Physical and Human Factors Affecting the Water Cycle

This lesson examines how physical factors (climate, geology, soil, vegetation, relief) and human activities (urbanisation, deforestation, agriculture, dams, water abstraction) alter the stores and fluxes of the water cycle at various scales. It addresses Edexcel A-Level Geography (9GE0) Paper 1, Topic 5, Enquiry Question 2: What factors influence the hydrological system over short- and long-term timescales?

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Physical Factors

1. Climate

Climate is the primary control on the water cycle because it determines the amount and timing of precipitation and the rate of evapotranspiration.

Climate Variable	Effect on Water Cycle
Temperature	Higher temperatures increase evaporation and transpiration rates. The Clausius-Clapeyron relation indicates that the atmosphere can hold ~7% more moisture per °C of warming.
Precipitation amount	Determines total water input to the system. Global precipitation averages ~1,000 mm/yr over land, but ranges from <50 mm/yr (Atacama Desert) to >11,000 mm/yr (Meghalaya, India).
Precipitation intensity	Controls whether water infiltrates or runs off the surface. Tropical convective storms deliver >100 mm/hr; temperate frontal rain delivers 2–10 mm/hr.
Precipitation seasonality	Determines periods of surplus and deficit. Monsoon climates receive 80%+ of annual precipitation in 3–4 months.
Wind	Increases evaporation rates by removing saturated air from water surfaces.
Solar radiation	Provides the energy for evaporation (~2,450 kJ needed to evaporate 1 kg of water at 20°C).

2. Geology

The permeability and porosity of underlying rock fundamentally control how water moves through a drainage basin.

Rock Type	Porosity	Permeability	Effect on Water Cycle
Chalk	20–45%	Moderate (porous, water moves through pore spaces)	High infiltration, large groundwater stores, sustained baseflow, subdued hydrographs. Springs emerge where chalk meets impermeable rock.
Carboniferous limestone	<5% (matrix)	High (permeable due to joints and fissures, not pores)	Rapid infiltration via swallow holes, underground drainage, dry valleys, resurgent springs. Very low surface drainage density.
Sandstone	15–25%	Moderate–high	Good aquifer; stores significant groundwater; major UK aquifer (Permo-Triassic sandstone).
Clay	45–60%	Very low (porous but NOT permeable — tiny pore spaces trap water)	Low infiltration, high overland flow, flashy hydrographs, high drainage density.
Granite	<1%	Very low (impermeable ignite)	Almost all water runs off the surface; rivers rise and fall rapidly; thin soils, moorland.

Exam Tip: Many students confuse porosity (the proportion of pore spaces in rock) with permeability (the ability of water to flow through connected pore spaces). Clay has high porosity but low permeability because its pore spaces are too small for water to flow through. Always define both terms and explain the difference.

3. Soil Type

Soil is the interface between the surface and bedrock. Its texture, structure, organic content and depth determine infiltration rates, water-holding capacity and throughflow rates.

Soil Property	Sandy Soil	Clay Soil	Loam
Particle size	0.06–2.0 mm	<0.002 mm	Mixed
Infiltration rate	High (25–50 mm/hr)	Low (1–5 mm/hr)	Moderate (10–20 mm/hr)
Water-holding capacity	Low (drains quickly)	High (retains water)	Moderate
Throughflow rate	Fast	Very slow	Moderate
Effect on hydrograph	Longer lag time, lower peak	Shorter lag time, higher peak	Intermediate

4. Vegetation

Vegetation affects the water cycle through interception, transpiration, infiltration enhancement and reducing overland flow.

Vegetation Type	Annual Interception Loss (% of rainfall)	Annual Transpiration (mm/yr)	Effect on Water Cycle
Tropical rainforest	10–20%	1,000–1,500	Massive transpiration returns moisture to atmosphere; 25–50% of Amazonian rainfall is recycled transpiration
Temperate deciduous forest	25–35%	400–600	Strong seasonal effect — interception highest in summer; lowest after leaf fall
Coniferous forest	30–45%	300–500	Year-round interception due to evergreen needles
Grassland	10–20%	300–500	Lower interception but root networks improve soil structure and infiltration
Bare soil / arable	<5%	Variable	Minimal interception; exposed soil susceptible to compaction and erosion; reduced infiltration

5. Relief (Topography)

Relief Factor	Effect on Water Cycle
Altitude	Higher altitude = lower temperatures, more precipitation (orographic effect), more snow storage, lower evapotranspiration
Slope gradient	Steeper slopes increase velocity of overland flow, reduce time for infiltration, increase erosion
Aspect	South-facing slopes (in Northern Hemisphere) receive more solar radiation = higher evapotranspiration; north-facing slopes retain moisture longer
Valley shape	Narrow valleys concentrate runoff; broad floodplains allow water to spread and infiltrate

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Human Factors

Human activities have increasingly modified the natural water cycle, often with unintended consequences.

1. Urbanisation

Urbanisation is one of the most significant human modifications to the water cycle. In the UK, approximately 7% of land area is urbanised, but in individual drainage basins (e.g. the River Rea in Birmingham), urban coverage can exceed 60%.

Effects of urbanisation on the water cycle:

Process	Natural (Rural)	Urbanised
Interception	25–40% by vegetation	Negligible (buildings intercept some, but route water to drains)
Infiltration	High in vegetated, permeable soils	Very low (concrete, tarmac, roofs are impermeable)
Overland flow	Relatively low (most water infiltrates)	Very high (60–90% of rainfall runs off impermeable surfaces)
Lag time	Long (water takes hours/days to reach channel)	Short (storm drains deliver water to channels in minutes)
Peak discharge	Moderate	2–5× higher than natural
Baseflow	Sustained by groundwater	Reduced (less groundwater recharge)
Evapotranspiration	High (vegetation transpires)	Reduced (less vegetation; but urban heat island increases evaporation from surfaces)
Water quality	Generally good	Polluted (road runoff, sewage overflow, industrial discharge)

2. Deforestation

The removal of forest cover has profound effects on the water cycle. Deforestation rates are highest in tropical regions: the Amazon lost approximately 17% of its forest cover between 1970 and 2020.