Human Impacts on the Hydrological Cycle (HL IB ESS OLD COURSE - IGNORE)

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

A mobile irrigation system sprays water over a dry, cultivated field with a forest in the background under a partly cloudy sky.
Agricultural irrigation has a significant impact on the hydrological cycle (photo by Przemyslaw Stroinski on Unsplash)

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

Flooded street in a village with brown water covering the road, surrounded by brick and stone buildings, and a church in the background.
Urbanisation has a significant impact on the hydrological cycle (photo by Chris Gallagher on Unsplash)
  • 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

Diagram showing a lake with arrows indicating inflows (river inflow, precipitation, groundwater inflow, surface run-off) and outflows (river outflow, evaporation, groundwater outflow, agricultural extraction).
Sustainable water harvesting
  • 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

Diagram showing an aquifer with arrows indicating inputs: precipitation and surface infiltration, and outputs: natural surface discharge, subsurface flow, and groundwater extraction.
Unsustainable water harvesting

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