4.1 Irrigation System Description
4.2.1 Soil Moisture Measuring Equipment
4.2.2 Flow Measuring Equipment
4.2.3 Irrigation Management Spreadsheet
4.4 Data Collection and Analysis
4.6.1 Milk Production
4.6.2 Seasonal Water Use
4.6.3 Energy Use
4.6.4 Water Stored in The Root Zone
4 Spray Irrigated Dairy Farm
4.1 IRRIGATION SYSTEM DESCRIPTION
This property is a dairy farm located in Te Pirita, Canterbury. The total irrigated area of the property is about 120-125 hectares out of a total farm area of 144 hectares. A Pierce towable centre-pivot is used to irrigate the property. The pivot is moved around six positions as indicated in Figure 2 below. Further details about the irrigation system are given in McIndoe et al, 1998.
Figure 2: Layout of sprinkler irrigation systemThe design audit carried out in 1997/98 showed that there were excessive pressure losses in some pipes. Plans have been made to improve the hydraulic performance of the system and to try to maximise the use of the well. Plans have also been made to use long-line sprinklers to irrigate the corners of the property not covered by the centre-pivot. However, these changes will not be implemented until the 1999/2000 irrigation season.
4.2 EQUIPMENT INSTALLATION
4.2.1 Soil Moisture Measuring Equipment
In January 1998, an Aquaflex™ unit with two sensing cables was installed in Paddock 1. Because the soil type was a shallow stony Lismore with about 20 cm of top soil, one sensor was placed at the bottom of the soil horizon at about 20 cm deep, and the bottom sensor was placed into the stony material at about 40 cm deep. Soil moistures and soil temperatures at the two depths were recorded every hour.
The computer chip in the data logger on this unit was upgraded to allow data to be read and downloaded by Bruce Rolls using a commercially available Palm Pilot hand-held computer. This gave Bruce the ability to take readings at any time and to transfer the information to his personal computer.
In September 1998, a second set of Aquaflex™ sensors was installed in Paddock 2. The soil type was similar to that of Paddock 1. The top sensor was placed at a depth of 20 cm and the bottom sensor was placed at a depth of 50 cm.
4.2.2 Flow Measuring Equipment
A prototype flow meter was developed in June 1998 and tested on a frost protection system throughout the winter months of 1998. It was installed on Bruce Rolls’ centre pivot irrigator in October 1998. As there had been no physical changes to hydraulic components on the pivot, the pressure-flow calibration curve obtained in the 1997/98 irrigation season was programmed into the meter to provide instantaneous flow rate and cumulative volume used.
The pressure-flow calibration equation was:
Flow (m3/h) = 76.626 x Pressure (m)0.316
4.2.3 Irrigation Management Spreadsheet
One of the problems of the previous season was that farmers found the worksheets provided with the Best Management Guidelines difficult to use and not particularly helpful in terms of making irrigation management decisions. An irrigation management spreadsheet that carried out a daily water budget based on rainfall, irrigation water applied, and average Lincoln daily evapotranspiration was developed by Lincoln Environmental and provided to Bruce Rolls. The use of average Lincoln daily evapotranspiration allowed a meaningful forward projection of soil moisture deficits to be made.
To update the water budget, the spreadsheet had facilities for entry of actual daily evapotranspiration (published weekly in the Christchurch Press). It also had provision for entry of actual rainfall (measured by Bruce Rolls on his property) and to convert Aquaflex™ soil moisture readings into soil moisture deficits and update the soil moisture status of the paddocks.
Running totals of soil moisture deficits based on soil moisture readings and on the daily water budget were kept in the spreadsheet to allow a comparison to be made between the two methods.
4.3 IRRIGATION PLAN
The general plan was to use the soil moisture measurements and water meter readings combined with the irrigation management spreadsheet as much as possible to minimise water use and to maximise production. The basic intention was to turn off the irrigation system as much as possible.
As no major changes to the irrigation system capacity had been made, it was known that system capacity would be limited in the peak demand months of December and January. Savings were expected to be made in the beginning and end of the season, and after significant rainfall.
The intention was to manage the system so that soil moisture deficits of no greater than 40 mm resulted. This figure was established from measurements made in the previous season. The spreadsheet, with its predictive function, would provide guidance on when to start irrigating and how long the system could be turned off after rainfall.
4.4 DATA COLLECTION AND ANALYSIS
The collection of data, interpretation of the data and management of the irrigation system was carried out by Bruce Rolls. Bruce made all irrigation decisions himself.
The data was made available to Lincoln Environmental at the end of the irrigation season (early June).
4.4.1 Soil Moisture Data
Bruce Rolls used the Palm Pilot to either read or download the soil moisture information on a regular basis, and graphed soil moistures and entered readings into the spreadsheet to monitor and plan his irrigation. This generally worked very well, except that the battery life of the Palm Pilot was short (several weeks) due to a programming problem in the unit. When the batteries went flat, the Aquaflex™ software was lost. Unfortunately, this is an internal problem in the Palm Pilot, and nothing could be done to rectify it short of replacing the unit with a later model. Reloading the software onto the Palm Pilot was carried out by Lincoln Environmental staff.
Although there were some small gaps in soil moisture data, the amount of data collected was more than sufficient for irrigation management.
4.4.2 Flow Measurement Data
The prototype flow meter installed on the centre-pivot provided instantaneous pressure, flow rate readings and cumulative volume used. Bruce Rolls generally read the unit when moving the pivot and recorded the information in the irrigation management spreadsheet.
The spreadsheet then calculated the average depth of water applied for that irrigation event.
The concept of measuring flow in this way was good, and the main problems encountered were technical problems such as flat batteries in the unit, and moisture in the pressure sensor, requiring it to be dried out. Although the pressure sensor was rated for these conditions, it did not appear to meet them.
4.5 IRRIGATION MANAGEMENT
4.5.1 Operation
Bruce Rolls started irrigating at the end of September 1998, perhaps a few days later than would have been desirable, but without suffering noticeable moisture stress. The return interval was initially about 16 days, but was reduced to 8 days in mid November.
Until mid November, the system was operated on the basis of one pivot rotation per day, applying 22-25 mm of water. The soil moisture readings indicated that more water could be applied less often, so the operation was changed to apply 30-36 mm every day and a half. This extended the minimum return interval from six days to nine days, but reduced the labour input by 50 percent.
In early December, limited system capacity and lack of rainfall meant that the system had to be operated continuously, except for a few days in January and February, through to mid March, when significant rainfall allowed the system to be turned off for about two weeks.
Another irrigation was required at the beginning of April. No further irrigation was required in April because rainfall kept soil moistures at reasonable levels.
Because of very little rainfall in May, irrigation probably should have taken place by the third week in May (based on the spreadsheet water budget), but the Palm Pilot battery was flat at that time. Bruce couldn’t update the soil moisture readings, and probably expected little response to irrigation anyway, as soil temperatures had cooled and growth had slowed. He started irrigation again at the beginning of June, but as it rained a couple of days later, he did not complete the irrigation cycle.
The irrigation season overall was very dry. There was less rainfall than in 1997/98, which was also very dry. Irrigation into May is extremely rare, as normally the last irrigation for the season occurs in March or April.
4.5.2 Maintenance
The pump and well operated as expected. Water levels had fallen significantly since the previous year, and towards the end of the season, there was a possibility that the pump was sucking air. With adequate winter rainfall, groundwater levels will recover, but the problem of lower than desirable irrigation system capacity remains.
The centre-pivot required minor maintenance, which was repaired under warranty.
The main problem was an underground electrical cable failure, which put the system out of action for several days. This was attributed to a problem during installation, and is not expected to occur again.
4.6 END OF SEASON INDICATORS
Six of the sixteen economic and environmental indicators of sustainable irrigation recommended in (MAF Technical Paper 00/03) have water use as one of their components. Of these, four key indicators have been calculated. They are cubic metres of water used per kilogram of milk solids produced (m3/kgMS), electricity used per cubic metre of water pumped (kWh/m3), Percentage of water flowing onto the farm (irrigation and rainfall) that is stored in the root zone (%), and depth applied / (evapotranspiration – rainfall), which is a measure of water applied versus water required.
4.6.1 Milk Production
According to Bruce Rolls’ production records, total milk production for the 1998/99 season was 142,000 kg MS.
4.6.2 Seasonal Water Use
Although total seasonal water use was not directly available from the flow meter mounted on the centre-pivot (because of the problems described in Section 3.5), the data collected was more than sufficient to calculate depth of water applied, and from that the total volume of water used.
In summary, 588 mm of water was applied over 120 hectares, resulting in a total seasonal volume used of 706,000 m3.
4.6.3 Energy Use
As in the 1997/98 season, water was being pumped into the system from two locations, a 64 kW pump on the property and a 101 kW pump on the eastern side of Two Chain Road. Only about 22 percent of the water pumped by the 101 kW pump (established in the previous years study) was used to supply the centre-pivot. The balance of the water was used to supply another irrigation system.
The total energy used (kWh) used to pump water to the centre-pivot was 268,523 kWh.
4.6.4 Water Stored in the Root Zone
By examining the soil moisture deficit immediately before and 24 hours after irrigation, it was possible to calculate the amount of water stored in the soil and compare it to the amount of water applied for each irrigation or rainfall event.
The same equation for calculation of soil moisture deficit as was used in the previous study was used once again:
Soil moisture deficit = 23 x Rt + 15 x Rb –609 mm
where Rt and Rb are the top and bottom readings for the Aquaflex™ sensor readings respectively.
The soil moisture deficit at the stress point was calculated to be about 42 mm and the total water holding capacity for the soil about 80 mm.
A summary of water use for the season is as follows:
Total irrigation water applied 588 mm
Total irrigation drainage 20 mm
Irrigation water stored in the root zone 568 mm
Total rainfall 343 mm
Total rainfall drainage 60 mm
Rainfall water stored in the root zone 283 mm
Total evapotranspiration (Lincoln) 973 mm
4.7 SEASONAL INDICATORS
Cubic metres of water used per kilogram of milk solids 5.0 m3/kgMS
Cubic metre of water pumped per kWh of electricity 2.6 m3/kWh
Percentage of irrigation water stored in the root zone 96 %
Percentage of rainfall stored in the root zone 82 %
Depth applied / (Evapotranspiration – Rainfall) 0.93
All of the above indicators except the energy indicator have improved significantly since last season. The reason that the kWh/m3 has deteriorated can almost certainly be explained by a lowering of groundwater levels. This was primarily caused by a lack of rainfall recharge in the 1998 winter. As a check, the well water level was measured in May 1999 and found to be 36.8 metres below ground level. This is significantly lower than any other measurements taken previously.
Contact for Enquiries
MAF Information Services
Pastoral House
25 The Terrace
PO Box 2526
Wellington, NEW ZEALAND
Fax: +64 4 894 0721
Contact this person
