5    Design Options

5.1    SUMMARY OF WATER REQUIREMENTS

In a cropping situation, the required irrigation system capacity depends on what kinds 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.

Based on typical crop types and areas, and considering planting and harvesting dates, the irrigation flow rates required for the months October to March have been calculated and these are detailed in Table 3. The figures have been calculated on the basis that average monthly evapotranspiration rates have been met and that the irrigation application system is 80 percent efficient. These figures have assumed that there is no effective rainfall. On this property, where the system capacity of the irrigation system is not able to meet the peak crop water requirements, soil moisture levels would tend to be lower and hence irrigation application efficiency would be higher than would normally be the case with a spray irrigation system that had a higher system capacity. As the irrigation system would typically operate for up to 22 hours per day, the flow rate figures in Table 3 have been calculated on the basis of watering for 22 hours per day. The monthly water requirement for each crop is detailed in Appendix I.

Table 3: Monthly flow rates required
 

Water Requirements - Months
 

Oct

Nov

Dec

Jan

Feb

Mar
m3/hour (a)

256

306

316

219

185

266
litres/second (a)

71.1

84.9

87.9

60.7

51.4

73.8
(a) Water requirement figures based on:
  • The irrigation system operating for 22 hours per day.
  • An irrigation application efficiency of 80%.
  • No effective rainfall.

The required flow rates detailed in Table 3 are substantially greater than the capacity of the well and irrigation system on the property. This imbalance is managed by priority watering of selected crops and not planting the full 145 ha in irrigated crops.

5.2    OTHER DESIGN ISSUES

    Subsequent to the initial farm visit in October 1997, the Van de Klundert’s purchased a second hand 240 m long side-roll irrigator. This enabled the pump to run without the need to throttle the system flow at the well head and increased the amount of water applied per kilowatt of electricity used. The side-roll irrigator was used to irrigate paddocks E, F and Z. It could have also been used to irrigate paddocks B and C but the high labour component required to shift the irrigator meant that it was not used in these paddocks.

    The addition of the side-roll irrigator increased the system flow rate. However, when the Rotorainer was operating on the higher elevation hydrants (i.e. paddocks A, H, J and L), there was an imbalance in the pressure requirements between the side-roll and the Rotorainer. As a result, a greater proportion of the flow was going to the side-roll. Restricting the flow to the side-roll could have rectified this. In addition to make best use of the side-roll irrigator, it would have needed to operate on up to three shifts per day and be used to irrigate a greater area than just paddocks E, F and Z.

    Two of the power line poles on the power supply line running between paddocks A and H have been replaced with taller poles which allow the Rotorainer irrigator to be moved directly between paddocks A and D.

    The property has a substantial amount of shelter trees planted along fence lines, which restricts changes that can be made to the irrigation layout. In addition, the Van de Klundert’s prefer to retain some smaller paddocks (for example paddocks E, F and Z) for speciality seed crops.

5.4    DESCRIPTION OF OPTIONS

    Three design options to improve the irrigation system performance were investigated, with the aim of identifying a preferred option.

5.4.1    Option 1

5.4.2    Option 2

    Option 2 consists of the modifications outlined in Option 1 plus increasing the flow that can be obtained from the system.

    Operation: Run the Rotorainer at 165 m3/hour and side-roll at 40 m3/hour. Rotorainer to run up to two 11-hour shifts per day. Side-roll to run up to three shifts per day. Total system flow 205 m3/hour.

    Enhancements required:
  • Replace the 150 mm diameter rising column at the well (111.4 m length) with a 200 mm diameter rising column. This would reduce the headloss in the rising column enabling the existing pump to provide sufficient head for the increased flow. (Cost $11,200).
  • Replace the 155 mm diameter Class B mainline in paddock A (250 m) with 175 mm diameter Class B mainline to reduce headloss to the highest elevation hydrants. At the time of replacing this mainline it is recommended that an additional hydrant be placed in paddock A, as the current lane spacings are too large. (Total cost including an additional hydrant is $6,800).
  • Install PRVs on each sprinkler on the side-roll to restrict the flow going to the side-roll. (Cost $550).
  • Standardise the nozzle size of the sprinklers on the side-roll to ensure even watering along the length of the side-roll. (Cost $50).
  • Maintenance checks to be carried out on the Rotorainer to enable it to irrigate on 11-hour shifts. (Cost would be dependent on maintenance work required).

    Total estimated cost of Option 2 is $18,600 + GST.

5.4.3    Option 3

    Option 3 consists of further increasing the system flow and purchasing an additional irrigator.

    Operation: Run the Rotorainer at 160 m3/hour on up to two 11-hour shifts per day. Purchase a Cipa hard hose boom irrigator to run at 60 m3/hour with a lane spacing of 45 m. The Cipa irrigator to run up to two 11-hour shifts per day. Under this option the side-roll would not be required. Total system flow 220 m3/hour.

    Enhancements required:
  • Purchase a Cipa hard hose irrigator complete with 350 m of 100 mm diameter hose, a 40 m boom and fitted with Nelson Rotator sprinklers. (Cost $36,000. If a purge compressor is required on the irrigator to empty the hose prior to it being towed out for next irrigation run, the additional cost is $2,500).
  • Install a booster pump at the top of the well to increase the head to enable the increased flow to be pumped. (Cost $10,000).
  • Replace the 150 mm diameter rising column at the well (111.4 m length) with a 200 mm diameter rising column. This would reduce the headloss in the rising column enabling the existing pump to provide sufficient head for the increased flow. (Cost $11,200).
  • Replace the 155 mm diameter Class B mainline in paddock A (250 m) with 175 mm diameter Class B mainline to reduce headloss to highest elevation hydrants. At the time of replacing this mainline it is recommended that an additional hydrant is placed in paddock A, as the current lane spacings are too large. (Total cost including an additional hydrant $6,800).
  • Maintenance check carried out on the existing Rotorainer to ensure that it would operate on 11-hour shifts. (Cost would be dependent on maintenance work required).

    Total estimated cost of Option 3 is $66,500 + GST.

5.5    CHOSEN OPTION

    In the short-term there would be a financial benefit in implementing Option 1, which has a very low capital cost of $600 + GST. Option 1 would ensure that the existing irrigators were able to operating more effectively on the property.

    Following discussions with the Van de Klundert’s, Option 3 was chosen as the preferred medium term upgrade option as it provided the greatest increase in the capacity of the irrigation system. They preferred to replace the side-roll irrigator with an irrigator that required less labour to shift between irrigation runs and that had greater flexibility. Their preferred option of an additional irrigator was a hard hose irrigator due to its ease of moving between irrigation runs and its potential for reduced crop damage by the drag hose. The Rotorainer irrigator with lane spacings of up to 100 m would be used for the longer runs (i.e. paddocks A, H, J, L, N, P, Q and R). The Cipa hard hose boom irrigator with lane spacings of 45 m would be used for the shorter run lengths (i.e. paddocks B, C, D, E, F, G, K, M and Z).

    The total estimated cost of implementing Option 3 is $66,500 + GST. In addition, Option 3 will require a new water permit to cover the proposed increased volume of water to be taken. A resource consent application for a water permit to take groundwater would need to be made to the Canterbury Regional Council.

    Option 3 will also require a change in the way the irrigators are managed. During periods of peak water requirement, the irrigators would be operated on two 11-hour shifts per day rather than one 22-hour shift. This would reduce the irrigation return interval and apply a smaller application depth more frequently to the irrigated area.

    While implementing Option 3 will increase the system capacity of the farm irrigation system from 187 to 220 m3/hour, this increase will not be sufficient to meet the irrigation demand of all crops in the peak of the irrigation season. To meet peak crop water requirements on the property, an additional water supply and associated irrigation equipment would be required.

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