1 Introduction
1.1 BACKGROUND
Within its responsibility of providing policy advice to Government, MAF Policy has identified the need to:
"Promote sustainable farm and orchard management planning including collaborative development of best management practice guidelines, and to provide environmental indicators or reporting systems for monitoring soil quality, water quality, air quality, biodiversity and energy use."
Within the objective of facilitating resource management there is a need to promote sustainable irrigation, and an Operational Research Programme has been formulated to meet this need. A program was designed with the aim of developing procedures that lead to sustainable irrigation and to have those procedures adopted and used by the New Zealand farming industry.
A study to develop a set of indicators that farmers could use to monitor the sustainability of their irrigation management practices was funded by MAF Policy (MAF Policy Technical Paper 00/03). This study was followed by the development of best management guidelines for sustainable irrigated agriculture. (MAF Policy Technical Paper 00/05).
Following on from this, two projects, the first to test the measurement and use of the Indicators and the second to test the use of the Guidelines, were implemented. The first project was completed by Agriculture NZ (MAF Policy Technical Paper 00/04), and Lincoln Environmental completed the second (MAF Policy Technical Paper 00/06).
1.2 RELEVANCE OF FLOW TO INDICATORS AND GUIDELINES
To enable the indicators to be calculated and the Best Management Guidelines to be used efficiently, both depths of water applied and soil moisture status must be determined throughout the season. This is a fundamental requirement of the process. Without being able to realistically determine both of these components, the process is of limited use.
Six of the sixteen economic and environmental indicators of sustainable irrigation recommended in MAF Policy Technical Paper 00/03 have water use as one of their components. Examples are production/m3 of water used, energy used per m3 of water pumped, and daily percentage of water flowing onto the farm that is stored in the root zone.
To calculate these indicators, each day the total volume of water used on a farm for irrigation and the rate at which it is taken must be measured. This enables total seasonal volumes and depths of water applied to be calculated. Without flow measurements, the key indicators of sustainable irrigation cannot be determined.
Traditionally, application depths have been estimated using data supplied from irrigator or sprinkler manufacturers, but this practice is compromised by:
- Pressure fluctuations in the water supply line;
- Poor data relating to travel/rotation speed of broad-acre irrigation systems and their running time;
- Poor data on the "as-installed" hydraulic characteristics of the irrigation system.
The best way to accurately calculate water use is to measure it with a flow meter.
1.3 FLOW MEASUREMENT
Totalising flow meters are used for measurement of irrigation water use in limited numbers in some areas of New Zealand, mainly in horticultural areas where irrigation systems tend to be relatively small. They are generally installed at the water supply or pump headworks. Their primary purpose is to provide information on water use to regional councils for resource management.
Typically, these flow meters require lengths of straight pipe equivalent to 8-10 diameters upstream and 4-6 diameters downstream of the meter for accurate readings. On a system using 200 mm diameter pipe, this means that approximately three metres of straight pipe is required. Shorter lengths of straight pipe expose the meters to turbulence, which makes them inaccurate.
When inspecting the irrigation systems of farms for the testing the Best Management Guidelines (MAF Policy Technical Paper 00/06), it was found that none of the irrigation systems on the farms visited were designed in a way that allowed installation of readily available flow meters at reasonable cost. The extent of this problem was not known before the project started. A recent study by Canterbury Regional Council in the Christchurch-West Melton district (McEwan et al., 1998) found that only 15% of installations were suitable for monitoring using a portable ultrasonic flow meter, particularly at system headworks (the pipework at the beginning of the system). A much smaller percentage of those sites were suitable for permanent flow meter installations.
Generally, only newer installations of irrigation system headworks have the required length of straight pipe. In most of the older systems, the mainline has an elbow immediately downstream of the gate valve, and often the mainline will branch out immediately at a tee underground below the elbow. To convert the headworks to allow a flow meter to be correctly installed, particularly on the larger systems, could be quite an extensive and costly exercise.
Although the general thinking by water managers such as regional councils is that the flow should be measured at the pump or the system headworks, this approach is more suited to resource management than to irrigation management. For irrigation management, flow should be measured at each irrigator or block, so that the amount of water applied to a specific area can be determined, regardless of the number of irrigation units or pumps on the system. In fact, the only time that measuring flow at the pump can be recommended for irrigation management is when there is only one pump and one irrigator on the system, and the flow out of the pump equals the flow out of the irrigator.
In addition to providing useful data for irrigation management, measuring flow on irrigators rather than system headworks has an additional benefit. In districts where water is not currently metered, there is little public support among the farming community to monitor water usage, because putting in flow meters is seen as the first step towards charging for water. Measuring flow at irrigators is seen as one step removed from the resource monitoring process, and is likely to be more easily accepted by farmers.
Unfortunately, installing flow meters on irrigation machinery is not as simple as it may appear. Travelling irrigators, for example, are not constructed in a way that allows easy installation of flow meters. Two problems arise. The first is that there is almost never a section of straight pipe on the machines long enough to install a meter correctly. The second is that most meters cannot be installed where they will be subjected to excessive forces (such as at the back of an irrigator). This makes installation of standard in-line meters on travelling irrigators difficult, and in many cases, impractical.
An extensive exercise to try to find practical and cost-effective solutions to the flow measurement problem was carried out. After all options were considered, it was decided that monitoring pressure rather than flow, and calibrating for flow would be the most suitable method, as it would not require any infrastructural changes. Pressure sensors are relatively inexpensive, and are easy to install, either at system headworks or on irrigators.
Although pressure can be measured using pressure sensors, additional equipment is needed to record the pressures, and calibration to calculate the flow from the pressure measurements must be carried out. The lack of a suitable commercially available product that could measure pressure and record flow rate or volume of water used led to the need for this project.
Contact for Enquiries
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