The Hydrosphere and Oceanography: Water Cycle, Ocean Dynamics and Coastal Processes
Domain 7 covers the water cycle, ocean circulation and the coastal processes shaping island shorelines. In an archipelago with limited aquifers and strong wave energy, the exam expects quantitative-style reasoning about residence times, upwelling, longshore drift and the balance between recharge and extraction.
The water cycle
Reservoirs and approximate residence times: oceans (~97% of water, ~3000 years), glaciers (~2%, tens to thousands of years), groundwater (years to millennia), lakes, soil moisture, atmosphere (12 days), rivers (days). Fluxes: evaporation, transpiration, condensation, precipitation, runoff, infiltration, percolation. On volcanic islands, infiltration is high in permeable lapilli and lava tubes, and runoff is concentrated in barrancos during flash floods.
Ocean dynamics
Surface currents are wind-driven (Ekman transport, gyres); the Gulf Stream and its branch, the Canary Current, are the relevant systems. Thermohaline circulation is density-driven (temperature and salinity). Upwelling occurs where winds diverge surface water and cold nutrient-rich water rises - the Canary upwelling supports fisheries. Tides are gravitational (Moon, Sun), with spring and neap cycles. Waves are wind-generated; fetch, wind speed and duration set height.
Coastal processes
Wave refraction concentrates energy on headlands and disperses it in bays. Longshore drift moves sediment by the swash–backwash angle. Coastal landforms: cliffs, platforms, sea arches, spits, beaches, dunes (Maspalomas), deltas and managed beaches with artificial sand. Coastal erosion is increasing with sea-level rise and reduced sediment supply.
Groundwater on islands
Freshwater floats on saline water (Ghyben–Herzberg principle): each metre of water table above sea level corresponds to about 40 m of freshwater below, so over-extraction causes saline intrusion. Extraction via galerías, pozos and nacientes; problems: over-abstraction, contamination by nitrates, desalination brine.
Worked example
An island aquifer has recharge of 5 hm³/yr and abstraction of 7 hm³/yr. What follows? Reasoning: the deficit of 2 hm³/yr must be met by mining stored groundwater, so the water table falls; as it falls, the freshwater lens thins and saline intrusion rises (Ghyben–Herzberg); in drought years recharge drops further and the trend accelerates. Answer: unsustainable extraction requiring reduced demand, recharge enhancement or desalination.
Traps that cost marks
Saying groundwater cannot be contaminated because it is underground. Confusing tides with waves. Claiming longshore drift moves sediment perpendicular to the coast. Ignoring the salinity balance in island aquifers.
- Atmosphere residence time of water is only ~12 days, so it is a fast flux.
- Canary upwelling = cold, nutrient-rich, high biological productivity.
- Ghyben–Herzberg: freshwater lens protrudes about 40 m below sea level per metre above.
- Wave refraction explains headland erosion and bay deposition.
- Barrancos behave as ephemeral channels with torrential flash floods.