The assessment process
Assessments begin with a discussion about irrigation practices, scheduling, and crop production levels. A plan of the irrigation system is also drawn up if not already available. This helps tailor the process for each business. This is followed with an irrigation system test.
Pressure and flow measurements, condition assessments of mainlines, sub-main and laterals and the pump are important as they provide a picture of the current system state compared to what the system should be capable of in terms application rates.
The technical team uses the information collected during the interview and confirms the following with the grower:
- block(s) layout
- irrigation shifts (which blocks are operated at the same time)
- layout of mainlines, sub-mains and laterals
- location of pump, dams and bores
- location of T junctions, elbows and pipe size changes
This information is overlaid onto an aerial photo of the property. With this map the grower identifies which blocks are to be tested by the team. Two or three blocks are usually selected, one of which should be furthest from the pump. This maximises the chances that measurements of the irrigation system operating under the most stress are captured.
At the block
Micro sprinklers and drip systems
At each block, measurements are taken so that the distribution uniformity (DU) of the micro sprinklers or drippers can be calculated. This is done by taking pressure and flow measurements at 16 points around the block. A high DU means the system is applying water evenly, and that any soil moisture monitoring gear installed is providing a representative indication of soil moisture status.
The flow rate of the drip emitters or micro sprinklers is recorded by measuring the output from an emitter over a set period of time so that the flow can be calculated in litres per hour (L/h).
Pressure is measured by inserting pressure point adapters into the poly line or dripline at the selected points around the block. A pressure gauge with a needle attachment is inserted into these adapters to take the readings. The adapters are left in the line for the grower to use for later pressure tests.
This data can then be used to calculate the total water flow to the block. The number of laterals running to any other blocks (and the total number of emitters and output) at the time of the test needs to be recorded in order to account for the total water flow for the particular irrigation shift.
Solid set systems
A grid of catch-cans is set up and the volume of water collected over a set period of time is measured to assess the distribution uniformity of a solid set system, like those used for vegetable crops. This grid collects the output from four sprinklers located on two adjacent lateral lines. Pressure and flow measurements are recorded while the system is running, at six locations around the block using a pressure gauge with an appropriate fitting and timing the flow into a 20L container. Two of these measurements should be at the closest and most distant point from the sub-main.
The measurement points and the catch-can grid location are plotted with GPS device. The total number of sprinklers operating is counted, including those which may be running in other blocks at the time of the test. The sprinkler type and spacing is also recorded.
Centre pivot systems
With centre pivot irrigation systems, like those used by dairy farmers and some vegetable growers, the assessment is essentially the same with pressure and flow being measured at the pivot point and pressure recorded at the end of the pivot.
Catch-cans are placed along the length of the pivot and the volume of water collected in each can is recorded after the pivot has passed over them.
The speed of the pivot is recorded by the time it takes the outer wheel to travel between two marked points (usually 10m). The flow at the pivot point is recorded using an ultrasonic flow meter or information from the controller. A properly set up pivot will have a pressure gauge at the pivot point and on the end of the last span.
At the Pump
Flow and pressure at the pump are recorded for each block that is being tested.
The flow can be recorded in litres per second (L/s), litres per minute (L/min), or cubic metres per hour (m3/h) at the pump outlet using an ultrasonic flow meter. To obtain an accurate reading the flow meter needs to be set up on a straight section of the pump outlet pipe that is some distance away from valves, elbows and filters to minimise turbulence. This measurement can be compared with readings on the controller and timed runs on the water meter.
The flow meter also records velocity and this is used to check that the mainline is operating within specification. Pressure is recorded from the installed pressure gauge or from the controller. There should always be a functioning pressure gauge at the pump or controller. Readings are taken separately for each block that is being tested. The pump specifications can generally be obtained from the compliance plate if the grower is unable to supply them.
Pump type, model, inlet outlet impeller ratio, motor model and revolutions per minute (rpm) details are required to check the pump efficiency and ensure that it is correctly matched to the irrigation setup. Other information needed includes inlet and outlet pipe size to and from the pump, water level above or below the inlet pipe, and the length of the inlet pipe. The pump or bore location is marked with the GPS device. A photograph of the pump and piping/filter arrangement is taken for reference.
During the test run for the selected block we need to know if water is flowing to any other points, such as tanks or dams. It is important to account for all of the water flow from the pump so that any significant differences in the flow between the pump and the block can be explained. Any visible leaks between the pump and the block which may account for some of these differences are noted.
Assessment results
On completion of the assessment, DAFWA staff will provide each grower with a report with an analysis of the results. This enables growers to benchmark their irrigation against the high performing businesses in their industry and identify priorities to help improve the return to their businesses per megalitre of water used in their farming operations. Contact Tilwin Westrup if you would like to be involved.