How does the water cycle change over time?

The inputs, transfers, flows, stores and outputs of the water cycle change over time for a variety of reasons. Some may be natural but others are anthropogenic (human-factors). Let's take a look at these natural and human-related factors.

Natural changes over time affecting water cycles

Storm events

These lead to an increase in both channel flow and surface runoff. Depending upon the drainage basin, flood events can occur.
Seasonal changes

Seasons with high levels of precipitation lead to increased surface runoff and channel flow. In contrast, drier seasons will lead to reduced river discharge and no runoff. In mountainous regions, increased channel flow and runoff can occur due to ice melt. Countries to the south of the Himalayas will face flooding during this time but also will utilise the extra water for agricultural and domestic use.

Ecosystem changes

Plant successions may change the dominant type of vegetation in an area. If vegetation dies off due to natural events there will be less absorption of water by plant roots, and less transpiration – which may reduce precipitation.

Human changes over time affecting water cycles

Climate change

Increasing global temperature is leading to a reduction in size of mountain glaciers and therefore future dependency on this water will become more of an issue as this input declines after a period of increased discharge. Potential drought conditions and associated economic and social impacts will be likely consequences.

Farming practices

Particularly in hotter climates, farming can have a significant effect on the water cycle. Irrigation for plants can lower channel levels in rivers together with groundwater levels if wells are the source for the irrigation.

Deforestation

Removing vegetation for agriculture, urbanisation or – most frequently in many rural developing countries – for fuel supply, an important water store and water-transfer capacity is lost. Soil moisture reduces, transpiration declines and micro-climates give less precipitation, leading to local river systems drying up.

Land use change

Change from natural landscape to urbanised landscape increases impermeable surfaces. This leads to an increase in runoff and a reduction in infiltration. Cities create drainage systems to take water quickly away from the urban environment. However, this can lead to flooding as river levels then receive this water too quickly in a large amount.

Water abstraction

Growth of global population and in particular in countries where climates are drier has resulted in increased water demand. Where precipitation levels are low, an alternative supply is ground water. This supply of water in porous rocks underground is known as an aquifer. Excessive abstraction of this water, where it is taken too quickly and does not recharge naturally, is unsustainable and can lead to groundwater stores being depleted.

River water is also used for water supplies for both domestic and industrial uses and that can affect river levels. In conjunction with limited groundwater reaching the river, the river can ultimately dry up.