Figure 9: Absolute and relative differences in irrigation amount, harvest, and crop water use efficiency (CWUE) between the full and the 75 % irrigation scenario (DI75-FI), and the full and 50 % irrigation scenario (DI50-FI) for apple orchards within the PHO during the period 2016-2022.

3.2.4 Irrigation and yield at the inter-annual and monthly scale

Yearly irrigation amounts, precipitation during the main irrigation season, and harvest averaged for all apple orchards in the PHO are shown in Figure 10. For the investigated 7-year period, irrigation ranges between 297 and 487 mm while precipitation is around 167-322 mm from May to October. Differences in precipitation drive the inter-annual variability in irrigation requirements whereby drier summer months, such as 2019-2022, result in higher irrigation demand compared to wetter years. Yield ranges between 32 and 55 t ha-1, with 2019 and 2020 being the years with the highest yields due to favorable meteorological conditions (high solar radiation and temperature). Notably, the effect of deficit irrigation on yield is strongest in these two years reducing yield by >12 t ha-1for the DI50 scenario. In contrast, both the DI75 and DI50 scenario have negligible effect on yield in the first three simulation years.