WaterSubject

We compare the net present costs of two approaches for managing irrigation-induced deep percolation under border-check irrigated pasture: (1) conversion from border-check irrigation to sprinkler irrigation to minimise deep percolation and (2) installation of a subsurface drainage system to extract excess deep percolation under the existing border-check system. Results for a dairy farm in northern Victoria, Australia, show that conversion to sprinkler irrigation is the more cost-effective approach. The net present cost of the second approach varies across an irrigation landscape, depending on the most suitable subsurface drainage and disposal system that can be used for a particular location. Where an aquifer is high yielding and of low salinity and thus drainage water is suitable for reuse on farm, tubewell drainage and farm reuse of drainage water provides a viable alternative to conversion from border-check irrigation to sprinkler irrigation. Where tubewell drainage or farm reuse is not feasible, sprinkler irrigation is more cost-effective than border-check irrigation with subsurface drainage.

Soil salinity, which is a major threat to the sustainability of irrigated agriculture and, develops when the capillary fringe of a watertable approaches the rootzone. A watertable rises when irrigation-induced deep percolation exceeds the natural discharge capacity of the underlying groundwater system. The management of irrigation-induced deep percolation is fundamental to the sustainability of irrigated agriculture.

Two approaches may be employed to manage irrigation-induced deep percolation: (1) minimising deep percolation to reduce the risk of a high watertable forming and subsequent rootzone salinisation , and (2) enhancing subsurface drainage to remove drainage water that has percolated below the rootzone. Land levelling and introducing more efficient irrigation technologies are examples of options for reducing deep percolation. Enhancing subsurface drainage can be achieved by installing subsurface drains or pumping groundwater.

The costs of managing deep percolation are dependent on site characteristics. Soil hydraulic properties influence deep percolation rates, while the underlying hydrogeological properties dictate the type of subsurface drainage and disposal system suitable for use. Soil and aquifer properties are often highly variable across an irrigated landscape. Thus, the most cost-effective option for managing deep percolation is likely to vary across such a landscape.

We compare two approaches for managing irrigation-induced deep percolation. In particular, we compare the cost of minimising deep percolation with the cost of implementing subsurface drainage in a landscape where soil and hydrogeological properties vary spatially. The comparison is specifically for irrigated dairy farming in northern Victoria, Australia, but the method of analysis can be applied to other irrigation regions.

We focus on the Shepparton irrigation region (SIR) of Australia (36°26?S, 145°16?E, altitude 114 m), which includes 560,000 ha, of which 280,000 ha are irrigated. Border-check irrigation accounts for nearly 90% of the irrigated area in the SIR, largely to support a pasture-based dairy industry. A high watertable caused by excess deep percolation threatens the sustainability of the irrigation industry in the SIR. The watertable level before the commencement of irrigation in the region was between 10 m and 30 m below the soil surface. The watertable quickly rose following the introduction of irrigation in the early 1900s. Between 20% and 50% of the region now has a watertable within 2 m of the surface, the actual area of shallow watertable varying from year to year as a result of climatic and irrigation influences. Active control of deep percolation is required to manage salinity risk in the SIR.

We consider a hypothetical dairy farm with 72 ha of perennial pasture, which is representative of dairy farms in the SIR. The dairy farm has a well-designed and managed border-check system. Annual farm water use is 11.3 ML/ha and annual pasture production is 17.4 t/ha of dry matter, based on experimental results reported. Annual deep percolation is 1.5 ML/ha, based on estimation by.

We assume that the shallow groundwater in the study farm is isolated from influences outside the farm. In reality, groundwater moves across farm boundaries. Thus, the results from this study should be more appropriately interpreted for a collection of farms rather than a single farm.

We compare the net present costs for managing deep percolation in a dairy farm in northern Victoria, Australia, by implementing the following four options:

• Option 1—conversion of border-check irrigation to sprinkler irrigation

• Option 2A—tubewell drainage with farm reuse

• Option 2B—tubewell drainage with disposal to an evaporation basin

• Option 2C—horizontal pipe drainage with disposal to an evaporation basin

Option 1 is the most cost-effective option for managing irrigation-induced deep percolation. The cost effectiveness of Option 1 mainly relies on an increased pasture production under sprinkler irrigation to give a positive return on investment. Where the aquifer is high yielding and of low salinity and thus drainage water is suitable for reuse on farm, Option 2A is a viable alternative to Option 1 for managing deep percolation. Option 2A has the advantage of a smaller initial capital cost compared with Option 1.

Option 2B and Option 2C are far more costly than Option 1 and Option 2A. Therefore, where tubewell drainage and farm reuse (Option 2A) is not feasible because the aquifer is low yielding and or of high salinity, Option 1 is clearly the preferred option. The analysis indicates that sprinkler irrigation may offer the greatest benefit when used in areas of low soil permeability, where aquifers are typically low yielding and saline.

In the SIR, about 30% of the irrigated perennial pasture area has existing subsurface drainage. In such area where farms already have subsurface drainage and reuse in place, conversion to sprinkler irrigation is not suggested. Approximately 40% of the remaining 100,000 ha is underlain by aquifers that are potentially suitable for the implementation of tubewell drainage and farm reuse (Option 2A). Even in this area, Option 1 should be considered in preference to Option 2A, especially if a farm does not already have a well-developed border-check irrigation system. The cost of managing salinity risk from irrigation varies across the SIR. There is an opportunity to encourage new irrigation development to move into areas where the cost of salinity management is likely to be low.