Mann, R. 1994. Tile Drainage and Evaporation Basin Research Project - Tile Drainage Component. Tragowel Plains Salinity Management Plan.

The document is an interim report pertaining to the first two years of operation of tile drains in the Tragowel Plains Irrigation District. The trial site selected for the project is located 10 kilometres west of Pyramid Hill on the property of Russell and Brian Smith.

The development of the irrigation industry in the Tragowel Plains has resulted in the salinisation of large parts of the region. The high saline watertables, and the associated degradation in soil structure, has led to a significant deterioration in the hydraulic conductivity of the soils in the Tragowel Plains area with an increased susceptibility to waterlogging. High, saline watertables are limiting agricultural productivity.

The upper soil profile is known as Mologa Loam, which is a duplex soil of the Red-Brown Earth great soil group. It is reported that the topsoil is quite fragile and prone to dispersion following cultivation and subject to slaking, due to low organic matter levels. The subsoil at the site is strongly sodic and subsequently prone to dispersion, resulting in a zone of low hydraulic conductivity at 30-40 cm within the profile. Soil hydraulic conductivities are in the order of 0.07 - 0.2m/day. Consequently, there are often extended periods of waterlogging during prolonged periods of high rainfall.
The watertable at the site is typically 1.8 to 2.2 metres below the surface in late summer and 0.5 to 1.0 metre in late winter. Groundwater salinity at drain depth is in the range of 40,000 to 50,000 EC units (24,000 to 30,000 mg/L TDS).
The water application rate was around 500mm rainfall and 340mm irrigation over the study period.

The tile drainqge system was installed to control groundwater levels and salinity. The specifications of the sub-surface drainage are:

• six tile drains were installed at an average drain depth of 1.8 metres to a total length of 440 metres;
• drain spacing was 60 metres on a slope of 0.1%;
• the tile drains are 100 mm slotted poly pipe, overlaid with 150 mm of coarse river sand;
• a header drain (150 mm unslotted poly pipe) transfers the water to a sump, from where it is lifted into an evaporation basin;
• installation was carried out using a specialised trenching machine; and
• the nominal drained area is approximately 16 hectares.

Within the 12 months of monitoring, 7.6ML/ha was applied to the drained area. Of this 2.14 ML/ha was recovered from the tile drains, representing a leaching fraction of 28%. A comparison of the watertable behaviour in the tile drained area with that of an undrained area indicates the watertable control at the site has not been significantly improved by the presence of the drains. In the drained area the water table was essentially flat between the drains, and the majority (around 70%) of the drawdown occurred within 1m of the drain. Similarly, there was only minor differences in soil salinity between the tile drained and undrained areas and no noticeable difference in crop yields.
In contrast, waterlogging was lower in the tile drained area in both years of observation (1992 and 1993). The differences however, do not appear to be significant and are likely to represent limited waterlogging control. It is concluded that improved waterlogging control will only be achieved through closer drain spacings.

A comprehensive economic analysis was undertaken to evaluate the tile drainage performance. Under the applied assumptions, it was demonstrated that the tile drainage system is not economically viable as a management option for irrigated cropping at the site and a break-even point is many decades away. Over the first ten years, the internal rate of return is estimated to be -20%. Such negative returns reinforces the notion that tile drainage systems in the irrigation regions of south-eastern Australia are dominantly restricted to horticultural areas where the protection of high value crops from watertables and waterlogging is an integral part of the production system. Furthermore, the requirement for evaporation basins in the Tragowel Plains, unlike other tile drained areas in Australia, significantly impacts upon the capital costs.

It is concluded that optimisation and security of yield for field crops in the Tragowel Plains environment is likely to be achieved by the adoption of cost effective soil management systems including good irrigation layout, fast watering, access to surface drainage, the use of raised beds and the sensible use of gypsum. If however, rising watertables and salinity are major concerns, control would be possible with tile drains at wider spacings than those used in this project and waterlogging control would diminish further from the current low level.

The study of tile drain installation at Tragowel Plains has indicated that soil type has a significant impact on the effectiveness of tile drainage systems, with limited drawdown even at short distances from the drains. It was alos suggested in this study that continual leaching of salts from above the drain level may also lwead to a reduction in hydraulic conductivty because of increased swelling of the sodic soils and increased dispersion. A similar effect was reported in an irrigation at Kerang nearby.
Typically, dryland areas unlike irrigated areas such as the Tragowel Plains, impart a relatively lower rate of economic return per hectare of land. Under these circumstances, for any level of improvement to crop productivity provided by the tile drains, there will be a comparatively lower rate of economic return in dryland areas.

Improvement to crop productivity is directly a function of the level of watertable control provided by the tile drains. Watertable control by way of any engineering option, including tile drains, will necessarily be more difficult in irrigated areas due to the application of surface water beyond plant or crop requirements. Tile drains installed in dryland areas are therefore expected to provide a comparatively higher level of watertable control which may translate into a higher level crop productivity.

The case study of tile drains installed in the Tragowel Plains demonstrated that this particular engineering option was not economically viable for the area investigated. As determined by the authors, a higher level of watertable control or the protection of higher value crops is required for tile drains to be considered economically feasible in this region. The economic viability of tile drains in dryland areas however, is uncertain and difficult to quantify, due to the counteracting factors of lower crop / pasture value and higher level of watertable control, comparative to irrigated areas.

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

• tile 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 tile drains; and
• a suitable disposal strategy.