Impacts of Urbanization - AP Environmental Science

SECTION 1

Historical Context & Motivation

For most of human history, people lived in small agrarian settlements dispersed across the landscape. The rise of urbanization—the concentration of populations into dense, built-up areas—began to accelerate during the Industrial Revolution, when factories drew laborers from farms into cities. By 2008, the United Nations reported that more than half of the global population lived in urban areas for the first time, and projections suggest that figure will reach 68 percent by 2050. Understanding urbanization matters to environmental science because this demographic shift drives habitat conversion, alters biogeochemical cycles, increases energy demand, and amplifies both pollution and vulnerability to natural hazards.

1800 • Pre-Industrial Baseline

Only about 3% of the world's population lives in cities. Land use is overwhelmingly agricultural, and ecological footprints are localized.

1900 • Industrial Expansion

Industrialization pushes urban share to ~14%. Cities like London and New York face severe air and water pollution from coal combustion and untreated sewage.

1950 • Post-War Suburbanization

Automobile-dependent suburbs spread rapidly in North America and Europe, introducing urban sprawl and converting farmland into low-density residential development.

2008 • Urban Majority Milestone

The global urban population surpasses 50% for the first time. Megacities (>10 million people) emerge across Asia, Africa, and Latin America, intensifying resource demands.

2050 • Projected Urban Future

An estimated 6.7 billion people—68% of humanity—will live in urban areas, making sustainable urban planning a central challenge for environmental policy.

This historical trajectory raises a core question for AP Environmental Science: How does converting natural and agricultural land into impervious, energy-intensive urban environments alter the flow of water, nutrients, energy, and biodiversity across landscapes? The sections that follow address this question by examining the ecological, hydrological, atmospheric, and social dimensions of urbanization.

SECTION 2

Core Principles & Definitions

Several foundational concepts underpin the environmental analysis of urbanization. Together they explain why cities—despite occupying only about 3 percent of Earth's land surface—exert disproportionate influence on global environmental systems.

Impervious Surfaces

Roads, rooftops, and parking lots prevent water infiltration, increase stormwater runoff, and raise local temperatures. The proportion of impervious surface cover is a key indicator of watershed health.

Urban Heat Island (UHI)

Cities absorb and re-radiate more heat than surrounding rural areas due to dark surfaces, waste heat from vehicles and HVAC, and reduced vegetation. The UHI effect can raise urban temperatures 1–3 °C above rural baselines.

Urban Sprawl

Urban sprawl is the low-density, automobile-dependent expansion of development into previously undeveloped land, fragmenting habitats and increasing per-capita resource consumption relative to compact development.

Stormwater Runoff & Non-Point Source Pollution

Impervious surfaces channel rainfall directly into storm drains, carrying oil, heavy metals, fertilizers, and sediment into rivers and estuaries as non-point source pollution—the leading cause of water quality degradation in the U.S.

Ecological Footprint

A city's ecological footprint extends far beyond its physical boundaries, encompassing farmland, forests, and water supplies that support its resource demands and absorb its wastes.

Two top-down development views contrast dispersed houses and curving roads with compact blocks, a transit spine and habitat patches. The companion assumes the same 1,000 residents and compares 100 ha with 40 ha of developed footprint. The same number of residents can occupy different developed footprints and travel networks. The hypothetical plan compares equal study areas without assigning universal travel emissions.

SECTION 3

Visual Explanation — Urban Water Cycle vs. Natural Water Cycle

The diagram compares water partitioning in a natural landscape (left) with an urban landscape (right). Notice how impervious surfaces shift the balance from infiltration and evapotranspiration toward surface runoff, reducing groundwater recharge and increasing flood risk and pollutant transport.

In a natural watershed, roughly half of incoming precipitation infiltrates into the soil, recharging aquifers and sustaining baseflow in streams. Vegetation returns a significant fraction to the atmosphere through evapotranspiration, and only about 10 percent becomes surface runoff. Urbanization reverses this ratio: impervious cover can push runoff to 55 percent or higher while slashing infiltration to 15 percent. The consequences ripple through the system—lower water tables, higher peak discharge during storms, degraded stream channels, and elevated concentrations of urban pollutants such as petroleum hydrocarbons, heavy metals, and thermal pollution in receiving waters.

Storm drain connects paved catchment to stream; vegetation and roots remain in permeable cutaway. Bright blue pore water is illustrative, not a measured saturation field. Separate exact panel: A narrow high triangular hydrograph and a broader lower triangular hydrograph have equal schematic area but different peaks and timing. Impervious cover can change both runoff timing and volume. Storm conditions, soils and drainage control the response; infiltrated water may enter storage, evapotranspiration or later streamflow.

Connected impervious surfaces can concentrate runoff into a sharper storm response.. Impervious cover can change both runoff timing and volume. Storm conditions, soils and drainage control the response; infiltrated water may enter storage, evapotranspiration or later streamflow. Impervious cover can change both runoff timing and volume. Storm conditions, soils and drainage control the response; infiltrated water may enter storage, evapotranspiration or later streamflow.

SECTION 4

How Urbanization Alters Environmental Systems

Hydrological Impacts

The conversion of permeable soil to impervious surface fundamentally restructures the local water budget. Engineers quantify this through the runoff coefficient (C), which represents the fraction of precipitation that becomes direct runoff. In the Rational Method—a standard tool for estimating peak discharge—peak runoff (Q) is computed as shown below.

RATIONAL METHOD

Q = C × i × A

Q = peak discharge (ft³/s or m³/s); C = runoff coefficient (dimensionless, 0–1); i = rainfall intensity (in/hr or mm/hr); A = drainage area. As impervious cover increases, C rises, directly increasing Q for any given storm.

Atmospheric & Thermal Impacts

Urban surfaces—asphalt, concrete, dark rooftops—have low albedo (reflectivity) and high thermal mass, meaning they absorb solar radiation during the day and re-radiate it as heat at night. Combined with anthropogenic waste heat from vehicles, industry, and air conditioning, this produces the urban heat island effect. The UHI intensity (ΔT) can be approximated by comparing urban and rural temperature readings, and it is influenced by city size, building density, vegetation cover, and regional climate.

UHI INTENSITY

ΔT_UHI = T_urban − T_rural

ΔT_UHI = urban heat island intensity (°C); T_urban = temperature measured in the city center; T_rural = temperature measured in surrounding rural area. Typical values range from 1–3 °C (annual average) to 5–12 °C on calm, clear summer nights.

Ecological & Biodiversity Impacts

Urbanization fragments habitats into isolated patches, reducing species diversity and disrupting migration corridors. Habitat fragmentation increases edge effects—altered microclimates, greater exposure to predators and invasive species—and reduces the effective area available to interior-dwelling species. The theory of island biogeography applies: smaller, more isolated habitat patches support fewer species. Meanwhile, cities tend to favor generalist, disturbance-tolerant species (pigeons, rats, coyotes) while displacing specialists, leading to biotic homogenization.

Matched exposed and tree-shaded street scenes with off-pavement sensor housings accompany an authored energy-pathway schematic. Dark paving can absorb and store solar energy, while vegetation provides shade and evapotranspiration when water is available. Air temperature, surface temperature and thermal comfort measure different aspects of heat exposure.

Urban heat depends on surface energy balance and measurement context.. Dark paving can absorb and store solar energy, while vegetation provides shade and evapotranspiration when water is available. Air temperature, surface temperature and thermal comfort measure different aspects of heat exposure. Dark paving can absorb and store solar energy, while vegetation provides shade and evapotranspiration when water is available. Air temperature, surface temperature and thermal comfort measure different aspects of heat exposure.

SECTION 5

Urban Sprawl vs. Smart Growth Strategies

Left: Urban sprawl disperses low-density development across the landscape, converting large areas of open land. Right: Smart growth concentrates mixed-use development, integrates green infrastructure, and limits outward expansion through urban growth boundaries.

Characteristic	Urban Sprawl	Smart Growth
Density	Low; single-family homes	High; mixed-use, multi-story
Transportation	Car-dependent; highways	Transit, walking, cycling
Land conversion	High; continuous expansion	Minimal; urban growth boundaries
Impervious cover	35–50% across large area	20–30% with green infrastructure
Per-capita CO₂ emissions	Higher (longer commutes, larger homes)	Lower (shared walls, transit, walkability)
Habitat impact	Severe fragmentation	Concentrated footprint; preserves surrounding land

Comparison of sprawl and smart growth development patterns

Smart growth strategies include transit-oriented development, which clusters housing and commercial uses around public transit hubs; urban growth boundaries (UGBs), which legally restrict development beyond a defined perimeter (as practiced in Portland, Oregon); and green infrastructure—bioswales, rain gardens, permeable pavement, and urban tree canopy—that mimics natural hydrological processes within the built environment. These approaches can simultaneously reduce stormwater runoff, lower UHI intensity, cut transportation emissions, and preserve habitat connectivity.

SECTION 6

Worked Example — Calculating Runoff Change from Urbanization

A 200-hectare watershed currently has 10% impervious cover (C = 0.3). A proposed development would increase impervious cover to 60% (C = 0.65). During a design storm with rainfall intensity i = 40 mm/hr, calculate the peak discharge before and after development and the percent increase in runoff.

Worked example

1
Step 1 — Convert area to compatible units

A = 200 ha = 2.0 × 10⁶ m². For the Rational Method in metric, Q (m³/s) = C × i × A ÷ 360, where i is in mm/hr and A is in hectares.
2
Step 2 — Calculate pre-development peak discharge

Q_pre = (C_pre × i × A) ÷ 360 = (0.3 × 40 × 200) ÷ 360

Q_pre ≈ 6.67 m³/s
3
Step 3 — Calculate post-development peak discharge

Q_post = (C_post × i × A) ÷ 360 = (0.65 × 40 × 200) ÷ 360

Q_post ≈ 14.44 m³/s
4
Step 4 — Calculate percent increase

% increase = ((Q_post − Q_pre) ÷ Q_pre) × 100 = ((14.44 − 6.67) ÷ 6.67) × 100

% increase ≈ 116.5%
5
Step 5 — Interpret the result

Urbanizing this watershed more than doubles peak discharge, substantially increasing flood risk, stream channel erosion, and pollutant transport to downstream water bodies. This illustrates why stormwater management infrastructure—detention ponds, permeable pavement, bioswales—is essential in development planning.

SECTION 7

Mitigation Strategies — Strengths & Limitations

Strategy	Strengths	Limitations
Green roofs	Reduce UHI, absorb stormwater, insulate buildings, extend roof lifespan	High upfront cost, structural load requirements, maintenance needed
Permeable pavement	Allows infiltration, reduces runoff, recharges groundwater	Clogs without maintenance; not suitable for high-traffic roads
Urban tree canopy	Shading reduces UHI 2–8 °C locally; sequesters CO₂; intercepts rainfall	Years to mature; requires space, water, and maintenance; can conflict with underground utilities
Transit-oriented development	Reduces per-capita emissions, limits sprawl, supports mixed-use walkability	Requires large infrastructure investment; may raise property values and displace low-income residents (gentrification)
Urban growth boundaries	Preserves surrounding farmland and habitat; encourages infill development	Can inflate land prices inside boundary; politically difficult to maintain as population grows

Mitigation strategies for urbanization impacts

SECTION 8

Connections to Broader Environmental Science

Urbanization intersects with nearly every major topic in AP Environmental Science, from climate change and energy to biodiversity and human health. The following table highlights how urbanization concepts connect to other units you will encounter on the exam.

APES Topic Area	Connection to Urbanization
Energy Resources & Consumption	Cities account for ~75% of global energy use and CO₂ emissions. Building efficiency, mass transit, and district heating reduce per-capita demand.
Global Climate Change	UHI amplifies heat stress; urban CO₂ and black carbon contribute to radiative forcing. Cities are also disproportionately vulnerable to sea-level rise and extreme weather.
Biodiversity & Ecosystem Services	Habitat fragmentation and biotic homogenization reduce biodiversity. Urban parks and wildlife corridors are critical for maintaining connectivity.
Water Pollution & Treatment	Stormwater runoff carries non-point source pollutants; combined sewer overflows release untreated sewage during heavy rain events.
Human Health & Environmental Justice	UHI disproportionately harms low-income communities lacking green space and air conditioning. Urban air pollution (PM₂.₅, ozone) causes respiratory and cardiovascular disease.

As global urbanization accelerates, especially in Africa and South Asia, the choices made about urban form, energy infrastructure, and green space in the next few decades will significantly shape cumulative greenhouse gas trajectories, water resource availability, and species conservation outcomes worldwide. The AP exam frequently tests your ability to trace these cross-topic linkages, so practice identifying how urbanization impacts cascade through environmental systems.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

Which of the following best explains why urbanization increases the frequency and severity of flooding in nearby streams?

A. Urban areas receive more total precipitation than surrounding rural areas due to the urban heat island effect.
B. Impervious surfaces reduce infiltration and increase the volume and speed of stormwater runoff reaching streams.
C. Urban vegetation absorbs less water than rural vegetation because it is exposed to higher levels of air pollution.
D. Urban groundwater pumping raises the water table, causing streams to overflow more easily.

Show answer (B)

B is correct. Impervious surfaces (roads, rooftops, parking lots) prevent precipitation from soaking into the soil, converting a much larger fraction of rainfall into surface runoff that reaches streams quickly, increasing peak discharge and flood risk. A is incorrect because while UHI can modestly increase local convective precipitation, this is not the primary mechanism for increased flooding—the key factor is reduced infiltration. C is incorrect because the flooding mechanism is about surface imperviousness, not pollution effects on vegetation. D is incorrect because urban groundwater pumping typically lowers water tables (causing land subsidence), not raises them.

Question 2: BASIC CALCULATION

A 50-hectare watershed with a runoff coefficient (C) of 0.25 experiences a storm with a rainfall intensity of 30 mm/hr. Using the Rational Method (Q = C × i × A ÷ 360, where Q is in m³/s, i is mm/hr, A is hectares), what is the approximate peak discharge?

A. 0.52 m³/s
B. 1.04 m³/s
C. 3.75 m³/s
D. 10.42 m³/s

Show answer (B)

B is correct. Q = (0.25 × 30 × 50) ÷ 360 = 375 ÷ 360 ≈ 1.04 m³/s. A (0.52) likely results from using half the area or an arithmetic error. C (3.75) results from omitting the ÷ 360 unit conversion and instead dividing only by 100. D (10.42) results from using C = 1.0 or otherwise neglecting the runoff coefficient entirely. Always check units and ensure all variables are included.

Question 3: INTERMEDIATE

A city measures an average summer temperature of 34 °C in its downtown core and 29 °C in the surrounding rural area. The city plans to increase urban tree canopy from 15% to 35%, and research indicates that each 10% increase in tree canopy reduces UHI intensity by approximately 0.8 °C. What would the predicted downtown temperature be after the canopy increase?

A. 31.2 °C
B. 32.0 °C
C. 32.4 °C
D. 33.2 °C

Show answer (C)

C is correct. The canopy increase is 35% − 15% = 20%, which equals 2 increments of 10%. Each increment reduces UHI intensity by 0.8 °C, so total reduction = 2 × 0.8 = 1.6 °C. Predicted temperature = 34 − 1.6 = 32.4 °C. A (31.2 °C) incorrectly uses a 3.5-increment multiplier or a larger per-increment reduction. B (32.0 °C) likely uses 1.0 °C per 10% increment instead of 0.8 °C. D (33.2 °C) applies only one increment of 0.8 °C, treating the change as 10% rather than 20%.

Question 4: APPLIED

A city planning department wants to determine whether installing permeable pavement in a commercial parking lot reduces the volume of stormwater runoff compared to traditional asphalt. Design a controlled investigation to test this question. Your response should include (a) a testable hypothesis, (b) identification of independent, dependent, and at least two controlled variables, (c) a description of the experimental procedure including data collection, and (d) a method for analyzing results.

Show answer

Hypothesis (1 pt): If traditional asphalt is replaced with permeable pavement in a parking lot, then the total volume of stormwater runoff will decrease because permeable pavement allows water to infiltrate through the surface. Variables (1 pt + 1 pt): Independent variable: pavement type (permeable vs. conventional asphalt). Dependent variable: volume of stormwater runoff (liters) collected at the outlet. Controlled variables include plot size (e.g., two 500 m² sections of equivalent slope), antecedent soil moisture, and storm characteristics (if using simulated rainfall). Procedure (1 pt): Install permeable pavement on one section and maintain conventional asphalt on an adjacent section of equal area and slope. Equip the drainage outlet of each section with a calibrated flow meter or collection basin. During each rain event (or simulated rainfall using sprinklers at a controlled intensity), measure total runoff volume over the event duration. Repeat across at least 10 storm events of varying intensity. Analysis (1 pt): Calculate mean runoff volume for each pavement type across all events. Use a paired t-test or Wilcoxon signed-rank test (if data are not normally distributed) to determine whether the difference in runoff between the two surface types is statistically significant at p < 0.05.

Question 5: CRITICAL THINKING

The following data show land cover and annual stormwater runoff for a watershed undergoing development over 30 years: Year 1990: Impervious cover = 12%, Annual runoff = 18 cm Year 2000: Impervious cover = 28%, Annual runoff = 29 cm Year 2010: Impervious cover = 45%, Annual runoff = 42 cm Year 2020: Impervious cover = 52%, Annual runoff = 48 cm (a) Describe the trend in the data and explain the causal mechanism. (b) Calculate the average increase in annual runoff per percentage point of impervious cover added between 1990 and 2020. (c) Calculate the runoff increase per added percentage point of impervious cover for each decade. Do the supplied data show a slowing rate? (d) Recommend one specific management strategy and explain how it would reduce runoff in this watershed.

Show answer

(a) Trend & Mechanism (1 pt): Both impervious surface cover and annual stormwater runoff increase steadily over the 30-year period. The causal mechanism is that impervious surfaces such as roads, buildings, and parking lots prevent precipitation from infiltrating into the soil, forcing a larger fraction of rainfall to become surface runoff. As impervious cover increased from 12% to 52%, annual runoff rose from 18 cm to 48 cm. (b) Calculation (1 pt): Average increase = (48 − 18) ÷ (52 − 12) = 30 ÷ 40 = 0.75 cm of annual runoff per percentage point of impervious cover added. (c) Decadal rates (1 pt): 1990–2000: 11/16 = 0.6875; 2000–2010: 13/17 ≈ 0.7647; 2010–2020: 6/7 ≈ 0.8571 cm of annual runoff per added percentage point of impervious cover. The rate increases slightly, so the data do not show slowing. These values alone do not establish why the rate changes. (d) Management strategy (1 pt): Installing bioretention rain gardens throughout the watershed would reduce runoff by capturing stormwater in vegetated depressions filled with engineered soil media. Water infiltrates through the media, is taken up by plants, or percolates to groundwater, thereby decreasing the volume and peak flow of stormwater reaching streams.