Figure 8: Mean and standard
deviation (SD) of average yearly irrigation, soil moisture in the root
zone (0-60 cm), harvest, and crop water use efficiency (CWUE) for apple
orchards within the PHO between 2016-2022 under full irrigation (FI).
3.2.3 Effect of irrigation deficit scenarios
The effect of deficit irrigation on total irrigation amounts, harvest,
and CWUE of apple orchards in the PHO for the moderate irrigation
deficit scenario, DI75, and the more severe deficit scenario, DI50, are
shown in Figure 9. Yield differences between the FI and the DI75
scenario are almost negligible, ranging from a decline of maximum 3 t
ha-1 (5 %) to even slight increases in yield.
However, the DI50 scenario resulted in a clear decline of simulated
yield with up to 12 t ha-1 corresponding to a 30 %
reduction in yield compared to the FI scenario. Nonetheless, orchards
located at high altitudes and in the southeast on clay-rich soils are
still barely affected by the higher water deficit (<5 %
decline in yield). Overall, annual water savings are highest in the
plain, averaging 100-125 mm for DI75 and 210-250 mm for DI50. CWUE shows
a differing pattern between both scenarios. While in DI75, CWUE declines
slightly in the central part of the plain by around 1 kg
ha-1 mm-1 (2 %), there are large
areas that show an increase in CWUE of similar magnitude. The decline in
CWUE is concentrated on the orchards growing on soils with a high
percentage of sand. For DI50, on the other hand, CWUE is almost
exclusively showing a decrease of up to 8.8 kg ha-1mm-1 (17 %), though again CWUE for orchards in the
higher altitudes and the ones located on soils with higher clay content
are less affected.