatedâ€™â€'systemtanksdepend. onThethe no runoff from their own catchment. They are not connected to any other tank.
3. Grouped tank: The grouped tanks, as the name implies, consist of a series of tanks connected together such that outflow from the upper tank is stored in the lower one for irrigation.
They usually have limited depth of 5 to 10 m and fill up two or three times in the rainy season and redistribute to some extent the available supplied and tide over breaks in the monsoon. The maximum flood discharge from the catc formula. The length of escape weir is worked out from the formula, Q= CLH3/2, where C is a coefficient of discharge with value for broad crested weir and 1.84 for short crested weir. Water losses from tanks are enormous as the usually have more wetted area for the given storage capacity. The water losses due to evaporation and absorption are taken as 1.75 m per year in Southern India and about 1.25 m in Mumbai.