Water Resources
What this domain tests
Water resources questions examine the global and local water cycle, water scarcity, storage and management, and the impacts of abstraction, dams and pollution. You must interpret water budgets, hydrographs and maps of supply and demand.
Core rules and concepts
- The water budget: precipitation = evapotranspiration + runoff ± change in storage. A positive storage change means surplus; negative means deficit.
- Groundwater is stored in aquifers; the water table is the upper level of saturation. Over-abstraction can lower the water table and cause subsidence or saltwater intrusion.
- Physical water scarcity is low supply relative to demand; economic water scarcity is lack of investment, infrastructure or management.
- Dams store water and generate hydropower but displace people and trap sediment; desalination is energy-intensive and expensive.
Worked example
A catchment receives 1,000 mm of precipitation. Evapotranspiration is 600 mm and runoff is 250 mm. Storage change = 1,000 − 600 − 250 = +150 mm. This is a surplus, so groundwater and soil moisture may recharge. If evapotranspiration rose to 850 mm, storage change = 1,000 − 850 − 250 = −100 mm, a deficit that could reduce river flow and groundwater. Reasoning: keep all units in mm and apply the budget equation in order.
Traps that cost marks
Do not confuse runoff with groundwater recharge. Do not assume all precipitation becomes usable supply. Do not describe dams as only beneficial or only harmful. Do not ignore evaporation losses from reservoirs.