Earth science

Weathering, soil, and the water cycle

Weathering, erosion, soil profiles, and the water cycle.

Basics

Weathering

Physical weathering splits rock with ice wedges, roots, and temperature swings. Chemical weathering turns feldspar to clay, dissolves carbonate, and oxidizes iron red. More surface area speeds chemistry. Warm wet climates favour chemical weathering; cold dry ones favour physical. Weathering is in place; erosion includes transport.

Erosion

Water, wind, ice, and gravity move weathered debris. Rivers round and sort grains downstream; slope and discharge set carrying power. Glaciers scrape and dump moraines; wind builds dunes and loess. Without plants the same rain strips far more soil. Where sediment stops you get floodplains, deltas, and coasts.

Soil

Soil is mineral grains, organic matter, water, and air. A common profile is O (organic), A (topsoil), B (accumulation), C (parent). Clay holds water and nutrients; sand drains. pH and cation exchange set which ions plants can use. Soil takes long to make; tillage and hard rain can lose a thin A horizon quickly.

Water cycle

Evaporation and transpiration lift water; clouds return it as rain or snow. Some runs off, some infiltrates to groundwater and returns in springs and streams. The ocean is the largest reservoir; much of the fresh water is ice. Residence time is short in air, long in ice and deep groundwater. Plants and soil aid infiltration and blunt flood peaks.

Formulas

Water budget (catchment)

P = E + R + ΔS

Precipitation = evapotranspiration + runoff + storage change.

Symbols

P precipitation
E evapotranspiration
R runoff
ΔS change in soil/groundwater storage

Darcy flow (sketch)

q = −K dh/dx

Groundwater follows the hydraulic gradient. K is hydraulic conductivity.

Symbols

q specific discharge
K hydraulic conductivity

Stream power (sketch)

Ω = ρ g Q S

Larger discharge Q and slope S raise the power to cut the bed and move grains.

Symbols

Q discharge
S channel slope

Residence time

τ = V / I

Storage V divided by inflow (or outflow) I — a mean stay.

Symbols

V reservoir volume
I inflow or outflow rate

Key table

Topsoil	decades to millennia to form; one storm can strip it
Fresh fraction	~2.5% of Earth’s water; mostly ice and groundwater
Atmospheric residence	water vapour typically ~10 days