Changing River Environments » Infiltration and Percolation

What you'll learn this session

Study time: 30 minutes

The definitions and processes of infiltration and percolation
Factors affecting infiltration rates in river catchments
How infiltration and percolation influence river discharge
The relationship between infiltration, percolation and the hydrological cycle
How human activities can alter infiltration and percolation rates
Case studies showing the impacts of changes to infiltration and percolation

Understanding Infiltration and Percolation

When it rains, water doesn't just flow across the surface of the land. Much of it soaks into the ground through processes that are vital to the water cycle and river systems. These processes - infiltration and percolation - significantly influence how rivers behave and how water moves through the environment.

Key Definitions:

Infiltration: The process by which water on the ground surface enters the soil.
Percolation: The movement of water through soil pores and rock fractures after infiltration has occurred.
Infiltration Rate: The speed at which water enters the soil, usually measured in mm per hour.
Permeability: The ability of a rock or soil to allow water to pass through it.
Porosity: The percentage of rock or soil that consists of void spaces.

💧 Infiltration Process

Infiltration begins when water hits the soil surface. The rate at which water can enter the soil depends on several factors:

Soil type and structure
Soil moisture content
Vegetation cover
Slope gradient
Rainfall intensity and duration

Sandy soils generally have high infiltration rates (20-30mm/hr), while clay soils have much lower rates (1-5mm/hr).

⬇️ Percolation Process

Once water has infiltrated the soil, it percolates downward due to gravity. This process is affected by:

The size of pore spaces in soil and rock
The presence of impermeable layers
Rock fractures and joints
Soil compaction

Water may continue to percolate until it reaches the water table, where it becomes groundwater, or it may move laterally as throughflow.

The Role of Infiltration and Percolation in River Systems

These processes are crucial in determining how quickly rainfall reaches river channels and affects river discharge. When water infiltrates and percolates rather than running off the surface, river flow becomes more regulated and less flashy.

🌊 Surface Runoff

When infiltration capacity is exceeded, water flows over the surface as runoff. This reaches rivers quickly, causing rapid rises in discharge.

↔️ Throughflow

Water that moves laterally through soil layers reaches rivers more slowly than surface runoff, moderating discharge peaks.

⏱️ Baseflow

Groundwater that slowly seeps into river channels provides a steady supply of water even during dry periods.

Factors Affecting Infiltration Rates

Natural Factors

🌱 Vegetation

Plants significantly enhance infiltration by:

Creating root channels that water can follow
Adding organic matter that improves soil structure
Protecting soil from raindrop impact
Slowing surface flow, giving water more time to infiltrate

Forested areas can have infiltration rates up to 10 times higher than bare soil.

🧱 Soil Properties

Soil characteristics dramatically influence infiltration:

Texture: Sand > Silt > Clay (infiltration capacity)
Structure: Well-aggregated soils have higher infiltration
Moisture content: Dry soils initially absorb water faster
Organic matter: Increases pore space and infiltration

Human Impacts on Infiltration and Percolation

Human activities can dramatically alter natural infiltration and percolation processes, with significant consequences for river systems:

🏙️ Urbanisation

Impermeable surfaces like concrete and asphalt prevent infiltration, increasing surface runoff by up to 45% in heavily urbanised areas. This leads to flashier river regimes and increased flood risk.

🚜 Agriculture

Soil compaction from heavy machinery can reduce infiltration rates by 70-80%. Removal of natural vegetation and ploughing also disrupt soil structure, while drainage systems alter natural percolation pathways.

🌲 Deforestation

Removing forest cover can reduce infiltration rates by 35-50%. This increases surface runoff, soil erosion and the risk of flash flooding in river catchments.

Case Study Focus: The Pontbren Project, Wales

The Pontbren Project in mid-Wales demonstrates how land management can affect infiltration and river discharge. In this upland sheep farming area, farmers planted native woodland and hedgerows and created shelter belts for livestock.

Key findings:

Areas with trees had infiltration rates 60 times higher than grazed pasture
Tree planting reduced peak flows in streams by 40% during heavy rainfall
Soil under trees contained 30% more organic matter, improving structure and water-holding capacity
The project reduced localised flooding and soil erosion while providing wildlife habitat

This case study shows how natural solutions can enhance infiltration and percolation, reducing flood risk downstream while providing multiple environmental benefits.

Measuring and Monitoring Infiltration

Geographers and hydrologists use several methods to measure infiltration rates:

🔬 Field Techniques

Ring infiltrometers: Metal rings inserted into soil with water added to measure infiltration rate
Rainfall simulators: Devices that create artificial rainfall to measure infiltration under controlled conditions
Soil moisture sensors: Electronic devices that monitor water content at different soil depths

📊 Analysing Results

Infiltration data helps geographers:

Predict how catchments will respond to rainfall events
Identify areas at risk of surface runoff and erosion
Plan sustainable land management practices
Model the effects of climate change on river systems

Sustainable Management of Infiltration and Percolation

Understanding and managing infiltration and percolation processes is essential for sustainable river management and flood prevention:

🌿 Natural Solutions

Afforestation in upland areas
Wetland restoration
Contour ploughing on farmland
Cover crops to improve soil structure

🏗️ Urban Techniques

Permeable paving
Green roofs and rain gardens
Sustainable Urban Drainage Systems (SUDS)
Retention ponds and infiltration basins

📝 Policy Approaches

Catchment-based management
Incentives for farmers to improve soil health
Planning regulations requiring SUDS in new developments
Public education about water management

Connecting to the Wider Hydrological Cycle

Infiltration and percolation are just two components of the complex hydrological cycle. They connect surface water to groundwater systems and play a crucial role in:

Maintaining river baseflow during dry periods
Filtering and purifying water as it moves through soil and rock
Recharging aquifers that may be used for water supply
Regulating river discharge and reducing flood peaks
Supporting plant growth and ecosystem health

Case Study: The Changing River Thames

The River Thames catchment has undergone significant changes in infiltration and percolation patterns over the past century:

Urban expansion in the Thames Basin has increased impermeable surfaces by 35% since 1960
This has reduced infiltration and accelerated runoff, contributing to more frequent flooding in places like Oxford and Reading
The Thames Catchment Management Plan now includes measures to increase infiltration, such as:

Creating 20 hectares of new wetlands near Aylesbury
Installing permeable paving in urban redevelopment projects
Working with farmers to improve soil structure and reduce compaction

These measures have helped reduce peak flows by up to 15% during heavy rainfall events

Summary: Why Infiltration and Percolation Matter

Understanding infiltration and percolation is essential for:

Predicting and managing flood risk
Ensuring sustainable water supplies
Protecting soil from erosion
Adapting to climate change impacts on river systems
Planning sustainable development in river catchments

By studying these processes, geographers can help develop solutions to water management challenges and contribute to more sustainable river environments.

🧠 Test Your Knowledge!