So, anyone in Pittsburgh or nearby areas has noticed by now that it is HOT out! Temperatures have been reaching the 90s for the last couple of days and probably will continue to do so for the next couple of days. This temperature map from the National Weather Service has perhaps poorly chosen colors (that yellow doesn't look so bad!) but you can see from the temperature readings that it is expected to be uniformly hot throughout the region.
This may seem nasty but believe it or not it is actually hotter and more humid in parts of Vermont than in Pittsburgh.
This is where the second half of 'slow water' kicks in.
Remember all that water the rain gardens were soaking up during the last period of thunderstorms? Some of it was used up by plants and released into the atmosphere. A lot of it is absorbed into the groundwater. In Schenley Park, Panther Hollow Run is almost dry, and the little bit of water in it right now certainly comes from groundwater, since it hasn't rained in a while.
An interesting thing to think about is where water goes once it soaks into the ground. I mentioned at one point that water that falls on Squirrel Hill and other urban neighborhoods mostly is routed into the combined sewage system. Some of the water, however, soaks into the ground in lawns or landscaped areas (or rain gardens!) and enters the groundwater. After that, where it goes is determined by areas underground that water can not soak through, like bedrock, or sometimes certain kinds of compacted clay. In Pittsburgh the bedrock is generally not all that far underground, so the water probably emerges later in streams or one of the rivers, rather than sinking into an aquifer deep underground. Most of the water that does soak into the ground in Squirrel Hill and Oakland and isnt used by plants probably turns up later in Panther Hollow, Four Mile Run, or the Monongahela River - after being filtered by the soil. It takes a long time for water to soak through soil, and water can take months or even years to emerge after soaking in (in some areas, such as parts of California, there is water coming out of springs that probably fell as rain or snow as long ago as the Ice Age!). Thus the ground acts as a sponge, absorbing rain and releasing it later. The more people use rain gardens, and use rain barrel water to water their gardens, the more water enters the groundwater rather than the sewer system.
On a related note, people often ask why we choose plants for rain gardens that tolerate dry conditions as well as flooding. After a week of 90 degree temperatures and no rain, even the rain gardens are going to be quite dry. Well-chosen plants will not require extra water during heat waves like this one, or at least, will be drought-tolerant enough that water from a rain barrel will keep them alive.