Results (continued from last month)
Volume
Approximately 290 and 500 mm irrigation were applied to the drip and microjet irrigation treatments, respectively, in year 1 (Figure 1).
There was a particularly dry spring in year 1 such that newly planted trees relied almost entirely on water delivered by the irrigation system.
Total ETo and rain during the season in year 1 was 1089 and 170 mm, respectively.
In year 2, slightly more irrigation was applied to all treatments as tree size increased, however, the amount applied to the drip treatments was still substantially less than the microjet treatments.
Total ETo and rain during the season in year 2 was 1026 and 250 mm, respectively.
Frequency
Average irrigation frequency was similar in spring and autumn but increased in summer (Table 1).
The drip-standard treatment was irrigated every 2 to 3 days in mid-summer whereas the drip-pulse treatment was irrigated once to twice daily.
Microjet treatments were less frequent than drip treatments. Average irrigation frequency did not differ substantially between years due to the dominance of soil evaporation in the calculation of irrigation requirement (see equation for ETc).
Stem water potential
Midday stem water potential (Ψleaf) was measured every 7–14 days between 1300 and 1500 h from 144 to 226 days after bud burst in year 1; and from 76 to 222 days after bud burst in year 2.
There was no significant difference between treatments in season average Ψleaf.
Microjet-pulse tended to be lower in year 1 and was significantly lower on the first two measurement days. Lower Ψleaf in the microjet-pulse treatment was attributed to greater soil evaporation and understorey transpiration from frequent wetting of the soil surface.
There was a general trend for less tree growth in the microjet-pulse treatment most likely due to transient periods of water stress associated with a shallower wetting pattern (Table 2).
Although not significant, leader growth was lowest in year 1 and pruning dry weight was lowest in years 1 and 2 in the microjet-pulse treatment.
Canopy radiation interception, as measured by effective area of shade (EAS), increased to an average maximum of 17% by early autumn in year 2.
There was no significant difference in EAS between irrigation treatments.
Such values of EAS meant that the calculation of irrigation requirement (Equation 1) was dominated by soil evaporation and understorey transpiration particularly in the microjet irrigation treatments. This balance between Ke and Kcb will change as the tree canopy increases in subsequent years and intercepts more radiation.
Soil water content
Soil water content was highest close to the emitter at 40 and 55cm depth in the drip-standard treatment (Fig. 2).
Soil was consistently drier at 10 and 25cm for all positions away from the emitter in the drip-standard treatment.
In contrast, soil water content in the microjet-standard treatment was similar at 10, 25, 40 and 55cm depths and did not decrease until 70cm from the emitter.
These results show a distinct smaller wetted soil volume in the drip-standard treatment where the plant available water after an irrigation event was confined to approximately 40–50cm in the horizontal plane towards the mid-row whereas plant available water in the microjet treatment extended beyond 100cm.
CONCLUSIONS
This study established that a narrow wetting pattern under drip irrigation compared with a wider wetting pattern under microjet irrigation can be successfully used in young pear orchards planted in a duplex soil where the emitter lateral is offset 0.25m from the tree base.
No evidence was observed that a reduction in horizontal wetted volume (i.e. drip irrigation) impacted leader vegetative growth, pruning dry weight, light interception or water status in young pear trees in the first two years after planting, despite minimal rainfall during the growing season.
Approximately 40% less irrigation was applied in the drip irrigation treatments resulting in a water saving of 2.1 ML/ha/y and this was attributed to reduced soil evaporation and understorey transpiration.
The results also suggest that the microjet-pulse treatment was vulnerable to water stress and more frequent irrigation with the same amount of water may need to be applied to counter additional soil evaporation and understorey transpiration from shallow wetting of the soil surface.
The next step is to measure fruit bud development, precocity and yield in response to the different irrigation treatments.
Acknowledgements
This project was funded through the Productivity Irrigation Pests and Soils research program by Horticulture Australia Limited using the apple and pear industry levy and matched funds from the Australian Government. Additional financial support was provided by the Department of Environment and Primary Industries Victoria.
See Tree Fruit November 2014
