Drainage basin hydrology is a key focus in IB Geography, particularly within Option A: Freshwater – Drainage Basins. Hydrology refers to the movement, distribution, and storage of water within a drainage basin. A range of physical processes control how water enters, moves through, and leaves the system, influencing river discharge and flood risk.
One of the first processes affecting drainage basin hydrology is interception. This occurs when precipitation is temporarily held by vegetation such as leaves, branches, and stems. Interception reduces the amount of water reaching the ground immediately and slows down runoff. Dense vegetation cover increases interception, reducing flood risk and soil erosion.
Once water reaches the ground, infiltration occurs. Infiltration is the process by which water soaks into the soil. The rate of infiltration depends on soil type, soil moisture content, vegetation cover, and rainfall intensity. Sandy soils generally allow faster infiltration, while clay soils have lower infiltration rates. When infiltration capacity is exceeded, surface runoff increases.
After infiltration, water may move downward through the soil by percolation. Percolation transfers water from soil moisture storage to groundwater storage. This process is important for maintaining river baseflow, especially during dry periods. Groundwater moves slowly through rock layers and contributes to long-term river discharge.
Surface runoff is a key process shaping drainage basin response to rainfall. It occurs when rainfall intensity exceeds infiltration capacity or when soils are already saturated. Surface runoff reaches river channels quickly, leading to rapid increases in discharge and increasing flood risk. Urban areas with impermeable surfaces experience particularly high levels of runoff.
Water can also move laterally through the soil as throughflow. Throughflow occurs when water moves downslope through the soil towards the river channel. It is slower than surface runoff but faster than groundwater flow. Throughflow contributes to rising river levels during and after rainfall events.
The final hydrological process is river discharge, which refers to the volume of water flowing past a point in the river over time. Discharge reflects the combined effects of all inputs, stores, and flows within the drainage basin. Discharge patterns are often shown using storm hydrographs in IB Geography.
In IB Geography, it is important to recognise that hydrological processes are influenced by both physical and human factors. Climate, vegetation, geology, and land use all affect how water moves through a drainage basin. Human activities such as deforestation, agriculture, and urbanisation can significantly alter hydrological processes and increase flood risk.
Overall, drainage basin hydrology is shaped by interconnected processes that control water movement and storage. Understanding these processes allows students to explain river behaviour, flooding, and water management strategies clearly.
RevisionDojo helps IB Geography students master drainage basin hydrology by linking processes, systems, and exam-style explanations into clear and confident answers.
