The water cycle and groundwater resources: WACE Year 12 Earth and Environmental Science

What this dot point is asking

SCSA wants you to explain how water cycles and is stored, and to apply this to managing groundwater as a resource. Water is renewable as a global flow but locally limited, which is the tension management must resolve, especially in a drying southwest WA climate.

Reservoirs and fluxes

The water cycle redistributes water among reservoirs of very different size and turnover time.

• Oceans hold the vast majority of Earth's water.
• Ice caps and glaciers store most fresh water.
• Groundwater is a large, slow store held in rock and sediment.
• Surface water and the atmosphere are small but rapidly cycling reservoirs.

The fluxes between them are:

• Evaporation and transpiration move water into the atmosphere as vapour, powered by solar energy.
• Condensation forms clouds, and precipitation returns water to the surface.
• Infiltration soaks water into the ground, recharging aquifers, while runoff carries it across the surface to rivers and the sea.

Groundwater and aquifers

Groundwater is water held in the pore spaces and fractures of rock and sediment below the water table.

• An aquifer is a body of rock that stores and transmits useful amounts of groundwater.
• Recharge is the infiltration of rainfall and surface water that replenishes the aquifer.
• The water table is the upper surface of the saturated zone, and it falls when extraction outpaces recharge.

Perth depends heavily on the Gnangara Mound, a large shallow aquifer beneath the northern suburbs, for part of its water supply.

Managing groundwater sustainably

Groundwater is renewable only if extraction stays within the rate of recharge. In southwest WA, declining rainfall has reduced recharge while demand has grown, so the Gnangara Mound's water table has fallen, drying wetlands and stressing dependent ecosystems. Management responses include:

• licensing and capping extraction to match estimated sustainable yield,
• monitoring water-table levels and ecosystem health,
• recharging aquifers artificially with treated water, and
• reducing reliance on groundwater by adding sources such as seawater desalination, which now supplies a large share of Perth's drinking water.

Residence time and why some water is barely renewable

Each water reservoir has a residence time, the average time a water molecule stays before moving on. Atmospheric water turns over in about nine days, river water in weeks, but deep groundwater can have a residence time of thousands of years, and water in ice caps even longer. This matters for management because the longer the residence time, the slower the recharge: a deep confined aquifer holding ancient water may receive almost no modern recharge, so pumping it is effectively mining a non-renewable store even though water as a whole is renewable. The shallow Gnangara Mound recharges from local rainfall on a timescale of years to decades, which is why a multi-year rainfall decline shows up quickly as a falling water table. Matching the extraction strategy to the aquifer's residence time and recharge rate is the heart of sustainable groundwater management.