Assess the importance of natural variation and human impact on changes in the water cycle
over time [20 marks]
Natural variation and human impact both have significant effects on changes in the water
cycle over time. Seasonal flooding, land use change and water abstraction are all factors
that contribute to changes in the water cycle. However, as populations are growing and
availability of water and space is running out, it could be seen that humans are having a
greater impact on the water cycle than natural variation.
Land use change for deforestation is impacting the water cycle significantly over time. As the
population is growing, there is a higher demand for food, which requires more farmland as
more crops and animals are required. As a result, more land is needed and so deforestation
is occurring at a faster rate. Replacing trees with crops reduces interception and
evapotranspiration and increases infiltration. This creates a positive feedback cycle as there
is less water in the atmosphere available for precipitation, so less water is held in the
hydrosphere. However, man has tried to counter this by using afforestation to replace the
deforested areas and in turn increase interception and evapotranspiration. This means that
more water is held in the atmosphere and the biosphere so river flow increases. Therefore
human impact has a large impact on the water cycle over time, but this can be reduced
through afforestation.
Water abstraction is another method which impacts changes in the water cycle over time. As
populations increase, there is a greater need for water held in the lithosphere. This puts
increasing pressure on water sources over time and sometimes, when these sources are
depleted, we access underground stores for water for human use. For example, aquifers are
used, particularly during long dry periods. When the outputs of water in the aquifer become
greater than the inputs, then there is a decline in the global long-term water stores as the
river sources dry out and there is less water held in the hydrosphere. Irrigation is the main
cause of groundwater exploitation. In the US, the Ogallala aquifer is being depleted so
quickly that it cannot cope with the human demand for water. This aquifer is responsible for
30% of the irrigation in the US, therefore the water loss threatens the existence of towns. It
also supports 1/6 of global grain produce, therefore the effects of the streams and rivers
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