Grades 3–5 · Systems
How does stormwater move through a neighborhood?
Rain runs across a roof but can enter the spaces in open soil. That surface difference helps determine whether neighborhood rain soaks in, collects temporarily or flows toward drains. The same incoming water can follow several routes.
Water entering soil is called infiltration. Water moving over the surface is runoff. A hollow, pond or other storage space can hold water that has not yet soaked in or drained away. These destinations compete for the same incoming water; storage does not make it disappear.
The illustration accounts for twelve invented water units during one event. Its simplified order sends two into the ground, holds four in storage and sends four through the drain. Two units remain beyond the listed capacities. The four destinations add to twelve, so all the incoming water is accounted for.
The twelve invented rain units divide into Soaked 2, storage 4, drainage 4 and Excess 2. Stored water remains in the account because holding it temporarily does not remove it from existence.
In the diagramWhere does the event water go?Rain: 12 water unitsSoak capacity: 2Soaked 2Store cap 4Drain cap 4Excess 2Soak 2 + store 4 + drain 4 + excess 2 = 12
During real storms, soaking, filling and draining happen together. Wet soil has less space available, and drains carry limited amounts in a given time. Water can accumulate while some still leaves; ground shape and timing determine where it gathers, beyond the simple volume account.
About this explanation
Starting knowledge: Add and subtract stated water amounts and compare available capacity.
Sources
- US Environmental Protection Agency — Urbanization and Stormwater Runoff
- US Environmental Protection Agency — Urbanization: Stormwater Runoff
- US Environmental Protection Agency — Nonpoint Source: Urban Areas