McFarlane, D. and Cox, J. 1990. Seepage interceptor drains for reducing waterlogging and salinity. W.A. Journal of Agriculture, Vol. 31, p. 66-69.

The design and effectiveness of three different types of seepage interceptor drains are assessed for three trial sites in Western Australia, namely, Narrogin (reverse and WISALTS drains), Mt Barker (reverse and conventional drains) and Cuballing (reverse drains).

These investigations have been undertaken in sloping areas where waterlogging is a key issue and crop yields are reduced. There is a further concern that the ponded water may result in increased recharge which can lead to saline discharge in the short or long term.

The area is typified by development of duplex soils, which have sandy topsoils over well developed clay subsoil horizons. It appears that seepage occurs along the interface between the sand topsoil and the subsoil.

Reverse bank interceptor drains (constructed at grade of 0.6-0.8%) have been installed with the spoil on the upslope of the drain. Conventional interceptor drains, are constructed at a slope of 0.4%, with the spoil on the downstream side of the drain. The drains are generally cut by grader but using a bulldozer when the clay subsoil is encountered. The construction needed to evaluate the possible extent of rilling and siltation.

WISALTS Banks have been constructed on a lower slope, with the concern that rilling may develop and that seepage waters can be lost by seepage.

The amount of rain that was removed by the drains was 1.1 to 7.3% in a rainfall zone of 370-470mm/yr. More rainfall was intercepted in the cooler months when crops used little water. In the dry years the study suggested that little water was removed by drains, and there was concern that the drains may intercept the little volume of water available to crops.

The high flows were partly due to the low storage capacity of the soils and the shallow impermeable subsoils.

The general result is that most drains had reduced waterlogging across the slope. Downslope of the drain the effects are greatest. The mean reduction in of waterlogging was 67%.

The effect on salinity is only that there is an assumed reduction in recharge, although there is no documentation of water tables at any of the sites.

Crop yields generally increased with reduced waterlogging after drainage in areas where there was a high probability of waterlogging. Drain spacing is important with, drains at 60m spacing providing the highest returns at Mt Barker. Drains were of marginal value when there is less frequent waterlogging.