When water flows continuously through a canal, losses take place due to seepage, deep percolation and evaporation. These losses are sometimes known as transmission losses. These should be properly accounted for; otherwise lesser quantity of water will be available for cultivation at the tail end. Water losses in canals can be broadly classified under three heads:

1. Evaporation losses: The loss due to evaporation is generally a small percentage of the total loss in unlined canal. It hardly exceeds 1 to 2 percent of the total water entering into the canal. The evaporation losses depend upon: (i) Climatic factors such as temperature, humidity and wind velocity and (ii) Canal factors such as water surface area, water depth and velocity of flow. Maximum loss is there in summer months when temperature are high and wind velocities are also high. Similarly, losses are maximum in unlined canals due to wider water surface area, shallow water depth and low velocity. The average evaporation loss per day may vary between 4mm to 10mm.

2. Transpiration losses: The transpiration loss takes place through lot of vegetation and weeds growth along the bank of canal. However, this forms a extremely small part of loss

3. Seepage losses: Seepage losses constitute major portion of loss in an unlined canal. The seepage losses are due to: (i) absorption of water in the upper layer of soil below the canal bed and due to (ii) percolation of water into the water table, thus raising the water table. If however water table is much lower, seepage losses are only due to absorption. Percolation losses are always much more than the absorption losses.

Rate of water loss:

Canal losses may be expressed in any one of the following methods:

(a) As cumecs per million square metre of the wetted perimeter.

(b) As depth of water lost per day over the wetted perimeter.

(c) As the percentage of the canal discharge.

(d) As the percentage per kilometre length of the canal.

Out of these four methods, the first method is the simplest, and is quite popular.