The Water Cycle

Evaporation is the process by which liquid water is heated (by the Sun) and turns into water vapour, rising from the surface of the sea, lakes, rivers or wet ground into the atmosphere.

• (a) Condensation — water vapour cools and turns into liquid droplets, forming clouds.
• (b) Precipitation — water falls from clouds to the ground as rain (or snow, sleet, hail).

The Sun heats the oceans, causing evaporation of water into water vapour; plants also release water vapour by transpiration.

The warm, moist air rises and cools, so the vapour condenses into tiny droplets that form clouds. Water then returns to the surface as precipitation — rain, snow, sleet or hail.

When precipitation reaches the land, some is intercepted by vegetation before it lands. Of the water that reaches the ground, some infiltrates (soaks down into the soil) while the rest flows over the surface as surface run-off.

Underground, water moves by through-flow through the soil and by ground water flow through the rock, eventually returning to rivers and the sea.

There, the water can evaporate again, so the cycle is continuous and closed — the total amount of water stays the same.

Trees normally intercept rainfall on their leaves and branches, and lose water to the air by transpiration, which slows the rate at which water reaches the ground.

When the trees are removed, less rain is intercepted and less water is returned to the air by transpiration, so more water reaches the soil surface.

The exposed soil quickly becomes saturated (and is often compacted), so less water can infiltrate into it.

The water that cannot soak in therefore flows over the surface — surface run-off increases, and it reaches rivers more quickly.

Human activities change the balance between the flows of the water cycle, generally shifting water away from slow, underground routes towards rapid surface run-off.

Deforestation. Trees normally intercept rainfall and return water to the air by transpiration. Clearing forest reduces both, so more water reaches the soil; the bare, often compacted soil allows less infiltration, so more water becomes surface run-off. Consequences: rivers rise faster and higher, increasing the risk of flooding, while fast-flowing water over bare soil causes soil erosion and the loss of fertile topsoil.

Urbanisation. Building towns and cities replaces natural, permeable ground with impermeable surfaces such as concrete, tarmac and roofs. Water cannot infiltrate, so almost all of it becomes rapid surface run-off, channelled quickly into drains and rivers. Consequences: rivers respond very quickly to rainfall, causing sudden flash flooding in built-up areas.

Over-abstraction of groundwater. Pumping water out of the ground faster than it is replenished lowers the groundwater store. Consequences: wells and springs can dry up, river flow falls, and near coasts salt water may seep into the supply, threatening water security.

Overall, most human activities reduce infiltration and increase surface run-off, so water reaches rivers faster and in greater quantity, raising flood risk — while abstraction depletes long-term water stores. This is why managing land use (protecting forests, using permeable surfaces and sustainable drainage, and limiting abstraction) is central to managing the water cycle.

Introduction. Flooding happens when more water reaches a river than it can carry. This can be caused both by human changes to the water cycle and by natural events such as heavy rainfall. Whether human activities are the main cause depends on which factor most increases surface run-off and how avoidable the flooding is, so this essay weighs human against natural causes before reaching a judgement.

The case that human activities ARE the main cause.

• Deforestation. Trees intercept rainfall and return water to the air by transpiration. Removing forest reduces both, and the bare, compacted soil allows less infiltration, so far more water becomes surface run-off and reaches rivers quickly — raising flood risk and eroding soil.
• Urbanisation. Replacing permeable ground with impermeable surfaces (concrete, tarmac, roofs) almost eliminates infiltration. Rain runs straight off into drains and rivers, which rise rapidly, causing flash flooding in cities.
• Other human factors. Straightening rivers, building on floodplains and (over a longer term) human-driven climate change, which can increase the intensity of rainfall, all add to the risk. Because these changes increase run-off in many places at once, human activity has driven a clear rising trend in flooding.

The case that natural factors are at least as important.

• Extreme rainfall and storms can overwhelm even an undisturbed, well-vegetated catchment — when rainfall is intense enough, the soil saturates and run-off rises regardless of land use.
• Snowmelt in spring, and prolonged wet seasons, deliver huge volumes of water naturally.
• Physical geography — steep slopes, naturally impermeable clay or rock, and large drainage basins — makes some areas flood-prone whatever humans do.
• Major floods occurred long before modern deforestation and urbanisation, showing nature alone can cause severe flooding.

Judgement. The decisive criteria are scale and avoidability. Where flooding follows land-use change — deforested hillsides, paved cities on floodplains — human activity is clearly the main, and largely avoidable, cause, and the rising frequency of floods worldwide is strongly linked to human alteration of the water cycle. However, the trigger of many individual floods is an extreme natural rainfall event that humans did not create, even if they made its impact worse. The fairest conclusion is therefore that human activities are increasingly the main cause of the rising trend in flooding, because they raise surface run-off and reduce infiltration, but they usually act by amplifying natural events rather than replacing them. This is why effective management — reforestation, permeable surfaces, sustainable urban drainage and avoiding floodplain building — targets exactly the human factors we can control.