Downstream control with limited demand

From the cultivator's viewpoint, the most desirable situation would be to have water available "on demand", preferably demand by the individual. During periods in which supply of water to the system is not a constraint, such demands could be met, as far as the hydraulics of the operation are concerned, by a "level-top" arrangement of the tertiary. The tertiary in this case is virtually an elongated pool, or stepped series of pools if gradients so require, the levels in which are maintained constant, regardless of amount of withdrawal, by float operated gates at the head of each reach. Each cultivator has access to the tertiary, and within limits of the capacity of his outlet he can take water whenever he pleases and in whatever amount. This is "downstream control" both in the hydraulic sense, the gate at the head of each reach being controlled by the level downstream of the gate, and in the popular sense, supply being determined from the downstream end of the system (the irrigator) rather than being dictated by upstream agency.

However, when supply to the system is limited, there can be problems with this utopian system. The total demands on the tertiary are reflected, reach by reach, up to head of the tertiary, at which point they become demands on the secondary or sub-secondary. If they are to be met, the gate on the outlet from the secondary must respond to the level in the first reach of the tertiary, maintaining it at its design elevation by discharging into the tertiary whatever flow is required to achieve that objective. The total demand of all the tertiaries served by a secondary becomes the demand at the head of the secondary, and this is passed on to the primary and eventually reaches the head of the primary, which is the outlet from the reservoir. Here a basic problem may arise. The reservoir may have insufficient water in storage to continue supplying at that rate or storage may be being held back for critically important irrigation at the beginning of the next season. Under these conditions, the controlling factor becomes supply rather than demand. Restrictions in water use must be passed on down the line, finally reaching the cultivator and unrestricted demand has to be substituted by limited demand or managed demand.

There are in fact two types of constraint on the downstream demand type of operation. One is in the hydraulics of the system, particularly in the control of the primary and secondary canals. While theoretically an infinite degree of hydraulic flexibility can be provided with capability of instant response to changes in demand regardless of length of canal the required control structures and their management can become very sophisticated. Work continues on more advanced computer-assisted dynamic operation of canal systems and float operated automatic controls. It can be assumed that most present problems will eventually be solved. More intractable is the second type of problem, the management of deficiencies in supply to cultivators while still preserving some degree of freedom in demand.

If the interests of the individual cultivator could be submerged into a collective common interest, a tertiary command of many small holdings with varying cropping patterns and varying water requirements could be served very well by a level-top demand-type of tertiary system. It would be communally managed with due regard to upstream supply constraints and benefitting from the flexibility in water distribution within the tertiary command which such a system provides. Decisions on the timing and amount of irrigation on each unit would presumably be on rational grounds.

However, for various reasons such a communal situation is not generally in prospect in South Asia, although it is approximated in some sugar-cane cooperatives, and the problem remains of how to regulate the use of a restricted supply of water by 8 group of cultivators acting as individuals. One possible solution is the Water Users Association discussed in a later chapter. Levying water charges in proportion to use, with rates possibly scaled upwards if consumption exceeds a certain rate or during periods of critical supply, could provide some degree of control. However, water meters have generally been short-lived in smallholder installations and collection of water charges remains a problem.

To summarize, the downstream demand type of system, while providing complete hydraulic flexibility in withdrawals from the tertiary, nevertheless, requires operational restraint during periods of limited supply. The same considerations apply to demand operation of the secondaries. When there is limited supply to the primary canal it would be unreasonable to respond to unrestricted demand in any secondary, although the hydraulic control at the head of each secondary is designed with that capability. While limited upstream supply may prevent full utilization of the hydraulic flexibility provided by downstream control, such constraint can be minimized for short-term operation (e.g. 24-hourly or weekly) by provision of pondage at some point on the primary canal, if a suitable site can be found.

To reiterate, the key question determining whether a demand-responsive type of system would be workable is whether cultivators would exercise restraint in withdrawals during periods of restricted supply. If not, and in the absence of effective metering at the farm turnout, a demand system in the form of a level-top tertiary can become a "license to steal". The alternative system employing rotational supply within the tertiary command has a tertiary channel of much smaller capacity than the level-top. Even if the rotation breaks down and many cultivators divert from the tertiary simultaneously rather than singly, the total flow which can be diverted is strictly limited to the capacity of the tertiary intake.

However, a limited demand system does provide the ultimate degree of flexibility for irrigation of diversified cropping, in circumstances where the social structure and the character of the cultivator permit its use. The latter factors should be the starting point in consideration of possible application of a demand system in a smallholder situation.