How does the hydrological cycle operate as a system, and what causes river regimes to vary and basins to flood or run dry?
The global hydrological cycle as a closed system of stores and fluxes, the drainage basin as an open system, the water budget and storm hydrographs, and the physical and human factors that drive floods and drought.
An Edexcel A-Level Geography answer to water cycle processes and budgets, covering the global hydrological cycle as a closed system of stores and fluxes, the drainage basin as an open system, the water budget, storm hydrographs, and the physical and human factors driving floods, drought and ENSO.
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What this dot point is asking
Edexcel wants you to explain the global hydrological cycle as a closed system of stores and fluxes, explain the drainage basin as an open system governed by the water budget, interpret storm hydrographs, and explain the physical and human factors that make basins flood or run dry, including ENSO.
The global hydrological cycle as a closed system
Stores differ enormously in residence time, the average time a water molecule stays before moving on. Deep groundwater and ice can store water for thousands of years, while the atmosphere turns over in about nine days. The fluxes that move water between stores are evaporation and transpiration (together evapotranspiration), condensation, precipitation, interception, infiltration, percolation, throughflow, groundwater flow, surface runoff and meltwater. The whole cycle is powered by two drivers: solar energy, which evaporates water and powers atmospheric circulation, and gravity, which returns water to the surface and moves it downslope.
The drainage basin as an open system
The basin's behaviour over a year is captured by the water budget (water balance), written as , where is precipitation, is runoff, is evapotranspiration and is the change in storage. A soil-water graph shows a surplus and recharge when precipitation exceeds potential evapotranspiration, then utilisation and deficit when it does not.
Storm hydrographs and basin response
The storm hydrograph plots river discharge against time after a storm. Key features are the rising limb, the peak discharge, the lag time (between peak rainfall and peak discharge), the falling limb and the baseflow that the storm flow sits on. A flashy hydrograph has a short lag time and high, narrow peak, typical of urban, steep or impermeable basins; a subdued hydrograph has a long lag and low, broad peak, typical of permeable, vegetated, gently sloping basins.
Factors driving floods and drought
Over the short term, storms raise discharge and cause floods; over the medium term, seasonal monsoon and snowmelt swell rivers; over the long term, glacial cycles and climate change reshape regimes. Flooding follows intense or prolonged rainfall, rapid snowmelt and impermeable saturated soils, amplified by human deforestation, urbanisation (flashy hydrographs) and river engineering. Drought comes in four types: meteorological (rainfall deficit), hydrological (low river and reservoir levels), agricultural (soil moisture too low for crops) and socio-economic (demand exceeds supply). Underlying much of this variability is ENSO: El Nino and La Nina phases shift Pacific circulation and trigger teleconnections that bring drought to Australia and Indonesia while flooding parts of the Americas.
Examples in context
Example 1: the River Tees, north-east England. The Tees rises on the impermeable, high-relief Pennines, where steep slopes, thin soils and high rainfall give a flashy upper-basin hydrograph and rapid response to storms. Downstream the gradient falls and the river meanders across a wide floodplain, where flood-control schemes (the Tees Barrage, embankments) manage discharge. It is a classic teaching basin for contrasting upland flashy response with lowland storage and human management.
Example 2: the Pakistan floods, 2010. An exceptionally strong monsoon dumped record rainfall on the Indus catchment, and deforested, steep Himalayan headwaters funnelled water rapidly downstream. Around a fifth of the country was inundated, over 20 million people were affected and about 2,000 died. It shows physical triggers (monsoon, relief) interacting with human factors (deforestation, floodplain settlement) to produce a catastrophic flood.
Try this
Q1. Define lag time on a storm hydrograph. [2 marks]
- Cue. The time between peak rainfall and the resulting peak river discharge.
Q2. Explain why urbanisation produces a flashier storm hydrograph. [4 marks]
- Cue. Impermeable surfaces and drains cut infiltration and speed overland flow, shortening lag time and raising peak discharge.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel Paper 1 (style)12 marksAssess the role of human activity in changing flood risk within drainage basins.Show worked answer →
AO1 sets out how the drainage basin operates as an open system and how the storm hydrograph reflects the route water takes to the channel. AO2 then weighs human against physical drivers. Human activity raises flood risk by accelerating runoff: urbanisation seals surfaces so infiltration falls and overland flow rises, giving a flashy hydrograph with short lag time and high peak discharge; deforestation removes interception and transpiration; river engineering such as channel straightening speeds water downstream onto vulnerable communities. The UK 2007 summer floods showed urban drainage overwhelmed, and Pakistan 2010 showed deforested Himalayan catchments funnelling monsoon water onto the Indus floodplain.
A balanced judgement notes that physical factors often dominate the trigger: prolonged or intense precipitation, snowmelt, impermeable geology and antecedent saturation. Human activity rarely creates floods alone but it amplifies the magnitude and frequency of an essentially physical hazard. The strongest answers reach a supported judgement using a located basin. AO1 supplies basin processes; AO2 weighs human amplification against physical triggers.
Edexcel 20198 marksExplain how the water balance varies through the year in a temperate basin.Show worked answer →
Use the water balance equation and walk through the year. In winter, precipitation exceeds evapotranspiration, so there is a surplus: soil storage refills (recharge) and, once at field capacity, the excess becomes runoff, raising river discharge. In spring, soil moisture is at a maximum. In summer, high temperatures raise potential evapotranspiration above precipitation, so plants and evaporation draw down soil storage (utilisation) until a deficit develops, lowering discharge. In autumn, precipitation again exceeds evapotranspiration and recharge resumes.
Strong answers link the balance to the seasonal energy budget and quote months, and connect surplus to flood risk and aquifer recharge, deficit to irrigation demand and low flows. AO1 supplies the budget terms; AO2 explains why the seasonal pattern arises.
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Sources & how we know this
- Pearson Edexcel A-Level Geography (9GE0) specification — Pearson Edexcel (2016)