CAIE A-Level Geography

## The drainage basin as an open system A **drainage basin** (catchment) is the area of land drained by a river and its tributaries, bounded by a ridge of high ground called the watershed. It is an **open system**, because both matter (water, sediment) and energy cross its boundary. Water enters as inputs, is held in stores, moves between them by transfers, and leaves as outputs. The balance is expressed as a water balance: precipitation = river discharge + evapotranspiration ± change in storage. When inputs exceed outputs the stores grow (soil moisture rises, the water table climbs); when outputs exceed inputs the stores shrink. ### Inputs and outputs The main input is **precipitation**: rain, snow and hail. The outputs are: - **evaporation**, the change of liquid water to vapour from lakes, rivers, soil and wet surfaces; - **transpiration**, water vapour lost through plant stomata, and **evapotranspiration**, the two combined; - **channel flow** (discharge) to the sea or a lake; - in a broader view, condensation of vapour in the atmosphere above the basin, which returns water to the air as cloud. ### Stores - Atmosphere: water vapour and cloud. - Vegetation: water held on leaves, stems and bark (**interception store**) and water taken up by roots. - Surface: puddles, lakes, ponds and the water in the river channel. - Soil: moisture held between soil particles. - Bedrock: **groundwater** stored in saturated rock below the **water table**, the upper surface of the saturated…