Inside This Unit: The Full Breakdown
Land and Water Use covers how humans use and impact terrestrial and aquatic resources, including agriculture, forestry, mining, urbanization, and water management. Sustainable practices aim to meet current needs without compromising future generations.
Why it matters
Land and water use is one of the most applied topics on the AP Environmental Science exam. You must understand farming practices, water management, and urbanization impacts, and be able to evaluate sustainability of different approaches.
Key concepts
- The Green Revolution increased agricultural yields through high-yield crop varieties, synthetic fertilizers, pesticides, and irrigation — but created environmental tradeoffs.
- Irrigation methods vary in efficiency: flood irrigation is least efficient; drip irrigation is most efficient. Water diversion can devastate downstream ecosystems.
- Deforestation reduces biodiversity, increases erosion, disrupts water cycles, and releases stored carbon. Sustainable forestry practices include selective cutting and replanting.
- Urbanization creates impervious surfaces that increase runoff and flooding, concentrates pollution, and creates urban heat islands.
Agriculture and Food Production
Modern industrial agriculture feeds billions but creates significant environmental impacts. The Green Revolution of the 1960s-70s dramatically increased crop yields through high-yield varieties, synthetic fertilizers, pesticides, and mechanized irrigation. However, these practices have consequences: synthetic fertilizers cause eutrophication when they run off into waterways, pesticides harm non-target organisms and can bioaccumulate in food chains, monoculture reduces genetic diversity and increases vulnerability to pests, and heavy irrigation can cause salinization and aquifer depletion. Sustainable alternatives include integrated pest management (IPM), crop rotation, polyculture, organic farming, and no-till agriculture. Genetically modified organisms (GMOs) offer potential benefits like pest resistance and drought tolerance but raise ecological and ethical concerns.
Forestry and Mining
Forests provide ecosystem services including carbon storage, biodiversity habitat, watershed protection, and timber. Deforestation — primarily for agriculture, logging, and development — eliminates these services and is a major source of CO₂ emissions. Clear-cutting removes all trees from an area, causing severe erosion, habitat loss, and stream sedimentation. Selective cutting removes only mature or targeted trees and is less damaging. Prescribed burns mimic natural fire cycles and reduce wildfire risk. Mining extracts valuable minerals and fossil fuels but causes habitat destruction, water pollution (acid mine drainage), air pollution, and land subsidence. Surface mining (strip mining, mountaintop removal) is particularly destructive. Reclamation laws require restoration of mined land, but full ecosystem recovery is often incomplete.
Water Use and Urbanization
Freshwater is a limited resource: only about 1% of Earth's water is accessible fresh surface water and groundwater. Agriculture accounts for about 70% of global freshwater use. Aquifer depletion occurs when withdrawal exceeds recharge — the Ogallala Aquifer in the U.S. Great Plains is a major example. Dams provide hydroelectric power, flood control, and irrigation water, but they disrupt river ecosystems, block fish migration, and trap sediments. Desalination of seawater is energy-intensive but increasingly used in water-scarce regions. Urbanization converts natural land to impervious surfaces (roads, buildings, parking lots), increasing stormwater runoff, reducing groundwater recharge, and concentrating pollutants. Urban heat islands form because dark surfaces absorb more heat than vegetation. Green infrastructure (permeable pavement, green roofs, rain gardens) can mitigate these urban impacts.
AP exam tip
AP Environmental Science free-response questions often ask you to propose solutions to land or water use problems. Always connect each solution to a specific problem, explain the mechanism, and acknowledge at least one tradeoff or limitation of your proposed solution.
Connections to other units
- Unit 1 (Ecosystems): Land use changes directly alter energy flow, nutrient cycling, and ecosystem services.
- Unit 3 (Populations): Human population growth drives the demand for agricultural land, water, and urban development.
- Unit 8 (Aquatic and Terrestrial Pollution): Agricultural runoff, mining waste, and urban stormwater are major pollution sources.
