To reduce useless transpiration of water, and contribute to the preservation of a water resources by stretching a shade screen a shade net having constant transmissivity around constant height from the water level of a dam lake, a reservoir or the like.

A shade screen and a shade net A having constant transmissivity are stretched around constant height from the water level B of a storing reservoir or the like around constant height to shield direct sunlight. The stretching method may utilize a support F, a helicopter D, advertising balloons C, C, a living tree E or the like. The useless natural transpiration of water by direct sunlight is relieved, and they does not cause injury to an ecosystem. Thereby water shortage caused by the dam lake, the storing reservoir or the like of each region in a low water season is relieved, and they can inexpensively and surely contribute to the preservation of water resources.