Norman, C.P. 1995. Effect of groundwater pump management on reclaiming salinised land in the Goulburn Valley, Victoria. Australian Journal of Experimental Agriculture. Vol. 35. pp. 215-222.

A strategic plan for watertable and salinity control combining surface drainage, subsurface drainage, and on-farm activities is being implemented in the Goulburn Valley Region of northern Victoria, within the Girgarre Project Area (GPA). The GPA is a part of the Shepparton Irrigation Region Land and Water Salinity Management Plan (SIRLWSMP).
This paper reports the results of a public groundwater pumping scheme undertaken in the GPA. The scheme was instigated by the Rural Water Corporation of Victoria (RWC) to provide salinity control over 1,000 ha of irrigated farming land. The main enterprise in the project area is dairy production from irrigated pasture.

The incidence of shallow, saline watertables and resulting soil salinisation is causing a significant loss of productivity on dairy farms in the Goulburn Valley Region of northern Victoria. Also threatened is the Murray River, with further deterioration of its quality as drainage flows from salinised land become more saline. The project was initiated after a wet winter in 1981 led to serious salinisation and water logginf problems.

The Girgarre Project Area has an annual rainfall of approximately 490 mm with a mean annual evaporation of 1,420 mm. The soils in the area are red brown earths (Dr2.33) and the principal soil types are Lemnos loam, Goulburn loam, Congupna clay loam and Goulburn Clay loam. All of these soils have thin topsoils (< 0.15 m) overlying sub-soils with 50 to 70% clay fraction (< 2 µm).
Three aquifers were identified within the trial site at Girgarre, Victoria. These were deeper than 10 metres and existed as channel and point bar deposits of the ancestral rivers and older prior streams. Pump tests indicated the aquifers have a high degree of interconnection and transmissivities of up to 500 m2/day.

The GPA includes a 30 ha evaporation basin and three RWC operated pumps drawing groundwater from beneath surrounding properties. The project design was based on the subsurface drainage strategies of the SIRLWSMP. Two of the pumps (T102 and T103) extract moderately saline groundwater (EC of 5 and 9.5 dS/m, respectively) and discharge this into the main drainage system flowing through the area. The pumps, T102 (commissioned in February 1996) and T103 (commissioned in May 1985) each extract 1 to 1.5 ML/day. Until the end of the 1990-91 irrigation season, both pumps operated continuously. Since 1991, the pumps have operated intermittently for only 2 periods of 8 weeks each year, in late winter and in late summer. The remaining pump, T101 was commissioned in September 1987 and discharges highly saline groundwater (EC 18 dS/m) at 0.5 to 2.0 ML/day into an evaporation basin.

Reclamation was evaluated on 5 paddocks. 3of these were sown to irrigated perennial pasture 300-1200m from a pump, another was soen with annual pasture 250m from a pump (T103), and the remaining paddock a dryland cropping paddock 250m from pump.T103.

Prior to pumping, the GPA had shallower watertable depths than the surrounding district. The introduction of pumping gradually reversed this trend and by the beginning of the 1987 - 88 irrigation season, the percentage of bores with shallow watertable depths was lower inside the project area than in the surrounding area. This downward trend inside the project area continued while there was continuous pumping until the completion of the 1990 - 1991 irrigation season. However, in August 1992, 16 months after continuous pumping from T102 and T103 had ceased, water levels had reverted to pre 1986 levels, with a higher percentage of bores inside the project area having shallower watertables than outside.
Generally, the drawdown resulting from groundwater pumping in the GPA during September and May in any year was evident up to approximately 600 metres from the pumps.

Average root-zone (0-60cm) soil salinity significantly declined from 1985 to 1992, although there was no significant change following 1988. By 1989, the root zone soil salinity within the area of pump influence has decreased below the theoretical threshold for yield reduction of perennial pasture. Leaching of salts form the root zone occurred dring both summer and winter due to the combined effects of irrigation with good quality water and from rainfall. Pasture Dry matter (DM) production in a number of areas increased significantly from an average of 6 t DM/ha in 1986 to 17 t DM/ha in 1993. However the paddock which was at a greater distance from the pump showed a slower rate of reclamation because of lower effect of the pumps and greater fluctuation of the water table.

In the dryland paddock, there was a also slower rate of reclamation, measured by both soil salininty and increasing yield, despite the paddock being within 250m of a pump. Lack of irrigation meant that rainfall events had a significant control on water table and soil salinity especially in periods of intermittent pumping after 1991

The findings from the monitoring and reclamation paddocks, highlighted that, with controlled watertables, soil salinity levels are likely to rapidly decrease and lead to increases in pasture production. Since 1991, when the pumping was more intermittent, the watertable was for prolonged periods, shallower than recommended. However this change does not appear to have adversely affected soil salinity or pasture production levels on the irrigated paddocks, although average soil and watertable salinity levels have shown short term fluctuations due to increased variability in leaching and capillarity. The improved salt balance in the project area from 8 years of salt export, in addition to the fresher watertables, would help maintain this status quo through the intermittent pumping strategy.

Another reason for the maintenance of reduced soil salinity levels in the project area is the rapid watertable response to relatively short periods of groundwater pumping. The work has shown that maximum drawdown in the project area can be achieved after only 12 weeks of continuous pumping with a decline of up to 0.5 metres occurring within 350 metres of the pump site in the first fortnight.
It is concluded that the continuous pumping strategy was initially successful in achieving both watertable and salinity control, whilst the intermittent strategy continued to maintain only a degree of salinity control.

Typically, dryland areas, unlike irrigated areas such as the Goulburn Valley region of northern Victoria, impart a relatively lower rate of economic return per hectare of land. Under these circumstances, for any level of improvement to pasture productivity provided by the groundwater pumps, there will be a comparatively lower rate of economic return in dryland areas.

Improvement to pasture productivity is directly a function of the level of watertable control provided by the groundwater pumps and the level of leaching of the salt store in the soil profile. Watertable control by way of any engineering option, including groundwater pumping, will enable leaching by rainfall although this may be a slower rate than that which could be obtinaed from the same prcedure in irrigation areas. An option where possible could be re-use of the pumped groundwater for a leaching fraction if the water is of suitable quality. Groundwater pumps installed in dryland areas are therefore expected to provide a comparatively higher level of watertable control which may translate into a higher level pasture productivity.

The case study of groundwater pumps installed in the Girgarre Project Area demonstrated that this particular engineering option was technically feasible, on a regional basis, due to reductions in water level and soil salinity as well as improvement to pasture production. However prior to considering implementation, the economic viability of the engineering option, in a dryland context requires evaluation.

The following are key determining factors for the successful implementation of groundwater pumps in dryland areas:

• adequate transmissivity and groundwater pumping system to maintain a particular drawdown of the watertable over a particular area;
• ability to allow leaching of the soi slat store
• a suitable disposal strategy for poor quality extracted groundwater;
• a level of economic return from the land subject to the influence of the groundwater pump; and / or