In a water-stressed country such as South Africa, it’s difficult to imagine any farmer complaining about having too much water.
However, across the country’s irrigation schemes, lands are deteriorating due to waterlogging caused by years of incorrect water and soil management.
The source of the water may be long-term irrigation, percolation from precipitation, or the movement of water from high-lying areas.
Globally, 500 000ha of agricultural land are falling out of production every year due to poor drainage. It is estimated that in South Africa, 240 000ha are affected by rising water tables and salinisation, and the problems appear to be growing.
Drainage problems in South Africa occur mainly in the following areas:
- Along the Orange River in the Northern Cape, especially Vaalharts, Douglas and Upington;
- The winter rainfall areas of Robertson, Worcester, Swellendam, Ceres and Wellington in the Western Cape;
- Pongola and Nkwalini in KwaZulu-Natal;
- The Gamtoos Valley, Sundays River Valley and Fish River Valley in the Eastern Cape;
- The Loskop and Hartbeespoort Dam irrigation schemes in Limpopo;
- Mainly where irrigation is concentrated.
Neil Lecler, of the School of Engineering at the University of KwaZulu-Natal and a specialist agricultural engineering consultant, notes that the Vaalharts Irrigation Scheme is a particularly sobering case study.
“At Vaalharts, the water table was 24m below ground level at the inception of the scheme in 1938. In 2011, it was reported that the water table had risen by more than 22m to an average of 1,6m below ground level. The raised water table has resulted in serious issues at the scheme, including salinity, and has highlighted the ‘slow poison’ consequences of insufficient drainage.”
Mechanics of drainage systems
Felix Reinders, chairperson of the Global Framework on Water Scarcity in Agriculture, explains that the purpose of agricultural drainage is to remove excess water from the soil in order to enhance crop production.
“In some soils, the natural drainage processes are sufficient for growth and production of agricultural crops, but in many other soils, artificial drainage is needed for efficient agricultural production.
“The main objective of subsurface drains (those installed below the ground) is to aerate the plant root zone for the favourable growth of crops. They also improve soil moisture conditions for tillage, planting and harvesting, and extend the growing season.
“Sufficient drainage also acts to remove toxic substances such as salts from the root zone.”
By contrast, anaerobic conditions (a lack of oxygen) and excessive soil water result in:
- The denitrification (nitrogen lost as gas into the air) of nitrogen fertiliser;
- A change in the soil biota to pathogenic populations;
- Reduced mineralisation of organic nitrogen present in the soil;
- Reduced availability of other nutrients, such as phosphorus;
- Lower soil temperature and, in some sugar cane areas, yield losses due to slower germination and ratooning, particularly where residue conservation is practised;
- Increased pest and disease problems;
- Less opportunity to perform field operations;
- Greater potential for compaction damage;
- More difficulties with planting during optimal times, with lands often being too wet to plant.
Ignore at your peril
Considering the important role that drainage plays in crop production, insufficient drainage is not a problem that farmers can ignore. Yet, says Lecler, it is often downplayed.
“Drainage problems often take many years to develop, so the implications of poor drainage may not be immediately obvious. Also, drainage problems have been relatively difficult and expensive to address. Digging trenches and laying pipes in an appropriate envelope and at an appropriate grade isn’t easy or cheap. Using survey equipment, complex calculations, pegs and graders to shape the surface is extremely tedious and difficult.
“Many agricultural projects have neglected effective surface and subsurface drainage, and the temptation to do so continues, particularly [in light of] expensive irrigation upgrades or installations. However, the fundamental laws of the balance between salt and water remain, and substantial portions of irrigated lands have become degraded over time.”
Lecler points out that the consequences of water ponding on the surface can be even more severe than those of waterlogging.
“Water ponds form when there’s poor surface drainage and excessive run-off, for example, from compaction (uncontrolled traffic), high sodium levels, inappropriate sprinkler irrigation nozzles and stand times, and excessive rain.”
He adds that crop losses due to ponding or waterlogging are caused by a lack of oxygen in the soil, as well as increasing levels of salts that become concentrated in the ponded areas after the ponded irrigation water has evaporated.
“Studies have shown that yields of most crops begin to decline after about three days of ponding, and often a 50% reduction is reached after five to eight days. Often, the crops die if the ponding lasts for more than two weeks.
“Even areas that experience short periods of intermittent ponding can eventually become unproductive due to the concentration of salts, especially when high sodium levels exacerbate infiltration problems.”
Consider oxygen and water levels first
While farmers generally look at improving nutrient levels to achieve higher yields,
Lecler recommends that they first investigate the water and oxygen levels in crop roots.
“In most cases, water, oxygen and radiation are the primary determinants of crop yield. Nutrients receive a lot of attention in various precision agriculture initiatives, but their effects on yield are often ancillary to water and oxygen.
“This was highlighted in numerous precision agriculture studies done under the auspices
of the Grains Research and Development Corporation in Australia. Soil characteristics
were mapped and compared with similarly detailed maps of crop yields. Although the focus was on variable-rate nutrient applications, the researchers found that addressing surface drainage or ponding issues was, in most cases, the best or only variable that correlated with crop yields.
“The trials provide strong evidence to prioritise water and oxygen in order to improve yield.”
Yield benefits from improved drainage can be dramatic. Lecler notes the case of a pilot field in Mozambique, where sugar cane production had been unviable due to poor drainage.
Drainpipes were installed using a GPS-controlled tile plough, sugar cane planted, and a crop of nearly 90t/ha achieved.
In similar vein, Reinders references a study in Pongola, KwaZulu-Natal, where the installation of subsurface drains in a sugar cane field resulted in yield jumping from 70t/ha to 111t/ha (see graph).
Grain farmers have also seen dramatic outcomes. Reinders says that results from a 13-year study showed that average yields on lands with adequate surface and subsurface drainage were double those of lands without drainage infrastructure.
“We’ve also seen a five-year study where treatments included irrigation only, drainage
only, irrigation and drainage, and a control.
In the irrigation-and-drainage treatment, average yields increased by nearly double those of the irrigation- only and drainage-only treatments which, on their own, had already yielded significant gains. This improvement was most likely due to adequate drainage assisting with managing any negative effects from non-uniform irrigation applications and/or imperfect irrigation scheduling.”
GPS software: the game changer
Lecler says that while addressing drainage issues in the past was difficult and costly, new GPS-driven technology and the associated software had resulted in improved and cost-effective drainage.
“Land forming (3D shaping) with continuously variable grades (CVGs) or curved surfaces can be achieved relatively easily and cost- effectively. Shaping the surface with CVGs allows ideal surface drainage to be achieved with minimal disturbance of the topsoil.
“Older technologies, such as laser-levelling, often scalped the topsoil and resulted in serious problems. Equipment used for shaping, rather than levelling, the surface includes GPS control, where the height of each side of the scraper blade is set to a desired level. Advanced software and technologies like LiDAR [light detection and ranging] for mapping existing topography and crop heights enable relatively rapid assessment of crop yield, ponding and surface drainage issues, and the associated design of new surfaces.
“Tile ploughs, also with GPS grade control, allow for precise, relatively easy and cost-effective installation of subsurface drains. With a tile plough, a specially designed and manufactured drainpipe is ‘pulled’ into the ground at the appropriate level and grade.”
Lecler advises that when installing drainage, it is especially important to use the appropriate equipment, and operators must have sufficient knowledge and experience.
“For example, inappropriate drainpipes with slots that are too large or insufficient can cause problems.
“In Ontario [in California] in the US, where the use of tile ploughs is relatively prevalent, contractors and their equipment must be licensed. This helps protect farmers from fly-by-night chancers who might use inappropriate equipment and materials, or who don’t have appropriate knowledge and experience. A similar licensing approach is being considered by the South African Institute of Agricultural Engineers for implementation in South Africa.”
To drain or not to drain?
Lecler says cost-effective technologies that permit surface drainage without scalping the topsoil, and the relatively rapid installation of subsurface drainpipes, are game changers and can sustain both dryland and irrigated agricultural production.
Reinders advises farmers to consider the yield and profit benefits that come with improved drainage when investing in drainage systems.
“The demand for the use of irrigation water and technologies to improve efficiency is derived from the whole-farm profitability of farming irrigation crops. The major reason for installing drainage is to improve the productivity of farmland. Higher yields translate into more returns; therefore the investment decision is based on whether the higher crop returns will justify the investment in drainage.”