The drainage basin hydrological system & water balance

Step back and look at a whole river catchment as a single working machine. In this AS lesson we meet the drainage basin as an open system, then take its four parts one at a time — the input of precipitation; the stores of interception, surface, soil moisture, groundwater and channel; the flows of stemflow, infiltration, overland flow, throughflow, percolation and groundwater flow; and the outputs of evapotranspiration and river discharge. We draw the full systems diagram, see how the balance shifts from the humid tropics to arid Namibia to a temperate climate, dig underground to find where groundwater is born beneath the water table, and calculate a real water balance with P = E + Q ± ΔS. Finally we look at the many ways people modify the whole cycle — dams, abstraction, urbanisation, irrigation and deforestation — using Namibian examples throughout, from the Cuvelai and Ohangwena aquifers to the Hardap and Von Bach dams.