Hydrosphere and Water Resources
This domain covers the water cycle, surface and groundwater behaviour, water quality and the management problems of a Mediterranean country. PAU questions favour hydrograph interpretation, aquifer reasoning (especially coastal salt intrusion) and quality indicators.
The cycle and surface water
Inputs are precipitation, outputs are evapotranspiration, runoff and infiltration. A drainage basin is the fundamental unit. The hydrograph records discharge against time: lag time, baseflow and peak flow. Urbanisation raises peaks and shortens lag because impermeable surfaces replace vegetation and soil; forests and wetlands store water and flatten the curve. In Catalonia the Ter and Llobregat supply Barcelona, with reservoirs such as Sau, Susqueda and La Baells acting as regulation, while the Ebro is the largest river but largely outside the internal basins.
Groundwater
Aquifers need porosity (storage) and permeability (transmission); clay has high porosity but very low permeability. In a free aquifer the water table is the upper surface; in a confined aquifer the water is under pressure and rises in piezometers. Darcy's law: Q = K·A·i, discharge proportional to hydraulic conductivity, cross-section and hydraulic gradient. Recharge comes from rainfall infiltration, discharge through springs, rivers and abstraction. Over-abstraction lowers the water table, compacts fine sediment, and dries springs and wetlands.
Water quality
Indicators: BOD₅ and COD for organic load, dissolved oxygen, nitrates and phosphates, electrical conductivity, turbidity, heavy metals, faecal coliforms. Eutrophication arises from excess nitrogen and phosphorus, causing algal blooms, oxygen depletion and fish kills. Pollution sources are point (outfalls, slurry tanks) and diffuse (agricultural fertiliser, urban runoff). Salinisation from seawater intrusion affects the Empordà and Tordera coastal aquifers when pumping reverses the freshwater–seawater gradient.
Management
Abstraction control, groundwater recharge, artificial wetlands, drip irrigation, reuse of treated water, desalination (El Prat), demand management and the Water Framework Directive goal of good ecological status for water bodies by set deadlines. Drought planning in Catalonia relies on reservoir thresholds that trigger staged restrictions.
Worked example
A coastal aquifer near a tourist town shows rising chloride and conductivity in its wells. Reasoning: pumping has lowered the water table below sea level, and since freshwater is less dense than seawater, the saline interface migrates inland and upward. Diagnosis: saltwater intrusion, not a pollution spill. Solutions: reduce abstraction, relocate wells inland, recharge with treated water, and monitor conductivity regularly.
Traps that cost marks
Equating porosity with permeability; calling seawater intrusion a form of chemical pollution from industry; reading a hydrograph peak as the rainfall peak (there is a lag); forgetting evapotranspiration in the balance.