Water accounting in surface irrigation network
Water metering methods
Water flow rate and runoff volume can be measured by direct and indirect methods.
Direct (in-situ) method of measurement (volumetric and by gravimeter) is applied for check measurement of low water flow rate, calibration of precision water metering instruments.
In the irrigation and drainage practice, indirect methods are mainly used, which can be broken down into the following groups:
- the metering methods that use the water flowrate determination methods from the components of the watercourse channel section and flow rate (channel method);
- the methods based on the hydraulic principles of water discharge through an orifice or structure crest (hydraulic method); and
- the methods of direct measurement of the total flowrate in the stream cross-section (area) (electromagnetic, ultrasonic, etc. methods).
Channel water metering method
The main point of the channel method consists in the determination of water flow rate directly in the channel through the effective cross-section area and average flow velocity.
Water flow rate is calculated in the following order. The surface width is to be measured in the selected section line and verticals are marked out for measuring the velocity. If the canal width is less than 5 m, 3-4 verticals are to be taken; 5-6 verticals are to be taken if the width is from 6 to 20 m; at greater width, 7-8 verticals are to be taken. Depending on the water depth in a given canal, they measure 1-3 velocity values at each vertical. With three points, velocity is determined at the level of 0.2, 0.6, and 0.8 of the depth from the surface; with two points, at the level of 0.2 and 0.8 of the depth; with one point, at the level 0.6 of the depth, if it does not exceed 0.3 m.
An average velocity is to be defined, and then the water flow rate is determined. After finding the specified relation, water metering boils down only to the measurement of water level Н by a gauge and determination of flowrate (discharge) Q from the relation Q = f(H) as well as to periodic check measurements in order to check this relation.
The canal section where the flowrate is measured has to meet the following requirements:
- the water flow behavior in this place should be characteristic of a possibly larger canal section;
- the section should be rectilinear over a distance of (4-5)Csw (Csw means surface width of canal);
- the influence of the variable backwater of control structures (water check) on the canal must be low.
The channel flow measurement method is used for water metering at the reference hydrological stations of irrigation sources, at the head sections and balance metering points of main and other large canals as well as for calibration and checkout measurements. However, it is a very long-lasting process, and the flow measurement errors with channel deformation because of erosion or siltation may come to 10% and over.
Hydraulic (slope-area) water metering method
Water metering by the hydraulic (slope-area) method is carried out either by means of hydrometric facilities (retaining works, water checks, water-discharge facilities) available on the canals or by using special water-gauge facilities (devices).
The main point of this method consists in the creation of some barrier (resistance) on the way of a uniform water flow to overcome which the flow spends/losses a certain head. By measuring this head and effective water cross-section they calculate water flow by a relevant formula.
As a rule, to create conditions for measuring water flow they resort to flow constriction (restriction). When using the hydraulic structures, which are already built on the canal, for this purpose, they undergo calibration.
Calibration of hydraulic structures
The calibration method implies the finding out of the relationship between the water flow rate and basic components of the flow and structure. Only operable and correctly built hydraulic structures undergo calibration. Moreover, such structures should have permanent discharge nature (waterlogged, submerged, pressure, free-flowing) and smooth water breakthrough in the absence of pumps.
The calibration of hydraulic structures is performed by the hydrometric (stream-gauging) and hydraulic method.
The principle of hydrometric calibration method is based on the measurement of flow rate through the structure with different heads at its sill and following determination of the Q=f(H) relation. This method is similar to the method of channel water-metering in canals described above.
The hydrometric method can be used only in limited cases, viz. for open structures with permanent discharge regime and non-regulated ones, i.e. with no sluices: check drops; chutes; fumes; aqueducts; etc.
In all other cases, the hydraulic calibration method is applied; it consists in the specification of the relationship between the discharge through the structure and hydraulic parameters of this flow by the above-mentioned formula.
The purpose of the hydraulic calibration method is to find out the values of the discharge coefficient which depends on both the type and design characteristics of the structure and conditions of the discharge through it and quite often on the discharges passing through. Each structure has its individual values of this coefficient; therefore, adoption of tabulated values at water metering can result in serious errors. All the other elements of the formulae are easily determined by measurements on the site.
Both with the hydrometric and hydraulic calibration method, the discharge through the structure can be specified either by using propeller-type flowmeters or by measurement by using the water gauges installed on the canal.
The determination by using water gauges is simpler, more accurate and faster; however, they can be applied only at relatively low flow rates.
Water meters in the irrigation network
The water meters used in the open irrigation network can be divided into the meters foe measuring transit flows and facility for measuring the flow rates in water apportioning sections.
The first group includes weirs with thin wall (polygonal and curved/arched) and water-measuring sills.
Water-metering orifices, water-metering adaptors, resistors in pipes fit into the second group: rings, diaphragms, contractions, etc. Water-metering orifices and adaptors serve for water measurement in canals, while diaphragms and contractions are applied both on canals and in closed irrigation pipelines.
Source: lib.cef.spbstu.ru
