Over thousands of years this has resulted in salt storage accumulation averaging about 1,000 tonnes under each hectare of typical farmland. Agricultural crops and pastures that were planted after clearing used less water than the native vegetation they replaced. This has resulted in increased run-off and increased greater percolation of water beyond the root zone. This accumulates as groundwater and causes the water table to rise. Once rising water levels are within one 1 or two 2 metres of the surface, evaporation of the water occurs and there is an accumulation of salt at the soil surface and in the root zone.
Salinity is a major environmental threat facing Western Australia.
1.8 million hectares in the south-west agricultural region are already affected by salinity to some extent. These projections are shown in Table 1. The estimated time to reach equilibrium varies between zones and catchments in the region but it could take as little as 30 years in the southern and western areas, to and as long as 50 to 100 years in the east and north. Observations of increasing land and stream salinity were first reported many years ago. In 1907 Government Analyst E. A. Mann suspected that there was a relationship between clearing and the development of land salinity.