Centre pivot test results on the Tallangatta Focus Farm
When measuring the Coefficient Uniformity (CU) a value of less than 90 per cent is considered unacceptable and hence a closer look is required to rectify the problem. A value above 90 per cent is considered acceptable and the crop will cope with the distribution difference.
Results uniformity test: The test conducted on the 38 hectare centre pivot on the Focus Farm resulted in a below standard result of 73 per cent. The results from the uniformity test can be seen in Figure 1 below.
Figure 1: The results from the "catch can method" showing the average application rate at 9.0mm and the variation around this.
Reasons for poor uniformity
It is expected in this case the poor result is due to blocked nozzles as a result of sucking water from a water hole and some filter issues. Once these have been unblocked the system will be retested. Other possible causes could be incorrect operating pressure, worn or incorrect nozzle placement.
Consequences of poor uniformity
Having low coefficient uniformity can have two effects or a combination of these:
- Under watering occurs: If planning to apply 9 millimetres in total, some areas under the pivot might only get 4 millimetres while other areas get 14 millimetres. The areas getting 4 millimetres are going to have reduced yields due to moisture stress. Calculating the exact reduced yield is difficult however applying half the required water generally reduced yield by more than half.
- Over watering occurs: If the low uniformity is compensated for and 14 millimetres of water is applied pumping costs will increase as well as water use . Doing some rough calculations changing from a 73 per cent to 90 per cent coefficient uniformity means applying an extra 1.5ML/ha over an average irrigation season. For this 38 hectare centre pivot the increase in running cost is likely to be $2,812 per annum;
- $1,672 in electricity costs @ $0.17/kwatt.hr; and
- $1,140 in water cost @ $20/megalitre.
Frequency of irrigation and application rate
The typical amount of water required can be determined from historic climatic conditions. Using historic daily evaporation figures for each month and multiplying by a crop factor (for pasture 0.8) the evapotranspiration rate or the amount of water you need to apply daily can be calculated. Knowing the amount of water the centre pivot applies per turn, allows the number of days between each run can be calculated.
Table 1 shows the average daily evaporation for each month for the location of the Focus Farm, and hence the average daily irrigation required.
|Daily evaporation (mm)||Crop factor||Daily transpiration (mm)||Average number of days required between runs when application rate is 9mm (days)|
For example in January (without rainfall) 5.8mm/day will need to be applied (0.058ML/ha). If the measured application rate of 9 millimetres is consistently applied per turn, the centre pivot will have to have one complete run every 1.6 days to apply enough water. Compared to April only 2.2 millimetres/day is required, with a 9 millimetres application rate the pivot only needs to do one turn every 4.2 days. These results are only considering the evaporation rates and do not take into account any rainfall over the irrigation period and the average evaporation rates are used as an indication only; the evaporation rates can increase and decrease from these on any given day.
For more information or if you are interested in testing your irrigation system, please contact Dennis Watson, DEPI Rutherglen (02) 6030 4567.