Key messages

1. Sub-surface drainage has resulted in improved production results at both demonstration sites located in Cranbrook & Perillup. An initial economic payback period of 2-to-7-years has been estimated depending upon trial site location and the associated field of influence.
2. The two demonstration sites had exceptional yield responses in 2021 & 2022, with values circa 47% yield improvement in barley during 2021, and 39 – 73% yield improvement in canola for 2022. Lower yield gains of 3.7 - 12.2% were achieved in 2023 with wheat, due to a heat-stressed environment.
3. Whilst yield improvements were seen across both demonstration sites, 2023 yields at the Perillup site were impacted by heat & drought stress during flowering & grain fill.
4. Sub-surface drainage is a waterlogging management solution that requires substantial upfront investment from growers, with fully “installed” costs typically estimated at around $15,500 - $19,000/km*.

Aims

This trial aims to assist growers in making informed decisions around the construction of sub-surface drainage to reduce the impacts of waterlogging on crop production and farm profitability. By 2024, 45% of growers whose properties are affected by waterlogging will have a good understanding of the yield benefits and time to return on investment of installation of on-farm sub-surface drainage and the ability to implement on-farm.

Introduction

Waterlogging is a common problem within the southwest region of Western Australia, with approximately 3 million hectares of land ranked with moderate to high susceptibility to waterlogging. Left unmanaged, waterlogging can lead to soil structural decline and has the potential to create nutrient deficiencies & toxicities, cause root death and reduced plant growth, or, in the worst case, result in plant death. Subsurface drainage is one control method for mitigating waterlogging; however, research in this space in a modern context is limited, and recent studies on the potential Return on Investment (ROI) are unknown.

Established in early 2021, the Stirlings to Coast Farmers (SCF) Sub-Surface Drainage project investigates methods of managing waterlogging within the Albany Port Zone using slotted pipe buried at depth at two demonstration sites (Cranbrook & Perillup). Both demonstration sites had a control (undrained) and a treatment (drained) treatments, with a range of monitoring activities conducted between 2021 and 2023 including plant counts, biomass, soil and yield analysis through to harvest 2023. The trial measurements provided growers with insights into the potential returns on investment, helping them enhance overall farm profitability

Method

This research project established two subsurface demonstration sites in the Albany Port Zone, with demonstration sites located West of Cranbrook (established 2021) and at Perillup (established 2022). Each paddock was comprehensively surveyed using RTK GPS survey equipment, elevation data & production data were utilised to create the final sub-surface drainage design.

Drainage Downunder installed a 100mm slotted pipe (no-sock) at each demonstration site at depth (varying depths between 600 – 1400 mm below surface level) in the pre-selected trial area. The process involved running a mechanical chain-saw-styled trencher to excavate the soil, inserting the slotted pipe into the trench, then approximately 500 mm or over (depending on soil type/texture/clay percentage)  of Limestone caprock was applied over the pipe to allow water permeability, finally the remainder of the trench was backfilled with excavated soil. Laser levelling was used in conjunction with pre-recorded GPS data ensuring sufficient fall in the pipe and allowing the water to flow without impediment.

Treatments at each site comprised sub-surface drainage (slotted pipe) installed at depth across a minimum area of 2 hectares, a ‘control’ region prone to waterlogging (no pipe installed) and a non-waterlogged site located upslope. The control and upslope sites were located within the same paddock to enable comparative yield analysis between treatments. This project raised grower awareness of the technical requirements in designing and planning sub-surface drainage. Additionally, the data generated demonstrated the potential return on investment from installing sub-surface drainage.

Field measurements collected included plant counts, yield, weed burdens, and NDVI assessment to determine production-side benefits of implementing sub-surface water management (SSWM).  At the completion of the project a Benefit Cost Analysis (BCA) was conducted for both demonstration sites by Dr Elizabeth Petersen from Advanced Choice Economics Pty Ltd. The BCA examined the capital outlay of implementation and included potential repairs & maintenance expenditure, which was spread over a very conservative twenty-year lifetime. This lifetime was based on best-practice cost-benefit principles from the Australian Federal Government; however, it is anticipated that the lifetime of the pipe will be much greater than this. The analysis also considered crop rotations and additional input costs (fertiliser and chemical), which may be required due to increased productivity for the sub-surface drainage treatments.

Results

The use of sub-surface drainage has shown a positive effect on yields achieved across both demonstration sites throughout 2021- 2023, however the responses were highly seasonal.  In 2021 Flinders barley observed a 47% yield increase at Cranbrook, followed by 45Y98 canola achieving a 72.5% yield increase in 2022. At the Perillup demonstration site in 2022, 45Y95CL canola achieved a 39% yield improvement between drained/un-drained treatments. However, in 2023, seasonal conditions were not as favourable, and the benefits of implementing sub-surface drainage were limited to -10kg/ha to +420kg/ha for Dennison wheat at Cranbrook and +180kg/ha for DS Bennett wheat at Perillup. The 2023 growing season represented the final year of monitoring these trial sites under the GRDC research project.

Figure 1. Drone imagery capturing waterlogging conditions experienced on 5th October 2022. There is a visible reduction in plant health evidenced in the control region to the left of the drainage lines. (Source: Stirlings to Coast Farmers Inc.)

All three seasons resulted in an above-median rainfall, however rainfall distribution during the individual years was completely different. Rainfall during 2021, tracked above the median from 21st of January, while 2022 started drier for both sites. Growing season rainfall between April to late September 2022 was just below median distribution patterns, then tracked above median rainfall thereafter. The final year of the trial (2023) experienced a below-median rainfall until the season broke on approximately 3^rd June at both sites, then rainfall tracked above the median for the remainder of the year.

Yield benefits were positive across both demonstration sites, with 2021 providing exceptional results at the Cranbrook site despite being a 99-percentile rainfall year, which saw approximate a 1.05 t/ha barley yield benefit in the drained sites compared to that of the control (undrained) region of the paddock. Similarly, in 2022 at Cranbrook, canola yields were higher in the drained regions (2.97 t/ha and 3.02 t/ha) compared to that of the un-drained control, which achieved 1.74t/ha. See Table 1 below.

Across the Perillup drainage site in 2022, canola achieved a 1.46 t/ha average yield benefit above the baseline (control) yield of 3.74 t/ha. The yield results for 2023 also were positive, however, yield benefits in the drained region were limited to 0.18 t/ha above the control yield of 4.95 t/ha sown to DS Bennett wheat. At Perillup, the 2023 season start was challenging for the early April-sown wheat due to limited rainfall at the beginning of the season. This, coupled with heat stress experienced in mid-late September during the critical grain fill period, resulted in below-average yields.

Table 1. Summary of yield differences and rainfall for each of the demonstration sites.

A benefit-cost analysis (BCA) was completed for each demonstration site by Dr. Elizabeth Petersen from Advanced Choice Economics Pty Ltd, the assessment was based on a limited 20-year time horizon., however again, it is anticipated that the life of the pipe will be much greater than this.  The analysis also considered the zone of influence from the drainage, installation costs, and the area covered, it was determined that the payback period ranged from 2 years at the Cranbrook site and approximately 8 years at the Perillup site.

Benefit-cost ratios varied between 5.6x and 1.8x for the Cranbrook and Perillup sites, respectively, indicating that for every dollar spent by the grower, they would receive a benefit of approximately $5.60 at Cranbrook and $1.80 at Perillup from the installation.

Financial sensitivity analysis was completed for both demonstration sites, this queried the effects of a range of measures impacting the financial viability and profitability of the installation. Factors examined included an increase or decrease in anticipated yields, installation costs, area of impact, maintenance costs and productivity improvements. In all instances, the Net Present Value (NPV) remained positive in all scenarios, which indicated that sub-surface drainage is likely to be cost-effective under a range of future economic conditions. These results are least sensitive to changes in maintenance and additional agronomic costs; however, they are most sensitive to future expected prices, yields and productivity improvements.

Conclusion

Implementing sub-surface drainage has resulted in improved production results at both demonstration sites located in Cranbrook & Perillup, with an initial economic payback period estimated between 2-to-7-year period, dependent upon trial site location and the associated field of influence.

The two demonstration sites had exceptional yield responses during 2021 and 2022, at Cranbrook in 2021 barley achieved a 47% yield improvement, in 2022 canola achieved a 39% and 73% yield improvement at Perillup and Cranbrook respectively, Results were lower at both sites during 2023, wheat yielding benefits of 3.7% and 12.2% at Perillup and Cranbrook respectively due to a heat-stressed environment. It is important to note that despite 2023’s limited yield response due to seasonal conditions (heat stress at the Perillup site), there were no significant differences or adverse effects on final yield between the drained & undrained trials when the season finished drier. NDVI values captured throughout the whole trial period showed that drainage performed well during the season, with higher levels of plant biomass recorded consistently in the drained treatments. Throughout the project, growers learned how to calculate a simplistic ‘back of envelope’ potential ROI analysis utilising the case study at Cranbrook. This involved utilising measured and analysed yield in both the drained and un-drained regions, calculating the ‘lost yield opportunity’, and considering installation costs, zone of influence, median grain prices and potential payback periods. They were then introduced to a comprehensive Benefits Cost Analysis (BCA) completed externally in 2024.

These learnings, in conjunction with positive values seen in the sensitivity analysis conducted with the BCA, have provided growers with increased confidence to investigate the implementation of sub-surface drainage on their own farms, even if applied on a relatively small scale. However, Petersen (2024) acknowledged that “our results need to be validated with further trial sites, over a longer period of time to consider a range of years with low to high growing season rainfall, and where the variety of crop types are repeatedly tested”, particularly if results are to be extended into different regions or environments.

Acknowledgments

The author would like to extend their appreciation to both trial site hosts, the Allison & Preston families, as well as Dan Smith & Paul Hooper from Drainage Downunder for their technical support and advice throughout the project, and Dr Elizabeth Petersen (Advanced Choice Economics) for the economic analysis of the drainage sites.

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support.

Contact details

Philip Honey
Stirlings to Coast Farmers
75 Albany Hwy, Mount Melville WA 6330
0428 768 589
philip.honey@scfarmers.org.au

Online Farm Trials Project Page - https://www.farmtrials.com.au/trial/33620