Deplete, drain, exhaust, impoverish, bankrupt are comparable when they mean to deprive a thing in whole or in part of what is essential or necessary to its existence or potency.
Deplete is often used as though it implied merely a reduction in numbers, in quantity, or in mass or volume; it may be used specifically to suggest the potential harm of such a reduction or the impossibility of restoring what has been lost before such consequences are evident; thus, bloodletting depletes the system, not only by reducing the quantity of blood but by depriving the system of elements essential to its vitality and vigor; an epidemic depletes an army when it reduces the army not only in size but in effective strength, especially at a time when that strength is needed.
Drain when precisely employed retains its basic implications of slow withdrawal of liquid (as by straining, seepage, or suction) until the substance which is drained becomes dry or the container which holds the liquid is emptied; hence it connotes a gradual depletion and ultimate deprivation of the figurative lifeblood of a thing or the essential element of its existence or well-being.
Exhaust (see also TIRE) is very close to drain, but it stresses emptying or evacuation rather than gradual depletion. Unlike drain, which usually implies loss without compensating gain, exhaust need not suggest ultimate loss of what is removed; thus, a mine is exhausted when all its ore has been removed for refining; a soil is exhausted, or drained of nutrients, by growing crops on it without adequate fertilizing; but, a person is drained of vitality when overwork or illness reduces him to a weak or ineffective state.
Impoverish implies a depletion or a draining of something as essential to a thing as money or its equivalent is to a human being; it stresses the deprivation of qualities essential to a thing’s strength, richness, or productiveness.
Bankrupt stresses such impoverishment of a thing that it is destitute of qualities essential to its continued existence or productiveness; it connotes a complete or imminent collapse or breaking down.