Land & Water Use

What this unit covers

The topics below follow the published Env. Science course framework for Unit 5. This unit is worth 10–15% of the exam, so budget your time against that rather than against how long the unit takes to teach.

Lessons in this unit

• Agriculture & Feeding the World13 min · 3 objectivesDescribe the trade-offs of the Green Revolution and industrial agriculture · Compare irrigation methods and explain how they lead to soil salinization · Calculate the percent change in crop yield from agricultural data
• Mining & Resource Extraction12 min · 3 objectivesDistinguish surface mining methods from subsurface mining · Explain the environmental impacts of mining, including acid mine drainage · Calculate the mass of metal recoverable from ore of a given grade
• Fishing & Overharvesting12 min · 3 objectivesExplain the tragedy of the commons using fisheries as an example · Describe overfishing, bycatch, and the promise and problems of aquaculture · Apply the concept of maximum sustainable yield to a fishery
• Urbanization & Ecological Footprints13 min · 3 objectivesDescribe urban sprawl and its environmental consequences · Explain how impervious surfaces alter local hydrology and heat · Interpret ecological footprint data to judge whether resource use is sustainable
• Irrigation, Salinization, and the Cost of Growing Food17 min · 3 objectivesCompare irrigation methods on efficiency and salinization risk · Explain waterlogging and salinization as consequences of irrigation practice · Evaluate the Green Revolution's gains against its costs
• Fisheries, Tragedy of the Commons, and Sustainable Yield17 min · 3 objectivesExplain the tragedy of the commons and identify it in resource scenarios · Define maximum sustainable yield and explain why fisheries overshoot it · Evaluate policy responses to overharvesting

Formulas in Unit 5

Every term in Unit 5

All 35 terms we publish for Land & Water Use, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.

Tragedy of the commons

A shared, unregulated resource is overused because each user gains the whole benefit while sharing the cost — the model behind overfishing and air pollution.

Clearcutting

Removing all trees in an area. Fast and cheap, but causes erosion, habitat loss, higher stream temperature and reduced water quality.

Green revolution

High-yield crop varieties with heavy irrigation, fertilizer and pesticide use. Raised yields substantially while increasing water use and runoff.

Monoculture

Growing one crop over a large area. Efficient to plant and harvest, but vulnerable to a single pest and depleting to soil nutrients.

Irrigation methods

Flood irrigation is cheap but wasteful and causes salinization; drip irrigation delivers water at the root and is the most efficient.

Salinization and waterlogging

Repeated irrigation leaves dissolved salts as water evaporates, and excess water raises the water table into the root zone.

Integrated pest management

Combines biological controls, crop rotation, targeted pesticide use and monitoring, reducing pesticide volume and resistance.

Pesticide resistance

Survivors of a treatment reproduce, so the pest population becomes resistant. The pesticide selects for it; it does not create it.

Concentrated animal feeding operations

Produce meat cheaply but concentrate waste, require antibiotics that drive resistance, and contaminate nearby water.

Overfishing and bycatch

Harvesting faster than stocks reproduce; bycatch is the non-target species caught and discarded, often including turtles and dolphins.

Aquaculture trade-offs

Reduces pressure on wild stocks but concentrates waste, spreads disease to wild populations, and often requires wild-caught fish as feed.

Urban sprawl

Low-density development that increases car dependence, impervious surface and runoff while consuming farmland.

Urban runoff and impervious surface

Pavement prevents infiltration, so rainfall runs off carrying oil, metals and nutrients directly into waterways, and causes flash flooding.

Sustainable practices

Crop rotation, no-till farming, contour plowing, terracing, windbreaks, cover crops and rotational grazing all conserve soil.

Desertification

Productive drylands degrading to desert through overgrazing, deforestation and poor irrigation. Roughly a third of global land is at risk.

Maximum sustainable yield

The largest harvest that can be taken indefinitely, occurring near half the carrying capacity where population growth rate is fastest.

Terracing and contour plowing

Terracing cuts steps into slopes and contour plowing follows elevation lines; both slow runoff and reduce erosion.

Windbreaks and cover crops

Rows of trees slow wind erosion; cover crops hold soil between plantings and add organic matter when tilled in.

Mining methods and impacts

Surface and strip mining remove overburden and cause severe habitat loss and acid mine drainage; subsurface mining is safer for land but far more dangerous for miners.

Acid mine drainage

Exposed sulfide minerals react with water and oxygen to form sulfuric acid, which lowers stream pH and mobilises heavy metals.

Selective cutting vs clearcutting

Selective cutting removes mature trees individually, preserving canopy, soil and habitat structure at higher cost per board-foot. Clearcutting is cheap and efficient but removes the canopy at once, causing erosion, stream siltation and habitat loss.

Prescribed burn

A deliberate low-intensity fire that consumes accumulated fuel, releases nutrients and favors fire-adapted species. A century of suppression built up the fuel loads that make modern wildfires catastrophic, so burning is fire prevention rather than fire.

Slash-and-burn agriculture

Clearing and burning forest releases a nutrient pulse from ash, but tropical soils hold few nutrients themselves, so yields fall within a few seasons and the plot is abandoned. Sustainable at low population density, destructive when rotation time shortens.

No-till farming

Planting through crop residue instead of plowing. It preserves soil structure, cuts erosion and retains moisture and carbon, at the cost of greater dependence on herbicides for weed control — a trade-off worth stating on a free-response question.

Crop rotation and nitrogen

Alternating a legume with a nitrogen-demanding crop restores soil nitrogen biologically, and rotating crop families breaks pest and pathogen cycles. It is the cheapest available substitute for both fertilizer and pesticide.

Strip cropping and alley cropping

Strip cropping alternates bands of row crop and cover crop across a slope so the cover strips trap soil the row strips lose. Alley cropping grows crops between rows of trees, which cut wind, hold soil and provide a second harvest.

Why drip irrigation reduces salinization

It delivers a small volume directly to the root zone, so far less water evaporates from the soil surface. Salinization is caused by evaporation leaving dissolved salts behind, so cutting evaporation cuts the salt left in the field.

Aquifer depletion and subsidence

Pumping faster than recharge lowers the water table, raises pumping costs and dries shallow wells. Where the aquifer sediment compacts as water leaves, the land surface itself sinks — permanently destroying the aquifer's storage capacity.

Dams: the full ledger

Benefits are flood control, water storage, irrigation and low-carbon electricity. Costs are displaced communities, blocked fish migration, sediment trapped behind the dam so downstream deltas erode, and evaporative losses from the reservoir surface.

The trophic cost of meat

Feeding grain to livestock inserts an extra trophic transfer, so roughly 90% of the energy is lost before it reaches a person. The same cropland feeds far more people directly as grain, which is why diet appears in land-use questions.

Feedlots vs free range

Feedlots produce meat cheaply and quickly but concentrate manure into a water-pollution problem and drive routine antibiotic use. Free-range systems spread the manure load and reduce disease pressure, using far more land per animal.

Bycatch reduction devices

Turtle excluder devices, circle hooks, and modified net mesh let non-target animals escape. They work because bycatch is a gear problem before it is a behavior problem, which is why regulation targets equipment.

Bottom trawling

Dragging weighted nets across the seafloor takes target fish along with everything else and flattens the structural habitat — corals, sponges, burrows — that juvenile fish depend on. The habitat damage outlasts the catch.

Marine protected areas

No-take zones let stocks rebuild and large breeding females return, and the surplus spills over into surrounding fished waters. They work only where enforcement is real, and their benefit takes years to appear.

Individual transferable quotas

Each fisher holds a defined share of the total allowable catch rather than racing for it. Because a share is a claim on a percentage of future catch, holders gain directly when the stock recovers, aligning private and collective interest.

What examiners penalize here

• For percent-change problems, the number-one error is dividing by the new value instead of the old. Anchor on "change ÷ original × 100." And remember: a doubling is +100%, a tripling is +200% — the increase is one less multiple than the factor of growth.
• For ore-grade calculations, convert the percentage to a decimal before multiplying (0.5% → 0.005). Then notice the waste-to-product ratio — AP loves to ask you to interpret how much waste rock is generated, which ties the math directly to the environmental impact.
• For maximum sustainable yield questions, compare the harvest rate to the reproduction (replacement) rate. Harvest ≤ replacement is sustainable; harvest > replacement causes decline. Numbers make it concrete — always state whether the catch is above or below the yearly recruitment.
• For ecological footprint questions, the rule is simple: footprint > biocapacity = deficit/overshoot (unsustainable); footprint < biocapacity = reserve (sustainable). Dividing footprint by biocapacity gives the overshoot factor, a common follow-up calculation.
• The tragedy of the commons is about **incentive structure, not character**. Answers that explain it by saying fishers are greedy miss the point and usually miss the policy question that follows, because the fix is to change who bears the cost — not to appeal to restraint.

Practice Env. Science

Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.

Questions about this unit

All 9 units of AP Environmental Science

1. Unit 1 · The Living World: Ecosystems
2. Unit 2 · The Living World: Biodiversity
3. Unit 3 · Populations
4. Unit 4 · Earth Systems & Resources
5. Unit 5 · Land & Water Use
6. Unit 6 · Energy Resources & Consumption
7. Unit 7 · Atmospheric Pollution
8. Unit 8 · Aquatic & Terrestrial Pollution
9. Unit 9 · Global Change