In this article we will discuss about:- 1. Meaning of Tanks 2. Classification of Tanks 3. Network.
Meaning of Tanks:
Tank is a general term used for surface water storage of moderate size. The storage may have come into being due to interception of rain water in a natural depression or a man-made excavation. Such water bodies are popularly called ponds.
Alternatively water storage may be done by closing the opening of a natural saucer shaped land form by constructing a bund or small sized embankment. The storage so Constructed is called a tank. The tank bunds are mostly constructed with earth to keep cost of construction low and commensurate with the benefits envisaged.
Water storages of large size are not called tanks but they are referred to as reservoirs. Such reservoirs are formed in the river valleys by constructing a barrier or a dam using either masonry concrete or earth depending upon site conditions. Technically bund is a miniature form of a dam.
Tank irrigation has been in vogue in our country since ancient times. Due to simplicity in construction it was a very popular mode of conserving rain water. In south India where river flows are monsoon fed, tank irrigation assumes special importance. In the plains of Uttar Pradesh, West Bengal and Orissa as also on the plateaus of Madhya Pradesh tank irrigation is extensively practiced.
Classification of Tanks:
Irrigation schemes benefitting less than 2000 hectares of Culturable Command Area (C.C.A.) are called minor schemes. All tank irrigation schemes come under this category. Adopting criteria of area benefitted tanks could be classified as small, medium and large.
Small tanks are those benefitting 20 hectares area and below. Medium tanks may be taken to be those benefitting areas between 21 and 500 hectares while tanks capable of irrigating more than 500 hecatres may be termed large.
Roughly bunds with less than 12 metres height create tanks. From the consideration of height of bunds, upto 4.5 metres height bunds generate small tank.
Medium tanks are formed by bunds upto 9 metres height. It may, however, be remembered that this classification is very approximate because shape and size of the tank is not dependent on the height of the bund alone. It is equally affected by the topographical features of the region.
Network of Tanks:
The tank irrigation system may exist with each tank as a separate entity or in the form of a group of tanks in a series or tanks with inter-connection.
In tank irrigation system following types of network exists:
(i) Isolated tanks;
(ii) Tanks with inter-connection; and
(iii) Tanks in series.
(i) Isolated Tanks:
When a tank is fed by an independent free draining catchment and also when the surplus flows do not form network inflow into another tank, the tank irrigation system is called isolated tank system. Mostly large and medium- large tanks are constructed as isolated tanks with independent catchment area as well as distinct command area. Also in the plains and on plateau land tanks exist in isolation.
(ii) Tanks with Inter-Connection:
Sometimes a group of tanks may be so situated that they could be inter-connected to receive flows through, as well as deliver flows to, other tanks in the group severally. It thereby implies that the tanks have a combined catchment.
Any surplus received by a tank from the catchment lying above is transferred to other tanks. Depending upon prevailing hydro-meteorological condition the tanks are capable of feeding each other in reverse. Thus optimum water utilisation is achieved.
(iii) Tanks in Series:
Such tanks are located alongside the river drainage channels. They are fed by inflow drains and serviced by escape or outflow drains. The tanks in upper reaches get their supplies from the catchment through inflowing drains. It then lets its surplus flow down through an escape or outflow drain which contributes to the inflow of the tank lower down in the series.
Thus while the uppermost tanks have substantial free catchment, the tanks lower down have limited free catchment falling between two tanks. The tanks lower down in series get inflows immediately after rainfall from their free draining catchments. But supplies from already intercepted catchment are received only after the upper tanks are filled to capacity. Most of the small tanks existing in southern States of the country form series network.
The advantage of this system is that, surplus water from the upper tank does not go waste but is picked up by the lower tanks. Thus optimum water gets conserved.
It, however, suffers from one disadvantage relating to safety. Since each tank has vast combined catchment, in case of breach in the upper tank, lower tanks also become prone to severe flooding endangering safety of the tank bund. To avoid this, breaching sections are provided at appropriate places in each tank.