Uchimizu is an ancient Japanese custom of sprinkling water on surfaces such as roads and sidewalks to cool the air during the summer through evaporation.
Translation of the text on the cover: [Emblem of Minato] この歩道は – This sidewalk 保水性道路です。– is a water-retentive sidewalk (or: a sidewalk designed to retain water). 打ち水に – with the practice of uchimizu (打ち水) ご協力ください。– Please cooperate / Please help.