Drainage of irrigated land

Other methods of drainage include underground or tile drains and grids of borehole pumps which extract water from aquifers, thus lowering the water table. These methods have been developed largely to cope with increasing salinity. In the early years of irrigation, there was little knowledge of soils and soil profiles. Salt problems occurred in the Murrumbidgee Irrigation Area within a few years of the commencement of irrigation in 1912, mainly as a result of the development of perched water tables above sub-soils of low permeability and high salt content. Subsequently, most of these areas were protected adequately by the installation of tile drains.

In other areas, high water tables and salting developed much later. Research showed that these areas were underlain by continuous aquifers which had been gradually over-charged by irrigation. This required a different approach, and vertical tube-wells were installed over wide areas to reduce water table levels and salinity.

More recent irrigation schemes have been preceded by comprehensive soil, ground water and contour surveys and the necessary drainage works have been incorporated in the overall planning, thus minimising the likelihood of serious salting and water table problems. In the Wakool Irrigation District in New South Wales for example, where tube-well drainage commenced in 1964, a network of 45 wells protects an area of about 47,000 ha. The average salinity of the water extracted is 12,000 mg/l and the system can remove some 300,000 t of salt each year. Commercial grade salt is harvested from evaporating basins and the remaining liquid waste is injected into an aquifer at a depth of about 300 m.

Disposal of saline drainage water in such a manner exemplifies an aspect of the salinity problem not appreciated in earlier years. Drainage schemes traditionally discharged to natural swamps or rivers, without thought of the potential effect on downstream water users. In addition, the development of high water tables led to increased lateral discharge from aquifers into rivers, especially in periods of low stream flow.

The River Murray is particularly susceptible to this form of pollution because of the proximity of many irrigation areas, its great length and its low velocity. Salinity reached excessively high levels in the Murray in the 1967-68 drought, following a succession of dry years, and some drains entering the river from Victoria were diverted to evaporating basins in order to reduce the salt level by some 40,000 t per annum.

Extensive studies by consultants in 1970 identified several possible schemes for reducing saline inflows from surface and ground waters, and a number of interception works have since been constructed in New South Wales, Victoria and South Australia.

Australian Academy of Technological Sciences and Engineering