Topic 5.10 Notes – Impacts of Urbanization

What Urbanization Is

Urbanization means two things happening together:

• a growing proportion of people live in cities
• developed land physically expands

The key environmental idea is simple. Natural land gets replaced with built land, and that changes how ecosystems work. Water soaks in less, habitats get broken up, and resource use becomes concentrated.

A common mix-up is urbanization versus urban sprawl.

• Urbanization = growth of urban populations and development
• Urban sprawl = low-density spread outward into suburbs and rural land

A city can grow in a compact, high-density way or in a low-density outward way. That matters because compact development often uses less land and less energy per person, even if the city’s total resource use is still huge.

Keep four impacts connected in your head through the whole topic:

• resource depletion
• hydrologic-cycle disruption
• increased atmospheric carbon
• habitat loss and fragmentation

How Urbanization Changes Water Flow and Water Supply

On natural land, rain can:

• infiltrate into soil
• percolate downward
• recharge groundwater
• return to the air by evaporation and transpiration
• move as surface runoff

Urban land shifts water away from infiltration and toward runoff. That is why hydrographs for developed watersheds usually show a faster, taller spike after rain.

Hydrograph of forested vs. non-forested runoff

Impervious surfaces

Impervious surfaces are human-made surfaces that block water from reaching soil.

Illustrative examples you should know:

• roads
• buildings and rooftops
• sidewalks
• parking lots
• traditional asphalt and concrete surfaces

Runoff and flooding

This cause-and-effect chain gets tested a lot:

1. More impervious surface
2. Less infiltration and groundwater recharge
3. More surface runoff
4. Water reaches streams and storm drains faster
5. Higher peak discharge and more flooding

Urban streams are called flashier because they have:

• shorter lag time
• higher peak discharge
• quicker drop after the storm

That fast runoff also erodes stream banks and carries pollutants quickly.

Groundwater depletion

Cities use lots of freshwater from rivers, reservoirs, and especially aquifers. Depletion happens when water is withdrawn faster than recharge replaces it.

Results can include:

• lowered water table
• dry wells
• land subsidence from sediment compaction

This connection matters a lot. Urbanization can increase water demand and reduce recharge at the same time.

Saltwater intrusion

Saltwater intrusion is saline water moving into a freshwater aquifer, usually in coastal cities.

A coastal aquifer cross section helps you picture what is happening underground near the shoreline.

Saltwater intrusion in a coastal aquifer

Sequence:

1. Heavy groundwater pumping
2. Freshwater pressure drops
3. Seawater moves inland or upward underground
4. Wells become brackish or salty

This is not seawater flooding over land. It happens underground.

How Urbanization Affects the Carbon Cycle

Urbanization raises atmospheric carbon in three main ways.

Fossil-fuel combustion

Transportation, electricity, heating and cooling, industry, and construction burn coal, oil, and natural gas. That moves stored geologic carbon into the atmosphere as CO2. Sprawl usually increases driving, so it often increases emissions too.

Landfills

Cities generate lots of waste with organic carbon.

• aerobic decomposition releases CO2
• anaerobic decomposition in landfills releases methane and CO2
• burning landfill gas still produces CO2

Vegetation removal

When land is cleared for development:

• fewer plants remove CO2 by photosynthesis
• less carbon is stored in biomass and soils

What Urban Sprawl Changes

Urban sprawl is movement from high-density urban areas to low-density suburbs spreading into rural land.

Land conversion and habitat fragmentation

Sprawl replaces:

• forests
• wetlands
• grasslands
• farmland

This splits continuous habitat into smaller patches. That means:

• more edge habitat
• less interior habitat
• blocked wildlife movement
• more wildlife-vehicle collisions

Transportation and infrastructure impacts

Sprawl separates homes, stores, and jobs, so people depend more on cars. It also needs more:

• roads
• parking lots
• sewer lines
• water lines
• electricity infrastructure

That raises both environmental costs and public costs per person.

Why These Impacts Matter and How They Are Reduced

These effects reinforce each other. Sprawl adds impervious surface, runoff rises, recharge falls, pumping increases, coastal aquifers face saltwater intrusion, and driving raises CO2 emissions.

Common ways to reduce impacts:

• compact development and public transit
• permeable surfaces and less unnecessary pavement
• protecting recharge areas
• water conservation and managed aquifer recharge
• urban trees and green spaces

Tradeoffs matter too:

• compact development can concentrate heat, traffic, and pollution if poorly planned
• green space and recharge projects need land, money, and maintenance
• no single solution fixes every urban problem