PURPOSE: To easily adjust the balance between frequency of starting or stopping a pump and width of water-level change in a water distributing tower, only by setting a specific curve.

CONSTITUTION: In a specific curve of L-Q its horizontal axis shows a water-level of a water distributing tower 2 and vertical axis shows a target value of delivery flow rate, and the level LM shows a daytime speed control range determined from the relation between the target value of pump delivery flow rate of a variable speed pump 10 operated in the daytime when a large flow rate is required and the water- level of the water distributing tower 2, while LN shows a nighttime quantity control range of constant speed pumps 4 operated in the night with a small flow rate. Then, for instance, at nignt when the water distributing tower 2 is at a high water-level, the target value of a total delivery flow rate is not so largely changed even if the water-level is largely changed, and this means low sensitivity of the L-Q curve, then the target value of total delivery flow rate is not changed even for a change of the water-level of the water distributing tower 2, capable of decreasing frequency of starting and stopping the pump.