Abstract: A main valve (1) is connected into a pressurized water pipe for an irrigation installation and has a valve body (4) formed as a differential piston and with a through bore (5), said body being operated in both directions by means of the water pressure, the piston (4) in one end position closing the water pipe and in its other end position keeping it open. The piston (4) is guided to said end positions by means of a control valve (7) arranged to open or close a duct (6) starting from the major cylinder bore, thereby to cause the water present to actuate the piston (4). The control valve (7) is in turn controlled by rainfall onto, and evaporation from an upwardly open bowl (8), and a drop means (9) active during irrigation. The bowl (8) is part of a tipping means with two tipped end positions, controlled by the quantity of water in the bowl (8).