Would it be possible to design a paving stone with a reservoir of some kind under it that would let water drain through the stone into the reservoir and then very gradually seep into the ground around the stone? In a dry climate, the goal would be to create an area of a lawn that was part green and part paving. The paving stones would hopefully make better use of natural rain and (unnatural) watering by capturing water that would otherwise run off or evaporate and releasing it slowly underground. I'm visualize paving stones of about 1 ft x 1 ft and about the same area of grass in between them. My thoughts on such a design: The paving known as "pervious concrete" could be used for the paving stone itself, or one could cast as stone with channels around the edge and small drain holes in them to let water pass through. A reservoir of only water under the stone might breed mosquitoes or bad odors. Perhaps the reservoir must actually be a bed of absorbent material like sand or clay. A critical goal of the design is to slowly release water to the areas adjacent to the stone instead of losing the water by letting it percolate into the ground directly under the stone. The bottom and upper sides of the reservoir should be impervious to water. If the reservoir is concrete, perhaps those areas of the reservoir can be sealed. I don't have a good intuiton for how quickly water would seep thorugh the unsealed parts of a concrete reservoir. (How fast you want the reservoir to empty would depend on the climate.) Do concrete swimming pools need a special coating to keep the water from seeping out? Or is the coating just to keep the concrete from getting into the water? In some soils and climates, perhaps no elaborate reservoir is needed. Perhaps you could simply bury a relatively impervious slab of concrete 10 inches or so directly under the paving stone and accomplish the goal. ------ My thoughts are just for a homeowner's type project. Anyone who wants to try this as an academic investigation or commercial venture is welcome to the idea.