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| Monitoring Water Taken for Irrigation | In New South Wales, the extraction of water from floodplains is controlled by the Department of Industry and Environment, through Water NSW. The whole process is undergoing change to ensure the process is managed in a sustainable fashion which is in turn fair for all water users. Under the new rules, those who take water from a floodplain under license, must measure their take. NSW Water collect the data from all storages and can then aggregate it up to gain a picture for what is happening across a region or catchment. TOIP’s ML417M1 Local Intelligence Device (LID) is available as a Primary or Secondary measurement device. | |||
| Storage Volume | Life is simple for farmers who have water delivered to their door in a pressurised pipe. The amount of water used can be measured accurately and economically using any of a wide range of flow meters. But where irrigation water is collected from a river, stream or floodplain and then pumped into an on-farm storage for subsequent use, the picture is far more complex. In a swimming pool, with a uniform rectangular shape and vertical sides, it is easy to calculate the quantity of water it contains: multiple the width by the length and then multiply the result by the depth. When the water is in an on-farm storage the principle is the same, but the process is more involved: the sides are not vertical and the length and width change with level. The solution traditionally used has been to use a surveyor to accurately measure the profile of the storage, giving the dimensions at specific heights and saving them - along with the amount of water calculated in each slice - in a rating table. The rating table may also be called a storage curve. Rather than rely on traditional surveying techniques, the storage curve can also be obtained using LIDAR. In another simplification, rather than rely on a lookup table with hundreds or thousands of lines, the storage curve can be expressed as a formula formulas usually as an exponential equation. |
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| Storage Meters | The term Storage Meter is used to describe what is normally called a water level sensor. But in this application it must be a sensor which can perform at the accuracy required to give a reliable volume figure. Water NSW has a process under which manufacturers can submit their sensors for approval as storage meters. The list of approved storage meters can be viewed on the DPIE web site or on the sites of Irrigation Australia Limited (IAL) and the Australian Hydrographers Association (AHA). |
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| Hydrostatic and Radar Level | Although there are a wide variety of level sensors, those which can offer the accuracy required for use in this application typically come from the following families: Radar Level Sensors: these are mounted above the water surface on a gantry or platform. The sensor sends short bursts of high frequency signals down to the water and these are reflected back when they reach the surface. If the time the pulse takes to travel out and back is measured, it can be used to determine the distance to the water. As wind will create ripples on the water surface, radar sensors must employ averaging to get rid of the noise produced by the ripples: the sensors will take a series of measurements and then take the average. |
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| Submersible Sensors: these may also be called hydrostatic level sensors. They are installed on the bottom of the storage and employ a pressure sensor to measure the weight of the column of water sitting above the sensor. The pressure can then be converted to an equivalent height. Submersible sensors are not impacted to the same degree by ripples. Regular maintenance is needed to ensure the sensing ports in the sensor do not become blocked. Pressure sensors may be one of two different types: vented and un-vented. Vented sensors have a breather tube which vents to atmosphere, via a small tube which runs through the cable. The vent ensures atmospheric pressure is applied to one side of the sensor element and water pressure to the other. The difference between the two is the pressure exerted by the column of water above the sensor. This is then converted to an equivalent depth. The breather tube makes the cable bulkier and more expensive. Un-vented sensors, although easy to work with, are not permitted under the project guidelines. |
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| Local Intelligence Device (LID) | On its own, the storage meter or sensor is of no use - a bit like having a seat but no car. What is needed is something which can power the sensor, take the level measurement and store it. In the past, this was the job of a data logger. But today, these have been replaced by Remote Telemetry Units or RTUs - which in the language of the DPIE have become a Local Intelligence Device or LID. This unit will have its own power supply to drive the sensor, have electronics to read the sensor and memory to store the readings taken over time. It will then have some communications which is capable of sending the data to a remote computer system for analysis and display. The most common - and most useful - method for data transmission is to send the data over the mobile telephone network. The computer system to which the level readings are sent, is referred to as the Data Acquisition System or DAS. |
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| Just as vendors of storage meters must submit sensors for approval against the standards laid down by DPIE, LIDs must too be tested against a set of strict rules. These govern the type of sensors which can be connected, how long the unit can run without charge, how many readings can be stored and how and when they are sent. LID vendors can take one of two approaches: firstly by offering an integrated package of LID and storage meter or secondly by offering a flexible LID which can be used with a range of storage meters. Regardless of which approach is taken, the LID must be installed by a Duly Qualified Person (DQP) who has been certified as competent by either IAL or AHA. |
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