The Ritual Bath (Heb. "miqwah") is a tradition amongst Jews dating back to the time of Moses. While it is certain that a body of water is needed to render bodily cleanness from defilement, as it is written (Lev. 11:36): "Nevertheless, a fountain or pit, wherein there is plenty of water, shall be clean," the prescribed quantity of water in that same bath or ablution is something that can only be known by way of a tradition passed down unto us from the time of Moses (So have we learned in the Palestinian Talmud, Hagigga 7a). So, too, the laws governing the making of ritual baths have largely been passed down by way of Oral Tradition. What things would disqualify one's immersion (e.g. interposing objects between the bather's skin and the water, which prevent the water from completely covering his skin: such as modeling clay, paint, dough beneath one's fingernails, gum resins, bandages attached to one's skin, etc.) are also teachings delivered orally from Sinai, as we learn in the Babylonian Talmud, Sukkah 5b.
Generally speaking, natural springs and cisterns containing a reservoir of water are capable of rendering cleanness to most bodily defilements. If the ablution were man-made, it must contain a minimum of 40 seahs  of rain water for it to render cleanness to a defiled body. (40 Seah = a liquid measure equivalent to the capacity of 5,760 eggs, or water which can be fitted into a small tank measuring 21.06 inches x 21.06 inches square, with a depth of 63.96 inches). Of course, most ritual baths contain well in excess of this quantity of forty seahs.
So, too, man-made reservoirs must only contain rain water, a prerequisite for the ablution to render cleanness to bodily defilement, or else contain water that has been diverted into it by way of a channel from a natural spring, river or lake. Ready-made tanks were never used, seeing that one of the requisites in making a ritual bath is that the place of immersion be stationary rather than portable. Natural impressions in the ground are stationary, or anything similar to them, such as man-made cisterns. Tanks, on the other hand, are considered vessels. Had water been collected therein, they would render the water invalid for purification, or what is known as "drawn water."
As noted, the water collected within the cisterns, or within man-made reservoirs, must be rain water (not drawn water, or pumped water), and allowed to drain naturally by way of conduits or sluices into the ablution, without the aid of buckets or pails since these implements would technically give the water the name of "drawn water." Had cylindrical pipes been used to carry water, this would be fine, since cylindrical pipes were never designed to hold water like ordinary vessels, but only to transport water. (Moreover, a pipe is opened at both its ends, having no receptacle for holding water). They can be fixed into the flooring and into the walls for transporting water directly unto the reservoir. (If valves are put into the pipes for controlling the water intake or flow, these should only be placed at the extreme top of the pipes so as to avoid creating thereby a vessel that would hold water when the valves are shut).
Where rain water is scarce, it is possible to make two reservoirs: the one containing a store of rain water; the other containing drawn water. These two baths or pools should be built one alongside the other, and small holes drilled into their sides and later plugged-up so that they can be opened up, when needed, one into the other. In this way, it is not necessary to fill-up the immersion pool each time after its drainage with fresh rain water. Rather, after refilling the immersion pool with drawn water, all that is needed is to open up the plugged holes and to allow the waters in both pools to mix together  (the one containing rain water, the other drawn water), and in this way, the reservoir of rain water by its "kissing action" of the drawn water, turns its water into rain water, having the same viable force as the other water, and capable of rendering objects clean that had once been defiled.
According to Rabbi Joseph Qafih (who served with Israel's Chief Rabbi emeritus, Rabbi Ovadiah Yosef, in Jerusalem's High Court of Appeals) whenever they wished to build a ritual bath (Heb. miqwah) in Yemen, they would carve out a conduit on the roof of one's house, or what resembles a canal for conducting water. Afterwards, they'd plaster the floor of the roof. Once this had been done, the carved impression takes on the name of "paved conduit" (Heb. mesillah). By way of these lime-coated conduits on the roof tops, water collected from falling rain would run its course directly into the reservoir of stored rain water (Heb. otzar hamayim), usually a cistern built into the ground floor (rather than a portable tank), since the oral law requires that the reservoir be stationary rather than portable. Any portable tank that contained rain water would automatically render such water as "drawn water," and invalid for use in purification, even though it had fallen from the sky. In some cases, the rain water which ran down the sides of the wall from the roof top was conducted thence by the aid of grooves carved into the floor of the courtyard, until it emptied itself into the ablution of stored rain water. Once the ablution was filled to its capacity, excessive water was drained away at its opposite end by a sluice leading away from the ablution at a slight graded angle.
Built directly adjacent to the reservoir of rain water, he said, was another pit containing water that had been drawn by buckets, or the like, from a well or cistern. It was built into the ground floor, in shape like unto a cistern with stairs descending down into it, and plastered so that the walls would be impermeable. It was used for immersion. They would open the plugged-up holes on the sides of each cistern in order to allow the waters to mingle, known as "the kissing action." The store of rain water would turn the other pool into rain water. So was this process repeated whenever the immersion pool became brackish or foul and needed changing. They would simply drain out the immersion pool and refill it with drawn water, and repeat the process known as "the kissing action," turning the drawn water into rain water, without ever diminishing from the power and efficacy of the first pool.
In Yemen, they never made use of the gutter or fixed drainage pipes that were fixed onto the roof top for conducting water, since these had the status of a vessel which would render invalid the immersion pool. 
Water used in an immersion pool may be heated. During the Temple period, priests would heat the water by means of throwing red-hot rocks (taken directly from a fire) within the pool. Around the beginning of the 20th century CE, ritual baths in San'a (Yemen) were built beside public baths, and were heated by a subterranean vault made of red brick beneath the actual bath, wherein was stoked a copper furnace. The fire was brought to feed through a double arched opening in the wall having a common center, and built closest to the baths, thereby heating the water. Today, baths may be heated either electrically or by radiators.
 A seah is equivalent to the volume of 144 medium-sized eggs.
 According to the Mishna (Miqwaoth 6:7), the hole which was to be opened between the immersion pool ("miqwah") and reservoir of stored rain water, allowing the waters to intermingle, was to be at least the size of the spout of a skin-bottle, meaning at least two fingerbreadths wide in diameter, or 4.5 cm. (Some put this at 5 cm.).
 Had artisans first carved out the conduit, such as out of wood or ceramic ware, and only later laid it down upon the roof top, it would render invalid all rain water which passes through it, seeing that the conduit before being laid down was considered a vessel, giving the status of "drawn water" to the water that fell therein (cf. Tosefta Miqwaoth 4:1 and Baba Kama 67a).