In Catholicism, how much water is necessary for someone to be baptized?
For example, if the only water present is a single droplet on a leaf, is that sufficient to baptize or is more needed? How much more?
Or if there is not even a droplet of water, but it is a humid day, and there is water in the atmosphere, is the atmospheric water that is continuously in contact with the skin sufficient, or is more required?
The Catholic Church has always taught that the three valid forms of Baptism are immersion, pouring, and sprinkling. Evidence that the Church has validated the form of pouring instead of immersion is demonstrated by the Didache which was written around A.D. 70:
"Concerning baptism, baptize in this manner: Having said all these things beforehand, baptize in the name of the Father and of the Son and of the Holy Spirit in living water [that is, in running water, as in a river]. If there is no living water, baptize in other water; and, if you are not able to use cold water, use warm. If you have neither, pour water three times upon the head in the name of the Father, Son, and Holy Spirit."
These instructions were composed either while some of the apostles and disciples were still alive or during the next generation of Christians, and they represent an already established custom.
Perhaps the "minimum" amount of water required to effectively administer the sacrament is impossible to quantify. In order to answer the question as accurately as possible, lets assess which situation would call for the smallest amount.
These days, the rules for administering the Sacrament of Baptism are clearly given in the Rituale Romanum:
The form for baptism is as follows: I baptize you in the name of the Father, and of the Son, and of the Holy Spirit, and it is absolutely essential. In no circumstance can it be altered, and these words must be pronounced simultaneously with the pouring of the water. The water is to be poured on the head with a triple ablution (or the head is to be immersed three times), each time in the form of a cross, saying the words simultaneously.
The Rituale Romanum also decrees that in the event of a complicated birth an emergency conditional baptism should be administered to the infant:
No child is to be baptized while still enclosed in the mother's womb, as long as there is a probable hope that it can be properly brought forth and then baptized. If only the head of the child has come forth and there is danger of its dying, it should be baptized on the head; if afterward it is born and lives, baptism may not be repeated conditionally. If another member of the body makes its appearance and there is danger of death, the baptism should be conferred conditionally upon that member; if the child lives after birth it must be rebaptized conditionally. Should a mother die in confinement, the fetus should be extracted by those obliged thereto by their profession, and if there is a certainty that it lives, it should be baptized absolutely, otherwise conditionally. A fetus baptized while in the mother's womb must be rebaptized conditionally after birth.
One should see to it that every abortive fetus, no matter of what period, be baptized absolutely if it is certainly alive. If there is doubt about its being alive, it should be baptized conditionally.
Considering that a new born (or, unborn in the case of an abortion) infant/fetus is about as small as any recipient might be, I would say, "However much it takes to roll 3 beads of water across a baby's forehead.
Charles Alsobrook has quoted authoritative sources in his answer, but there is another side to the question (or the answer).
The Didache indicates that the minister should use as much water as possible1 — a river is mentioned first. And in every case the water must be moving, either of its own accord or having been enlivened by pouring.
It's not a question of the minimum amount of water but of its quality. A drop from a pipette and running across the skin is enough: it's moving. If only a drop is available at all, it's enough.
However, the water does need to move, because it needs to be directed to the candidate. Relying on the humidity in the air is not enough, because it's not moving; but once dew has formed and can be used on an individual, any amount [the larger the better] will do.
1 As much as possible is used as a sign of God's generosity in giving life to man. The Sprinkling in the Shrine of Our Lady at Walsingham [Norfolk, UK] is a good example. There's a little bit about it on their website (bottom left of the page).
From "Moral Theology" by Fr. Heribert Jone (a book that was recommended to me by a traditional Catholic priest):
"A definite quantity of water is not required; it suffices that the water flow over the one being baptized. Some authors hold the flowing of one or two drops insufficient." (Section 467)
I take the first of the quoted sentences to be the majority opinion and Fr. Jone's opinion, while the second sentence reports a minority opinion.
Also, in view of a comment on the question, suggesting saliva, I should add that Fr. Jone includes saliva in a list of "certainly invalid matter". In case anyone's interested, the list (in Section 464 of the book) is "milk, blood, amniotic liquid, tears, saliva, foam, fruit juices, wine, beer, oil, thick broth, ink."
By the present authorized ritual of the Latin Church, baptism must be performed by a laving of the head of the candidate ... It is to be noted that it is not sufficient for the water to merely touch the candidate; it must also flow, otherwise there would seem to be no real ablution. At best, such a baptism would be considered doubtful. If the water touches only the hair, the sacrament has probably been validly conferred, though in practice the safer course must be followed. If only the clothes of the person have received the aspersion, the baptism is undoubtedly void.
