Evapotranspiration of high performance apple tree orchards

Information on orchard water use is crucial for irrigation scheduling and on-farm water management to ensure growth and sustainability of the fruit industry. A greater WRC/Hortgro Science funded study proposes a multi-disciplinary approach to establish unstressed water use by high performing orchards and orchards at different growth stages from planting to full-bearing age and to develop models to extrapolate the results of the study to other apple cultivars and growing regions in the country. The ARC/Hortgro Science project forms an integrated part of this study by monitoring soil water status, estimating water use according to a soil water balance and assisting in apple orchard water balance modelling.

Data were collected over three growing seasons (2014/15, 2015/16 and 2016/17) in two production regions. Soil physical properties (texture, chemical status, soil water retention curves) of twelve orchards in the Koue Bokkeveld (KBV) and Elgin, Grabouw, Vyeboom and Villiersdorp (EGVV) regions were determined. Soil water content sensors were installed and calibrated, soil water content monitored throughout the season and evapotranspiration (ET) calculated according to a soil water balance for two of the four selected micro-irrigated orchards per season due to equipment limitations.

The maximum monthly averaged soil water balance based ET in low canopy cover non-bearing orchards, medium canopy cover intermediate bearing orchards and high canopy cover full bearing orchards, respectively, ranged between 2.5 mm and 3.8 mm; 4 and 5.3 mm and 3.9 and 5.2 mm.  Differences in orchard water use were mainly caused by variation in tree size, presence/ absence of weeds and cover crops and irrigated area wetted width. Increased ET from the work row of full surface compared to short-range micro-sprinkler irrigated orchards can be attributed to water use of tree roots, weeds and cover crops. Under-irrigation of full surface micro-irrigated apple trees may result in a twofold increase in water usage from the work row area compared to that of sufficiently watered orchards. The KBV had higher evaporative demand and lower rainfall compared to EGVV, creating the expectation that more irrigation will have to be applied to orchards of comparable properties. However, the total amounts of irrigation applied did not increase from non-bearing to full-bearing in similar production regions, nor was more irrigation applied in KBV than in EGVV for orchards of comparable properties. These anomalies were best explained by the soil water dynamics, which indicated that, even though farmers attempted best irrigation practices, dry and/or overly wet conditions were present periodically in all six orchards.

Measures to improve on-farm irrigation scheduling are necessary. Knowledge regarding soil water holding capacity and water requirements of individual orchards can enable accurate irrigation scheduling. Properly calibrated continuous logging soil water content equipment can aid to improve irrigation scheduling. Longer irrigation intervals and deeper irrigations can reduce evaporative losses. Cover crops and weeds increase orchard water use and should be managed to minimise orchard water loss. Proper soil preparation and a well-developed root system will enable trees to access water from the work row area during drought conditions.