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.