Re: Palm, date palm, coconut tree.

I assume you mean liters of water needed for evapotranspiration per day.
Palm trees, as desert plants can survive on very little water per day.
However the absolute mininmum amount of water depends on how big the tree
is, whether natural subsurface water or moisture is available, and how
harsh the environment, especially wind and temperature and for how long a
time these conditions have and will exist. An absolute amount of water is
not possible to calculate without knowing these variables.
Coconut trees on the other hand grow in more tropical climates where water
is not limiting and if grown in desert climates, would require much larger
amounts of water, again depending on how big the tree is and how harsh the
environment, especially wind and temperature.