Ground Cover North : Ground Cover 058 October-November 2005 - North
25 Salinity Digging deep to beat salt n Deep open drains are increasingly seen as an answer to encroaching salinity in Western Australia, where it is predicted that salinity's grip could rise from the 10 per cent of farm land now affected to 30 per cent within two or three decades. In a desperate effort to try to forestall the situation, somewhere between 12,000 and 30,000 kilometres of drains are now said to be snaking their way around the wheatbelt. While many drains appear to be working, CSIRO's Dr Riasat Ali, who heads a GRDC- supported project evaluating the impact of deep drains on crop productivity, says others have been constructed with limited planning and construction guidelines and, usually, little understanding of downstream effects and linkages. Dr Ali, with the Department of Agriculture WA (DAWA), is working with growers to evaluate more than 70km of deep drains in the Wakeman sub-catchment near Narembeen, where most of the valley floor was cleared in the 1930s, with hillside clearing as recent as the 1970s. Salinity developed in the 1960s after a period of high rainfall and is mainly within the valley floor. Land use is dominated by wheat and barley in rotation with pastures, lupins, canola and legume crops. Dr Ali and his team selected four sites in drained areas and two in areas proposed to be drained, installing an array of instruments -- transects of shallow and deep piezometers (instruments for measuring pressure), stream gauging stations, flow measuring devices, automatic water samplers, weather stations and rain gauges. They aim to study and evaluate the impact of deep open drains on groundwater levels, root zone salinity, crop productivity, quality and quantity of drainage discharge and drain design and effectiveness. Biannual soil sampling, up to 1.6 metres deep and 200m from the drain, has already been conducted over five years to assess changes in root zone salinity, soil moisture and pH profiles. This study found the impact of deep open drains on groundwater levels is significant, often extending more than 300m away from the drain. Post-drainage salinity at most sites remained below the threshold for barley and wheat throughout the five-year monitoring. "This implies deep open drains improve soil surface salinity by lowering groundwater levels and leaching of salts from the soil profile into the drain following significant rainfall," Dr Ali says. "Soil pH increased slightly in the soil surface and deeper layers but remained unchanged in the middle layers of the profile." During the project, Dr Ali observed some areas adjacent to the drain which were previously dominated by salt-tolerant vegetation could now be successfully cropped. "Land previously considered unproductive due to high salinity was cropped following drain construction, with varying success, depending on drain depth and construction. "In 2003, a reasonable barley crop grew close to a drain where, before the drain, groundwater levels were only half a metre and soil surface layer salinity was very high." Dr Ali has developed the following guidelines for growers: Guideline 1: A deep open drain should be more than 2m deep and/or be constructed in permeable materials Most trial drains were 2m to 3m deep and impacted on water tables up to 200m to 300m away from the drain, and water levels were below 1.5m of the soil surface during most of the post-construction monitoring period. Guideline 2: A deep open drain should cut through the confining ferricrete layer which exists in the wheatbelt. Deep open drains of more than 2m were more effective than predicted, probably due to porous material in the top 3m and a ferricrete layer of varying density and thickness between 1.5m to 3m of the soil profile. Guideline 3: A leveed groundwater drain (a deep open drain that entrains only groundwater flow) requires less construction and maintenance costs and is therefore more effective than an open drain (one that entrains surface and groundwater). Dr Ali observed open drains silted more than leveed groundwater drains due to transportation of sediments by surface runoff into the open drains. Therefore, unless de-silted regularly, open drains become less effective. Generation of unstable flow velocities, especially in winter, caused more erosion of side slopes and banks in open drains. Guideline 4: A drain should have enough capacity to handle the designed flows. Open drains generally lack capacity to handle surface run-off from significant rainfall events and overflow, causing silting and erosion. As an open drain involves both surface and groundwater, its designed flow rate should be based on the likely maximum five- or 10-year rainfall event. Drains based on this flow rate will probably incur higher construction costs unless adjacent streams are also considered as conveyors of surface runoff. However, silting is likely to be much higher and it is therefore preferable to construct a leveed groundwater drain at least at the farm-scale level. Experience with the Narembeen drains and others in the eastern wheatbelt, suggest a design flow of 50 kilolitres per kilometre a day should be considered. Guideline 5: A properly designed drain should have adequate side slopes based on the type of soil and drain. A drain's side slopes depend on soil types and Dr Ali recommended determining the natural angle of repose of soils before selecting slopes for a drain. In the Wakeman sub-catchment, most drains were constructed as trapezoidal sections, with steeper than recommended side slopes due to digging equipment limitations. Minimum side slopes in leveed groundwater drains should be 0.5:1 and 1:1 for open drains. Guideline 6: The longitudinal gradient of a drain should reduce risk of silting, erosion and unstable velocities. According to Dr Ali, drain gradient is generally governed by the landscape topography where it will be constructed: "In flatter areas, gradient is determined by the minimum drainage depth requirements in the upstream section of the drain and by the lowest level of discharge at the outlet. At trial sites, gradients were measured annually, averaging about 0.3 per cent." Guideline 7: Flow velocity in a drain should be between the permissible maximum and minimum. Maximum permissible velocity, or greatest mean velocity without causing drain erosion, varies greatly and is best estimated through landholder experience and judgment. If poorly designed, maximum velocity becomes an issue in open drains. For better control of the flow velocity, leveed groundwater drains are preferable, as the controlled velocities reduce erosion and silting. To avoid silting, average flow velocity should be more than the minimum permissible. Guideline 8: Embankments should prevent entry of surface water, be properly shaped and placed 3m to 4m from drain edges to avoid scouring embankments back into drain edges. This strip should have reverse gradient Susan Hall reports on the increasing use of deep drains to remove saline ground water from crop land, and the need for careful planning before sending in the bulldozers so rain falling over it does not flow into the drain. The embankment also should have gradient away from the drain to avoid its erosion and washing into the drain. Other issues to address include considering where the water is drained to and the quality of that water; legal requirements; and catchment plan impact on neighbours and on public assets. A drain designed and constructed following Dr Ali's guidelines is likely to be more effective and less expensive to maintain. GRDC Research Code CSO204 For more information: Dr Riasat Ali, 08 9333 6329, Riasat.Ali@csiro.au OCTOBER/NOVEMBER 2005 GROUND COVER The Corporation has a range of Awards aimed at developing and maintaining the skills and information bases required to achieve the GRDC's objectives. These Awards provide selected growers, researchers and others directly involved with the Australian grain industry opportunities for professional development. RESEARCHER DEVELOPMENT Undergraduate Honours Scholarships (UHS) Ten awards, each of $6000, are available annually for students undertaking full-time study at a recognised tertiary institution, who demonstrate an excellent record of undergraduate achievement and a program of research directed at strategic priorities in the Australian grains industry. Grains Industry Research Scholarships (GRS) Available to students proceeding to postgraduate study in a field relevant to the GRDC. The award will normally be for three years with an annual tax-free stipend of $25,000. An additional amount of up to $5000 may be available for operating expenses. Senior Fellowships (SF) Available to senior R&D personnel seeking to enhance their experience and potential to contribute to the work of the GRDC at an institution in Australia or overseas. Support will be for up to 12 months. Preference may be given to applicants who have matching funds from other sources and who propose working in an institution other than their current one. Support will generally be up to a maximum of $50,000. Visiting Fellowships (VF) Available to overseas researchers who are able to enhance GRDC programs. Activity may occur at an Australian host institution or company. Support may be for up to 12 months and a maximum of $17,500. In-Service Training (IST) Available to younger scientists, technical staff and others engaged in work relevant to GRDC objectives who may not be eligible for other forms of support. Funding will be considered for travel, secondment or interchange between institutions and will be available for a maximum of six months. SCHOOL-LEAVING STUDENTS Agricultural Training Awards (ATA) Twelve scholarships, each of $5000, are available annually to school-leaving students undertaking full-time study who can demonstrate an excellent record of school achievement at a recognised institution. Preference is given to applicants who indicate an understanding of the knowledge and skills that will be required for a successful career in the Australian grains industry. Closing date for all training award applications is 28 October 2005. All referees' reports must also be received by this date. No late applications will be accepted. Further information and Application Forms can be found at 'For Researchers' / 'Applying and Reporting' section of the GRDC's website at www.grdc.com.au The Grains Research & Development Corporation invests in research and development on behalf of the Australian grains industry and the Australian Government TRAINING AWARDS 2006 Dr Riasat Ali (left) and John Byrne measure the flow in Latham Drain, near Narembeen.
Ground Cover 059 December-January 2006 - North
Ground Cover 057 August-September 2005 - North