Losses of water in canal

 

Losses of water in canal

During the passage of water from main canal to the agriculture field water can be lost either by evaporation from the top surface or by seepage through the wetted surface of the canal cross section.

These losses are generally very high and of the order of 25 to 50% of the water diverted into the main canal.

Canal distribution system

A- Evaporation losses

The water lost by evaporation is generally very small as compared to the water lost by seepage through certain channels.

Evaporation losses are generally of the order of 2 to 3% of the total losses.

These losses depends upon factors such as temperature,wind velocity,humidity etc.

B-Seepage losses

Seepage losses are further classified into following types.

1- Percolation[join ground water table]

In percolation there exist a zone of continuous saturation from the canal to the ground water table and a direct flow is observed.

Almost all the water lost from the canal joints the ground water reservoir.

The losses of water depends upon the difference of top surface of water level in the canal and ground water depth.

2- Absorption

In absorption a small saturated soil zone exist around the canal cross section and there is no direct flow line is observed in this case.

In absorption the rate of loss depends upon the distance between the water surface level of the canal and the bottom of the saturated zone.

Note- Generally the percolation losses are observed in permeable soils where as the absorption losses in porous soils.

Factors affecting seepage

1- Type of seepage whether percolation or absorption.

2- Soil permeability-if soil permeability is high then losses will be more.

3- The condition of the canal-Seepage through a silted canal is less than that from a new canal.[If the canal is carrying water from a sediment carrying river then that sediment will settle @the bottom and block the bottom of the canal and will block the seepage pores thus seepage losses will reduce].

4- Amount of silt carried by the canal i.e. more the silt lesser will be losses.

5- Velocity of the canal water i.e. more the velocity less will be the losses [ as the water will flow fast and seep less].

6- Cross section of the canal and its wetted perimeter[if wetted perimeter is more then losses will be more].

• Next to china, India has the largest area under irrigation. The irrigated area is constantly and continuously increased, so as to ensure assured irrigation and to avoid crop failures due to famines and non availability of water. 
• To secure the benefits of irrigated land, a tremendous amount of capital has been invested in this country in the irrigation projects. So much so, that an expenditure of about rs.115000 crores has been incurred in our country on major and medium irrigation project since independence and upto the end of ninth five year plan. This data confirms the fact that irrigation water is a costly commodity, and as such, there should be no wastage during its carriage from the reservoirs to the fields. 
• Most of the canals, constructed in India to irrigation water, are unlined, and hence a large part of the costly irrigation water is lost in percolation and absorption as seepage loss. No doubt, there are  regions where the soil is such that seepage losses are very small, and there is no justification for lining them, but at the same time, it is also true that there are areas where 25 to 50 % of the water is lost in seepage. This is very serious loss and proportionately reduce the irrigation potential of the supplied water. such seepage loss of the costly irrigation water must, therefore, be minimised. The seepage can be avoided or minimised by lining the irrigation canal.
• By lining the canal, we mean that the earthen surface of the channel is lined with a stable lining surface, such as concrete, tiles, asphalt, etc. Depending upon the type of lining adopted,the seepage loss can be reduced to 2 to 5 % of their original values by lining the canals. Apart from the reduction in seepage losses, there are various other advantages of lining of the canals. All these advantages are described below in details.

Advantages of lining

Seepage control

The seepage losses are considerably reduced if the channel are lined. A lined canal costs about 2 to 2.5 times as much as unlined canal, but where seepage is heavy, the savings of costly irrigation water may itself be sufficient to fully justify the capital expenditure of lining. It should also be kept in mind that heavy seepage losses in canals would necessitate the construction of large reservoirs and bigger dams. Prevention of seepage by lining would, therefore, reduce their impounding capacity, and hence,lower the construction costs of these works. The idea of the extent to which seepage takes place in different kinds of channels can be obtained from the table.

Prevention of water logging

Uncontrolled seepage through unlined canals, often raises the water table in the surroundings fields up to or near to the ground level, as to bring the crop plant roots within the capillary fringe. This, in turn, brings up the alkali salts near to the ground surface, which soon renders the land unfit for cultivation. such a land usually called thur, and the phenomenon of rise of water table is known as water logging.

• It is well known that large tracts  of land in punjab have been rendered uncultivable  in this manner. Thus, the very canal, built to increase food production, have within a few years, caused the ruin of a large portion of agriculturable land. Lining of canals prevent seepage and, thus, protects cultivable land. Combined with land drainage schemes, lining helps to reclaim water logged areas.