Definition: The hydrological cycle, or water cycle, is the continuous circulation of water between the atmosphere, lithosphere, hydrosphere and biosphere through processes such as evaporation, condensation, precipitation, infiltration, runoff and groundwater flow.

Important points

• The hydrological cycle is a continuous, cyclic and dynamic process driven mainly by solar energy and gravity.

• Solar radiation causes evaporation from oceans, rivers, lakes and soil surfaces.

• Plants release water vapour into the atmosphere through transpiration.

• Water vapour cools and condenses to form clouds.

• Water returns to the Earth's surface as precipitation in the form of rain, snow or hail.

• A part of precipitation flows over the surface as surface runoff and reaches rivers, lakes and oceans.

• Another part infiltrates into the soil and moves downward through percolation.

• Water reaching the saturated zone becomes groundwater and is stored in aquifers.

• Groundwater moves under hydraulic gradients and may discharge into springs, rivers, lakes and oceans.

• Water is temporarily stored in glaciers, lakes, reservoirs, soil moisture, aquifers and oceans.

2. Major Processes of the Hydrological Cycle

1. Evaporation

Conversion of liquid water into water vapour due to solar energy. It occurs mainly from oceans, lakes, rivers and wet soil.

2. Transpiration and Evapotranspiration

Transpiration is the release of water vapour from plant leaves. Evapotranspiration combines evaporation and transpiration.

3. Condensation

Water vapour cools and changes into tiny liquid droplets or ice crystals, forming clouds and fog.

4. Precipitation

Water returns from the atmosphere to the Earth's surface as rain, snow, sleet or hail.

5. Infiltration and Percolation

Infiltration is the entry of water into the soil, whereas percolation is its downward movement through soil and rock towards the groundwater zone.

6. Groundwater Flow and Discharge

Groundwater moves through interconnected pores and fractures and may emerge as springs or contribute to river baseflow.

3. Diagram of the Hydrological Cycle

weather.metoffice.gov.uk

For your handwritten examination diagram, label these features:

• Sun and evaporation

• Condensation and clouds

• Precipitation

• Surface runoff and river flow

• Infiltration and percolation

• Soil moisture and unsaturated zone

• Water table and saturated zone

• Aquifer and groundwater flow

• Groundwater discharge into rivers and oceans

• Transpiration by vegetation

4. Role of Groundwater in the Hydrological Cycle

Groundwater is the water present below the Earth's surface within the saturated zone of soil, sediment and rock formations. It occupies interconnected pores, joints, fractures and solution cavities.

Important M.Sc.-level points

1. Subsurface storage: Groundwater acts as a major natural reservoir that stores infiltrated precipitation for periods ranging from days to thousands of years.

2. Recharge: Rainfall, snowmelt, seepage from rivers and lakes, and irrigation return flow contribute to groundwater recharge.

3. Infiltration and percolation: Groundwater links atmospheric precipitation with the subsurface geological environment through the movement of water across the soil and unsaturated zone.

4. Baseflow contribution: Groundwater supplies water to rivers during periods of little or no rainfall, maintaining streamflow in dry seasons.

5. Spring formation: Where the water table intersects the land surface, or groundwater encounters suitable geological structures, water may emerge as springs.

6. Groundwater discharge: Groundwater flows into rivers, wetlands, lakes and oceans, completing important pathways in the hydrological cycle.

7. Evapotranspiration support: Shallow groundwater can supply moisture to plant roots, supporting transpiration and ecosystem productivity.

8. Flood moderation: Infiltration and subsurface storage may reduce immediate surface runoff and delay some stormwater reaching streams, depending on soil conditions and antecedent moisture.

9. Drought regulation: Stored groundwater can sustain wells, vegetation and river baseflow during prolonged dry periods.

10. Geological control: Lithology, porosity, permeability, fractures, faults and aquifer geometry determine groundwater storage, movement and discharge.

11. Water-quality regulation: During subsurface movement, water undergoes filtration, adsorption, ion exchange and mineral dissolution. These processes may improve or deteriorate water quality.

12. Human water supply: Groundwater provides water for drinking, irrigation, industry and domestic use, linking the natural hydrological cycle with human activities.

13. Groundwater–surface water interaction: Rivers may gain water from aquifers or lose water to them, depending on hydraulic-head differences.

14. Long-term storage and delayed response: Groundwater often responds more slowly to rainfall than surface runoff, creating a time lag between recharge and discharge.

15. Human impacts: Excessive pumping, urbanisation, deforestation and contamination can alter recharge, water-table levels, baseflow and the natural balance of the hydrological cycle.

5. Groundwater Movement: Important Concepts