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UNIT 7

Saltland Toolbox

 

7.3  Managing surface water

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Waterlogging & Drainage

Waterlogging in soils is simply an excess of water, held above field capacity – water that cannot drain away because of a high watertable or an impermeable layer in the soil. Plant growth can begin to be affected by waterlogging when average watertables are less than about 0.5 m deep in winter.

By filling the soil pores with water instead of air, waterlogging has the effect of reducing the oxygen supply to plant roots. This lack of oxygen induces an energy deficiency which reduces the ability of roots to perform many of their normal functions. Most importantly in the context of salinity, energy is required by roots in order to prevent salt from entering them, so waterlogging reduces the ability of plants to prevent salt uptake. In other words, while salinity and waterlogging are both ‘detrimental’ to plant health, in combination they are considerably more difficult for plants to tolerate.

Waterlogging is a common companion for dryland salinity, because by definition, many saline sites have shallow watertables and are located in the lowest positions in the landscape. Reducing waterlogging is often required for the successful establishment and persistence of saltland pastures, and to reduce the soil pugging and compaction associated with grazing. Therefore, it is not surprising that drainage is often one of the things that needs to be considered when attempting to improve the productive capability of saltland. For more information on drainage, including legal considerations, see Section 7.3.

There are two forms of drainage to consider:

  • Drains that either divert surface water from flowing on to saline areas, or that remove surface water from saline sites. These are often collectively called ‘shallow’ drains.
  • Drains that intercept the watertable and drain water from it, thereby lowering the watertable and reducing both salinity and waterlogging. These are often collectively called ‘deep’ drains.

These options can be further examined by reviewing the attached factsheet.

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Shallow drains (includes raised beds)

Shallow drains of various forms reduce the effect of inundation and surface waterlogging by either diverting surface water that would otherwise run on to the salty site, or by collecting surface water from the site and enabling it to run off. Raised beds, which some farmers have found very beneficial, also fit into this form of drainage.

Either way, the effect is to prevent surface water from ponding on the site, and/or from infiltrating into the saline soil, further raising the watertable. Some farmers have found this approach particularly rewarding – see “Intensive surface drainage equals increased production”. If the watertable rises, then more salts are likely to be concentrating at the soil surface in spring and summer as water evaporates from the site. For more information, see Surface Drainage: Contour Drains or Surface Drainage: Shallow Drains and Banks

Shallow drains are most likely to be effective where:

  • topography is relatively flat and soils are impermeable (eg clays) or highly variable;
  • surface water can be diverted without large costs; and
  • natural drainage lines are ill-defined or discontinuous.

Problems with shallow/surface drains can include:

  • if the drainage water is saline, then disposal to streams (or neighbouring properties) can be problematic, although surface water is never as saline as subsurface drainage and in most jurisdictions surface water is not regarded as ‘drainage’ water;
  • concentrating surface flows can initiate soil erosion.

Raised beds are a specific form of drainage that require land to be engineered in such a way as to allow the seed-bed to be elevated and drainage to occur from close-spaced gutters. Beds are typically 2 – 3 m wide and may be up to 1000 m long in ideal locations. Though well regarded in some areas of Australia for cereal cropping, research in SGSL did not indicate that raised beds were a viable proposition for saltland pastures because the added pasture growth usually did not justify the added cost.

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Deep drains

Deep drains can intercept a rising watertable and control groundwater levels so as to prevent rising watertables from causing salinity. Alternatively, deep drains can be used to lower already high watertables so as to bring saltland back into production. Deep drains can also be configured to remove surface water, thus reducing inundation and waterlogging. Typically, deep drains are dug to a depth of >1.5m and are often controversial because of the difficulties associated with disposal of the drainage water which can have some very ‘hostile’ properties with extremely low pH levels and associated heavy metals. Some of the options and challenges are outlined in Surface Drainage: Deep DrainsEngineering – options for salinity control and Evaluation of the impacts of deep open drains in the Narembeen area, wheatbelt of WA.

The effectiveness of deep drains depends on:

  • their location in the landscape, being most effective on the lower parts of the landscape such as valley floors and coastal plains, where the slope is generally less than 0.5%;
  • drains being located in permeable soils, so that the impact of the drain extends a reasonable distance from the drain itself, that are structurally stable - so that the drains do not collapse;
  • a safe and legal disposal option for the drained water.

Drainage water may be highly saline and/or acidic and so may impact significantly on neighbours, waterways and the environment in general. It is not surprising therefore that there are many rules and regulations regarding deep drains though some farmers have had good success with deep drainage.

Alternatives to deep drains include pumps and siphons that bring water from the watertable to the surface – all are faced with the same problems of water disposal and legal restrictions as deep drains.

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Legal considerations

It is not possible in a general publication such as this website, to outline all the state and regional rules and restrictions relating to deep and shallow drains. The key point is that no drainage operation should be attempted without first checking the legal requirements because they vary so much from place to place – in some areas (eg the Upper South East of SA) drains have been installed in an integrated government program to which farmers can connect their property drains under license issued by the regional Drainage Board, while in other areas it may be impossible to get permission for drainage works. For example, in some catchments in Victoria, you can only drain a paddock area as long as the drainage water is directed into a holding dam on your own property. In addition, you are not allowed to change flow direction - for example if the natural water course is running north/south you cannot change it to flow east/west.

Even where installing deep drains is legal, anyone installing a deep drain may be liable under common law for any damage caused to neighbours by either the construction or the operation of the drain.

Check the local rules and regulations before contemplating any (shallow or deep) drainage option for saline land!

The Million Hectares project (funded by the Grains Research & Development Corporation) compiled a very comprehensive overview of drainage options.

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