17. Canal Discharge Curves

To rate a flow section, discharge measurements at a current-meter station should be taken over a wide range of canal flows to ensure accuracy in preparing velocity, area, and discharge rating curves (section 1). Water is usually turned into the canals at gradually increasing rates as demand increases during the irrigation season. Thus, measurements for all flow stages in the canal often can be obtained during one season.

The canal bed at a well selected current-meter station is generally permanent in character, and a permanent rating curve could be made if not for sediment accumulations or for growths that occur in the canal during the irrigation season. The sediment and the growths both decrease the discharge capacity of the canal for all flow depths, and the effect is usually most pronounced during the latter part of the irrigation season. This change in flow capacity of the canal for a given depth of flow must be taken into consideration when computing the quantity of water carried by the canal. If the canal is cleaned during the season, the relationship of discharge to gage height is again disturbed. The changing relationship of discharge to gage height in irrigation canals caused by changing boundary conditions is the chief source of error in flow measurements.