Water and Carbon Cycles
Natural and Human Changes to the Water Cycle
Introduction
The water cycle is dynamic; it changes continuously over a range of temporal and spatial scales. These variations result from natural processes (such as storm events and seasonal changes) and human activities (including farming practices, land use change, and water abstraction).
Understanding how these factors influence the magnitude of stores and flows helps explain fluctuations in river discharge, soil moisture, and groundwater levels, and how equilibrium within the drainage basin system is maintained or disrupted.
1. Natural Variations in the Water Cycle
a) Storm Events
Short, intense rainfall events can temporarily transform the local water cycle.
| Process | Effect on the Water Cycle | Timescale |
|---|---|---|
| Heavy precipitation | Rapid surface runoff increases; infiltration and throughflow decrease as soil becomes saturated. | Hours–days |
| Increased discharge | River levels rise quickly, creating a flashy hydrograph. | Hours–days |
| Reduced infiltration | Rainfall intensity exceeds infiltration capacity, producing infiltration-excess overland flow. | Hours |
| Post-storm adjustment | As rainfall ceases, throughflow and baseflow gradually restore equilibrium. | Days–weeks |
Example:
During the Boscastle flood (2004), 60 mm of rain fell in two hours on steep, impermeable slopes. Surface runoff dominated, and river discharge increased sharply within one hour.
b) Seasonal Changes
Seasonal variation affects the balance between inputs, flows, and outputs within drainage basins.
| Season | Typical Conditions | Water Cycle Response |
|---|---|---|
| Winter (Temperate Regions) | Low temperatures, minimal vegetation cover. | High precipitation, low evapotranspiration → soil saturation, increased runoff and river discharge. |
| Spring | Rising temperatures and vegetation growth. | Snowmelt adds to discharge; increasing transpiration moderates soil moisture. |
| Summer | High temperatures, strong vegetation growth. | High evapotranspiration creates soil moisture deficit; lower river flow. |
| Autumn | Falling temperatures and decaying vegetation. | Soil moisture recharge begins; higher rainfall refills stores. |
The water balance shifts through the year, from surplus in winter to deficit in summer, influencing seasonal river regimes.
c) Long-Term Natural Variation
Over decades to millennia, the global water cycle is influenced by broader climatic and geological processes:
- ENSO (El Niño-Southern Oscillation) – alternating warm (El Niño) and cool (La Niña) phases shift rainfall and drought patterns globally.
- Glacial-interglacial cycles – water is redistributed between ice and ocean stores, lowering sea levels during glacials.
- Volcanic eruptions – ash and aerosols can cool global temperatures temporarily, reducing evaporation and precipitation.
- Vegetation succession – ecosystem development or clearance changes interception, infiltration, and evapotranspiration rates over time.
2. Human Impacts on the Water Cycle
a) Farming Practices
| Practice | Impact on Water Cycle | Resulting Change |
|---|---|---|
| Irrigation | Adds water artificially to soils. | Increases soil moisture and evapotranspiration, but may reduce downstream river flow. |
| Ploughing and soil compaction | Reduces infiltration by compacting soil. | Greater surface runoff, lower groundwater recharge. |
| Field drainage systems | Removes excess water rapidly. | Reduces soil water storage; increases flood peaks. |
| Crop type and timing | Influences interception and transpiration. | Affects seasonal soil moisture balance. |
| Deforestation for agriculture | Lowers interception and evapotranspiration. | Flashier runoff, reduced atmospheric moisture feedback. |
Example:
In eastern England, intensive arable farming and field drainage have reduced soil water storage, increasing flood peaks in the Great Ouse basin.
b) Land Use Change
| Change | Impact on Water Cycle | Hydrological Consequence |
|---|---|---|
| Urbanisation | Impermeable surfaces limit infiltration; storm drains channel water rapidly. | Increased surface runoff, shorter lag times, and higher flood risk. |
| Deforestation | Reduces interception and transpiration. | Increases surface flow and erosion. |
| Afforestation | Increases interception, infiltration, and evapotranspiration. | Enhances water storage and delays peak discharge. |
| Wetland drainage | Removes a natural storage buffer. | Loss of temporary storage and reduced groundwater recharge. |
Example:
In the Thames Basin, urban expansion has increased impermeable cover, contributing to higher storm runoff and flooding during the 2014 winter storms.
c) Water Abstraction
| Type of Abstraction | Effect on the Water Cycle | Potential Impact |
|---|---|---|
| Groundwater abstraction | Lowers the water table and reduces aquifer storage. | Decreases baseflow to rivers; may cause subsidence or saltwater intrusion in coastal areas. |
| Surface water abstraction | Water removed from rivers and reservoirs for supply. | Reduces river discharge, increases pollution concentration during low flow periods. |
| Seasonal variation in demand | Summer irrigation and domestic use peak. | Worsens water deficits and can dry up smaller tributaries. |
Example:
In southern England, groundwater abstraction from chalk aquifers has reduced baseflow in rivers like the Kennet, especially during dry summers.
d) Climate Change and the Water Cycle
Global warming is intensifying the hydrological cycle by altering evaporation, precipitation, and storage patterns.
| Process | Effect on the Water Cycle |
|---|---|
| Rising temperatures | Increase evaporation and atmospheric water vapour content, intensifying rainfall events. |
| Changing precipitation patterns | Wet regions becoming wetter; dry regions drier. More frequent intense storms and droughts. |
| Cryospheric melt | Accelerated glacier and ice-sheet melting increases sea levels and ocean storage. |
| Enhanced feedbacks | Warmer oceans release more moisture → positive feedback on global precipitation. |
Example:
In the Himalayan region, rapid glacier melt has increased short-term river flows but threatens long-term water security for millions downstream.
3. Integrated Impacts – Balancing Natural and Human Factors
- Storm events and seasonal cycles drive short-term fluctuations in flows and storage.
- Human activity, such as deforestation, farming, and abstraction, alters the long-term balance and may amplify natural extremes.
- Climate change acts as a global multiplier, intensifying natural variability and disrupting regional equilibrium.
Example:
In the Murray-Darling Basin (Australia), natural droughts have been worsened by irrigation and water diversion, reducing river discharge and groundwater recharge.
4. Temporal Scale of Change
| Timescale | Example Processes | Effect on the Water Cycle |
|---|---|---|
| Short-term (days–weeks) | Storm events, flash floods. | Temporary increase in surface runoff and discharge. |
| Seasonal (months) | Evapotranspiration and rainfall cycles. | Predictable fluctuations in soil moisture and river flow. |
| Decadal (years–decades) | ENSO cycles, land use change. | Shifts in rainfall patterns and groundwater levels. |
| Long-term (centuries–millennia) | Glacial cycles, global warming. | Redistribution of water between stores; sea-level change. |
Summary
- The water cycle changes continually in response to both natural processes (storms, seasons, long-term climate shifts) and human activities (farming, urbanisation, abstraction).
- These changes affect the magnitude of water stores and the rate of flows within drainage basins.
- Human modification often amplifies natural extremes, increasing flood risk, drought severity, and long-term water stress.
Exam Tip
When answering questions on changes in the water cycle:
- Distinguish natural variation (storm, seasonal, long-term) from human modification (farming, land use, abstraction, climate change).
- Use named examples (Boscastle 2004, River Kennet, Murray–Darling Basin).
- Refer to timescales and link to systems concepts (inputs, flows, stores, outputs).
- Include both short-term impacts (storm response) and long-term trends (climate change).