Possible alignments of an irrigation canal

Canals are classified by alignment according to the ground they follow.

1. Watershed (ridge) canal

Aligned along the ridge line between two drainage basins. Water can flow by gravity to both sides, there is no cross-drainage work, and the whole area can be commanded.

        ridge
   fields /\ fields
    ____ /  \ ____
   command  canal  command

2. Contour canal

Runs along a contour line with a very small slope; used in hilly areas. It irrigates only the land on its lower side. It meets streams, so it needs cross-drainage works and is liable to landslides.

    hill (higher side)
   ===== canal along contour ======>
    irrigated area on one side only

3. Side-slope canal

Runs roughly at right angles to the contours, on the side slope of the ground, between a watershed and a drainage line. It meets no cross-drainage, but is liable to be damaged by runoff from the hill side, so it is protected by catch drains.

     ridge
       \  canal  (perpendicular to contours)
        \ -->
         \
      drainage line

Comparison

In plains the main canal is generally a watershed canal; in hills contour or side-slope canals are used.