Journal of the Department of Agriculture, Western Australia, Series 4

Volume 19 Number 4 1978 Article 8

1-1-1978

Effects of surface drainage on dryland salinity

P R. George

T. R. Negus

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Recommended Citation George, P R. and Negus, T. R. (1978) "Effects of surface drainage on dryland salinity," Journal of the Department of Agriculture, Western Australia, Series 4: Vol. 19 : No. 4 , Article 8. Available at: https://researchlibrary.agric.wa.gov.au/journal_agriculture4/vol19/iss4/8

This article is brought to you for free and open access by Research Library. It has been accepted for inclusion in Journal of the Department of Agriculture, Western Australia, Series 4 by an authorized administrator of Research Library. For more information, please contact [email protected], [email protected], [email protected]. Effects of surface drainage on dryland salinity

By P. R. George, Soil Research and Survey Branch and T. R. Negus, Officer in Charge, Narrogin Office.

Areas which are salt-affected are often also flooded. Although flooding is not the basic cause of salinity, surface drainage may improve conditions for plant growth, and this article describes suitable Fig. 1.—Profile of W-drain methods.

Surface drainage is used to prevent or reduce flooding problems. It is rarely, if ever, claimed that surface drainage alone will control soil salinity, although under certain conditions it is used in other parts of the world with sub-surface drainage to give better control of watertables (see article "The dryland salinity problem in North America). Surface drainage may be useful where: • The topography is extremely flat and the soils are relatively impermeable. • There are depressions that hold water. • Large amounts of runoff from surrounding higher land accumulate. • Overflow from streams or rivers is likely. Fig. 2.—Levee which does contain the water but has not stopped the spread of salt • The natural drainage lines are ill-defined or discontinuous. In agricultural districts, flood-prone areas are also mainly those where salinity occurs. This may be a cause of the belief that flooding is a major cause of soil salinity. However this oversimplifies the cause of salinity. Other articles in this Journal explain the importance of subsoil sources of salt which are brought to the surface by capillary rise from shallow watertables. Because of the association between salinity and flooding, surface drainage is commonly used to reduce flooding, and hopefully salinity also. It reduces water­ logging and thus improves condi­ tions for plant growth. This allows maintenance of good plant cover which protects the soil surface and can reduce the rise of salts to the surface. The value of ground Fig. 3.—Interceptor bank

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Journal of Agriculture Vol 19 No 4 1978 cover in reducing the rise of salt to areas can flow directly into the land. Unlike W-drains they do not the surface from the watertable drains from each side. They are pose a barrier to cultivation and cannot be over-emphasised. cheap and convenient to construct water can be easily led into the Surface drainage practices have and work most effectively when the drain. Spoon drains can be rapidly largely developed out of experience area they are draining is smooth installed but require regular and observation. No controlled with few irregularities to retain maintenance to be effective. studies have been done on the surface water. effectiveness of the various drainage W-drains are frequently Levees schemes in the wheatbelt. recommended to remove excess Levees are built along major creeks This article reports on the main surface water from salt-affected to prevent water flooding adjacent surface drainage techniques used land, to provide better conditions pastures and crops for extended in the wheatbelt and observations for growth and establishment of periods (Fig. 2). of their effectiveness, mainly in salt-tolerant vegetation. They do Levee systems can be single-sided relation to soil salinity. For the achieve this, but fencing to control or double-sided, and to be most purpose of this article, surface water grazing on the saltland is still of effective, adjacent areas outside the is taken also to include shallow, prime importance. levees need supplementary drainage perched groundwater which, on W-drains have not been observed to to enable surplus 'local' water to sloping areas, flows laterally along stop or reverse the spread of drain into the creek. This usually less permeable material such as a saltland. However, on relatively is not done and waterlogging and layer of clay in the soil profile. porous soils where they have flooding from local runoff and Generally this perched water will be reduced flooding and hence seepage tributaries of the main creek occurs. within 1 metre of the surface, but down to the water table, the area Extremely low gradients in the this varies. This definition allows over which the saline watertable main channels often make it difficult discussion of interceptor banks and comes near to the surface may be to provide gravity inlets into the interceptor drains. reduced. In this situation the creek but venturi inlets which rely drains may have stopped the salt on higher flow velocities in the main W-drains from spreading as far as it would drain to suck in this local water have spread without treatment. have proven effective. W-drains are used mainly in flat Such situations are uncommon in areas and consist of two closely Western Australia. Levee systems, if well sited and spaced parallel single drains, the built, can contain large volumes of spoil from each being placed Spoon drains are a modification of water flowing from upper catchment between the drains (Fig. 1). W-drains. A special spinning areas. In this way flooding from The advantage of W-drains is that digger mounted on a tractor is used local water sources and rainfall is the spoil does not have to be to dig a shallow drain and thinly not worsened by flood waters from spread and water from surrounding spread the spoil over the adjacent further up the catchment. Levees have not, however, been observed to directly reduce soil salinity. Grade banks Grade banks (commonly referred to as contour banks) are built on sloping land and are designed to reduce the speed of overland flow of water and lead runoff to safe waterways. This reduces erosion risks. Grade banks and associated contour cultivation encourage the retention and absorption of as much water as possible where it falls. This reduces runoff, and helps prevent flooding and waterlogging of low-lying areas and erosion of sloping areas. Retaining water on the slopes where it falls will reduce the need for drains and levees in low-lying areas. One case where grade banks have Fig. 4.—Waterlogging above and below an interceptor bank. Note the healthier crop reportedly reduced the area of in the background saltland is on an area of lieht. 113

Journal of Agriculture Vol 19 No 4 1978 gravelly soil near Narembeen. Some interceptors, especially those interceptor bank system built by Mr Clearing from 1962 to 1967 caused which are built on a grade and H. Whittington are now being 8 ha of saltland and it is claimed lead water to a definite disposal compared with a conventional soil that grade banks installed after point have been observed to reduce conservation system on an area of 1967 have been associated with a waterlogging downslope from the saltland near Dangin. reduction in saltland to 0.4 ha. banks. However, at other sites Similar reports are heard from well-clayed, level interceptors have farmers in the wetter parts of the been observed to fill with water and wheatbelt who have contoured their then overflow around one or both Conclusion higher slopes. ends. Keeping water on the slopes where Obtaining a continuous seal on the it falls is much better than relying Interceptor drains interceptor lines has proven difficult on drainage schemes to control Interceptor drains are installed to and bad leakage can occur at poorly excess water once it reached collect relatively shallow sub-surface sealed points. Extensive water­ low-lying areas. water flow coming from upslope, logging for prolonged periods has The need for surface drainage, and prevent it reaching the area to been observed in such situations continues to exist however, because be protected. They are suitable (Fig. 4). farmers in the lower parts of large for cutting off and diverting Where interceptors are effective in catchments are unable to control underground water supply from cutting off shallow sub-surface flow what happens on remote higher seepages on sloping areas if the and draining it away to a creek, the lands. Also natural drainage seepage is caused by impermeable resulting reduction of waterlogging systems are frequently unable to rock or clay at shallow depth. and flooding may shrink the contain large amounts of water The amount of flow intercepted by marginal areas of salt-affected land resulting from unusually high a drain depends on its depth and also allow cultivation and rainfall. relative to the thickness of the better establishment of plant cover. In poorly drained low-lying parts waterbearing layer. On steep Level interceptors, if they result in the incident rain may in itself be slopes open drains are more enough to cause waterlogging. effective than covered tube drains, deep infiltration of water into and an open drain cut into the sloping land, will increase Grade banks, level banks, W-drains impervious layer intercepts nearly watertable levels and thus expand and levees can all help alleviate all the flow. valley areas permanently under the waterlogging and flooding problems influence of shallow, saline and improve plant growth. Interceptor banks groundwater. Improved plant cover will protect the soil and hence reduce the rise Interceptor banks (Fig. 3) for Department of Agriculture experience has shown that grader of salts to the surface by capillary controlling soil salinity are being action. actively promoted by some farmers. built interceptor lines are effective The treatment is based on the in removing shallow sub-surface Reducing flooding may in a limited premise that the prime cause of water and bulldozer construction number of cases reduce the amount wheatbelt salinity is shallow, sub­ is not necessary for them to of water infiltrating into the surface water flow which function properly. watertable and by doing this, accumulates in low-lying areas. Moreover, as the aim of interceptor restrict the area of soil salinity. The banks are therefore intended drains is to stop water from sloping Generally however, surface drainage to intercept this flow and hold it, areas accumulating on low-lands, will not reclaim saltland because of together with runoff water, on the the water is best intercepted with its insignificant effect on ground slopes. one drain immediately upslope from water levels but will improve plant To do this the downslope banks of the area to be protected. The growth on waterlogged saline areas. the drains are lined with subsoil clay water intercepted should then be led Interceptor drainage has a role in in an attempt to make them away and not allowed to seep back controlling salinity but it always has impervious and thus prevent into the soil where it can continue to be applied with the mechanism of downslope movement of the shallow to cause damage. salt encroachment in mind. seepage water. Banks can be up to Grader construction of one Intercepted water must be led away 0.9 m deep. strategically placed drain and disposed of where it cannot do Some banks are made on the true considerably reduces the cost of further harm. There is no point in contour and don't run water while interceptor drainage. Contour intercepting winter shallow sub­ others are built on a low grade to banks however still may be needed surface flow where deeper, run the intercepted water to a on higher land to control run-off permanent groundwater is the waterway. and erosion. The effects of an major cause of salinity.

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Journal of Agriculture Vol 19 No 4 1978