What is a "mound"?
Superficial or unconfined (shallow) groundwater sometimes forms "mounds" where the water table slopes away from a high central area with groundwater flowing outward to oceans or rivers. The centre of the Gnangara "Mound" is 70 metres above sea level at its highest point and covers approximately 2,200 km2.
Aquifers
Geological formations such as those composed of sand, sandstone and limestone, containing useable quantities of groundwater, are called aquifers. The aquifer closest to the ground surface is called the shallow, unconfined or superficial aquifer (the upper surface is the water table). There are also deeper, confined (sometimes called artesian) aquifers where the water is confined under pressure between relatively impermeable layers. On the Gnangara Mound, there are three aquifers- the superficial aquifer, the Leederville aquifer (confined) and the Yarragadee aquifer (confined).
Recharge
Recharge is the term used to describe the addition of water to the water table as a result of direct rainfall or run-off caused by rainfall. On the Gnangara Mound, recharge has significantly declined. This is due to two reasons:
- A decrease in annual rainfall over the past 30 years; and
- The existence of high density (very thick) vegetation (both native and pine plantations) over some sections of the Mound, that results in a greater usage of groundwater, increase in transpiration and evaporation as well as preventing rainfall from flowing down into the superficial aquifer.
Bores/Wells
Bores or wells can be constructed into aquifers to pump water to the surface for consumptive or irrigation purposes.
Drawdown
Drawdown describes the effect on the water table of pumping (removing) water from an aquifer.