Tropical climates and drainage basins

H2 Geography - syllabus 9173, 2027

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Explain tropical rainfall, track water through a drainage basin, and judge how well floods can be managed.

• Tropical climate zones and climographs

  Tropical climates share strong heating. Rainfall amount and seasonality separate the humid and arid climate zones.

• Hadley cells and the ITCZ

  Rising moist air favours rain near the ITCZ. Descending air suppresses rain in the subtropics; seasonal migration shifts these belts.

• Summer and winter monsoons in Asia

  Seasonal heating changes regional pressure and winds. Rainfall depends on the air's route over water and the terrain it meets.

• Tropical cyclones and rainfall

  Cyclones need ocean heat, moisture and an organised circulation. Their rain can continue inland after the strongest winds weaken.

• ENSO and Pacific rainfall

  ENSO couples ocean temperatures and atmospheric circulation. It shifts the main tropical Pacific rain zone rather than warming every place equally.

• Drainage-basin water budgets

  Inputs minus outputs change storage. A river can continue flowing by drawing down water stored earlier.

• Water storage in drainage basins

  Storage depends on available space, previous rainfall and human use. A large store is not necessarily an empty store.

• Infiltration, runoff and baseflow

  Water can take rapid surface routes or slower underground routes. Soil, rock, wetness and land use determine which routes dominate.

• Storm hydrographs and lag time

  A hydrograph shows discharge through time. A short lag and high peak suggest rapid delivery, but several basin and storm conditions can cause them.

• Causes of fluvial flooding

  A fluvial flood occurs when discharge exceeds the channel's bankfull capacity and water covers normally dry land.

• Flood impacts and vulnerability

  Flood impacts depend on depth, speed, duration and timing, and on people's ability to avoid, withstand and recover from them.

• Managing fluvial floods

  Effective flood management combines measures and checks where risk is reduced, transferred or left behind.