How deforestation disrupts drainage basin processes

Deforestation — the large-scale removal of trees from forested areas — has significant effects on the movement and storage of water within a drainage basin. Trees play a key role in intercepting precipitation, slowing the transfer of water to river channels, and supporting infiltration into the soil.

Effects of deforestation on drainage basin processes

• Reduced interception - Without a tree canopy, less rainfall is caught and stored on leaves and branches, meaning more water reaches the ground surface directly
• Increased direct runoff - The removal of dead plant material (leaf litter) from the forest floor reduces the surface's ability to slow water movement, so more water flows across the surface rather than soaking in
• Reduced infiltration - With less vegetation cover and organic matter to hold moisture, less water is able to infiltrate into the soil
• Soil erosion - Exposed land is more vulnerable to erosion by water and wind, which damages soil structure and further reduces its capacity to store rainwater
• Declining groundwater levels - As infiltration decreases, less water percolates down to recharge groundwater stores, causing the water table to drop over time

The effects of land use change on water movement

Changes in how land is used — whether through agriculture, urbanisation, or infrastructure development — alter the pathways and speed of water movement through a drainage basin.

Agricultural land use changes

• Ploughing - Breaks up the soil surface, creating spaces for water to infiltrate more easily, which reduces the volume of surface runoff
• Livestock farming - Animals such as cattle compact and trample the soil through repeated movement, decreasing its ability to absorb water and increasing direct runoff

Urbanisation and impermeable surfaces

Urban development introduces impermeable surfaces — materials such as concrete, tarmac, and roofing that do not allow water to pass through them. These surfaces prevent infiltration entirely, forcing all precipitation to flow across the surface as direct runoff.

Key consequences of urbanisation for drainage basins:

• Water passes through the drainage basin system much more rapidly
• Urban drainage systems channel rainwater directly into rivers, increasing river discharge quickly
• The speed and volume of water reaching river channels increases the likelihood of flooding

How water storage reservoirs alter the water cycle

Water storage reservoirs are artificial lakes created by constructing dams across river channels. They are built to increase the amount of freshwater available for human use, such as drinking water supply, irrigation, and hydroelectric power generation.

Effects of reservoirs on drainage basin processes

• Reduced river discharge - Dams trap water behind them, decreasing the volume of water that flows downstream and ultimately reaches the ocean
• Increased evaporation - Reservoirs create large areas of standing water with a wide surface area, which increases the rate of evaporation compared to a flowing river channel

The impact of water abstraction on water stores

Water abstraction refers to the process of removing water from natural stores — such as rivers, lakes, reservoirs, and underground aquifers — to meet human demand. This is particularly significant in areas of high population density where demand for water is greatest.

How abstraction affects water stores

• Depletion of surface stores - Abstracting water from rivers and lakes reduces the volume held in these stores, potentially lowering water levels significantly
• Groundwater depletion - Underground aquifers are a major source of abstracted water, but extraction often occurs at a faster rate than the aquifer can be naturally recharged through percolation, leading to long-term decline in groundwater levels
• Seasonal pressure - During dry seasons, abstraction rates increase further as demand rises for both consumption and irrigation, placing even greater strain on already depleted stores

Case study: Amazonia

Amazonia is the world's largest tropical rainforest, covering approximately 38% of the South American land mass. It provides a clear example of how natural drainage basin processes operate and how human activity can significantly disrupt them.

Natural water cycle processes in Amazonia

• High rainfall - Large amounts of evaporation occur over the Atlantic Ocean, and prevailing winds blow this moist air westwards towards the Amazon basin, producing very high levels of precipitation
• High evapotranspiration - Warm tropical temperatures drive high rates of evapotranspiration from the dense vegetation, which returns moisture to the atmosphere and contributes to local convectional precipitation
• High interception - The dense, multi-layered canopy of the rainforest intercepts a large proportion of rainfall, reducing the amount and speed of water flowing into river channels

How deforestation disrupts Amazonia's water cycle

Extensive deforestation takes place in Amazonia for purposes such as timber harvesting and the conversion of forest to farmland. This disrupts the natural water cycle in two significant ways.

Increased flood risk:

• The removal of the tree canopy eliminates interception, so more water reaches the ground surface directly
• The soil becomes saturated more quickly, and excess water moves to rivers as saturated overland flow
• This rapid transfer of water to river channels increases the risk of flooding

Increased drought risk:

• Deforestation reduces the rate of evapotranspiration, as there are fewer trees to release moisture into the atmosphere
• Less water vapour reaches the atmosphere, meaning fewer clouds form and rainfall decreases
• Over time, this reduction in precipitation increases the risk of drought in deforested areas

The case of Amazonia demonstrates how deforestation can simultaneously increase the risk of both flooding and drought within the same drainage basin, by disrupting the balance between interception, runoff, and evapotranspiration.