A-Level Geography: The Water Cycle and Drainage Basins

The water cycle is half of the compulsory 'water and carbon cycles' core theme in the A level Geography subject content, and questions on it appear in the physical geography paper every year. This material trains the reasoning behind it rather than a list of definitions.

It starts with systems thinking. The global hydrological cycle is a closed system: energy from the Sun drives it, but the amount of water on Earth is effectively fixed. A drainage basin, by contrast, is an open system that gains water as precipitation and loses it as runoff and evapotranspiration. You practise using the vocabulary of inputs, outputs, stores and flows, and telling positive feedback from negative feedback and dynamic equilibrium in a real catchment setting.

Next come the processes inside a drainage basin: interception, stemflow and throughfall, infiltration, percolation, throughflow, groundwater flow and channel flow. Particular attention goes to the two ways overland flow is generated: infiltration-excess (Hortonian) flow, when rain falls faster than the soil can absorb it, and saturation-excess flow, when the soil is already full. Short scenarios describe invented catchments in words, so you learn to recognise each process from its conditions.

The quantitative part covers the water balance, P = Q + E ± ΔS, runoff as a percentage of rainfall, and the soil moisture budget with its stages of utilisation, deficit, recharge and surplus. Every number you need is given in the task. You then read storm hydrographs through their causes: why impermeable surfaces and steep slopes give a flashy response, why woodland and permeable rock subdue it, and how a snow-fed river regime reflects storage in the cryosphere. Human impacts include urbanisation, land-use change and groundwater abstraction, with its effect on baseflow in chalk streams.

The material offers a quiz with worked explanations, flashcards for the key terms, a printable written sheet of short explanations and calculations, and an oral exam in which an examiner asks one question at a time and gives brief feedback at the end.

• Apply systems concepts (open and closed systems, stores, flows, feedback, dynamic equilibrium) to the water cycle
• Explain the relative size of global water stores and the meaning of residence time
• Identify drainage basin flows, including infiltration-excess and saturation-excess overland flow
• Calculate a water balance and a runoff percentage from given data
• Describe the soil moisture budget through the year
• Explain how physical and human factors shape storm hydrographs and river regimes

Practice material written by Zestly, based on the 'water and carbon cycles' core theme of the DfE GCE AS and A level subject content for geography (2014) and board specifications such as AQA A-level Geography 7037, section 3.1.1.2, and Pearson Edexcel A level Geography 9GE0, Topic 5.