Berend, looks like you are well organised.So on reading your latest noite you seem to be seeking knowledge about the minimum volume of water that you will need for the plantings.
Here is a research paper about "These results evidence the need for better management of orchards irrigation water in the region,
and the current study provides for reliable information on the Kc of tree crops to support improving the management of local orchard systems
and the preservation of soil and water resources.
Aimed at these resources and the sustainability of their use, simulated alternative irrigation schedules were performed,
which identified possible water savings of 20 mm in case of olives, up to 855 mm for citrus."
use of water for irrigating tree crops in Southern Portugal
Another "Mediterranean Olive Orchards under Climate Change:"
Climate change in the Mediterranean
"The Mediterranean Basin is considered a climate change “hotspot,” as future projections hint at considerable warming and drying trends.
Changes in olive tree suitability have already been reported over the last few decades. "