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Automation and water accounting in irrigation systems

Principles of automation of water supply in irrigation systems

Irrigation system is a compound complex of hydraulic structures intended for water supply to certain plots at the designed time and in the required quantity. This problem is complicated by that the water supply mode changes too frequently and hence requires rearranging the operation of the structures in the course of their usage. Consequently, almost continuous observation of a number of units and particular structures is needed. The problem consists in that the distances between the particular units and structures are considerable.

At present, irrigation systems engages automatic control of hydraulic structures and devices by using electronic, radio and remote control devices. The means for automatic control of water supply at particular hydraulic structures are represented by a great number of instruments and devices recommended by different authors; some of them can be applied for wide usage, but need to be improved further.

The main objects of measurement at every hydraulic structure are water level and discharge. The water measurement capacity of a hydraulic structure usually provided for by the design, allows measuring water level and calculating water consumption during operation. However, the degree of the difficulty of such measurements and calculations may differ from case to case.

At free outflow from under a sluice gate or through a spillway, the water discharge can be easily determined. To determine water discharge it is enough only to know the water level before the sluice gate or spillway and the sluice gate opening size. At submerged outflow in the similar case, the downstream water level is also should be known; therefore estimation of discharge becomes more complicated and the accuracy becomes lower because a variable coefficient of submergence is inputted.

The gates on the structure must be handled to maintain the discharge passing through the structure during operation steady. To automate this process, a number of various designs of water gauge facilities and devices the operations of which is based on well-known hydraulic principles have been worked out.

With a Tainter gate, when the resultant water pressure flows through the geometric center and the real gate rotation axis does not coincide with the geometrical axis, a moment appears that facilitates opening or closing the Tainter gate depending on the position of real rotation axis in respect to the geometrical center: if the real axis is located lower than the geometrical one, the moment of force of hydrostatic pressure facilitates opening the gate; if it is higher, the force of hydrostatic pressure tends to press the gate to the apron.

These as well as many other principles at one or another embodiment are often employed in different automatic devices and yield good results during operation.


Automation of irrigation systems. Degree of water distribution automation

The word automation implies implementation of hands-free technological operation. However, the scale of automation may vary over a very wide range. For example, the following can be automated:

  • some part of the process of measurement of a certain parameter of the object;
  • the whole process of measurement of one parameter or a set of parameters of the object;
  • an engineering procedure at one object as a whole;
  • system of objects, complex of the systems of single-type objects;
  • a sector of the national economy (on the scale of any region, republic or country in whole), etc.

At that, it may happen that not all of the engineering procedure operations or objects components become automated. Therefore, the concepts of automation degree and stage have been introduced. Automation degree can be partial, complex, and overall.

With regard to the process of water distribution in irrigation systems, automation degree has the following differentiation.

Partial automation covers only some of process operations or system elements. Consequently, water distribution automatic control process does not lock in the system. At that, in particular system components, e.g. at headworks, an automatic control process can be implemented in full with close cycle; however since system-wide water distribution is considered, the automation deems to be partial.

Complex automation consists in the automation of the entire package of water distribution operations, apart from control operations. The control process completes through the operator. In case of need, the water consumption mode is set and changed by a dispatcher independently from permanent operating personnel. Acquisition of the information about the condition of a controlled unit and transmission of control instructions are performed by using remote control devices.

Overall automation implies optimal automatic execution of the water distribution process is performed by computing and controlling machines without direct human input or participation of a dispatcher with special devices.

The final aim of the automation of any engineering procedure is to achieve top gear, viz. overall automation. However, in real conditions automation is implemented step by step.

Automation of the water distribution process in irrigation systems depending on the equipping with automatic devices can be divided into the following stages:

Stage I (partial automation): structures are equipped with local automation equipment (automatic controllers or local programming units) for controlling the required parameters (levels, discharges) as well as with control and measurement instruments. The permanent operating personnel carry out change and monitoring of the water distribution regime. The dispatch office personnel works out a water consumption regime. The communication between the dispatcher and operating personnel (for acquisition of data and transmission of control commands) is provided by means of telephone, radio, or special messengers.

Stage II (partial automation): remote centralized control devices are supplemented in addition to the available equipment of the first stage. Automated data convey to the dispatching station enhances the control efficiency and allows carrying out systematic control of the water distribution process.

Stage III (complex automation): here, the second stage of automation is additionally equipped with centralized remote control devices. Line operating personnel is not involved in the control of the structure operation. All the structures are automated. The control process is closed; it is completed through a dispatcher. The dispatch office personnel processes the information received from remote control devices, determines the optimum regime of water distribution, forms control instructions and communicates them to local automation devices.

Stage IV (complex automation) differs from the third stage by the use of computing technique to help the dispatcher in data processing and identifying the optimum water distribution regime. The decision concerning a mode change and transmission of control instructions is taken by the dispatcher.

Stage V (overall automation): the water distribution process is implemented without human input, i.e. automatically by means of controlling machines. Thus, a higher automation step is achieved by step-by-step equipping of irrigation systems with remote control devices. Local automation devices (hydraulic automatic controllers and other devices) serve as the basis for the implementation of automatic control.

However, the most important thing consists in determining advisable degree of automation with a glance of specific characteristics of the technological procedure.

The main feature of irrigation systems is associated with a great number of water distribution hydraulic structures scattered over a large territory, which are to be automated. Overall automation of the water distribution process in such systems, at least at this stage, is out of the question. So far it is unfeasible from the technical point of view.

The determinant factors for selecting the degree of water distribution automation in irrigation systems are technical and economic efficiency as well as the level of training of the operating personnel.

For different sections of irrigation systems different degrees of automation should be provided for. So, in mountain irrigation systems (at the current level of technological expansion), Stage I, partial automation, is quite acceptable for the on-farm network; Stage II, for the inter-farm network; and Stages III and IV (overall automation), for large main canals, including head water intake unit. At that it is understood that with the lapse of time the automation degree of each section can be raised in case of need by additional equipping with relevant automation hardware.

Therefore, when implementing any stage of the automation of the irrigation system or its sections, it is necessary to foresee the possibility of their transition to a higher step of automation without reconstruction. However, the expected highest automation step at a given section should be exactly determined to avoid unwanted complication of the object. For example, it hardly makes sense to control and operate from a water outlet to a temporary irrigation ditch (at surface irrigation methods or at sprinkling irrigation by mobile machines) equipped with an automatic hydraulic unit of constant discharge the dispatching station, because control of such a water outlet reduces to its commissioning at the beginning of a water application process and its turning off when the water application is over; this operation can be done by an irrigator itself. There is a reason behind this example, because such structures constitute the majority in the water distribution network.

Source: Kriulin, K.N. Land reclamation: operation and automation
of irrigation and drainage systems. Saint Petersburg, Publishing House
of the Saint Petersburg State Polytechnic University, 2004

Selected bibliography