In India, research into improving water resource harnessing is a continuous process. One of the main focuses has been to achieve better control of water consumption on surface irrigation schemes.
Under the impetus of the World Bank, the study and implementation of new techniques no longer targets controlling scheme discharges but rather the actual volumes delivered to farmers. This requires enhanced monitoring and control of hydraulic parameters throughout the scheme networks, i.e. stored volumes, flows and levels.
A first series of full-scale experiments was successfully implemented in Maharashtra using financial backing provided jointly by France and the World Bank. The technical options adopted for the scheme were original insofar as they associated proven techniques, some of which are at the forefront of modern industrial technology (centralised process control), and others much more traditional (float gates, duckbill weirs, baffle distributors). The originality of the concept adopted resides in the fact that the entire control system for the project was designed to provide an integrated and uniform solution.
The very positive results achieved by this pilot project have led the Maharashtra authorities to undertake a much larger operation (over 200 000 ha) on an extension zone. Henceforth, the Majalgaon project will be an essential reference for controlling water consumption on gravity irrigation schemes.
No system of canal control can be designed without considering the entire irrigation network on which it is to be installed.
Indeed, an open-channel irrigation network is a complex system of which the main canal is just one part. For instance, a 10 000 ha irrigated area will roughly consist of:
In practice, networks are operated globally, i.e. each supply configuration involves operation of all the regulators and field outlets. Steady flow conditions take time to stabilize (six hours in the case of a 10 000 ha network). Any change in the flow conditions must therefore be scheduled in advance in order to define the adjustments to be made at all the control points on the network.
Lastly, operation of the network has to overcome a series of constraints:
There are other constraints that we shall mention for reference only, such as problems of social order, safety and reliability of supply, scarcity, etc.
All these reasons explain why control has to be designed globally by integrating water management from the farm turnout up to the resource. An analysis has to highlight the specific constraints which particular canals will have to deal with, and the performances that it will have to achieve.
In the absence of an overall approach, there is a high risk of obtaining a theoretically effective control system which in practice is unsatisfactory and does not fundamentally improve overall network efficiency.
We will use the Majalgaon irrigation area modernization pilot project to describe the application of the different control techniques from design through to construction and initial monitoring of the system, including the tender phase. We will also describe the control facilities at the different levels from the supply network up to the main canal.
The Majalgaon project
The Majalgaon irrigation area is part of the Jayakwadi scheme (240 000 ha), one of the largest irrigation areas in Maharashtra. The limited water resources have reduced the first stage irrigation area to 58 000 ha, although it is planned to ultimately irrigate 119 000 ha.
The first phase of the scheme was financed by the World Bank and began in the 1970s. Its main components were:
The main and secondary canals (design discharge over 500 l/s) have been lined in order to reduce water losses.
Use of water before modernisation
In this part of India, rainfall is concentrated in irregular storms during July to October (monsoon). Rainfall averages 600 to 700 mm per year. The farmers are able to cultivate three crops a year:
The type of crops may be seasonal, such as sorghum, sunflower or peanut, or perennial, such as sugarcane. A large part of the area is cultivated during the rains, whereas the more profitable crops that require investments are grown during the two other seasons. As a result, the crops are varied over the irrigation area due to the different economic capacities of the farmers.
The area irrigation is operated by the Maharashtra Irrigation Department. The traditional method was based on on / off flow, i.e. a period during which the flow was close to the design flow for the canal followed by a period during which the canal was closed. The irrigation units were supplied proportionally by concrete structures which allowed no room for adjustments. The stream available at the unit was shared on a rotation basis.
Figure: Majalgaon system general layout
This type of organization led to sudden changes in the flow conditions which were difficult to control. The fixed facilities (i.e. structures contributing to flow control but without the possibility of adjustment, such as calibrated orifices) could not allow the volumes supplied to match the demand, since the needs were not uniform in space due to the extent of the unit sizes and the farm practices, which varied from one farmer to another.
The upshot was that the Irrigation Department decided to modernize the system.
In order to improve supply efficiency and to thereby have sufficient water resources available at the farm, the government of Maharashtra, on the advice of the World Bank, decided to improve the control method and facilities used in the area. In particular, the improvements included:
Such objectives can only be achieved using an appropriate control method. This is why the control project was developed as a pilot research and development project using French government and World bank funding. This project covers:
Control of the supply network
The four distribution canals break down into two lined canals which supply 1500 ha and two earth canals which supply 300 ha.
The aims of each control system on these canals were defined with the Irrigation Department to ensure accurate measurement and adjustment of the flows while allowing operating flexibility. These terms can be more explicitly defined as follows:
To satisfy these constraints, sharing of water between the four sectors takes places as follows:
All the equipment installed on the distribution networks is manufactured locally.
To guarantee a flexible supply, the canals must be operated at different flows corresponding to the users' needs. The structures set up have to guarantee reliable supply even during transient flow conditions.
To do this, upstream control relies on the association of two types of equipment:
Baffle distributors guarantee that the flow is controlled within ±5 percent even when the level in the canal varies within a range from 15 cm (for structures whose design flow is less than 100 l/s), to 1.10 m (when the structure discharge is above 5 m3/s).
These baffle distributors are unrelated to the downstream conditions and associate three successive flow discharge conditions to keep the flow nearly constant when the level varies upstream of the structure.
The flow is adjusted by opening or closing small gates of different widths (the flow released is proportional to the width of the gate), so that the required discharge is obtained through the opening of a proper combination of gates.
Duckbill weirs are very long-crested weirs constructed downstream of flow control structures. These weirs have a V or W shape to maximise the crest length. This configuration allows them to control the water level in the canals. For example, the level can be kept within a range of 15 cm when the flow varies between nil and maximum by constructing weirs with one meter crest length per 100 l/s flow.
The combined use of these two facilities gives the operator total control of the adjusted and delivered flows. The irrigation schedule can be defined in advance based on the requirements of users and implemented by adjusting the control facilities at the different points of the irrigation network.
Control of the Ganga Masla Branch canal
The Ganga Masla canal is a branch on the network. It supplies 20 secondary canals. The propagation time for a flow modification between the head and the downstream end of the branch canal is two hours. It is long enough to allow a single team equipped with a four-wheel drive vehicle and two-way radio to operate this canal.
The upstream control principle is applied on the Ganga Masla canal by using baffle distributors associated with duckbill weirs to control the levels on the one hand, and flow limiters associated with spillways on the other. The following structures have been constructed and installed on the canal:
Most of the hydro-mechanical equipment (in particular AVIO gates, baffle distributors) has been imported from France.
Main canal control
Upstream control cannot be used on the Majalgaon Right Bank canal in view of its size: the number of users, the number of structures and the canal natural transit times prevent control based solely on forecasts of distribution without risking shortages at off-takes and excess releases at the dam.
Computerized centralized control, or dynamic regulation, has therefore been used on this canal. The structures are controlled by software installed at the general control centre. This associates anticipation (open loop) of demand based on irrigation programmes prepared each month and real-time adjustments (closed loop) after comparison of the volumes present in the canal with target volumes (needed to correctly supply off-takes while maintaining a safe level in the canal).
The canal, initially designed for a discharge of 83 m3/s, is currently used at 50 m3/s. This results in over-capacity of the canal in volume. This surplus capacity enables the control system to cope with significant differences between the scheduled adjustments and the actual off-takes. The additional volume required is initially taken from the water stored in the canal before the control system adjusts the settings to meet actual needs. The same applies to the opposite situation, when the storage capacity is used for storing excess volumes of water.
These additional storage volumes authorise longer periods between successive adjustments. The system can then operate automatically or manually (manual operation of gates) depending on the availability of the equipment.
The architecture of the remote control system comprises the following:
At regulators (10):
At the general control centre:
Operation of the system is based on these three types of canal. The staff employed on the distribution networks and on the Ganga Masla Branch canal before modernization have been kept on. The organization follows a cycle which is repeated every 15 days or every month as follows:
The main canal is operated and maintained at three levels:
The equipment was commissioned in December 1996. Since then three missions of 15 days each by three Société du Canal de Provence engineers have been organized to assist Irrigation Department personnel taking over the entire system.
Problems have been solved at several levels, some examples of which are given below:
With these problems behind them, the Irrigation Department has sufficient control of the system to start it without assistance by making a few modifications to equipment, and ensure operation of the canal during an irrigation season.
On the Ganga Masla branch canal and the supply networks, structures have been executed by applying new construction methods to scrupulously comply with the dimensions and elevations established at the design stage. The principles behind operation of the structures was explained to the Irrigation Department personnel and to farmers.
After more than one year of operation, farmers are appreciative of the flexibility and accuracy which these new systems have brought to irrigation. They are asking for an extension of the project to neighbouring networks so that all the area can benefit from these improvements.
This case illustrates the synthesis which needs to be developed to install an effective canal control system. Indeed, it will be necessary
The pilot project has demonstrated that the approach proposed can be adapted to the conditions prevailing in India. The first results have highlighted the improved efficiency which can be expected from this approach and motivated the Irrigation Department to pursue their efforts to reorganize the irrigation area. As a result, the department plans to extend this method of operation to other canals on the Jayakwadi area. The second phase envisaged will confirm and detail the organization implemented to make best use of the improvements of the structures.