The water cycle Revision Notes

The water cycle is a natural system that continuously circulates water through the atmosphere, land, and oceans. It ensures the distribution and renewal of freshwater resources essential for life. Water changes state and location through various processes, driven primarily by solar energy and gravity.

Understanding the water cycle requires examining each process and how it contributes to the movement and storage of water in different environments.

• Water moves in liquid, solid, and gaseous forms.
• Solar energy powers evaporation and transpiration.
• Gravity drives precipitation and run-off.

Precipitation occurs when water vapour in the atmosphere condenses into droplets heavy enough to fall as rain, snow, sleet, or hail. This is the primary way water returns from the atmosphere to the Earth's surface.

Before reaching the ground, some precipitation is intercepted by vegetation, buildings, or other surfaces. Interception temporarily holds water, reducing the amount that immediately reaches the soil and affecting the timing and volume of water entering the ground or flowing over the surface.

For example, a dense forest canopy can intercept a significant portion of rainfall, which may evaporate directly back into the atmosphere without reaching the soil.

• Precipitation forms clouds and releases water to the surface.
• Interception delays water reaching the soil, reducing immediate run-off.
• Vegetation type and density influence interception rates.

Infiltration is the process where water soaks into the soil from the surface. The rate depends on soil type, saturation, and land cover. For instance, sandy soils allow faster infiltration than clay soils.

Once in the soil, water can move laterally as through-flow, travelling slowly downhill through the soil layers towards streams or rivers. This movement is important for maintaining base flow in rivers during dry periods.

Some water percolates deeper to recharge groundwater, moving through permeable rock layers as groundwater flow. This underground movement can be slow, taking days to years to reach discharge points such as springs or wells.

• Infiltration controls how much water enters the soil.
• Through-flow moves water sideways within soil layers.
• Groundwater flow transports water underground over long distances.

Surface run-off occurs when water flows over the land surface towards rivers, lakes, or oceans. It happens when the soil is saturated, frozen, or impermeable, or when rainfall intensity exceeds infiltration capacity.

Run-off can cause soil erosion and flooding if excessive. Urban areas with concrete surfaces increase run-off due to low infiltration.

For example, after heavy rain on a compacted field, water quickly runs off into nearby streams, raising water levels rapidly.

• Run-off moves water quickly over land.
• Soil saturation and surface type affect run-off volume.
• Run-off contributes to river flow and potential flooding.

Evaporation is the process where liquid water changes into water vapour from surfaces like lakes, rivers, and soil. It requires energy, mainly from the sun.

Transpiration is the release of water vapour from plant leaves during photosynthesis. Together, evaporation and transpiration are called evapotranspiration, which returns water to the atmosphere.

Condensation occurs when water vapour cools and changes back into liquid droplets, forming clouds. This process is essential for precipitation to occur, completing the cycle.

• Evaporation removes water from surfaces to the atmosphere.
• Transpiration releases water vapour from plants.
• Condensation forms clouds from cooled vapour.