4.1 Water systems

Hydrological cycle

The hydrological or water cycle is the continuous water movement within the Earth’s atmosphere, surface, and underground. This process can be shown using a systems diagram with stores as boxes and flows as arrows.

Solar radiation and gravity drive water movements in the hydrosphere. Heat causes water to evaporate and water vapour to form. Gravity causes water to drain through the soil and into rivers to the sea.

Main Stores in the Hydrological Cycle

1. Ocean (96.5%)
2. Surface Freshwater: Lakes, rivers, and oceans (0.02%)
3. Glaciers and ice caps (1.7%)
4. Groundwater: Aquifers and underground reservoirs (1.7%)
5. Biosphere: Water within living organisms (0.0001%)

Main Flows in the Hydrological Cycle

1. Evaporation: Water from oceans, lakes, and rivers turns into vapour due to solar radiation.
2. Transpiration: Water vapour is released by plants during photosynthesis.
3. Condensation: Water vapour cools to form clouds.
4. Precipitation: Rain, snow, or hail falling to Earth.
5. Runoff: Water flows over land into rivers, lakes, and oceans.
6. Infiltration: Water seeps into the soil, replenishing groundwater.
7. Percolation: Water moves deeper into the ground.

Human impact on water systems

Human activities significantly impact our water systems by altering the natural process and polluting water storage.

• Withdrawals - Water is unsustainably extracted for domestic use, and irrigation in agriculture and industry. Irrigation is the largest consumer of freshwater, accounting for about 70% of global use. Such over-extraction caused groundwater depletion and reduced river flows.
• Change of water flows (the speed and where it flows) - Building roads and channelling rivers underground and construction of dams and water reservoirs changes natural river flow patterns.

• Point Source and Nonpoint Source Pollution: Some examples of Point source pollution can be factories and sewage treatments and some examples of Nonpoint source pollution can be excessive fertilizers or chemical discharge from agricultural lands and acid drainage from abandoned or active mines. Non-point source pollution is harder to control due to its widespread nature and difficulty identifying them.
• Climate change - Climate change alters the natural water system by increasing evaporation, melting glaciers and ice caps causing higher sea levels and altering natural precipitation levels.

Properties of water (HL)

Some properties of water are listed below.

1. Water is transparent allowing light to pass through making photosynthesis by aquatic plants and algae possible.