Moving beyond the basic arithmetic answer of 800 mm, lets consider how a tree actually grows.
There are a number of factors to consider. Here are a few of them.
• A small tree needs less water than a large tree.
• However, young tree also has a shallower and less-established root system than an older tree.
• Older trees may (or may not) get their roots deep enough access year-round moisture.
• Trees only benefit from water during their seasons of active growth, not during dormancy.
• In much of the temperate world trees are dormant in Winter.
• However in Mediterranean climates , such as Athens or California, dormancy may occur in either Winter or Summer depending on the species.
• Trees can survive with much less water than what is required for maximum growth, or 'ideal' growth whatever that might be.
When determining how much water (total or added) any species or cultivar tree might need, also consider the following.
Size of the tree.
Age of tree
Climate parameters (temperature, precipitation, wind, etc.)
Location in the landscape (bottom land or slope, exposure to sun and wind, etc.)
Soil conditions (water retention, drainage, level of water-table, etc.)
Amount of growth (or crop production) that is expected or desired.