Water Cycle, Oceans, and Groundwater
Water Cycle, Oceans, and Groundwater
This domain tests the pathways and stores of water, streamflow, groundwater movement, and ocean circulation.
Core rules and concepts
- The water cycle includes evaporation, transpiration, condensation, precipitation, infiltration, runoff, and storage in ice, groundwater, lakes, and oceans.
- A watershed is the land area draining to a stream; divides separate watersheds. Stream discharge = cross-sectional area × velocity.
- Porosity is the fraction of void space; permeability is how easily fluids move through connected pores. Clay can have high porosity but low permeability.
- Groundwater flows from high water table to low water table, generally perpendicular to water-table contours. An aquifer yields water; an aquitard restricts flow.
- Surface ocean currents are wind-driven; deep currents are density-driven. Tides result mainly from the Moon’s gravity, with the Sun modifying range.
- Urban impermeable surfaces increase runoff and reduce infiltration; pumping wells can lower the water table and cause saltwater intrusion near coasts.
Worked example
A 100 cm³ saturated soil sample holds 25 cm³ of water. Porosity = 25 ÷ 100 = 25%. If the sample drains in 10 minutes, permeability is high. A clay sample could also have 25% porosity but drain very slowly, showing that porosity and permeability are different properties.
Traps that cost marks
- Treating porosity and permeability as the same property.
- Drawing a flat water table; it follows topography, rising under hills and falling toward valleys.
- Assuming groundwater always flows downhill at the surface; it flows down the water-table gradient.
- Confusing tides with surface currents.