Last reviewed 1 Oct 2026 · 8 min read
What is agricultural drainage?
Drainage is the artificial removal of excess water — from the soil surface or from the root zone — so that crops can grow in a healthy, well-aerated soil. Where drainage is poor, the water table rises, the root zone becomes saturated and the plants suffer.
Why drainage is needed
- Excess rainfall on flat or low-lying land stays on the surface (surface ponding).
- Canal seepage and over-irrigation raise the water table (a major cause of waterlogging in canal commands).
- Flat topography and low permeability of the soil slow natural drainage.
- Poor outlets — rivers and natural drains that are silted or higher than the field.
- Rising groundwater in deltas and coastal belts, and seawater intrusion.
- Artesian pressure from deeper aquifers.
Effects of poor drainage
| Effect | Explanation |
|---|---|
| Oxygen deficiency | Roots need oxygen for respiration; saturated soil has none, so roots die and growth stops |
| Waterlogging | Water table within about 1–1.5 m of the surface for long periods |
| Salinity and alkalinity | Evaporation from a shallow water table leaves salts in the root zone |
| Poor soil temperature and structure | Cold, sticky soil, delayed sowing and poor tillage |
| Denitrification and toxic compounds | Loss of nitrogen; formation of hydrogen sulphide and other toxins |
| Reduced yields and land abandonment | In India, millions of hectares are affected by waterlogging and salinity (check the latest figures) |
A soil is waterlogged when the water table rises so close to the surface that the root zone is saturated, cutting off air to the roots. A water table within about 1.5 m of the ground (for deep-rooted crops) or 0.5–1 m (for shallow-rooted crops) is often treated as the danger zone.
Objectives of drainage
- Remove excess surface water quickly after rain or irrigation, preventing ponding.
- Lower the water table below the root zone and keep it there.
- Control salinity by leaching salts and carrying them out of the root zone.
- Improve aeration and soil temperature, allowing timely tillage and sowing.
- Reclaim waterlogged and saline lands and make them productive.
- Improve trafficability for machines.
Classification of drainage
| Type | Purpose | Methods |
|---|---|---|
| Surface drainage | Removes excess water from the land surface | Open drains, land grading, bedding, ridges and furrows |
| Sub-surface drainage | Removes excess water from the root zone and lowers the water table | Tile (pipe) drains, mole drains, open ditches, tube-well (vertical) drainage, relief wells |
| Main (outlet) drainage | Carries the water away to a river, sea or lake | Outfall drains, pumping stations |
Surface drainage
Surface drainage is used where rainfall or irrigation runoff collects on the surface because the land is flat, depressional or of low permeability. It consists of field drains that collect water and main drains that carry it to an outlet.
Common methods
| Method | Description | Suitable for |
|---|---|---|
| Random drains (ditches) | Shallow open drains connecting the low spots of the field | Undulating or irregular land with isolated depressions |
| Parallel field drains (ditches) | Parallel shallow ditches at a regular spacing (30–200 m) with a common outlet; the land between is graded to the drains | Flat land of uniform soil |
| Bedding system | The land is formed into beds (raised strips) separated by dead furrows that act as drains; the beds slope to the furrows; direction of beds follows the grade | Flat, poorly drained soils with a high water table; rice and vegetable fields |
| Cross-slope (diversion) drains | Drains laid across the slope to intercept runoff from upland before it reaches the field | Sloping land; protects lower fields |
| Interceptor (cut-off) drains | Intercept seepage from an upland or canal | Seepage-affected land at the foot of a canal or hill |
| Land grading and levelling | Smoothing the surface to a uniform slope (0.05–0.2 %) so water flows to the drains without ponding | Almost all drained land; improves irrigation efficiency |
| Field ditches in tidal areas | With sluice gates or flap gates to keep tidal water out | Deltas and coastal belts |
Features of a good surface drainage system
- Works by gravity, with a reliable outlet lower than the field.
- Drains are shallow and broad (side slopes flat enough for machines to cross) so that they do not interfere with farming.
- Drains are laid along the natural lines of flow, with minimum field-boundary disruption.
- Includes culverts and crossings, erosion control, and regular maintenance (weed and silt removal).
Design of surface drains
Design discharge
The drain is designed for a design (peak) discharge from the drainage area. It depends on the rainfall, runoff and the drainage coefficient.
- Drainage coefficient (D) — the depth of water to be removed from the area in 24 hours (mm/day) or the discharge per unit area (m³/s per km² or per ha). It is chosen from the design storm (for example the 1-in-5 year, 1-day or 2-day rain), the runoff coefficient and the crop's tolerance to ponding. Typical values for agricultural surface drains are of the order of a few tens of mm/day (higher for rice-growing regions and heavy-rainfall areas).
- Discharge: with in m/day, in m² and in m³/s. In practice the Rational method ( in m³/s, in mm/h, in ha) or empirical flood formulae are also used for larger watersheds.
Hydraulic design of an open drain
A trapezoidal cross-section is usual. Design uses Manning's formula:
where = velocity (m/s), = roughness coefficient (about 0.025–0.035 for earth drains in good condition; higher where weeds grow), = hydraulic radius, = bed slope, = flow area, = wetted perimeter.
Design considerations:
- Permissible velocity — the flow must neither erode the bed and sides (too high) nor allow silting and weeds (too low): about 0.3–0.6 m/s minimum to avoid silting; maximum depends on the soil (about 0.5–1.0 m/s for sandy to clay loams).
- Side slopes — flatter for loose soils (about 1.5 : 1 to 3 : 1, horizontal : vertical).
- Bed slope — usually between 0.03 % and 0.2 % for flat land.
- Depth — the drain must be deep enough for the outlet and to lower the surface water; the freeboard is 0.15–0.3 m.
- Bottom width and depth are chosen for the most efficient section and easy cleaning (the best hydraulic section has ).
A field of 50 ha is to be drained at a drainage coefficient of 50 mm/day.
Discharge: .
A trapezoidal drain with bottom width 1.0 m, side slopes 1.5 : 1, depth of flow 0.5 m, bed slope 0.0004 and = 0.03 has ; ; m. , , which is a little short of 0.289 m³/s, so the section is enlarged slightly (a flow depth of about 0.52 m would carry the design discharge).