BIVAL control, also called 'continuous volume control', with a constant water level in the middle of the downstream channel as the target. ELFLO control, also called 'downstream control in inclined channels', with a constant water level at the other end of the downstream channel as the target. It is applied to distribution systems under proportional control with a fixed distribution of discharges and to upstream controlled systems with permanent orders for the delivery of water throughout the irrigation season.
Selecting the most appropriate operating method for an irrigation supply system, such as proportional, upstream and downstream control, is quite complex. If not, the original operational objective of the delivery system should be revised and another method of operation may be pursued. Under 'dictated delivery', tertiary unit water user associations have no role in water distribution.
An alternative control method could be the ELFLO control, which is also responsive, but requires a delay for the dynamic channel storage adjustment. Filling the negative storage leads to a time difference between the water release at main works and the actual water availability at the tertiary supply. The political and social aspects can have an essential say in the final choice of operating method.
Choosing a mode of operation on existing systems can become quite complex when the operational objective changes gradually.
Many 'supply-based' (dictated delivery) systems under proportional and upstream control have been developed for conservation irrigation (eg, India, Pakistan). These systems cannot easily be reshaped into 'crop-based' (regulated and on-demand delivery) systems. First, the relative capacities of these systems must be increased significantly (eg, from 0.3 l/s.ha to 1.2 l/s.ha), and water availability must be ensured.
BIVAL control would reduce these reconstruction efforts, but introduces telemetry and electromechanical gates. ELFLO control would even avoid all reconstruction work, but requires telemetry and electromechanical gates and operates with delay and operational losses.
Operation Techniques in Canal Systems
This is the least expensive type of distribution and covers a large part of the world's irrigation area, especially in older irrigation systems. The principle is that the irrigation module circulates within a block from one user to another, the duration of irrigation for each user depends on the irrigated area. Within a block the channel size is set in the module and upstream of the block is set in the continuous flow request of the command area.
This module is based on the user's ability to efficiently control water application at field level. With furrow irrigation the module can be somewhere between 25 l/s and 50 l/s, with basin or border irrigation this can go up to 100 l/s. The module is the same regardless of the size of the field, and the adaptation to water needs is obtained by adapting the duration of the turn to the irrigated area.
One block is designed so that at the end of the cycle, after the last user has been irrigated, the module is reissued to the first user. The size of the block is derived from the module's capacity to meet peak water demands. The regulated demand system was developed mainly in the USA, it is a good compromise between rotary and open access.
The dimensions of the channel are designed to allow little more than the maximum continuous flow. Users submit their questions to the manager in advance in various ways (currently telephone or internet) with a requested dismissal (Q) and time (start and duration). He then informed each user how the distribution would respond to the original request.
The operation consists of manipulating gates and structures to produce the agreed service and deliver it to the users. Irrigation system operations may seek to control discharge, volumes, or a combination of discharge and volume. Available storage is dependent on the variation of water depth in the system, and therefore discharge discharge should be somewhat independent of upstream water level, i.e. outlet structures (the delivery structures in this analogy) should have a low sensitivity.
Flow increase at head 2. Gate opening at the 1st turnout
New steady state3. Flow increase at successive
Flow decrease downstream 2. Gate adjustment
With BIVAL, the sensor is placed in the middle of the range, and with ELFLO, in the lower part. This parameter combines the adjustability of control structures and the degree of automation of operation. The difference is due to the exponent of the precipitation loss variable in the flow equation, 32 and 12, respectively (see sensitivity chapter).
This property is site-specific and highly dependent on the maintenance of the system and the discipline of the users. The response of a reach to any planned or unplanned disturbance to the input is related to the topography of the channel section. On-line storage works in two conflicting ways that are important to the time delay involved in operations.
In that case, the objective for operation is to keep the water surface at the discharge level of the side thrusters. The Bottom-Up (BU) operation consists of implementing gate adjustments from the tail of the systems. These estimates can then be used to identify approximate values for PTL at each cross-structure of the channel.
Main channel discharge control (DIC) is a specific procedure for controlling the outlet structure of intermediate reservoirs in STO subsystems. Operating procedures for the reaches upstream of the storage may be fixed frequency or time-delayed operation (TLO). This is characterized by operating each cross regulator to maintain a specified discharge downstream via a meter located downstream of the X-reg.
In the true downstream control, the downstream range is under the influence of the downstream controller, the flow is submerged1. If positive disturbances occur, water levels rise and part of the amount of unplanned disturbances can be temporarily stored in the canal areas. Any positive disturbance that occurs in the system causes an increase in the water depth in the channel and increases the effective delivery to the drainage.
For the Oversupply to Decrease and Undersupply to Decrease strategies, the goal is to increase the effective use of the disruptions by making additional online deliveries. The second is a Cut-Back (CB) of the main supply to compensate for additional water availability in the system due to the storage of the disturbance.