Hydrosphere and Water Resources
The water cycle and oceans
Water moves through evaporation, transpiration, condensation, precipitation, infiltration, runoff and storage. Oceans hold most of Earth's water; only a small share is fresh, and much of that is locked in ice. Know ocean properties: salinity, temperature decreasing with depth, currents driven by wind and density, tides caused by the Moon and Sun, and waves driven by wind.
Freshwater systems
Rivers are described by regime, discharge, drainage basin and watershed; a river's regime reflects climate (spring snowmelt, summer monsoon, winter rain). Lakes form in tectonic, volcanic, glacial and karst basins. Groundwater occupies aquifers, and the water table rises and falls with recharge. Glaciers store water and shape land.
Water resources and management
Water stress arises from uneven distribution, pollution and overuse. Know terms like irrigation, reservoirs, desalination, transboundary rivers and sustainable management. Bulgaria's rivers drain to the Danube and the Aegean; the Danube is a key European waterway.
Traps
Beware: confusing watershed (boundary) with drainage basin (area); assuming all lakes are tectonic; and mixing tides (gravitational) with currents (wind and density).
Rule of thumb
Link regime to climate: snowmelt-fed rivers peak in spring, rain-fed rivers peak in the wet season, and glacier-fed rivers peak in summer heat.