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examBoard: Cambridge
examType: IGCSE
lessonTitle: Surface Run-off and Infiltration
When rain falls on the Earth's surface, it can take several paths. Two of the most important pathways are surface run-off and infiltration. These processes are vital parts of the water cycle and have significant impacts on our environment, from water availability to flood risks.
Key Definitions:
The water cycle is a continuous process where water moves from the Earth's surface to the atmosphere and back again. It includes evaporation, condensation, precipitation, infiltration and run-off. Surface run-off and infiltration determine how much water flows over land versus how much enters the ground, affecting everything from groundwater supplies to flood risks.
Infiltration occurs when water soaks into the soil. The rate at which this happens depends on soil type, vegetation cover and soil moisture content. Sandy soils allow faster infiltration than clay soils. Infiltrated water may recharge groundwater supplies or move laterally as throughflow before eventually reaching streams and rivers.
Several natural and human-influenced factors determine whether rainwater will infiltrate the soil or flow as surface run-off.
Soil texture, structure and porosity significantly affect infiltration rates. Sandy soils have large pore spaces allowing rapid infiltration. Clay soils have small pores that slow infiltration, promoting run-off. Compacted soils reduce infiltration regardless of soil type.
Plants and their root systems create pathways for water to enter soil. Vegetation also slows the movement of water over land, giving it more time to infiltrate. Forested areas typically have higher infiltration rates than bare ground or urban areas.
Steeper slopes increase the speed of water movement, reducing the time available for infiltration and increasing run-off. Flat areas allow water to pond and infiltrate more effectively.
In June 2007, Hull experienced severe flooding after receiving 100mm of rainfall in 24 hours. The city's flat topography and high proportion of impermeable surfaces (over 90% in some areas) severely limited infiltration. Drainage systems were overwhelmed, resulting in surface run-off that flooded 8,600 homes and 1,300 businesses. This case highlights how urbanisation can dramatically alter natural infiltration processes and increase flood risk.
Human activities have significantly altered natural infiltration and run-off patterns, often with negative consequences.
Cities dramatically change how water moves across landscapes. Impermeable surfaces like roads, pavements and buildings prevent infiltration and increase both the volume and speed of surface run-off. A typical urban area may generate 5 times more run-off than a forested area of the same size.
Urban run-off often contains pollutants from roads (oil, heavy metals), gardens (pesticides, fertilisers) and litter. This polluted water flows directly into water bodies without the natural filtering that occurs during infiltration. Urban areas also experience more frequent flash flooding due to rapid run-off.
Sustainable Urban Drainage Systems (SUDS) aim to mimic natural processes by increasing infiltration and reducing run-off. Examples include permeable pavements, green roofs, rain gardens and retention ponds. These systems help manage flood risk while improving water quality and creating wildlife habitats.
Agricultural practices significantly affect infiltration and run-off patterns, with both positive and negative outcomes depending on management approaches.
Farmers in the South Downs have adopted contour ploughing (following the natural contours of the land rather than ploughing up and down slopes). This practice has reduced surface run-off by up to 30% during heavy rainfall events, decreased soil erosion and improved water quality in local streams. Additional measures like cover crops during winter months have further increased infiltration rates and reduced nitrate leaching into groundwater.
Changes to infiltration and surface run-off patterns can have far-reaching environmental impacts:
Increased surface run-off leads to more frequent and severe flooding. Urban areas are particularly vulnerable due to concentrated impermeable surfaces and drainage systems that can be overwhelmed.
Surface run-off carries pollutants directly to water bodies without the natural filtering that occurs during infiltration. This can lead to eutrophication, harmful algal blooms and reduced biodiversity in aquatic ecosystems.
High rates of surface run-off can wash away topsoil, reducing agricultural productivity and causing sedimentation in rivers and lakes that damages aquatic habitats.
Various approaches can help manage surface run-off and promote infiltration:
Green Infrastructure: Green roofs, rain gardens and urban trees increase infiltration in cities.
Permeable Surfaces: Permeable pavements allow water to pass through rather than run off.
Retention Basins: Temporary storage areas that collect run-off and release it slowly, allowing more time for infiltration.
Contour Ploughing: Following land contours rather than ploughing up and down slopes.
Cover Crops: Planting between main crop seasons to protect soil and improve infiltration.
Buffer Strips: Vegetated areas along water bodies that slow run-off and filter pollutants.
Afforestation: Planting trees to increase interception and infiltration.
The most effective approach to managing surface run-off and infiltration is often at the catchment (watershed) level. This involves coordinating actions across the entire area that drains into a particular river system. Natural Flood Management (NFM) techniques work with natural processes to slow and store water in the landscape, reducing flood risks downstream while providing multiple environmental benefits.
The town of Pickering in North Yorkshire had a history of flooding. Rather than building traditional flood defences, a natural approach was implemented. This included planting 40,000 trees upstream, creating 167 woody debris dams in streams and restoring heather moorland. These measures increased infiltration and slowed water movement through the catchment. During heavy rainfall in December 2015, the scheme prevented flooding that affected nearby towns, demonstrating the effectiveness of working with natural processes to manage surface run-off.
Surface run-off and infiltration are crucial processes in the water cycle that determine how water moves through our environment. Natural factors like soil type, vegetation and slope interact with human activities to influence these processes. Understanding and managing the balance between run-off and infiltration is essential for addressing challenges like flooding, water quality and water resource management. By implementing appropriate strategies in both urban and rural settings, we can work with natural processes to create more resilient and sustainable water systems.
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