Human Impacts on the Hydrological Cycle (HL IB ESS OLD COURSE - IGNORE)
Revision Note
Written by: Alistair Marjot
Reviewed by: Bridgette Barrett
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
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