PURPOSE: To increase a pondage in a reservoir by a method wherein a coefficient of flowing water, i.e. a factor for showing the pondage in the reservoir is artificially increased.

CONSTITUTION: Medium sized and large sized living trees 1 and left in a flow area A facing a reservoir, remaining assorted trees or weeds are removed, and then some waterproofing materials such as concrete, asphalts, vinyl etc. are coated on the earth surface. Rain falls in the area A are flowed into the reservoir B without immersing into the ground due to waterproofing materials 2, resulting in that a coefficient of flowing water is increased. The pondage in the reservoir is expressed by a relation of: (Pondage)=(annual rainfall)×(drainage area)×(coefficient of flowing water), and so it is possible to increase the pondage by increasing the coefficient of water flowing artificially.