McFarlane, D. Negus, T. and Ryder, A. 1990. Shallow drains for reducing waterlogging and salinity on clay flats. Western Australia Journal of Agriculture. Vol. 31, p. 70 - 73.

The use of W and spoon drains to manage surface water in large catchments is investigated with reference to two trial sites in Western Australia, Mt Barker and Wickepin.

Flooding, waterlogging and salinity are problems which commonly occur together on the broad valleys of the wheatbelt. Drainage lines often become poorly defined once they reach the valleys and flood waters spread out, causing inundation and waterlogging. There is a need to control this excess water as it contributes to salinity.

Geological and Hydrogeolgical setting

No technical specifications of the surface drains are provided in the article, however the following details are known:

• W drains have an excavated channel on either side of a central spoil bank and water enters the channels from both sides; and
• spoon drains have the spoil from the channel spread or scattered away from it so that the soil does not prevent water from flowing into the channel. If a scraper is used to construct broad spoon drains, the excavated soil can be used to fill in the depressions. Machinery can be worked across the drains which helps water flowing down cultivation furrows to enter the channel.

At Mt Barker it has been reported that a spoon drain removed 5% of annual rainfall in a below average rainfall year (535 mm), 19% in a slightly wetter year (548 mm) and 29% in an above average rainfall year (710 mm). Oat yields adjacent to the spoon drain ranged from 2 to more than 4 t/ha, whereas several areas more than 20 metres from the drain yielded 1 to 2 t/ha.

At Wickepin, flow in a spoon drain on a grade of 0.25% was monitored for three years with rainfall recorded at below the average of 415 mm. In 1986, 1.5% of the 345 mm of annual rainfall was removed by the drain. The corresponding figures for 1987 and 1988 were 1.3% (352 mm rainfall) and 1.5% (366 mm rainfall). Although these percentages are small, the run-off takes place in one or two storms and represents a large volume of water when spread over a large area. The drains at Wickepin have not impacted wheat yield to a significant extent, most likely due to the below average rainfall experienced during the monitoring period.
Groundwater levels at the Wickepin drainage site have risen appreciably since 1985 and areas are now salt affected. This indicates that the present drains were belated in their installation, not close enough or were only capable of delaying the onset of salinity.

It is indicated in the article that W and spoon drains are capable of reducing waterlogging on poorly drained flat land, however the effects of these drains in a other areas is largely unknown. It is likely that the performance of the surface drains will be largely site specific and should be evaluated, both in a technical and economic sense prior to considering implementation.

The following are key determining factors for the successful implementation of surface drains to control waterlogging and soil salinity in dryland areas:

• drain spacing adequate to maintain the water table at a particular depth below the ground surface;
• a level of economic return from the land subject to the drains;
• a level of economic value of the re-used water;
• ability to re-use the drained water; and
• a suitable drainage disposal or storage strategy.