pavements

Think about what happens when rainwater falls on an impervious surface in a big outdoor parking lot studded by the occasional tree:  The water dampens the surface, which instantly becomes saturated.  Only a minute percentage of water that penetrates the trees' canopies to reach their curb-bound planters becomes available to the trees' roots.  The rest almost immediately starts flowing to drain grates or perimeter drainage details and is lost to a stormwater-collection system. The trees are helped only marginally by the life-giving rain, and the water