Understanding the effect of canopy design on crop evapotranspiration for Scilate apple and Syrah grape in Hawke's Bay, New Zealand
Efficient and effective irrigation scheduling is strongly dependent upon accurate estimation of crop evapotranspiration (ETc), this requires understanding and incorporation of the effects that different soil conditions, orchard designs and trellis systems have on ETc. Orchard designs and trellis systems are designed to harness ≥ 85% light interception when fully grown, which is expected to lead to high yield and fruit quality. To investigate the effectiveness of such designs, an experiment in Hawke’s Bay, New Zealand, was carried out. Scilate apple trees of fifth-leaf vee and tall spindle, fourth-leaf planar, and Syrah grapes that were seven-years-old grown on silt-loam soil, and fourteen-years-old grown on stoney soil were used to investigate how light interception (LI), leaf area index (LAI), yield and fruit quality differed among these orchard designs, and ultimately affected evapotranspiration during the 2019/20 season.
Frequency domain reflectometers measuring soil water content (SWC), and tensiometers (matric potential) were used to continuously measure the soil water balance and estimate crop evapotranspiration. Light interception (LI-COR, Lincoln, Nebraska, USA) and manual LAI readings were taken over four periods from budburst to full canopy. Fresh weight yield was extrapolated from a sample per tree/vine (n=30), maturity was measured by starch pattern index in apples, and quality was estimated from dry matter content (DMC%), and soluble solids content. Hourly ETo was computed using the FAO standardised Penman-Monteith equation, and data collected by an automated on-site weather station. ETc was estimated for ‘well-watered’ conditions using extrapolated daily light interception measurements, and compared with measured changes in SWC.
The apple trellis systems showed variation in LI (vee: 56%, tall spindle: 50%, planar: 36%), and LAI (vee: 3.2, tall spindle: 1.6, planar: 1.3). The vee system had the largest fresh weight yield in tonnes per hectare (vee: 141, tall spindle: 108, planar: 54). The different aged vines and soil type also showed variability in LI (young Syrah: 36%, old Syrah: 22%) and LAI (young Syrah: 1.3, old Syrah: 0.7). This variability was particularly obvious in the SWC results, where tall spindle showed the highest ETc and change in SWC, whereas planar and vee the lowest which was attributed to an ‘over-irrigated’ environment causing waterlogging, and ultimately decreasing transpiration, despite vee having the highest LI and LAI. Differences in orchard design and trellis system caused changes in ETc and soil water content which can be illustrated by light interception and LAI. However, this thesis demonstrated that soil and groundwater heterogeneity can cause significant variability in results which needs to be accounted for when modelling, irrigating and growing. Future work suggestions are the inclusion of transpiration measurements using sap flow meters to differentiate drainage and groundwater effects from ETc, and to further clarify when the trees/vines are being over-watered.