Ferdowsian, R. Ryder, A. and Kelly, J. 1997. Evaluation of Deep Open Drains in the North Stirlings Area. Natural Resource Management Services. Technical Report 161.

This study evaluates the impacts of deep drains established 10 years earlier, on the property of K Pech in North Stirling, Western Australia.

Waterlogging and high water tables occurred in the area resulting in salt affected land and elevated salt export. Installation of drains installed on the property led to conflict with landholders downstream who received increased saline water from the drains.

The area considered occurs in the headwaters of Six Mile creek. It is situated on the Tertiary aged Pallinup Siltstone. The Pallinup sediments in this area are dominantly composed of fine silt and clay with intermittent fine to medium grained sand. The Pallinup Siltstone overlies the Precambrian basement rock, which forms part of the Yilgarn Craton. Regional groundwater studies have suggested that some discharge (40m3/day) could be derived from flow out of the North Stirling Ranges.

The following specifications of the deep open drains had the following specifications: are provided in the article:

• the drainage system comprises one main drain, 6.5 km in length and 7 km of lateral drains;
• each of the drains possess a trapezoidal cross ? section;
• the drain is moderately deep (1.8m)
• the bed of the main drain is 3 metres in width, with sideslopes of 1:1 (vertical : horizontal). The beds of the lateral drains are 1.2 metres in width, with sideslopes of 2:1; and
• the drains, with the exception of a 100m length are situated within the Pallinup Formation.

Preliminary assessment four years after the drains were installed suggested that there was some water table reduction in the vicinity of the drains and a decrease in waterlogging. Detrimental effects included erosion on either side of the drains, and some site access difficulties resulting from spoil piles from construction.

Further evaluation, based on aerial photographs, from 1985 and 1993, it was determined that suggested there had been a reduction in salt affected areas close to the deep drains. About 251ha was strongly salt affected in 1985 and this had decreased by 27ha 1993. The major area of reduction was close to the deep drains, but salinity has encroached to other areas further way from the drain., primarly due to the presence of sandy lenses within the Pallinup sediments has helped to expand the extent of impact of the drain. During this period, however, the salinity had encroached into other areas, further from the drain. In total, the salt affected area of land in the study area had reduced by 27 ha between 1985 and 1993. Approximately 8 ha of the recovered is land was successfully cropped in 1996.

A comprehensive economic analysis undertaken (at a discount rate of 8%) demonstrated that only if 40 ha of land was reclaimed would the investment cost be recovered. As the actual reclaimed area was only 8 ha, the drainage scheme is not expected to be a cost effective exercise. It should be recognised that the economic analysis did not account for any off-site effects of the drains, such as export of salt and silt to other properties or intangible gains such as the scenic value of reclaiming land.
As deep drains are expensive to construct and maintain, the authors recommend that a feasibility study be undertaken prior to construction. The environmental consequences of the deep drains should also be thoroughly considered prior to implementation.

Although water level and soil salinity were not monitored in the vicinity of the drainage network, evidence for the effectiveness of the drains was observed by the increase in crop productivity, (. 8ha has been reclaimed) and the unknown area prevented from being salinised because of the presence of the drain. However, as determined in the study, tHowever the increased crop productivity is not expected to off-set the capital and installation cost incurred for the drainage system. Furthermore, the limited benefits attained at this site, with respect to increased crop productivity, are likely to be far less in other areas with soil profiles comprising tight clays which will act to restrict the movement of groundwater.

The following are key determining factors for the successful implementation of deep drains in dryland areas:

• deep drain length / 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 deep drains; and
• a suitable strategy to dispose of the drainage water.