Agricultural drainage
Why fields need drainage, waterlogging and salinity, surface, subsurface, mole, vertical and bio-drainage, and a Hooghoudt drain spacing sum.
On this page
Irrigation brings water to fields; drainage removes what the crop cannot use. Without it, heavy rain or over-irrigation can leave soils saturated for days, starving roots of oxygen, and in dry regions rising water tables carry salts to the surface, slowly turning productive land into salty wasteland. Large areas of the world's irrigated land are affected by waterlogging and salinity, and drainage has been called the forgotten half of irrigation.
Drainage principles, systems and design are standard topics in agricultural engineering, soil science and irrigation courses.
Why drainage is needed
- Waterlogging: saturated soil lacks oxygen; roots of most crops suffer within a day or two, nitrogen is lost by denitrification, and diseases increase.
- Salinity control: in arid and semi-arid irrigated areas, a shallow water table rises by capillarity, evaporates and leaves salts behind. Drainage lowers the water table and removes the salty water leached through the root zone.
- Trafficability and timeliness: drained fields can be cultivated, sown and harvested on time.
- Warmer soils in spring in cool climates, allowing earlier planting.
- Better root growth and use of nutrients.
The critical depth of the water table, below which capillary rise of salts is small, depends on soil texture and climate, and is often taken as roughly 1.5 to 2 m in irrigated arid regions.
Types of drainage
Surface drainage
Removes excess water from the soil surface: land shaping and grading, field ditches, bedding (raised beds with furrows between), and broad-bed and furrow systems on heavy clay soils. It is the first step on flat land with slowly permeable soils.
Subsurface drainage
Removes excess water from the soil profile and lowers the water table:
- Pipe drains (tile or perforated plastic pipe), laid typically 1 to 2 m deep and some tens of metres apart, discharging into collector drains. Widely installed in Europe, North America, Egypt, Pakistan, India and elsewhere.
- Mole drains: unlined channels formed in clay subsoil by a bullet-shaped mole plough; cheap but short-lived.
- Open deep ditches, which take land out of production but are simple.
- Controlled drainage: structures that raise the outlet level when drainage is not needed, conserving water and reducing nitrate losses.
Vertical drainage
Tube wells pump groundwater to lower the water table over a wide area, often reusing the water for irrigation if it is fresh enough, as in parts of the Indus basin.
Bio-drainage
Deep-rooted, high-water-using trees such as eucalyptus planted in strips transpire excess groundwater. It needs no energy but works slowly and does not remove salts from the area.
Drainage design basics
- Drainage coefficient (or design discharge, q): the depth of water to be removed per day, often a few millimetres per day for subsurface drains in irrigated areas and more in humid regions.
- Hydraulic conductivity (K) of the soil, measured in the field, for example by the auger-hole method.
- Drain depth and the depth to an impermeable layer.
- Allowable water table height midway between drains.
Hooghoudt's equation, developed by the Dutch soil scientist S. B. Hooghoudt in 1940, relates these:
where L is drain spacing (m), q the drainage coefficient (m per day), h the water table height midway above drain level (m), Ka and Kb the hydraulic conductivities above and below drain level (m per day), and d the equivalent depth (m), a correction of the actual depth to the impermeable layer that accounts for flow converging on the drains.
Spacing subsurface drains
A field has uniform soil with hydraulic conductivity K = 0.8 m per day above and below the drains. The design drainage coefficient is 5 mm per day (0.005 m per day), the water table must be held 0.5 m above drain level at midpoint, and the equivalent depth is d = 1.5 m.
- First term: 8 × 0.8 × 1.5 × 0.5 ÷ 0.005 = 4.8 ÷ 0.005 = 960.
- Second term: 4 × 0.8 × 0.52 ÷ 0.005 = 0.8 ÷ 0.005 = 160.
- L2 = 960 + 160 = 1,120, so L ≈ 33.5 m.
If the soil were less permeable, with K = 0.3 m per day, L2 = 420, and L ≈ 20.5 m: more drains per hectare and higher cost. One hectare at 33.5 m spacing needs about 10,000 ÷ 33.5 ≈ 300 m of lateral drain; at 20.5 m, about 490 m.
Environmental considerations
Drainage water can carry salts, nitrates, phosphorus and pesticides into rivers and wetlands. Disposal of saline drainage water is a major problem in closed basins. Solutions include controlled drainage, reuse of drainage water on salt-tolerant crops, evaporation ponds, constructed wetlands, and careful irrigation that reduces the amount needing drainage in the first place. Wetland drainage for farming has also destroyed valuable habitats, and is now restricted in many countries.
Key points
- Drainage removes excess water to prevent waterlogging, control salinity, and improve trafficability and root growth.
- Surface drainage shapes land and removes ponded water; subsurface drainage (pipe, mole, ditches) lowers the water table; vertical drainage uses tube wells; bio-drainage uses trees.
- Design depends on the drainage coefficient, hydraulic conductivity, drain depth and depth to an impermeable layer.
- Hooghoudt: L2 = 8 Kb d h ÷ q + 4 Ka h2 ÷ q.
- Drainage water must be managed to protect downstream water quality.
What the exams ask
- School-leaving and entrance papers: causes and effects of waterlogging, and types of drainage.
- Degree and research-fellowship entrances: surface and subsurface drainage systems, critical depth, drainage coefficient, hydraulic conductivity measurement, and Hooghoudt's equation.
- Agricultural engineering examinations: drain spacing and depth design, outlet and collector design, reclamation of waterlogged saline lands, and controlled drainage.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
How do I know if my field needs drainage?
Signs include water standing for more than a day or two after rain or irrigation, yellowing crops in low patches, grey or mottled subsoil, a shallow water table in nearby wells, and white salt crusts in dry regions.
Why are drains laid so deep?
To keep the water table below the root zone and below the critical depth for capillary rise of salts. Shallower drains would need to be closer together to achieve the same effect.
Can drainage waste water?
Over-drainage can remove water that crops could have used. Controlled drainage raises the outlet when drainage is not needed, saving water and reducing nutrient losses.
Sources
- Ritzema, H. P. (ed.) Drainage Principles and Applications, 2nd edition. ILRI Publication 16, Wageningen, 1994.
- FAO. Materials for Subsurface Land Drainage Systems. FAO Irrigation and Drainage Paper 60, 2005.
- Smedema, L. K., Vlotman, W. F. and Rycroft, D. W. Modern Land Drainage. Taylor and Francis, 2004.
- Hooghoudt, S. B. "Bijdragen tot de kennis van eenige natuurkundige grootheden van den grond, No. 7." Verslagen van Landbouwkundige Onderzoekingen 46, 515–707 (1940).
- Michael, A. M. Irrigation: Theory and Practice, 2nd edition. Vikas Publishing, 2008.
More in agricultural engineering and water
Discussion
Questions, corrections and notes from readers on this article. Every comment is checked before it appears: keep it on the topic, and do not share phone numbers, links or contact details.