Water's always on the move

Planet Earth is often called the 'water planet'. This is not surprising, since more than three-quarters of the earth's surface is covered in water, with clouds forming the major visible feature of the earth's atmosphere.

Although water is one of the most plentiful substances on Earth, it would soon run out if it were not for a natural cycle that ensures its replenishment. Water is a vital liquid for every form of life. Therefore, the water cycle (hydrological cycle), must be one of the most important cycles operating on earth. Most people are familiar with diagrams of the water cycle, such as the one shown here.

A feature of the water cycle often overlooked is that a major part of the cycle takes place unseen beneath the soil.

Native bush and water balance

Plants intercept much of the rainfall over the dry interior of Western Australia before it reaches the water table. Observations have shown that the native vegetation, especially the mallee forms of eucalypts, catches the rainfall with its foliage and channels it along its leaves, branches and stems to the root zone. In an uncleared catchment near Newdegate, less than 1 mm of rainfall per annum escaped past the root zone.

Rainfall that reaches the soil surface may either infiltrate or run-off. Under native vegetation, most of it infiltrates, because of the more open soil structure. On cleared land, the soil surface may be compacted and have little water absorbing capacity, restricting rainfall infiltration. It then runs off, causing erosion.

The shallow water table is often called a perched watertable. It owes its existence to an impervious layer of soil. The deep watertable (aquifer) exists underneath this impervious layer, in freer draining soil. The soil between this impervious layer and the bed-rock has a high salt store; hence, the water in the deep watertable (aquifer) can be very saline. Further, since this watertable is confined, pressure will build up as more water enters the watertable (aquifer). The impervious layer does not form a perfect seal and leaks occur through 'preferred pathways' such as root channels and cracks, allowing salty water to be forced to the soil surface. This is the salinity we hear about today.

Farming and water balance

We know that the native vegetation is very efficient at intercepting and using rainwater. Once native vegetation is removed and replaced by annual crops and pastures, more rain water passes beyond the root zone and adds to and becomes recharge to the deep water table. Although the actual amount may be small - between 12 and 35 mm per annum in the 300 mm rainfall zone - over a period of years this net recharge is enough to cause the watertable to rise causing salinity problems in the way described above.

If, by changing farming methods, this extra water (recharge) could be used, then the onset of salinity could be contained. (For example, by growing high water-using crops and pastures, or by introducing alley farming systems.)

Without changes to current land use a water cycle balance will eventually establish itself, but this will take hundreds and hundreds of years. Clearly, we cannot wait that long. Revegetation of key recharge areas and the introduction of high water using crops is the key to establishing the balance before soil salinity has 'run its course'

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