Developing an Effective Irrigation Water-Use Meter

2 Objective of Project

This project was initiated to develop a prototype irrigation water use meter that could be easily installed and used with reliable results on all types of pressurised irrigation systems. The development of this meter was required to significantly increase the applicability and value to agriculture of irrigation performance measures currently being developed through MAF Operational Research Project contracts.

3 Description of Flow Meter

3.1 GENERAL FARMER NEEDS

To ensure the meter would meet farmer’s needs, a number of basic requirements had to be met. These were:

very robust;

easy to install;

battery-powered with a one-season battery life;

battery change-over could occur without loss of stored data;

waterproof;

lightning protected;

farmer friendly with simple user controls, and

low cost.

In terms of output, the meter was required to provide on-demand visual display of:

current pressure;

current flow rate;

accumulated volume.

The fundamental concept of the proposed flow meter is to use a pressure measurement (which is relatively easy to make) to calculate the flow rate (which is much more difficult to measure). The task of developing a satisfactory relationship between flow and pressure for the particular irrigator that the meter is fitted to is central to this concept. For the majority of irrigation machines, this relationship will be easily obtained by a simple field calibration, and as many machines use standard nozzle configurations, the information will be transferable to many farms. Equipment suppliers could easily provide the information to farmers. For more unique situations, a specific field calibration for that situation will be required.

3.3 HYDRAULIC CONSIDERATIONS

In terms of hydraulic considerations, the four factors considered in the development of the flow meter were:

The desirable level of accuracy for both total accumulated flow and the current flow rate;

The appropriate sampling interval used to interrogate the pressure sensor;

The appropriate time period used to update the accumulated total volume, and

The most appropriate form of the flow – pressure relationship used to transform the pressure readings into a flow rate.

For the initial development of the flow meter the following approaches to these issues were adopted.

3.3.1 Accuracy

The intended use for the proposed device is to aid farmers in the management of their irrigation systems. For this purpose an accuracy of plus or minus 5% for both total volume and current flow rate was considered to be adequate (for a total application depth of say 50 mm this would represent an accuracy to a depth of 2.5 mm).

3.3.2 Sampling Interval

Sampling interval relates to how often the software has to activate the pressure sensor and to make a reading. Choice of a suitable sampling interval has a considerable impact on the design of the proposed flow meter and on the resulting accuracy of the system. In particular it has a significant bearing on the overall power requirements and battery life.

The most appropriate interval is governed to a large extent by the expected deviation and rate of change of irrigator operating pressures. In all systems, the biggest deviation that will be experienced will be under start-up and shutdown conditions. Irrigation systems that have a number of different irrigators running at once may also experience additional pressure variations throughout any one particular set or run due to changing hydraulic conditions within the system. These changes will not be as extreme as the start /stop condition but may well take place over a longer time period.

As part of the Testing if Best Management Guidelines project conducted over the 1997/98 summer, several months of pressure records were obtained for a centre-pivot irrigator. This irrigator was operated as part of an overall system comprising two interconnected systems (separate wells, pumps and irrigators) in which the irrigator management was quite independent for most of the season. This lead to a very wide range of operating conditions in terms of the pressure supplied to the centre-pivot system. These variations were considered to be at the upper end of those likely to be experienced by most irrigation systems under normal circumstances and therefore this data set provided a very useful test for the selected sampling interval.

We concluded that the most appropriate sampling interval based on our knowledge of the typical range of conditions likely to be encountered for different irrigator types and operating systems is one minute, i.e. the meter should take a pressure reading every minute.

3.3.3 Updating Accumulated Volume

Although the meter may take a pressure reading every minute, updating accumulated volume every minute may be unnecessary and wasteful of power.

The installation of standard in-line flow meters is likely to be adopted for small flows on systems using small pipe sizes, as it will probably be lower cost. On this basis, a decision was made to record accumulated volume to a resolution of one cubic metre. For a very small system with a flow rate of 10 m³/h, the time taken to accumulate one cubic metre is six minutes. For systems with flow rates greater than 60 m³/h (as most of the systems using the metre will be), the time taken to accumulate one cubic metre will be less than one minute.

Updating accumulated volume when the volume increases by one cubic metre, but not more than once every minute has been adopted.

3.3.4 Flow Pressure Relationship

There were two main ways in which this issue could be addressed. The first is by use of a mathematical relationship (i.e. an equation) which relates flow to pressure. The second is by the use of a look-up table of pressures and corresponding flows, which could be used with an appropriate interpolation technique to estimate the flow for any given pressure. To determine the most appropriate technique, the theoretical flow-pressure relationship for a number of different irrigator types (i.e. guns, rotating booms, linear move systems, centre-pivots etc. was investigated). Issues such as attainable accuracy and ease of obtaining and entering the necessary data were other factors considered.

On balance, the use of a look-up table was preferred as it could be used to implement all types of pressure-flow relationships and did not rely on fitting data to mathematical equations.

Although the number of measured pressure-flow points is often limited, graphing the relationship enables additional points to be determined for the meter’s look-up table. As the meter uses linear interpolation of the look-up table data to determine flow rate for any intermediate pressure, the higher the number of pairs of points (maximum 20), the greater the accuracy.

Contact for Enquiries

MAF Information Services
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25 The Terrace
PO Box 2526
Wellington, NEW ZEALAND

Fax: +64 4 894 0721
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