Drainage basins are a core topic in IB Geography, particularly within Option A: Freshwater – Drainage Basins. A drainage basin is the area of land drained by a river and its tributaries. In geography, drainage basins are best understood as open systems, where inputs, transfers (flows), stores, and outputs interact continuously.
The main input into a drainage basin system is precipitation, which may fall as rain, snow, sleet, or hail. The amount, intensity, and type of precipitation strongly influence how the system behaves. Heavy rainfall over a short period is more likely to cause flooding, while gentle rainfall allows more infiltration into the soil.
Once precipitation enters the system, it is held in various stores. These include vegetation interception, soil moisture storage, groundwater storage, and channel storage within the river itself. These stores temporarily hold water and delay its movement through the system. The size and capacity of these stores affect how quickly water reaches the river channel.
Water moves between stores through flows (transfers). Key flows include infiltration, percolation, surface runoff, throughflow, and groundwater flow. Surface runoff occurs when rainfall exceeds the soil’s infiltration capacity, leading to rapid movement of water into river channels. Throughflow and groundwater flow move water more slowly, contributing to baseflow and sustaining rivers during dry periods.
The main output of a drainage basin system is river discharge, where water leaves the basin at the river mouth and enters the sea, lake, or another river. Evapotranspiration is another important output, returning water to the atmosphere through evaporation and plant transpiration.
In IB Geography, it is important to recognise that drainage basins aim to maintain a dynamic equilibrium. This means that although inputs and outputs vary over time, the overall system remains balanced under normal conditions. However, extreme weather events or human activities can disrupt this balance, leading to flooding or water shortages.
Human activities significantly affect drainage basin systems. Urbanisation increases impermeable surfaces, reducing infiltration and increasing surface runoff. Deforestation reduces interception and increases soil erosion, altering flows and increasing flood risk. Water abstraction and dam construction change river discharge patterns and storage levels.
Understanding drainage basins as systems allows IB Geography students to explain river processes, flooding, and water management more effectively. It provides a framework for linking physical processes with human impacts.
RevisionDojo helps IB Geography students understand drainage basin systems clearly by linking key concepts, diagrams, and processes into structured, exam-ready explanations.
