Sinclair Knight Merz 1997. Glenelg Sub-surface Drainage Investigation. Department of Natural Resources and Environment.

Sub-surface drains and extensive monitoring equipment were installed at two trial sites (Lyons and Neylon's) in the Dundas Tableland, south western Victoria. The main objectives of the project were to determine whether sub-surface drainage in recharge areas contributed to controlling dryland salinity on farms in the Glenelg Region and whether the drains would have any adverse impacts to receiving streams.

Land management issues

In the Dundas West LMU area (Neylon's), the soils are typically laterite to approximately 1 metre depth and thereafter the soils are a mottled heavy clay representing deeply weathered granodiorite. Below the clay horizon is the parent granodiorite.
In the Dundas East LMU (Lyons), the geology is typically laterite capping on tops of rises and deeply weathered rhyolite on lower slopes. The basement rock is rhyolite and on the flat, near the stream, the soil is sandy loam on the surface, with mottled heavy clay below one metre. Sandy veins in the clay are present from 1 to 1.5 metres in depth.

Sub-surface drainage, consisting of slotted PVC piping 65mm in diameter, were installed at two sites (Lyons and Neylon's) in the Dundas Tablelands in February 1993. The drains, installed in recharge areas, are approximately 1 metre in depth and spaced 35 to 40 metres apart. The area of monitored drains is 10 ha at Lyons and 12.3 ha in Neylon's.

Time series flow results indicate that the drains respond rapidly to rainfall events during the 3 year monitoring period. As the soil becomes saturated there is a corresponding rise in groundwater level, however, it is not until the groundwater level rises above the level of the drains that a constant baseflow is observed. This flow generally ceases by early summer, when the groundwater level falls below the level of the drain.
The groundwater hydrographs indicate that during the wetter seasons, groundwater is generally lower in close proximity to the sub-surface drains. In some instances there is up to one metre difference in groundwater elevations between bores close to sub-surface drains and bores 20 metres away, indicating that the drains are providing localised groundwater control, once groundwater rises to their level.

Drawdown dominantly occurs within one or two metres from the drains due to the low permeability of the soils at both sites which acts to restrict the lateral movement of water. This indicates that the current drain spacing is not adequate to lower overall groundwater levels.
The effluent from the sub-surface drains is of relatively low salinity and of lower salinity than the receiving streams in the area. The drains are therefore not expected to have an adverse effect on the receiving streams. Nutrients in the sub-surface drain effluent were not investigated and the authors recommend that nutrient concentrations should be considered to understand potential quality impacts to the streams.
The drain provides a localised volume of soil with significantly reduced moisture content which assists prompt capture of significant portions of the potential surface runoff. It is this removal of water from the soil profile which reduces surface waterlogging.

The study has shown that in this area, subsurface drains influence groundwater elevation only in their immediate vicinity. They are effective only when the water table rises to the level of the drain.
Drain flows respond rapidly to rainfall events, although there was a time lag of 1-2 days, and subsequently promptly decline after rain stops. The drains have proven to reduce waterlogging by the rapid removal of a proportion of surface run-off. The effect of reduced waterlogging on crop productivity has not been measured in the study, nor has the economic feasibility of the drainage network (at catchment scale) been evaluated.

The authors conclude that the low permeability of the soils limits sub-surface flow and hence only closely spaced drains would be capable of controlling groundwater recharge. The limited impact on lateral subsurface flow into the drains suggests that the influence of widely spaced drains would have minimal impact on overall recharge to the groundwater system and thus a reduction in groundwater discharge. However the ability of drains to reduce water logging by the rapid removal of part of the surface runoff, may have some unquantified beneficial effect on groundwater recharge.
The installation of subsurface drains as a recharge control technique, will clearly depend on the hydraulic conductivity of the soils and the potentila for interception odf the water table by the drains to lower the initial groundwater head.

The following are key determining factors for the successful implementation of sub-surface drains to control waterlogging and / or saline soils in dryland areas:

• drain spacing adequate to maintain the water table at a particular depth below the ground surface and/or to provide sufficient waterlogging control;
• 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.