The Hydrosphere, Oceans and Water Resources
The water cycle
The hydrosphere includes oceans (about 97% of water), glaciers, groundwater, lakes, rivers and the atmosphere. The hydrological cycle moves water via evaporation, transpiration, condensation, precipitation, infiltration, runoff and groundwater flow. Residence times differ enormously: oceans ~3000 years, glaciers longer, atmosphere only days.
Oceans
- Salinity reflects evaporation minus precipitation plus river input; about 35 g/kg on average.
- Thermohaline circulation is driven by density differences (cold, salty water sinks); it redistributes heat globally.
- Upwelling brings nutrient-rich deep water to the surface, supporting fisheries.
- El Niño–Southern Oscillation shifts Pacific pressure and trade winds, altering rainfall worldwide.
Water resources
Aquifers store groundwater; recharge depends on infiltration and geology. Porosity is storage capacity; permeability is flow capacity - a clay can be porous but impermeable. Over-extraction causes water-table decline, land subsidence and saltwater intrusion in coastal areas.
Traps
- Confusing porosity with permeability.
- Claiming most freshwater is in lakes and rivers - most is in glaciers and groundwater.
- Treating the ocean as a static reservoir; circulation and mixing matter.
- Ignoring that groundwater recharge can take decades to centuries.
Worked example
A catchment receives 1200 mm rain; 300 mm evapotranspires, 500 mm runs off, so 400 mm infiltrates. If 150 mm recharges the aquifer, recharge is about 12.5% of rainfall. Rule: precipitation = evapotranspiration + runoff + storage change, a water balance you can always apply.