A Level Geography: Core Physical Geography — Revision Notes

Condensed for the final weeks. For the full explanation, use the Core Physical Geography study guide.

Hydrology

The drainage basin system: inputs (precipitation), stores (interception, soil moisture, groundwater, surface storage), flows (throughfall, infiltration, throughflow, percolation, groundwater flow, overland flow), outputs (evapotranspiration, channel discharge).

The storm hydrograph is examined every series. Know what shortens lag time and raises peak discharge, and why:

Factor	Shorter lag, higher peak when
Rock type	Impermeable — no infiltration
Soil	Saturated or clay-rich — low infiltration capacity
Relief	Steep — faster overland flow
Vegetation	Sparse — less interception
Land use	Urban — impermeable surfaces and drains
Basin shape	Circular — water arrives from all sides together
Rainfall	Intense — exceeds infiltration capacity

Urbanisation is the standard case: tarmac prevents infiltration, drains deliver water to the channel rapidly, and vegetation is removed — so lag time falls and peak discharge rises. That is why flash flooding is an urban problem.

Antecedent conditions are worth a category of their own: soil already saturated from earlier rainfall has no remaining storage capacity, so a following storm behaves as though the ground were impermeable, regardless of the underlying rock type. A named rock contrast strengthens an answer here — impermeable clay or granite gives a flashy hydrograph, while permeable chalk gives a flatter one with a longer lag, since water can infiltrate and be released more gradually.

Channel processes and landforms

Erosion: hydraulic action, abrasion, attrition, solution. Transport: traction, saltation, suspension, solution. Deposition: when velocity falls — inside a meander bend, at the mouth, or as discharge drops.

The Hjulström curve contains one counter-intuitive result worth knowing: fine clay needs a higher velocity to be entrained than sand, because clay particles are cohesive and stick together. Once suspended, though, they stay in transport at very low velocities.

• Waterfall — hard rock over soft; undercutting forms a plunge pool and overhang, which collapses, so the fall retreats upstream leaving a gorge.
• Meander — faster flow on the outside erodes a river cliff; slower flow on the inside deposits a slip-off slope.
• Ox-bow lake — the neck narrows until breached in flood; deposition then seals the abandoned loop.
• Levée and floodplain — overbank flow deposits coarse material nearest the channel and fine material further away.
• Delta — requires a high sediment load and low wave and tidal energy.

Rocks and weathering

Weathering is in situ; erosion involves movement. Confusing them is the most penalised error here.

• Physical — freeze–thaw, exfoliation, salt crystallisation, pressure release.
• Chemical — carbonation, hydrolysis, oxidation, hydration, solution.
• Biological — root action, burrowing, chelation.

Chemical weathering is fastest in hot, wet climates, because reaction rates rise with temperature and water is required. Freeze–thaw needs repeated crossing of 0 °C, so it is most active in periglacial and high-altitude environments — not in the coldest places, where temperatures remain below freezing and no thawing occurs.

Mass movement, by speed: soil creep, solifluction, mudflow, slumping (rotational, curved failure plane), rockfall. Failure occurs when shear stress exceeds shear strength; water is usually the trigger because it adds weight and reduces inter-particle friction.

Atmosphere and weather

The energy budget: incoming shortwave radiation is reflected, absorbed or scattered; the surface re-emits longwave radiation, which greenhouse gases absorb. The tropics run a surplus and the poles a deficit, so energy is redistributed by atmospheric cells (Hadley, Ferrel, Polar) and ocean currents.

Adiabatic cooling — rising air expands and cools with no heat exchange with its surroundings.

Stability depends on comparing the environmental lapse rate with the adiabatic lapse rate: if rising air remains warmer than the surrounding air it keeps rising, giving instability and convectional cloud; if it becomes cooler it sinks back, giving stability.

Rainfall: convectional (surface heating), relief/orographic (air forced over high ground), frontal (warm air rising over cold).

Urban heat island: low-albedo surfaces absorb more radiation; concrete and brick store heat and release it slowly; less vegetation means less evaporative cooling; tall buildings reduce wind speed and trap longwave radiation; human activity releases heat directly.

Exam traps

• Confusing weathering with erosion.
• Describing a landform without the processes forming it.
• Saying freeze–thaw is fastest in the coldest environments.
• Reading a hydrograph without linking its shape to basin characteristics.
• Answering without annotated diagrams — they earn marks in this paper.
• Generic answers with no named examples or figures.

Self-test

1. Distinguish weathering from erosion.
2. Give four factors that shorten lag time, with reasons.
3. Explain the formation of an ox-bow lake.
4. Why is freeze–thaw not most active in the coldest climates?
5. Give two causes of the urban heat island.

Answers: 1. Weathering is the in-situ breakdown of rock; erosion involves the removal and transport of material. 2. Impermeable rock (no infiltration), steep relief (faster overland flow), sparse vegetation (less interception), urban land use (impermeable surfaces and drains) — also saturated soil, circular basin, intense rainfall. 3. Erosion on the outsides of adjacent bends narrows the meander neck until a flood breaches it; the river takes the straighter route and deposition seals off the old loop. 4. It requires water to freeze and thaw repeatedly, so temperatures must cross 0 °C regularly; where it stays below freezing there is no thawing and the process stops. 5. Low-albedo surfaces absorb more radiation and building materials store and slowly release heat; reduced vegetation means less evaporative cooling — also reduced wind speed and direct heat from human activity.