How Humans Change the Water Cycle

We cannot create or destroy water, so we do the next best thing: we move it, speed it up and block its path. Almost every human impact in this topic works the same way — it shifts the balance between water that soaks in and water that runs off. Get that one sentence and the rest of the page writes itself.

Three ways people rewrite the cycle

Irrigation

Irrigation means supplying water to crops artificially. It changes where water is and when it is there, which is exactly what a hydrological cycle is made of.

• Fields get far more water than rain would give them, so evapotranspiration rises above natural levels. More vapour goes into the air, and that can slightly increase rainfall downwind.
• Water is often applied faster than the soil can take it in. The surplus becomes surface run-off, carrying soil, fertiliser and pesticide into rivers, causing pollution and nutrient enrichment.
• The water has to come from somewhere — usually a river or an aquifer. So one place gains water and another loses it.
• In hot, dry regions, repeated irrigation can leave salts behind as the water evaporates. That is salinisation, and it slowly ruins the soil.

Deforestation

A forest works like a sponge with a lid. The canopy catches rain and slows it down (interception), roots hold the soil open so water can sink in, and the trees return a huge amount of water to the air.

Take the trees away and every part of that breaks:

• No canopy, so rain hits bare soil at full speed. Soil surfaces seal and compact, so infiltration falls and run-off rises.
• Less water soaking in means less groundwater recharge, so rivers run lower in dry spells.
• No trees means less transpiration, so less moisture returns to the atmosphere and the local climate can become drier.
• Fast run-off over loose soil causes erosion, and rivers fill with sediment.

Urbanisation

Cities replace soil and plants with concrete, tarmac and roof tiles. These are impermeable, which means water simply cannot get through.

• Impermeable surfaces stop infiltration, so groundwater recharge drops and wells and rivers suffer in dry periods.
• Rain becomes run-off within minutes, and drains are built to remove it as fast as possible. Rivers get a sudden surge instead of a slow release, so flooding downstream becomes more likely.
• Concrete and brick soak up heat, giving the urban heat island effect. Warmer air can hold more moisture and can nudge local rainfall patterns.

Steady state: the water budget of a lake

A water body is in a steady state when the water coming in equals the water going out. The lake still has water arriving and leaving all the time — it is busy, not still — but the level stays about the same. That is a dynamic balance, not a frozen one.

Sustainable and unsustainable harvesting

Sustainable water harvesting means taking water no faster than nature replaces it. Add up the inputs, add up the natural outputs, and whatever is left over is the most you can safely remove.

Go past that and the store shrinks. With a lake you see it happen — the shoreline retreats. With an aquifer you often do not, because the damage is underground, which is why over-extraction is so common. Keep taking, and:

• The water table drops, so wells must be drilled deeper and pumping costs more.
• Rivers and wetlands fed by that groundwater dry up, and the ecosystems in them go with it.
• Near a coast, sea water can seep in to replace what was taken (saltwater intrusion), which can ruin the aquifer permanently.
• The ground itself can sink as the emptied pores collapse.

Worked examples