6.1 Irrigation System Description
6.2 Summary of Case Study Findings
6.3.1 Soil Moisture Measuring Equipment
6.3.2 Flow Measuring Equipment
6.3.3 Irrigation Management Spreadsheet
6.5 Data Collection And Analysis
6.7.1 Crop Production
6.7.2 Seasonal Water Use
6.7.3 Water Stored In The Root Zone - Wheat
6.7.4 Water Stored In The Root Zone - Potatoes
6 Spray Irrigated Cropping Farm
6.1 IRRIGATION SYSTEM DESCRIPTION
Midway Farm, a 164 ha irrigated property located 4 km east of Darfield on the Canterbury Plains (see Figure 4), is owned and farmed by the Van de Klundert family. Approximately 145 ha is used for cultivation of crops and 10 ha of pasture for hay silage and grazing animals. Irrigation is primarily carried out using a Roto-Rainer rotary boom irrigator, although a sideroll system is used on occasions.
The property was the subject of a case study to investigate the financial benefits of improving the irrigation system. A full description of the irrigation system can be found in Borrie et al, 1998.
The layout of the paddocks and irrigation system is displayed in Figure 4.
6.2 SUMMARY OF CASE STUDY FINDINGS
The case study identified the following problems with the irrigation system:
- The irrigation system had insufficient capacity to meet peak crop water demands.
- To make allowance for the inadequate system capacity the Van der Klundert’s have adopted crops and planting dates within their cropping rotation to reduce the peak water requirement during the months of November to January.
- The existing irrigation equipment is not operating as efficiently as it could be.
- The well had additional capacity that could be used.
Improvements were made to the rotary boom irrigator to ensure that it operated correctly. The sprinkler nozzles on the side-roll were replaced where necessary to make them the correct size.
Although there were plans to improve the system based on the findings of the case study, no major infrastructural changes were made. The primary reason for this is because the adjoining property (unirrigated) was purchased, and the intention now is to extend the irrigation system to cover the additional property at a later date.
6.3 EQUIPMENT INSTALLATION
6.3.1 Soil Moisture Measuring Equipment
In early October 1998, an Aquaflex™ unit with two sensing cables was installed in a wheat crop in Paddock A. The area of Paddock is approximately 16 hectares. The wheat at this stage was about 150 mm high and well established as it was planted in the autumn of 1998.
The soil is a Lismore type with about 30 cm of well structured top soil and loose stony silt loam below that. One sensor was placed within the topsoil horizon at about 20-25 cm deep, and the bottom sensor into the stony material at about 55-60 cm deep. Soil moistures and soil temperatures at the two depths were recorded every hour.
Data was read and downloaded using a commercially available Palm Pilot hand-held computer. This gave the Van de Klunderts the ability to take readings at any time.
In early December 1999, two Campbell Scientific CS615 Time Domain Reflectometry (TDR) soil moisture probes were installed in a crop of newly planted potatoes in Paddock P. The paddock area is about 9.5 hectares. The soil type was similar to that in the wheat crop. The first probe was placed in loose soil just below the seed potato at about 200 mm from the top of the mound. The second probe was placed 200 mm below the first probe. Soil moisture data was recorded every hour using a Campbell data logger.
At the time the soil moisture probes were installed, it was not possible for the Van de Klunderts to download the information themselves, and Lincoln Environmental staff did the downloading on a weekly basis.
6.3.2 Flow Measuring Equipment
Both the wheat and potato crops were irrigated using the rotary boom irrigator.
A prototype flow meter similar to the one developed in June 1998 and installed on the centre-pivot on the sprinkler-irrigated dairy farm was fitted to the rotary boom irrigator in December 1998. This was used to provide current operating pressure, instantaneous flow rate and cumulative volume of water used. Delays in manufacture meant that the meter was not available until that time.
The following pressure-flow calibration equation was calculated for the rotary boom irrigator and programmed into the meter:
Flow (m3/h) = 9.045 x Pressure (m)0.5
6.3.3 Irrigation Management Spreadsheet
Given that the worksheets developed for and provided with the Irrigation Best Management Guidelines were found to be unsuitable, it would have been desirable to use an irrigation management spreadsheet for this farm as well.
However, unlike the dairy farms, which irrigate one crop (pasture) in a known rotation, cropping farms have a wide range of crops and water use patterns. Developing a spreadsheet would have taken significant resources, and a search was made to find a suitable system. Although comprehensive commercial packages were available, nothing suitable could be found in the public domain.
It was therefore decided to use the soil moisture information alone to schedule the irrigation on the wheat and potato crops, and to use that information as a guide for what should be done for the other crops.
6.4 IRRIGATION PLAN
In a cropping situation, the required irrigation system capacity depends on what kind of crops are grown and when they are planted or harvested. The Van de Klundert’s have recognised the fact that the existing irrigation system does not have sufficient capacity to supply the irrigation demand to all crops in the peak of the irrigation season. Therefore, to make allowance for this, they have adopted some crops and planting dates within their cropping rotation to reduce the peak water requirement during the months of November to January.
No formal method of irrigation scheduling had been used. The general plan was to use the soil moisture measurements and water meter readings to minimise water use and to maximise production for the wheat and potato crops, and to use that information as a guide to manage irrigation on the remainder of the property. Because irrigation system capacity was limited in the November, December and January period, the important decision related to what needed to be irrigated next, rather than whether to irrigate or not. It was hoped that the soil moisture data would help to determine whether the irrigation return intervals used on the monitored crops and whether the application depths applied were appropriate.
Unless significant rain fell, savings were expected to be limited to the beginning and end of the season.
Because soil moisture holding capacities and stress points were unknown, maximum allowable soil moisture deficits could not be specified at the beginning of the season. The intention was to visually interpret the soil moisture data to ensure that over-watering or crop stress did not occur.
For the wheat crop, this required managing the irrigation so that soil moistures in the lower soil profiles started close to field capacity at the beginning of the season and slowly tracked down as the season progressed.
The plan for soil moisture in the potato crop was to start off under dryish conditions to get the roots established, and then to increase soil moisture as the season progressed.
6.5 DATA COLLECTION AND ANALYSIS
6.5.1 Soil Moisture Data
The intention was for the Van de Klunderts to use the Palm Pilot to read and download the soil moisture information on the wheat crop on a regular basis, to graph the soil moisture, and to plan the irrigation from this. This did not work as well as expected. Although the Aquaflex™ soil moisture system worked very well, problems were encountered in loading the graphing software onto their personal computer. In addition, the battery life of the Palm Pilot was too short, which often resulted in loss of the downloading software. Lincoln Environmental staff were then required to reload the software onto the Palm Pilot.
Despite these problems, a full set of hourly soil moisture readings for the wheat crop was obtained for the period 13 October 1998 through to 26 March 1999.
Hourly soil moisture readings for the potatoes began to be recorded on 2 December 1998, and were recorded until 28 March 1999. As downloading the data required the use of a laptop, and the Van de Klunderts did not have access to one, Lincoln Environmental staff downloaded the data, plotted it and faxed the results through to them.
6.5.2 Flow Measurement Data
The prototype flow meter installed on the rotary boom irrigator provided instantaneous pressure, flow rate readings and cumulative volume used. The unit was read after an irrigation run, while the irrigator was operating at the top of the property. Based on the area covered, and the run time, the depth applied was calculated to be 45 mm. This was crosschecked with a rain gauge, which also showed an applied depth of 45 mm.
There is no doubt that the irrigator was operating under its recommended operating pressure, as the meter indicated that it was operating at 220-230 kPa. The recommended operating pressure is 280 kPa. The design audit carried out in the previous season showed that low pressures would be expected to occur in Paddock A at the top of the farm.
When the irrigator was operating at the bottom of the farm, pressures and flow rates increased, as expected. Flow rates and depths applied were about 10 percent higher than when at the top of the property.
During the course of the project, two problems arose with the flow meter that prevented a full set of water use figures from being obtained. The first was a leak in the seal of the case that allowed water to enter. The second was moisture in the pressure sensor, which caused inaccurate readings.
Both problems were associated with using components that should have operated satisfactorily under the conditions, but didn’t.
Given the limited data that was obtained from the flow meter, water use for the season is calculated on the basis of 45 mm of water being applied to the wheat crop per irrigation. The water use for the potato crop was on the basis of 55 mm of water being applied per irrigation.
6.6 IRRIGATION MANAGEMENT
Irrigation was started on the grass seed crops on 18 September 1998. All crops planted at that time were watered once, and then irrigation was stopped in October for two weeks.
After that time, most of the focus was on trying to keep up with the water demand through to February 1999. Limited system capacity and lack of rainfall meant that the system had to be operated continuously. The final irrigation took place in the first week in March. By that time, many crops had been harvested, and those remaining were approaching the end of their active growing season.
6.7 END OF SEASON INDICATORS
Three key indicators have been calculated. They are:
- cubic metres of water used per tonne of crop (m3/tonne);
- percentage of water applied to the crop (irrigation and rainfall) that is stored in the root zone (%);
- depth applied / (ET – rainfall), which is a measure of water applied versus water required.
6.7.1 Crop Production
The wheat crop averaged 6 tonnes per hectare in the monitored paddock.
The potato crop averaged 40 tonnes per hectare, although an additional 10 tonnes per hectare was obtained from areas sheltered from the northwest winds.
6.7.2 Seasonal Water Use
Over the irrigation season, the wheat was irrigated five times and the potatoes were irrigated six times. Depths applied were approximately 45 mm for the wheat and 55 mm for the potatoes. Volume of water used for each crop was as follows.
Wheat (16 hectares) 36,000 m3
Potatoes (9.5 hectares) 31,000 m3
6.7.3 Water Stored in the Root zone - Wheat
By examining the soil moisture deficit immediately before 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 equation for calculation of soil moisture deficit was calibrated using the soil moisture data collected over the November to mid January period, taking into account wheat crop factors, evapotranspiration and rainfall. The calibrated equation was:
Soil moisture deficit = 10 x Rt + 54 x Rb –943 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 80 mm and the total water holding capacity probably greater than 120 mm.
A summary of the data for Paddock A is as follows:
Total irrigation water applied 225 mm
Total irrigation drainage 1 mm
Irrigation water stored in the root zone 224 mm
Total rainfall (Sep-mid Jan) 137 mm
Total rainfall drainage 0 mm
Rainfall water stored in the root zone 137 mm
Total ET (Lincoln Sep – mid Jan) 550 mm
The above results show that the irrigation of the wheat was carried out very efficiently. To have the soil moisture in the lower layers slowly tracking down through the season is the result of good irrigation management.
It is possible that the crop came under a small amount of water stress in mid December and perhaps early January, indicating that the return intervals were perhaps longer than they should have been. Irrigation probably should have been applied three or four days earlier, but limited irrigation system capacity probably prevented this.
Rainfall had very little influence on soil moisture at the sensor level. This indicates that most of the rainfall was either intercepted by the crop, (which would work to suppress ET) or was stored in the surface soil layers.
6.7.4 Water Stored in the Root zone - Potatoes
The TDR probes used to measure soil moisture in the potato crop output data as volumetric soil moisture expressed as a percentage of total soil volume.
To convert these readings to soil moisture deficits expressed in millimetres requires either measuring or assuming a crop root depth for each probe.
Assuming that the potato root depth around the top sensor was about 250 mm and the potato root depth around the bottom sensor was also about 200 mm, the maximum total soil water content would be about 140 mm in the root zone of the crop. This is based on a 25 percent volumetric soil moisture content in the top layer and a 35 percent volumetric soil moisture content in the bottom layer. Note that the content in the bottom layer is higher than in the top layer. This is primarily because the top layer was loose cultivated topsoil and the bottom layer more compact silt. It is likely that as the growing season progressed, some soil settling occurred in the top layer, which would increase the maximum volumetric soil moisture content.
Stress points for the crop cannot be easily determined from the data. It appears that some slowing down of water use did occur when the top sensor reading fell below 10-12 percent and the bottom sensor fell below 20-22 percent. On this basis, the soil moisture deficit at the stress point would be about 60-70 mm. However, soil moisture deficits of not more than 25 percent of available soil moisture are normally applied to potatoes. This implies the maximum deficit should not exceed 35mm.
A summary of the data for Paddock P is as follows:
Total irrigation water applied 330 mm
Total irrigation drainage 7.5 mm
Irrigation water stored in the root zone 322 mm
Total rainfall (Dec-Mar) 145 mm
Total rainfall drainage 0 mm
Rainfall water stored in the root zone 145 mm
Total EVT (Lincoln Dec – Mar) 573 mm
The irrigation of the potatoes was carried out very efficiently, with very little water loss either from irrigation or rainfall.
The soil started dry as planned, and in general, soil moisture deficits were reduced as the season progressed. The exception is a period at the end of January, where soil moisture deficits were approaching 50-55 mm, and water stress may have occurred.
6.8 SEASONAL INDICATORS
Cubic metres of irrigation water used per tonne of wheat 375m3/tonne
Cubic metres of irrigation water used per kilogram of potatoes 81 m3/tonne
Percentage of irrigation water stored in the root zone 98%
Percentage of rainfall stored in the root zone 100%
Depth applied / (ET – Rainfall) for wheat 0.55
Depth applied / (ET – Rainfall) for potatoes 0.78
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
