AP Environmental Science

## Learning Outcomes

- Explain how urbanization increases stormwater runoff and flood risk
- Distinguish between infiltration, runoff, and groundwater recharge
- Compare urban and rural watershed responses after rainfall
- Interpret how impervious cover changes hydrographs, lag time, and peak discharge
- Apply the Rational Method qualitatively and through a worked example
- Identify links between flooding, erosion, and nonpoint source pollution
- Evaluate methods that reduce flooding from impervious surfaces

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand how impervious surfaces affect the hydrologic cycle, with a focus on the following syllabus points:

- Know that **impervious surfaces** such as roads, rooftops, parking lots, and sidewalks prevent water from soaking into soil
- Explain the chain: urbanization increases impervious cover, which decreases infiltration and increases surface runoff and flooding
- Understand that reduced infiltration also lowers groundwater recharge and can contribute to a lower water table
- Compare urban and less-developed watersheds in terms of runoff speed, peak flow, and hydrograph shape
- Recognise that storm drains move water quickly but do not eliminate flood risk and may worsen downstream flooding
- Identify that urban runoff often carries pollutants such as oil, metals, sediment, fertilizers, pesticides, and trash
- Understand that intense rainfall, drainage design, slope, and soil conditions also affect flood severity
- Know mitigation strategies including permeable pavement, green space, constructed wetlands, and retention or infiltration basins

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. Why do roads and parking lots increase flood risk after heavy rain?
2. How does infiltration differ from surface runoff?
3. What happens to groundwater recharge when a watershed becomes more urbanized?
4. How would an urban hydrograph differ from a rural hydrograph after the same storm?
5. Name two ways a city can reduce runoff from impervious surfaces.

## Introduction

Urbanization changes the water cycle. In a natural environment, much rainfall soaks into soil, moves slowly through the ground, and is taken up by vegetation. In a built-up area, much of that same rainfall lands on asphalt, concrete, and rooftops. Instead of entering the ground, it moves rapidly over the surface.

For AP Environmental Science, this topic matters because it links Unit 5 land use to Unit 4 watersheds and Unit 8 water pollution. A short exam answer often depends on getting the cause-and-effect chain in the right order: more **impervious surfaces** means less **infiltration**, more **stormwater runoff**, and greater flood risk.

> **Key Term: impervious surfaces**
> Surfaces such as roads, rooftops, parking lots, and sidewalks that prevent water from soaking into the ground.

### How impervious surfaces change the water cycle

In an undeveloped watershed, rainfall can follow several pathways. Some water evaporates. Some is taken up by plants and later released through transpiration. Some flows into streams as runoff. A large amount infiltrates into soil, percolates downward, and helps recharge groundwater.

Urbanization shifts that balance. Covering land with pavement and buildings blocks the soil surface. Water that once infiltrated now stays above ground. This raises the amount of water moving as runoff and reduces the amount entering aquifers.

> **Key Term: infiltration**
> The process by which water on the ground surface enters the soil.
>
> **Key Term: stormwater runoff**
> Rainwater or snowmelt that flows over land rather than soaking into the ground.

The most common student error is to say the water “disappears into drains.” That misses the key science. The water does not vanish. It is redirected. In cities, runoff is often channelled quickly into storm drains, pipes, and streams. That means two things happen at once:

- **Water quantity changes:** more water remains on the surface
- **Water timing changes:** water reaches channels faster

That second point is why flooding can become severe even if total rainfall stays the same. The same storm can produce a much larger and faster runoff pulse in a city than in a forested area.

Another result is reduced groundwater recharge. If less water infiltrates, the water table may drop over time, especially where groundwater is already being heavily used. So impervious surfaces are not only a flood issue; they also contribute to groundwater depletion.

> **Test Tip:** In FRQs, do not stop at “pavement increases runoff.” Add the mechanism: pavement prevents infiltration, reduces groundwater recharge, and sends water quickly into storm drains and streams.

### Why flooding risk rises: volume, speed, and peak flow

Flooding from impervious surfaces happens because runoff increases in both **volume** and **velocity**. When rain falls on soil with vegetation, friction from leaves, stems, litter, and porous ground slows the water. Some soaks in, so less is left to flow overland. On pavement, water meets a hard, smooth surface. It cannot infiltrate, and it often travels faster downslope.

This creates what hydrologists call a “flashy” response. Streams rise quickly after a storm, often with little lag time. Urban flooding is especially severe during short, intense storms because water reaches drains and channels before the system can handle it.

> **Key Term: peak discharge**
> The highest flow rate reached by a stream, river, or drainage system during a storm event.
>
> **Key Term: flash flooding**
> Rapid flooding that develops quickly after intense rainfall, often because runoff reaches streams or drains in a short time.

Think through the sequence carefully:

1. Rain falls on a developed watershed
2. Impervious cover prevents infiltration
3. More water stays at the surface
4. Storm drains and smooth surfaces move water quickly
5. Streams and drainage channels receive a sudden surge
6. Peak discharge rises
7. Water overtops channels, streets, or drainage systems

Flooding is not caused only by rainfall amount. That distinction matters. Two places can receive the same storm, but the more urbanized watershed may flood worse because land cover changed the hydrologic response.

Students also sometimes overclaim that impervious surfaces are the only cause of flooding. That is inaccurate. Flooding also depends on rainfall intensity, slope, soil type, preexisting saturation, drainage design, and whether channels are already full. Impervious surfaces are a major cause, but not the only one.

### Urban hydrographs and the Rational Method

A hydrograph shows how stream discharge changes over time after precipitation. You are unlikely to have to draw one perfectly from memory, but you should know the pattern.

In a rural or forested watershed, the hydrograph usually has:

- a longer lag time
- a lower peak discharge
- a broader, less steep curve

In an urban watershed, the hydrograph usually has:

- a shorter time to peak
- a steeper rising limb
- a higher peak discharge
  > **Key Term: hydrograph**
  > A graph showing how stream discharge changes over time, especially before, during, and after a storm.

This difference exists because urban runoff reaches streams faster and in greater quantity. That is the APES-ready comparison. If a prompt asks how urbanization changes stream flow, those hydrograph features are strong evidence.

A quantitative way to express this is the Rational Method:

$$
Q = C i A
$$

Where:

- $Q$ = peak runoff rate
- $C$ = runoff coefficient
- $i$ = rainfall intensity
- $A$ = drainage area

The runoff coefficient $C$ is crucial here. It represents the fraction of rainfall that becomes runoff. Impervious surfaces have high runoff coefficients, often above 0.80, because most water becomes immediate runoff rather than infiltrating.

#### Worked example

A small drainage area is 2 hectares. Rainfall intensity is 5 units. Compare two scenarios:

- **Scenario 1:** mostly vegetated land, $C = 0.20$
- **Scenario 2:** heavily paved urban land, $C = 0.90$

Step 1: Write the formula

$$
Q = C i A
$$

Step 2: Calculate vegetated case

$$
Q = 0.20 \times 5 \times 2
$$

$$
Q = 2
$$

Step 3: Calculate urban paved case

$$
Q = 0.90 \times 5 \times 2
$$

$$
Q = 9
$$

Step 4: Compare

The paved watershed has a peak runoff value of **9**, compared with **2** for the vegetated watershed. That means much more of the rainfall becomes rapid runoff, so flood risk is much greater.

This does not mean every real watershed behaves exactly like this simple model. The Rational Method is mainly used for small drainage areas. But for APES, the key reasoning is simple: increasing impervious cover raises the runoff coefficient, which raises peak runoff.

Common mistakes students make here:

- confusing rainfall amount with rainfall intensity
- assuming drainage area changes when only land cover changed
- forgetting that a larger $C$ means a greater fraction of rain becomes runoff
  > **Test Tip:** If you see a formula question, you often do not need advanced hydrology. State clearly that more impervious cover increases runoff coefficient $C$, which increases peak runoff $Q$.

### Flooding is linked to pollution and environmental damage

Impervious surfaces do more than increase flood risk. They also worsen water quality. Most urban storm sewer systems discharge runoff without treatment, so the water reaching streams may carry pollutants picked up from roads, lawns, rooftops, and parking lots.

These pollutants can include:

- gasoline and motor oil
- heavy metals
- trash and debris
- fertilizers and pesticides
- sediment
- pathogens from contaminated water
  > **Key Term: nonpoint source pollution**
  > Pollution that comes from many diffuse sources rather than a single identifiable discharge point.

This matters during floods because larger and faster runoff moves more material. A flood pulse can erode stream banks, carry sediment into rivers and reservoirs, and reduce water quality. Sedimentation can lower storage capacity in reservoirs and raise turbidity in streams. Warm runoff from asphalt can also increase stream temperature, which stresses fish and lowers dissolved oxygen availability.

There are also wider ecological effects. Urban runoff can fragment habitats, reduce vegetation cover, and shrink the food base for primary consumers. If less vegetation is present, fewer herbivores can be supported, and food webs may be simplified. Nutrient-rich runoff can also promote algal growth, which may lead to later oxygen depletion as algae decompose.

From a human health view, flooding can increase the risk of contaminated water supplies and disease exposure. That is why this topic crosses into both water pollution and public health.

### How flooding from impervious surfaces can be reduced

The main principle of mitigation is simple: restore some of the functions lost through urbanization. That means increasing infiltration, storing stormwater temporarily, or slowing its movement.

Important methods include:

- **permeable pavement** that allows water to pass through into underlying layers
- **green spaces and parks** that increase natural infiltration
- **constructed wetlands** that filter and slow runoff
- **retention or infiltration basins** that hold stormwater and allow it to soak into permeable soils
- **rain gardens and vegetated areas** that absorb and delay runoff
- **street sweeping** to remove debris and pollutants before storms move them into waterways

Permeable pavement is especially useful for APES comparisons. Conventional pavement sheds water. Permeable pavement allows some water to infiltrate downward, reducing direct runoff and local ponding. Retention and detention systems also help, but they work in slightly different ways. A basin may not eliminate total runoff, but it can reduce peak flow by releasing water more slowly.

Constructed wetlands are valuable because they provide two services at once: they lower flood risk by storing water and they improve quality by filtering pollutants through soils, microbes, and vegetation.

One point often missed by students: storm drains alone are not a full solution. They may move water away from one street corner, but they can worsen downstream flooding by delivering runoff rapidly into streams. Green infrastructure tries to slow, spread, and soak water rather than simply pipe it away.

Brownfield redevelopment can also reduce stormwater runoff when abandoned urban land is cleaned and reused thoughtfully, especially if it replaces more sprawling development patterns and adds better stormwater design.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Impervious surfaces include roads, rooftops, parking lots, and sidewalks
- Impervious surfaces prevent infiltration and increase surface runoff
- Urbanization reduces groundwater recharge and can lower the water table
- More impervious cover increases both the amount and speed of runoff
- Flood risk rises because runoff reaches drains and streams more quickly
- Urban watersheds usually have higher peak discharge and shorter lag time than rural watersheds
- Urban hydrographs are flashier, with steeper rising limbs and higher peaks
- In the Rational Method, increasing impervious cover increases runoff coefficient $C$ and therefore peak runoff $Q$
- Flooding from impervious surfaces also increases erosion, sedimentation, and nonpoint source pollution
- Urban runoff may carry oil, metals, trash, fertilizers, pesticides, and pathogens
- Warm runoff from asphalt can raise stream temperatures and stress aquatic life
- Mitigation includes permeable pavement, green space, wetlands, and retention or infiltration basins

## Key Terms and Concepts

- impervious surfaces
- infiltration
- stormwater runoff
- peak discharge
- flash flooding
- hydrograph
- nonpoint source pollution


Impacts of Urbanization - Impervious surfaces cause flooding