Canal Alignment should be done in such a way that

1. It should serve the entire area proposed to be irrigated.

2. Cost of construction including cross drainage works should be minimized.

3. A shorter length of canal ensures less loss of head sues to friction and smaller loss of discharge due to seepage and evaporation, so that additional area may be brought under cultivation.

4. An canal may be aligned as a contour canal, a side slope canal or a ridge canal according to the type of terrain and cultural area.

ALIGNMENT OF A CANAL.

It is essential to align a canal on the watershed (i.e. ridge) of an area in order that water can flow under gravity to fields on either side of the canal, as mentioned above. From the head work's the canal should be so aligned that it reaches the top of the ridge of the watershed in the shortest possible distance. It is, therefore, necessary to prepare a detailed contour plan of the area to project distinctly the ridges and valleys of the command area. So that the main branches and all dis-tributaries are aligned appropriately in accordance with the principal enunciated above.

According to the alignment (i.e. the path) they follow, irrigation canals are classified as:

1] Contour canals.

2] Watershed (or ridge) canals, and

3] Side slope canals.

Figure 2.1 shows the layout of a contour canal, watershed canal an side slope canal.

Contour canals.

A contour canal is aligned almost parallel to the contours of the terrain traversed (i.e., with very mild slopes as such). In figure 2.1 the contour canal is also the main canal. It is given a longitudinal slope, as said above, such that the required velocity of flow is generated. A contour canal can irrigate land only on one side of itself because the area on the other side is higher due to higher contour lines. These conditions generally occur in the head reaches of the canal. These situations usually prevail in a hilly country. A contour canal would cross the natural drainage lines since the line of flow of surface drainage lies at right angles to the ground contours. After it takes off from the river a main canal is a contour canal. But an attempt is always made to bring it to the main watershed as early as possible. Contour canals below the ridge will have to cross a very large number of drainage's.

WATERSHED OR RIDGE CANALS.

A watershed is an imaginary line that passed through all the highest points in the area such that the ground slopes downward on both sides of this line. A canal constructed on this watershed or ridge (of course, with a bed slope) is called a watershed canal or a ridge canal. In Figure 2.1 the two branch canal taking off from the main canal are also the examples of ridge canals. Here, cross drainage works are not required. The watershed at times may be so zigzag that the canal cannot possibly follow it exactly all the way. Any substantial variation shall necessitate the provision of cross drainage works. A watershed canal is the best canal as it can command areas on both sides to which water flows under

INTRODUCTION (WATER LOSSES IN CANAL)

Canal system designed and constructed transports water from the source like River to the farmer’s fields. But it has to take care about canal water supply with a minimum amount of water loss. Water losses can seriously reduce the efficiency of water delivery to fields. Water may be lost by seepage, leakage or both. The canal water tries to seep into the soil. Moreover, the canals are exposed to the atmosphere at the surface. The water also goes to the atmosphere in the form of vapor. The losses in irrigation canals are mainly,

1] Evaporation losses.

2] Absorption losses.

3] Percolation (seepage) losses.

4] Transpiration losses.

A. Seepage losses:

When water table is close to the ground surface a direct flow from the channel to the ground water reservoir occurs and there is a zone of continuous saturation from the channel to the water table or water table decreases due to absorption. [1] The loss due to seepage is the one which is most significant so far as irrigation water loss from a canal is concerned. Providing perfect lining can prevent seepage loss from canals but cracks in lining develop due to several reasons and performance of canal lining deteriorates with time. [3]

B. Evaporation (percolation) losses:

It is the transfer of the water from the liquid to vapor stage. As the canal water is exposed to the atmosphere at the surface, loss due to evaporation is obvious. It is of course true that in most of the cases evaporation loss in not significant. It is may range from 0.25 to 1% of the total canal discharge. [1]

The loss of water due to seepage and evaporation from irrigation canals constitutes a substantial part of the usable water. By the time the water reaches the field, more than half of the water supplied at the head of the canal is lost in seepage and evaporation. [4] Seepage loss is the major and the most important part of the total water loss. [5] Seepage and evaporation are the serious problems of water in an irrigation canal network.