There is a huge difference between the basic halacha and the practical application in this matter, and it is important to try arrive at a reasonable perspective.
The required volume of a cup used for netilat yadayim is a revi’it.[1] There is a well-known machloket as to the volume of a revi’it. The most prominent opinions are those of Rav Chayim Na’eh – that it is 86 cubic centimeters (cc. or milliliters – approximately 3 oz.) – and of the Chazon Ish – that it is 149 cc. (approximately 5 oz.).[2] While this is an important machloket regarding such issues as Kiddush, it is usually not crucial regarding the cup for netilat yadayim, as it is difficult to find a cup for that purpose that does not hold the volume of the most stringent opinion.
Regarding the amount of water needed for washing, one does not have to use an entire revi’it. However, there are advantages to using a revi’it. When one pours less than a revi’it on the hands, the water becomes tamei,[3] and if the water goes beyond the area that needs washing and then returns to the critical part of the hands, the hands become tamei again.[4] This requires one to keep his hands raised the whole time so that any water that runs off will not return. If, however, a revi’it is used at one time, even if it is one revi’it for both hands together, the water does not become tamei and it is not a problem if the water goes beyond the hand area and comes back.[5] According to most opinions,[6] when a revi’it is used, there is also no need to wash the hands twice.[7]
In truth, however, using even significantly more than a revi’it of water is beneficial, as water must reach the entire area that needs to be washed at one time.[8] There is significant discussion concerning how much of the hand must be washed. One opinion is that it is only the fingers, up to their connection to the palms. The other opinion, which is the accepted practice under normal circumstances, is to wash all of each hand, until its connection to the wrist.[9] The Bi’ur Halacha[10] advises that since one washes the entire hand, “it is prudent to be careful not to use an exact amount of water. Rather, he should wash with an abundance [of water], for if he uses exactly a revi’it, it is very likely that part of a hand will remain unwashed.”
There is a famous story (which comes in many versions) about Rav Yisrael Salanter, who was observed using a minimal amount of water for netilat yadayim. He did not want his fulfillment of the mitzva in the preferred way to come at the expense of someone else – i.e., the servant schlepping the water. This might support your suggestion that in our situation of limited water supply,[11] we should curtail our ritual use of water to the minimum amount required. Indeed, under the correct circumstances (it is likely that Rav Salanter usually used a larger quantity, and the case of the story was one in which someone was uniquely affected), this is a very laudable approach. But although it is true that using ridiculously large amounts of water, which might cause others to have to wait on line until the person finishes washing, is likely more a sign of psychological compulsion or ignorance than righteousness, we should avoid being judgmental.
Furthermore, we would urge even those who are sensitive to the benefits of conservation to employ some balance and perspective. We would guess that the average religious family expends less than 1% of its water usage on netilat yadayim. If this is indeed the case, cutting back on other uses by a mere 0.5% will save as much water as cutting back on netilat yadayim by 50%. If one is already cutting back on water usage to his maximum in general, and he wants to include netilat yadayim as well, his idealism is praiseworthy, and it is fine to concentrate on washing the entirety of his hands with less water than most people use. But the great majority of the members of our society, who use water with less idealism, should not make a special issue of water conservation regarding the moderate amounts of water used for netilat yadayim.