The Hydrosphere: Water Cycle, Surface and Groundwater, Resources and Hydrological Risk
This domain covers water storage and transfer, drainage-basin behaviour, aquifer mechanics and water management, with particular weight on karst and coastal aquifers, both critical in the Balearics.
The water cycle
Processes: evaporation, transpiration, condensation, precipitation, interception, infiltration, percolation, runoff, groundwater flow and discharge. Learn residence times: oceans hold the largest reservoir, glaciers second; groundwater outlasts surface water by orders of magnitude.
Surface water
A drainage basin is bounded by a divide. The hydrograph plots discharge against time: peak discharge, lag time, rising and falling limbs and baseflow. Short lag times and high peaks follow impermeable surfaces, steep slopes, sparse vegetation, small basins and intense storms.
Groundwater
- Porosity = total void space; permeability = ability to transmit water. Clay has high porosity and low permeability.
- Aquifer (confined or unconfined), aquiclude, aquifuge, water table, piezometric surface, recharge and discharge areas, springs and wells; pumping creates a cone of depression.
- Karst: limestone dissolves, producing dolines, sinkholes, poljes, caves and underground drainage; pollutants travel fast and with little filtration.
Water resources and risk
Demand from tourism, irrigation and urban growth exceeds recharge on many islands. Overexploitation causes falling water tables, land subsidence and seawater intrusion, followed by desalination and reclaimed water as substitutes. Reservoirs (for example Cúber and Gorg Blau in Mallorca) and aquifers together supply the population. Hydrological risks are droughts and flash floods in rambles and torrents.
Worked example
A pumping well operates near the coast. Pumping lowers the water table below sea level, the freshwater-seawater interface rises and migrates inland (Ghyben-Herzberg relation), and the well turns saline. The reasoning chain is: abstraction → piezometric drop → loss of the freshwater lens → intrusion. The remedy is to reduce abstraction, relocate wells inland or recharge the aquifer.
Traps that cost marks
- Treating porosity and permeability as the same property.
- Saying a confined aquifer has a water table; it has a piezometric surface that can rise above the top of the aquifer.
- Assuming reforestation increases flood peaks; it usually reduces them.
- Ignoring that karst offers almost no contaminant attenuation.