||Technology in Australia 1788-1988
Table of Contents
II Early European Settlements
III Assessment Of Available Water Resources
IV Water Supplies For Goldmining Development
V Irrigation Development
i Channels, weirs and barrages
ii Measuring farm supplies - the Dethridge wheel
iii Early pumping schemes
iv Irrigation techniques
v Drainage of irrigated land
vi Recharge of aquifer
vii Soil-plant-water relationships
viii Carry-over storages and security of supply
VI Farm And Stock Water Supplies
VII Urban Water Supplies
VIII Wastewater Management And Treatment
IX Water Quality Management
X Limnological And Water Quality Research
XI New Techniques In Water Resource Planning And Management
XIV List Of Abbreviations
XVI Plantations-high Productivity Resources
Drainage of irrigated landThe need for drainage in irrigation areas was not appreciated in the early years of irrigation development, reflecting the general ignorance of drainage problems in the countries from which our technology originated. By the 1920s, however, the waterlogging of soils and reduced crop yields, especially in intensively irrigated areas, showed that drainage was essential to survival. Surface drainage systems were designed using empirical formulae which soon proved inadequate in many areas after prolonged wet seasons. Costly litigation against water authorities for flooding led to amending legislation in respect to negligence in the design and operation of drainage systems, and to the development of new design rules based on catchment models and taking account of new construction techniques. These new rules, which have been progressively refined, were successfully tested in extensive litigation in 1977-78. They are believed to be unique to Australia.
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.
© 1988 Print Edition pages 164 - 165, Online Edition 2000
Published by Australian Science and Technology Heritage Centre, using the Web Academic Resource Publisher