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.
In this system assured supply of water throughout the crop period to irrigation requirement of the crops is made available to the command area through storage of water done at the dam or diversion of supply made by means of head works at the off take point of the canal. Perennial irrigation may be either direct or indirect, as follows:
In direct irrigation system water is directly diverted from the river into canal by construction of diversion weir or barrage across the river without attempting to store water. This method is practiced where the river has adequate perennial supply to feed the canal system at the times of crops periods.
It is also termed as storage irrigation. Here water is stored in reserved during monsoon period by construction of a dam across the river for supply into the off taking canals. Evidently indirect irrigation is adopted where the river is non â€'perennial or flow in the river is inadequate during lean period. Storage irrigation has greater irrigation potential the direct irrigation but is costly due to the cost of construction of dam.
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.
Inundation irrigation is done by a canal taking off from a river in flood without any diversion work. It depends on the periodical rise in water level of the river and the supply is drawn through open cuts in the river bank or creeks which are called heads. Owing to changes in the river course the heads have often to be changed. A regulator is, however, provided at the canal about 5 km downstream from the off take, where the discharge passing below in the canal is controlled and the surplus supply is escaped back into the river. Inundation canals usually flow only during the summer months and bring in large quantity of silt beneficial to crops.
Selection of off taken Point:
The various factors which govern the selection of offtake point are (i) Site which is least likely to silt, (ii) Site on a creek of suitable size is preferred, since it is more constant in discharge than main river and also subject to milder attacks by floods, (iii) Site at the tail end of a creek is preferred as the flow there has less silt and is less liable to changes, (iv) site on concave curve towards the downstream is preferred from minimum silt entry. A part of concave reach remains always within the range of outside curvature for low as well as high river discharges, otherwise a site in a straight reach free from erosion upstream is suitable, (v) site with stable and high river banks is suitable, (vi) A back water reach also offers a good site, (vii) Wider river section at the offtake point which embodies two benefits, low velocity in the river and as such low silt charge in the water drawn in the canal, and low variation in water level, and (viii) If other conditions are favourable, a site close to the command area is preferred.
Design of Inundation Canal:
The bed level in the channel is kept low enough to draw about half the full supply discharge of the canal in low river, i.e., in the middle of April and again in October, and narrow enough to limit the excessive high floods to the minimum possible. Full supply level is fixed with due consideration to the steady water level in the river during about 1 Â½ months, usually fair irrigation season. Beyond the head regulator, design considerations of unlined canals apply equally to the inundation canals. However, higher values of silt factor f and water surface slope are adopted than those allowed for canals off taking from permanent head works. Generally the
slope ranges between 0.20 and 0.25 m/km. Manningâ€™s, Chezy or Lac the design of inundation canal.
(i) Economical in cost being the simplest system of irrigation as permanent head works is not to be constructed,
(ii) Silty water carried by the canal has manorial value, and
(i) Highs maintenance cost, and
(ii) Seasonal irrigation.
(i) Irrigable area being fixed, the supply if available in excess cannot be use
(ii) Supply is uncertain if the river is non â€'perennial, and
(iii) By damming up whatever small supply available in the river or stream, the areas lying down are deprived of this water.
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.