Water in Australia
It is often said that Australia is the driest continent. This is because the land surface in Australia receives a lower average rainfall than any other continent, other than Antarctica, where most precipitation falls as snow. Average rainfall in Australia is 460mm, compared with the world average of 660mm. Also, in Australia we have a hot, dry climate with plenty of sunlight, which results in 87% of precipitation evaporating. For instance, SA receives 7.1% of the precipitation falling on the continent but produces only 3.4% of the runoff, however almost all of this is from the River Murray and the Glenelg River (i,e, the water originates outside of SA). The runoff from all other streams combined is only 0.43%. Of the remaining 13%, 10.6% is discharged from rivers and 2.2% ends up in groundwater. A remarkable 50% of surface runoff is flow to the oceans from Tasmania, or into the Gulf of Carpentaria or the Timor Sea.
Potential evaporation (i.e. the amount that would evaporate from an exposed water surface) in central Australia is 4,500mm, or more than 20 times the rainfall. Three quarters of the continent has evaporation rates higher than 2,500mm.
Underground water
Much of the underground water is on its way back to the ocean. It seeps into the soil profile after precipitation, or from streams and lakes, and begins a slow journey through soil and porous rocks, back to the sea. Some is trapped by impervious layers, and forms a basin. A little very old water was trapped in sedimentary beds during formation, and this is called ˜connate™ water.
When it first infiltrated into the soil, a considerable amount of water is retained in the spaces between soil particles. This portion also seeps gradually downwards, under the influence of gravity, and some eventually becomes groundwater. Some is evaporated from the soil surface or used, and eventually transpired, by plants. Even when soils look dry, they have ˜hydroscopic water™ “ a very thin layer around individual soil particles, held there by surface tension forces (remember the meniscus in the glass, from school science lessons). This water cannot be squeezed out of soil, and to get soil completely dry (say for laboratory analysis), it is necessary to bake soils in an oven. Hydroscopic water is very important for plants. They can extract it, with their fine root hairs.
Permeability of soil and bedrock depends upon porosity, or the total amount of pore space and the ˜connectedness™ of pores, to allow water to flow through the medium. Soft rocks like sandstone can be 30% or more porous, and therefore they permit good water storage and flow.
Soil and water storage
The best place to store water for crops is in the soil. Good soil is full of shrinkage cracks, wormholes, root holes, spaces between crumbs (or lumps) of soil and very fine spaces between the individual particles. The actual mineral part of a good soil is only around 45%. Organic mater accounts for 3-5% of soil. The remainder is air spaces or pores. After a saturating rain these pores are filled with water. If there are plenty of larger particles (i.e. sand) and aggregates (or lumps) and there is good connectivity between the pores, some water will quickly infiltrate into the deeper layers under the influence of gravity, leaving about half the soil pores (or 25% of soil) as soil air. Ideally there will be some smaller particles (i.e. silt and clay) to hold onto or trap soil in the smaller spaces between them, and on the surface of particles (hydroscopic water).
In normal conditions sandy soils drain quickly, because they have larger particles, that stack together with larger and better connected pores between them, and they require more frequent water applications. Clay soils trap and hold more water for a longer time, and require fewer applications. Clay particles hold onto much more water due to the very small spaces between them (small channels impede gravitational flow) and the very large surface area of particles that have hydroscopic water.
Humus particles are also very good at storing soil moisture. They have an open, lattice structure that traps water, and organic matter also holds hydroscopic water. Small improvements in soil organic matter can result in a very large increase in water storage (see the table below).
