Human Impact on the Hydrological Cycle (DP IB Environmental Systems & Societies (ESS))

Human Impact on the Hydrological Cycle

• Human activities, such as agriculture (specifically irrigation), deforestation, and urbanisation, have significant impacts on the hydrological cycle, altering the natural processes of surface run-off and infiltration

Photo by Przemyslaw Stroinski on Unsplash 

Agricultural irrigation has an impact on the hydrological cycle

The impact of agriculture and irrigation on the hydrological cycle

• Irrigation is the process of artificially supplying water to agricultural crops

  • It has a direct impact on the hydrological cycle by modifying the water distribution and availability in a region

• Increased irrigation leads to artificially high evapotranspiration rates as more water is supplied to plants than would occur naturally, resulting in increased atmospheric moisture levels

• This can lead to localised increases in precipitation downwind of irrigated areas, altering rainfall patterns in the region

• Additionally, excessive irrigation can result in increased surface run-off

  • When water is applied faster than the soil can absorb it, it flows over the surface, carrying sediments, fertilisers, and pesticides, leading to water pollution and nutrient imbalances

The impact of deforestation on the hydrological cycle

• Deforestation refers to the clearing or removal of forests, primarily for agriculture, logging, or urban development purposes

• Forests play a crucial role in the hydrological cycle

  • They act like natural sponges, absorbing rainfall and facilitating infiltration, which helps recharge groundwater and maintain stream flows

• When forests are cleared, surface runoff increases significantly

  • Without the tree canopy and vegetation to intercept and slow down rainfall, more water reaches the ground surface, leading to higher surface runoff rates

• Deforestation also reduces evapotranspiration rates

  • As trees are removed, there is less transpiration and evaporation occurring, resulting in reduced moisture release into the atmosphere

• Overall, deforestation disrupts the balance between surface run-off and infiltration, leading to increased erosion, reduced groundwater recharge, and altered stream flow patterns

The impact of urbanisation on the hydrological cycle

Photo by Chris Gallagher on Unsplash 

Urbanisation has an impact on the hydrological cycle

• Urbanisation involves the transformation of natural landscapes into urban areas with buildings, roads, and infrastructure

• Urban development dramatically alters the hydrological cycle by replacing permeable surfaces (such as soil and vegetation) with impermeable surfaces (concrete, asphalt)

  • Impermeable surfaces prevent infiltration, leading to reduced groundwater recharge

  • Instead of infiltrating into the soil, rainfall quickly becomes surface runoff, resulting in increased flooding and diminished water availability during dry periods

• Urban areas typically have efficient drainage systems designed to remove the excess water quickly

  • This further accelerates surface runoff, which can overload natural water bodies and cause downstream flooding

• Urban areas often experience higher temperatures due to the urban heat island effect

  • This effect, caused by the concentration of buildings and paved surfaces, increases evaporation rates, altering local precipitation patterns