Hydrosphere: water cycle, resources, quality and management
Domain 9 - Hydrosphere. The exam links the water cycle to aquifer behaviour, water quality indicators and the management conflicts typical of Mediterranean Spain.
Distribution and cycle
- 97.5 % of water is saline; of the 2.5 % fresh, about 69 % is ice, 30 % groundwater and under 1 % surface water.
- Fluxes: evaporation, evapotranspiration, condensation, precipitation, runoff, infiltration. Residence times: oceans about 3,000 years, glaciers millennia, groundwater decades to millennia, rivers days.
- A watershed (cuenca) collects runoff to a single outlet; the hydrograph shows peak discharge and lag time, modified by urbanisation (faster, higher peak) and vegetation (slower).
Groundwater
- Porosity stores, permeability transmits. Aquifers: free (water table), confined (potentiometric surface, artesian wells), perched.
- Darcy's law: Q = K·A·i, where i is the hydraulic gradient.
- Overexploitation causes water-table decline, subsidence, drying springs and saline intrusion; the Ghyben–Herzberg relation puts the freshwater–saltwater interface about 40 m below sea level for each metre of freshwater head above sea level.
Water quality
- Indicators: dissolved oxygen, BOD5, COD, nitrates and phosphates, conductivity, turbidity, pH, heavy metals, faecal coliforms, emerging pollutants.
- Eutrophication follows nutrient enrichment: algal bloom, oxygen depletion, fish kills. In the Valencian Community, the Albufera lagoon suffers eutrophication and nitrate loading.
- Legal framework: EU Water Framework Directive 2000/60/EC sets ecological and chemical status objectives through river basin management plans.
Management in Mediterranean Spain
- Supply: reservoirs, the Tagus–Segura transfer, the Júcar–Vinalopó transfer, desalination, treated-water reuse.
- Demand: irrigation dominates (roughly 60–70 % in the region), urban supply, tourism peaks, golf courses.
- Tools: metering and pricing, efficiency in irrigation, aquifer recharge, flood irrigation conversion, drought plans.
Worked example
An aquifer with K = 10 m/day, cross-section A = 100 m2 and hydraulic gradient i = 0.005 transmits Q = 10 × 100 × 0.005 = 5 m3/day. Doubling the gradient doubles the flow; raising K tenfold also doubles nothing else - the relation is linear in each term.
Traps
- Confusing porosity with permeability: clay is porous but nearly impermeable.
- Giving a confined aquifer a water table rather than a potentiometric surface.
- Attributing saline intrusion only to sea-level rise, when pumping is usually the cause.
- Treating BOD as a measure of suspended solids.