Lesson 2: Human impacts on the hydrological cycle

The Hydrological Cycle is a natural process, a closed-loop system that has remained unchanged for millions of years.

A closed-loop system is one that does not change. It does not have any new inputs and does not lose anything through outputs. There is the same amount of water on our planet now as there has been for millions of years.

Dinosaurs drank the same water as you are drinking today...

Despite this closed-loop, there are many ways the hydrological cycle can be impacted or changed by humans.

Mining for minerals such as gold, aluminium and more occurs around the world. The formation of these minerals tends to locate larger and easy to access deposits in the tropical regions of the world. These regions, due to the climate, are normally covered in

large rainforests (like the Amazon). To access these minerals deforestation first needs to occur, then huge open cast mines are excavated, looking like huge empty basins.

High-pressure hoses are used in these pits to blast away the dirt and rock (much like hydraulic action). This dirt and rock are then sieved for the small chunks of minerals found. The dirt is then carried and deposited (known as silt) into existing river and water systems.

Impact on Hydrosphere

• Lakes, rivers and reservoirs ‘silt up’ leading to reduced storage capacity in these areas.

• There is reduced vegetation cover which leads to:

  • increased run-off

  • lower evapo-transpiration and cloud formation altering the rainfall pattern

Irrigation is the process of diverting water away from the main water system and storing it in man-made lakes or waterways. This can then be used to water crops and allow animals to drink at a more consistent rate, improving crop yield and spread.

Impact on Hydrosphere

• It involves taking water from a river or underground store which can:

  • reduce river flow

  • lower water tables

• It can increase evaporation/ Evapo-transpiration by placing water in surface stores (ditches/canals)

• Crops remove water from the cycle as they grow

Deforestation is the process of cutting down and clearing large parts of forest or woodland areas to be used for another purpose. Normally this is for cattle ranching, mining or creating urban areas.

Impact on Hydrosphere

• Cutting down trees increases run-off

• It decreases evapo-transpiration (and therefore cloud formation)

• more extreme river flows as water is not intercepted

Urbanisation is the movement of people from the countryside to the city. More than this it is also the build-up and expansion of those cities as a result of increased population.

The important part of urbanisation for this topic is the construction of impermeable surfaces (roads, pavements and houses). This prevents water from infiltrating into the ground but quickly diverts it to drainage systems. Increased urbanisation also requires more water for use as drinking water, cleaning and garden maintenance.

Impact on Hydrosphere

• The removal of natural vegetation & replacement with impermeable surfaces & drains can speed up surface run-off and evaporation and can lead to higher river levels.

• It also decreases the amount of water that returns to groundwater storage, possibly reducing the water table.

• It requires taking water for use in the cities which reduces water in rivers (due to reservoirs) or groundwater (due to wells)