Capacity of primary and secondary canals and size of irrigation area

In determining the capacity of the primary canal, two factors are relevant:

(a) The availability of water (rate of flow) at the primary canal head, modified by storage if any, month by month throughout the year

(b) The pattern of varying demand for irrigation corresponding to the proposed composite cropping pattern, throughout the year.

The two factors are related, in that the cropping pattern may be designed to make best use of the seasonally varying irrigation supply. Conversely the operation of the storage reservoir, if any, will be designed around the varying seasonal irrigation needs of the selected cropping pattern.

With regard to the second factor, in designing the system it is convenient to work with a hypothetical area of 100 ha under cultivation with the proposed crop mix and with 100% irrigated crop intensity (also hypothetical) in the season of maximum irrigation intensity. The irrigation demand for this area is estimated at weekly intervals and plotted throughout the year. The 100 ha area is then scaled up progressively until the plot of irrigation demand touches the monthly plot of flow available. The corresponding peak annual demand for that scaled-up area gives the desired nominal maximum design rate of supply at the primary canal head If the irrigation system were based on a design rate of flow smaller than that value, the available supply would only be partially utilized throughout the year. If on the other hand, the system were to be based on a design rate of flow greater than that value, there would be a deficit in part of the year.

As discussed above, cropping pattern and reservoir operation may be varied to optimize water use, for instance to maximize water use in the high-flow season, or to minimize demand in the low-flow season. Hence, the procedure described is part of a trial and error process in project design.

It is emphasized that the desirable maximum design rate of flow at primary canal head determined above is not necessarily the same as the canal capacity. Canal capacity may be made greater than the nominal maximum design rate of flow for a number of reasons, including provision for possible future changes in cropping pattern (particularly in reservoir-controlled systems) or to accommodate unsteady flow in the canal. It is, however, a key parameter in the design of the remainder of the system.

So far no reference has been made to size of irrigation area or to actual irrigation intensity. The hypothetical 100 ha irrigated at an assumed 100% intensity in the peak season has been scaled up until demand matches the available flow in the critical period. The area so derived by this scaling-up process is the net service area which could be supplied at 100% intensity during that period. However, the area actually served could be made considerably greater, for social or other reasons. In this case, the irrigation intensity would be correspondingly smaller. It is noted that reducing the irrigation intensity through increasing the size of the service area does not change the design rate of flow or canal capacity at canal head.- It does, however, affect the figure commonly referred to as the canal "duty", which is simply the design maximum flow per hectare of area served by the system. If the peak design irrigation intensity is reduced by 20% by increasing the size of the service area the canal duty is similarly reduced. Provided that the cropping pattern is uniform, the canal duty in the primary and secondary canal system is uniform throughout (adjusted for losses), and the design flow at any point on a canal is proportional to the area commanded by that point on the canal. An exception is noted when a secondary is designed to operate rotationally, half the time on and half the time off, even in the season of peak delivery. Such operation is the exception but it is practiced in some systems with long unlined secondaries, in order to minimize conveyance losses. In such circumstances, the canal duty has to be doubled if the same seasonal delivery of water per unit of area is to be achieved, as with a system based on continuous operation in the peak period.

The same canal duty, adjusted for conveyance losses, is also applied at the head of the tertiary, provided that the cropping pattern is uniform. However, what distinguishes a tertiary from the higher-order canals is the fact that the design flow remains constant throughout the tertiary command. For reasons discussed in the next section, it does not reduce as the area served reduces, toward the outer margin of the tertiary command.