Selection of operation methods in canal irrigation delivery systems

P. Ankum, Associate Professor, University of Technology, Delft, The Netherlands

SUMMARY

On-farm irrigation scheduling requires a canal irrigation delivery system that provides water at the expected time, rate and duration. The selection of the proper operation method for this delivery system, i.e., proportional control, up- or downstream control, BIVAL or ELFLO control, is quite complex. The selection process may follow three iterative steps: (i) the operational objective of the delivery system has to be defined; (ii) an initial selection of the operation method follows directly from this operational objective; and (iii) an evaluation has to be made whether the actual features of the selected operation method are still acceptable. There is no ideal method of operation applicable to any system anywhere. The proper selection requires inputs from all parties concerned, as well as a multidisciplinary approach.

Dual-managed irrigation systems have two levels of management: (i) the distribution system or 'tertiary unit', where the water users are responsible for the irrigation scheduling, and (ii) the delivery system or 'main system', where an operation and maintenance (O&M) agency is responsible for the delivery scheduling. The operation of a delivery system (main system) has to meet the requirements of the distribution system (tertiary unit) at the 'tertiary offtake', where the responsibility over the irrigation water changes hands.

The on-farm scheduling is proposed by the water users, the agronomists, economists, and others, but has to be decided in a multidisciplinary forum. This agreed on-farm scheduling dictates not only the required scheduling in the canal irrigation delivery system, but also the required operation method of the delivery system (up- or downstream control, etc.).

OPERATION METHODS

Classification of operation methods

At present, different classifications for the basic operation methods in delivery systems are used by different authors. For instance, ASAE proposes for canal control (Burt and Plusquellec, 1990): (i) conventional upstream control; (ii) automatic upstream control; (iii) downstream control with level top canals; (iv) downstream control on sloping canals. ICID proposes for canal system operation (Goussard, 1993): (i) upstream control; (ii) close downstream control; (iii) distant downstream control. The World Bank proposes for control strategies (Plusquellec et al., 1994): (i) proportional control; (ii) adjustable flow-rate control; (iii) upstream control; (iv) downstream control; (v) remote monitoring; (vi) remote control.

It seems more practical to adopt a classification based on the location of the water level that is maintained by gate regulation. Thus, there are five basic operation methods (Ankum, 1993b):

· proportional control, in which the incoming flow is split according to predefined ratios. Thus, there is no constant water level in a canal reach;

· upstream control, with a constant water level at the upstream side of the regulator as the operational target. The outflowing discharge from the delivery system can be changed only after an intentional change of the inflowing discharge from the river by a system manager, a subsequent diversion at the bifurcations and after a certain 'time lag'. The time lag is related to its (negative) dynamic canal storage;

· downstream control, also called 'downstream control on level-top canals', with a constant water level at the downstream side of the regulator as the operational target. Such a delivery system is 'responsive' and would provide immediately the requested outgoing discharge because of its (positive) dynamic canal storage. Level-top embankments are required;

· BIVAL control, also called 'constant volume control', with a constant water level in the middle of the downstream canal reach as target. Hence, there is no effective dynamic canal storage in a canal reach. BIVAL control resembles downstream control and is also 'responsive' as it provides immediately the requested outgoing discharge. However, the canal embankments can be kept at a lower level, while telemetry and electro-mechanical gates are needed;

· ELFLO control, also called 'downstream control on sloping canals', with a constant water level at the other end of the downstream canal reach as target. It requires telemetry and electromechanical gates. ELFLO control is 'responsive' to changing outgoing discharges, like in downstream and BIVAL control. On the other hand, ELFLO control resembles an upstream controlled system as the embankments are similar, and as the change in outflowing discharge will only be effective after a certain 'time lag' because of the (negative) dynamic canal storage.

It is also possible to use different control methods at one location, such as composite control, where the gate is normally regulated under downstream control, but may change to upstream control for certain conditions (Goussard, 1993). Moreover, it is often very effective to use different operation methods at different locations in the main system, e.g., downstream control in primary irrigation canals and with upstream control at secondary irrigation canals.

Delivery system management and regulation of structures

The term delivery system management refers to the management of the delivery system as a whole, and deals with matching the outgoing discharges with the inflowing discharges from the river. The main system management can be (Ankum 1993b):

· no (day-to-day) system management, when regulation of the structures is not possible and/or required. It is applied in delivery systems under proportional control with a fixed splitting of the discharges, and in upstream controlled systems with standing orders for the water delivery during the whole irrigation season;

· central system management, when a 'water operation centre' has to match the incoming discharge from the river with the required delivery schedule. It is applied in upstream controlled delivery systems;

· responsive system management, when the delivery system itself adjusts to the changing outflowing discharges. It is a feature of downstream, BIVAL and ELFLO control (as well as composite control).

The term regulation refers to the actions of the controller, and focuses on maintaining the water level or the discharge at the target value. Regulation can be done manually by a human operator, by the water forces on a hydro-mechanical gate, by a float with programmable logic controller (PLC) of an electro-mechanical gate, etc. Moreover, regulation can be done by a passive regulator, such as a long-crested weir.

SELECTION PROCESS FOR THE OPERATION METHOD

Procedure

The selection of the most appropriate operation method for an irrigation delivery system, such as proportional, up- and downstream control, is quite complex. Moreover, the consequences of the selected operation method are often not well understood. Therefore, the selection is usually based on past experience. A systematic selection process is recommended here.

The systematic selection process for an operation method may follow iterative steps:

· the operational objective of the delivery system has to be defined first;

· an initial selection of the operation method follows directly from the operational objective, as will be discussed below.

Certain features of the control method are already determined by this initial selection, such as the hydro-dynamic performance, the design and constructional aspects, the operation and maintenance requirements, and some other aspects;

· an evaluation has to be made whether the features of the selected operation method are acceptable. If not, the original operational objective of the delivery system has to be revised, and another operation method may follow.

Operational objective

The operational objective of a delivery system is specified by three fundamental factors:

· the decision-making procedure on the water delivery to the tertiary offtake, i.e., 'who' decides on water delivery to the tertiary unit;

· the method of water delivery to the tertiary unit, i.e., 'how' that water is delivered to the tertiary unit;

· the method of water distribution through the main system, i.e., 'how' the water is distributed through the main irrigation system.

The decision-making procedure on the water delivery to the tertiary unit has to be selected first. Three options are possible. Under 'dictated delivery', the water user associations of the tertiary unit have no say in the water delivery. Alternatively, water user associations may request a changed water delivery, which is effected after endorsement by the water operation centre, and after some time ('arranged delivery'). Thirdly, the users themselves may decide on the water delivery, which is supplied immediately or after a time-lag ('on-demand delivery').

Also, there are three options for the delivery method to the tertiary unit. The traditional delivery method to a tertiary unit is 'split flow', where the available flow is diverted equitably throughout the system by 'proportional structures'. The second delivery method, 'intermittent (on/off) flow' to the tertiary units, is often applied in schemes under dryland crops, where a 'unit flow' is rotating between the individual fields. The choice of such an intermittent delivery is irrespective as to what decision-making procedure has been selected. A third delivery method, the 'adjustable flow' to the tertiary units, is often applied in schemes under paddy, when the peak discharge is required, e.g., during land preparation, and the discharges are gradually reduced during the off-peak period.

FIGURE 1 - The selection of the operation method in canal irrigation delivery systems

The distribution method through the delivery system can either be based on 'split flow', 'intermittent flow' or 'adjustable flow', while also 'rotational flow' can be applied when the (sub)secondary offtakes are intermittently supplied by the same 'rotating' flow.

It is obvious that only a few options of delivery scheduling can be combined into the operational objective of a delivery system (Figure 1).

The initial selection of the operation method

The logic choices for the operation method follow from the above operational objective (see also Figure 1):

· The operation method for dictated delivery is either proportional control or upstream control. Proportional control requires only structures throughout the whole system that split the flow proportional into fixed ratios. Upstream control is required for an intermittent and adjustable flow to the tertiary units.

· The operation method for arranged delivery is upstream control with a central system management. Downstream, BIVAL and ELFLO control can also be operated on a basis of arranged delivery when the water availability is less than the accumulated water needs. Then, the water requests from the individual tertiary units are checked first against the water availability, and the actual delivery is only done after endorsement by the O&M agency. Thus, a time-lag is introduced.

· The required operation method for on-demand delivery must be responsive. Downstream control and BIVAL control meet these requirements immediately. An alternative operation method might be ELFLO control which is also responsive, but will require a time-lag for the adjustment of the dynamic canal-storage.

EVALUATION OF THE OPERATION METHOD

Evaluation criteria

The criteria for evaluation of an initial selected operation method will vary for different irrigation projects. Also, different weighting factors may be applied. Generally, an operation method may be evaluated on aspects such as (see also Table 1): hydro-dynamic performance, hydrological and geographical setting, design and construction, operation and maintenance, economy, political and social aspects.

Evaluation on the hydro-dynamic performance

Systems with 'negative' dynamic canal storage (upstream, ELFLO control) have to adjust first the in-canal storage before a steady state is reached. The filling of the negative storage leads to a time-lag between the water release at headworks and the actual water availability at the tertiary offtake. Thus, these systems have long response time T_r during increasing discharges. The reverse will happen during decreasing discharges, as water flows out of the dynamic storage while it is not needed at the tertiary offtake. This is the operational loss.

In downstream control, the 'positive' (operational) in-canal storage provides immediately the increased discharge to an offtake, while the decreasing discharges can be saved in the canal reach for later use. In BIVAL control, the 'positive' (operational) storage always equals the 'negative' storage of the reach, so that such a canal functions like a pipeline without dynamic storage. Thus, systems under downstream control and under BIVAL control do not have time-lags nor operational losses. Efficient water use may not be a major factor in irrigation systems with re-use of drainage water, or when other systems depend on this return flow.

Irrigation systems with day irrigation only may have a low overall efficiency when the flow is not halted during the night. Systems with positive in-canal storage (downstream and BIVAL) will be suitable for an intermittent supply of 12 hours. The suitability for day irrigation only might be less relevant in run-of-the-river systems than it is for systems on storage reservoirs.

Evaluation on the hydrological and topographical setting

Operation methods under responsive management (downstream, BIVAL, ELFLO) fail during water shortages in the river. However, also upstream controlled systems may suffer from serious water problems in the tail-ends during unexpected water shortages, unless 'gate-proportional' operation is applied (Ankum 1993c).

TABLE 1 - Evaluation of operation methods in irrigation

Evaluation criteria		Proportional control	Upstream control	Downstream control	BIVAL control	ELFLO control
hydro-dynamic performance
	· short response time	na	- -	+ +	+ +	- -
	· high operational efficiency	na	- -	+ +	+ +	- -
	· high overall system efficiency	- -	-	+ +	+ +	-
	· suitable for day irrigation only	- -	-	+ +	+ +	+
the setting
	· performance under water shortages	+ +	+	- -	- -	- -
	· performance under sediment loads	+ +	+	- -	- -	+
	· applicable at steep terrains	+ +	+ +	- -	-	+ +
design and construction
	· simplicity structures and regulators	+ +	+	-	- -	- -
	· reliability structures and regulators	+ +	+	-	- -	- -
	· application of local materials	+ +	+ +	- -	-	-
	· operation without telemetric system	+ +	+ +	+ +	- -	- -
	· operation without electric power	+ +	+ +	+ +	- -	- -
operation
	· without a water operation centre	+ +	- -	+ +	+ +	+ +
	· without discharge measurement	+ +	- -	+ +	+ +	+ +
	· no operators at secondary offtakes	+ +	- -	+ +	+ +	+ +
	· no operators at tertiary offtakes	+ +	- -	-	-	-
maintenance
	· dependence on canal maintenance	+	+	- -	- -	+
	· maintenance at a low technology	+ +	+ +	-	- -	- -
economy
	· low costs of earthwork	+ +	+ +	- -	-	+ +
	· low costs of structures and gates	+ +	+ +	- -	- -	- -
	· local currency only	+ +	+ +	- -	- -	- -
political and social aspects
	· robustness against water-theft	+ +	+	- -	- -	- -
	· equity of water distribution	+ +	na	na	na	na
	· equal disturbances in supply	+ +	+ +	- -	- -	- -
	· flexibility of water delivery	- -	-	+ +	+ +	+ +
	· use of dependable rainfall	- -	-	+ +	+

na = not applicable
+ = good
+ + - very good
- = poor
- - = very poor

High sediment loads prohibit operation methods with stagnant water in the canal reaches, such as downstream control and volume control. Also upstream and ELFLO control will be a less suitable choice. The application of a sandtrap at the headworks may solve these sediment problems.

Evaluation on operation and maintenance requirements

Water operation centres are required for all upstream controlled irrigation systems where a 'flexible' supply of water should meet the changing demands. However, worldwide, it appears to be very difficult to create a well-functioning water operation centre. The question may arise whether more efforts should be made to establish water operation centres, or whether they be avoided by selecting other operation methods.

Field staff for system operation are especially required in upstream controlled systems, where they play an essential role. The flow diversion through these systems is frequently adjusted by the water operation centre, and the discharge regulators and the water level regulators have to be adjusted accordingly. The unsteady state of the system during the adjustments make proper gate settings a very time consuming activity. Discharge measurement throughout the system appears to be a burden for the day-to-day management, and is rarely well achieved.

Deferred canal maintenance on canals causes the roughness to increase, and leads to a steeper gradient of the water line in the canal reach. For proportional, upstream and ELFLO controlled systems, the tailwater level at structures becomes higher than the design water levels. This may not disturb the diversion of flow through the system as long as sufficient headloss and freeboard remain available at the regulators. Systems under downstream control and under BIVAL control may react differently. Here, the downstream water levels are kept constant, and the steeper water line 'rotates' around this target level. Thus, the upstream water level at structures becomes lower than the design levels. This causes the velocity to increase, and hence the friction losses in the canal, which leads to a further lowering of the water levels. It means that systems under downstream or BIVAL control are more vulnerable to poor canal maintenance.

Evaluation on economical, political and social aspects

The cost of the operation method will be a major determining factor in the selection. The creation of positive in-canal storage means that the embankments have to be raised above the design water level. This involves higher costs, especially for the longer distances between the regulators. The automation of the gate regulation as necessary under downstream, BIVAL and ELFLO control seems expensive in comparison with the manually operated gates. However, these incremental costs are only a minor component of the total project costs. The costs of the different control methods should be determined in the pre-design of the system, and have to be evaluated against the benefits and other aspects, such as organization, efficient water use, flexibility, and need for foreign currencies.

The political and social aspects might have an essential say in the ultimate selection of the operation method. Aspects may include: the robustness against water-theft and abuse, the equity of seasonable water distribution ('protective' irrigation), the equal distribution during disturbances in supply (upstream controlled systems), the flexibility of water delivery (downstream controlled systems), the use of 'dependable rainfall'.

THE ULTIMATE SELECTION

Observations on the ultimate selection

It is obvious that the selection of an appropriate operation method for delivery systems is not easy, and that all operation methods have their pros and cons (see also Table 1):

· Proportional control is the most simple and will suffice only in certain conditions of mono-cropping, extensive irrigation or when drainage water can be re-used. Systems under proportional control perform without system management, and react slowly to changes to supply. Equitable distribution is guaranteed for all discharges, when proportional structures with sufficient headloss are utilized.

· Upstream control is widely applied. These systems require strong central management, which is often hard to effect. The operation of the upstream controlled systems encounters difficulties because of the response time, while the operational losses may become high due frequently changing discharges.

· Downstream control does not require a central system management, and solves the above problems in response time and operational losses. However, earthwork and structures can become very expensive, while the in-canal storage, and thus the earthwork, is normally larger than required for good operation.

· BIVAL control is a good alternative to the downstream control at sloping canals. However, telemetry and electro-mechanical gates are introduced.

· ELFLO control is responsive, and needs even less earthwork than BIVAL control. However, it requires telemetry and electro-mechanical gates, while its in-canal storage behaves as in upstream control; with its response time and operational losses.

· A combination of the above control systems may sometimes lead to the optimum control method. For instance, it is possible to combine different control methods throughout the delivery system, or to apply regulating reservoirs or storage reservoirs.

Changing operational objectives

The selection of the operation method on existing systems may become quite complex when the operational objective has changed gradually. A mismatch between the changed operational objective and the operation method of the delivery system often occurs.

The objective of equitable water distribution is best obtained by proportional control. However, such a system is often not very efficient in water use. Efforts to improve the efficient use of water by applying gates in the structures will transfer the system into upstream control. The new objective of water efficiency might be obtained within the tertiary unit, but at the same time it introduces (new) operational losses in the delivery system. The original concept of equitable water distribution is deliberately eliminated and should no longer be used as a criterion to judge the performance of the central system management.

Arranged delivery in upstream control is obtained only when the central system management is active. However, it is often felt that such a water operation centre does not fit in the prevailing technical and social context. An upstream controlled system without central management can never meet an arranged delivery. It is just operated on a dictated delivery basis with standing orders, or is operated in a chaotic manner. Proportional control (dictated delivery) or ELFLO control (demand delivery with responsive system management) would often lead to better results.

Many 'supply-based' systems (dictated delivery) under proportional and upstream control have been developed for protective irrigation (e.g., India, Pakistan). These systems cannot easily be reshaped into 'crop-based' systems (arranged and on-demand delivery). Firstly, the relative capacities of these systems have to be increased substantially (e.g., from 0.3 l/s.ha to 1.2 l/s.ha), and the water availability has to be ensured. Secondly, it has to be acknowledged that upstream control for arranged delivery has to rely on a strong central management, while a slow response time and operational losses are unavoidable. Other alternative control methods with responsive management are normally not applicable. Downstream control is often prohibitive because of extensive reconstruction works on the canal banks. BIVAL control would reduce these reconstruction works, but it introduces telemetry and electro-mechanical gates. ELFLO control would even avoid any reconstruction works, but it requires telemetry and electro-mechanical gates and it performs with a time-lag and with operational losses.

CONCLUSIONS

On-farm irrigation scheduling is only possible when the delivery system provides the irrigation water at the expected time, rate and duration. The selection of the proper operation methods for these delivery systems is quite complex.

There is no ideal method of operation applicable to any delivery system anywhere. For instance, proportional control might be the best choice for irrigation scheduling focusing on equity. Upstream control might be considered for high efficiency at on-farm level. Downstream control provides a flexible irrigation scheduling at on-farm level. However, all operation methods have their weaknesses.

The selection process may follow the three iterative steps outlined above. The ultimate selection is closely related to the required irrigation scheduling. It requires inputs from all parties concerned, in a multidisciplinary approach.

REFERENCES

Ankum, P. 1993a. Operation specifications of irrigation main systems. In: Proc. 15th Congr. on Irrigation and Drainage, The Hague (Vol. 1A, question 44). ISBN 81-85068-34-8. ICID, New Delhi. pp. 119-130.

Ankum, P. 1993b. Canal storage and flow control methods in irrigation. In: Proc. 15th Congr. on Irrigation and Drainage, The Hague (Vol. 1B, question 44). ISBN 81-85068-35-6. ICID, New Delhi. pp. 663-679.

Ankum, P. 1993c. Some ideas on the selection of flow control structures for irrigation. In: Proc. 15th Congr. on Irrigation and Drainage, The Hague (Vol. 1B, question 44). ISBN 81-85068-35-6. ICID, New Delhi. pp. 855-869.

Burt, C.M. and Plusquellec, H.L. 1990. Water delivery control. In: Management of Farm Irrigation Systems. ASAE Monograph, The United States. pp. 373-423.

Goussard, J. 1993. Automation of Canal Irrigation Systems. International Commission on Irrigation and Drainage, ICID, New Delhi.

Plusquellec, H., Bun, C. and Wolter, H.W. 1994. Modern Water Control in Irrigation: Concepts, Issues and Applications. World Bank, Technical Paper No. 246. Washington D.C.