AP® Environmental Science · Unit 5 · Topic 5.10
Impacts of Urbanization
Urbanization replaces soil and plants with pavement and buildings, which creates urban heat islands and fast, flood-prone runoff, while changing a watershed's natural flow pattern.
A city stays warmer than the countryside around it, especially at night, because of what it's built from. Dark pavement, roofs, and masonry absorb sunlight all day and release that stored heat slowly after dark. Cars, air conditioners, and buildings add waste heat on top of that. Farmland cools off faster because plants shade the ground and release water vapor through evapotranspiration, which pulls heat out of the air. A city has far less of that vegetation, so its nights stay warm. This is the urban heat island effect.
Pavement changes water too, not just temperature. Soil and plant roots normally soak up rain and slow it down, letting it filter into the ground. Pavement is impervious: water can't sink in, so it runs fast across the surface into storm drains, carrying oil, salt, and trash with it. A watershed that goes from forest to suburb sees this shift directly — streams that used to rise and fall gently now spike hard and fast after storms, a pattern called flashy flow, with higher peak flows and lower base flow between storms since less water recharges groundwater.
None of this means density itself is the problem. A compact city with shared transit and a small footprint can have a lower per-person impact than the same population spread across a car-dependent suburb, because shorter trips, shared infrastructure, and a smaller developed area leave more surrounding land undeveloped. How a population is arranged matters as much as how large it is.
Key terms
- Urban heat island
- A pattern where city areas stay measurably warmer than surrounding rural land, especially at night.
- Impervious surface
- A surface like pavement or concrete that water cannot soak through, forcing rain to run off instead of infiltrate.
- Evapotranspiration
- The combined cooling process of water evaporating from soil and being released as vapor by plants.
- Flashy flow
- A stream flow pattern with sharp, fast peaks after rain and low flow between storms, common in urbanized watersheds.
- Per-capita impact
- The amount of environmental harm or resource use divided by the number of people, used to compare density patterns fairly.
Watch it explained
- How it worksWalk me through pavement turning a gentle rain into a fast flood inside a storm drain.
- How it worksHow do dark surfaces, waste heat, and missing trees build an urban heat island?
- Going furtherWhat happens to a stream's flow pattern when its watershed goes from forest to suburb?
- Common mistakeWhy isn't a dense city automatically worse for the environment than a spread-out one?
Check yourself
End of topic.