Abstract: A water distributor is disposed above an air-liquid contact pad, and is supplied with water from a supply line. The water is received within an elongated enclosed chamber having at least one elongated corner in its lower portion. A floor portion of the chamber is formed of an imperferate slightly flexible wall disposed generally tangential to the corner. A screen is disposed on the inner surface of the floor portion and is manually adjusted to engage with the said corner in a line contact relationship to form a water flow restriction or gap. The chamber above the restriction is sealed. During each cycle of operation, as water initially flows into the chamber, a miniscus forms at the gap which prevents water discharge. As the chamber fills with water, the air thereabove is trapped and pressurized and, together with the pressure of the accumulating head of water, gradually increases the pressure on the miniscus.