Human Impacts on the Hydrological Cycle (DP IB Environmental Systems & Societies (ESS)) : Revision Note

Human Impacts on the Hydrological Cycle

• Human activities have significant impacts on the hydrological cycle

  • They alter the natural processes of surface run-off and infiltration

• These activities include:

  • Agriculture (specifically irrigation)

  • Deforestation

  • Urbanisation

Impact of agriculture and irrigation

• Irrigation is the process of artificially supplying water to 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

  • This is because more water is supplied to plants than would occur naturally

  • This results in increased atmospheric moisture levels

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

• Excessive irrigation can also result in increased surface run-off

  • Water is applied faster than the soil can absorb it

  • This causes water to flow over the soil surface, carrying sediments, fertilisers, and pesticides

  • This leads to water pollution and nutrient imbalances

Impact of deforestation

• Deforestation refers to the clearing or removal of forests

  • This is primarily for agriculture, logging or urban development purposes

• Forests play a crucial role in the hydrological cycle

  • They act like natural sponges

  • They absorb rainfall and facilitate infiltration

    • This helps to 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

  • This leads to higher surface runoff rates

• Deforestation also reduces evapotranspiration rates

  • As trees are removed, there is less transpiration and evaporation occurring

  • This results in reduced moisture release into the atmosphere

• Overall, deforestation disrupts the balance between surface run-off and infiltration

  • This can lead to increased erosion, reduced groundwater recharge and altered stream flow patterns

Impact of urbanisation

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

• Urban development significantly alters the hydrological cycle by:

  • Replacing permeable surfaces (such as soil and vegetation) with impermeable surfaces (concrete, asphalt)

    • Impermeable surfaces prevent infiltration

    • This leads to reduced groundwater recharge

    • Instead of infiltrating into the soil, rainfall quickly becomes surface run-off

    • This results in increased flooding and diminished water availability during dry periods

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

  • This further accelerates surface run-off

  • This can overload natural water bodies and cause downstream flooding

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

  • This effect is caused by the concentration of buildings and paved surfaces

  • It can lead to increased evaporation rates

  • This can alter local precipitation patterns

Steady State of Water Bodies

• Understanding the steady state of a water body involves analysing the balance between inputs and outputs

  • This balance ensures that the water level remains constant over time

Flow diagrams of inputs and outputs

• Flow diagrams visually represent the water inputs and outputs for a water body

• Inputs: e.g.

  • Precipitation: rain, snow, or other forms of water falling directly into the water body

  • Surface run-off: water flowing over the land into the water body

  • Groundwater Inflow: water moving into the water body from underground sources

• Outputs: e.g.

  • Evaporation: water turning into vapour and leaving the water body

  • River outflow: water leaving the water body through rivers or streams

  • Groundwater outflow: water moving out of the water body into underground aquifers

  • Agricultural extraction: water that is extracted for irrigation

• For example, a lake that is at a steady state may have the following inputs and outputs:

  • Inputs: river inflow (80 units), rainfall (30 units), groundwater inflow (40 units), surface run-off (30 units)

  • Outputs: river outflow (80 units), evaporation (30 units), groundwater outflow (40 units), agricultural extraction (30 units)

  • Steady state: inputs (180 units) equal outputs (180 units)

• This is an example of sustainable water harvesting

  • Sustainable harvesting means taking water from a water body at a rate that does not exceed the rate of natural replenishment

  • Assessing the total inputs and outputs of a water body can help calculate sustainable rates of water harvesting

  • This ensures the harvested water amount does not disrupt the steady state

• If total outputs are greater than total inputs, then the water body will decrease in size

  • This may be due to unsustainable water harvesting for agriculture or for domestic and industrial purposes, e.g. water used in drinking, cleaning, heating and cooling systems, and manufacturing processes

  • Water may be extracted faster than it can be naturally replenished

• For example, an aquifer that is being unsustainably harvested (and therefore is not at a steady state) may have the following inputs and outputs:

  • Inputs: precipitation (70 units), surface infiltration (80 units)

  • Outputs: natural surface discharge (30 units), subsurface flow (70 units), groundwater extraction for domestic and industrial use (150 units)

  • Steady state disruption: inputs (150 units) are less than outputs (250 units), causing a water deficit of 100 units

• This is why groundwater extraction must be balanced with recharge rates—to prevent aquifer depletion