Figure 10: Yearly sum of
precipitation during the main irrigation season (May-Oct), irrigation,
and harvest averaged over all apple orchards within the PHO from 2016 to
2022, under full irrigation (FI) and the difference for the 75 % and
the 50 % deficit irrigation scenarios (DI75 and DI50).
Figure 11 shows the seasonal course of irrigation, precipitation, and
fruit growth in the apple orchards averaged over the PHO and the 7-year
period. The simulated irrigation season starts in April or May and lasts
until October with negligible amounts still applied in November for some
years. Monthly irrigation requirements increase sharply between April
and June until reaching their peak in August with on average 107 mm per
month. Accordingly, August is also the month in which the greatest water
savings occur for the deficit scenarios. After that, irrigation declines
rapidly. Fruit biomass increases steadily from April to harvest in
September with faster growth occurring in the earlier months. While
fruit growth is barely affected by a 25 % reduction in irrigation
(DI75), for the DI50 scenario it decreases sharply in August and to a
smaller extent in July and September. The reduced fruit growth results
in a yield loss of on average 0.5 t ha-1 for DI75 and 6.5 t ha-1 for
DI50.