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Necessity and methods of drainage of irrigated land

Short answer

Drainage prevents waterlogging, maintains soil aeration, controls salinity, and removes excess water. Methods include surface open drains (field, collector, main), subsurface tile/mole drains, vertical pumping from wells, and interceptor drains along canals or slopes.

Explain why the drainage of irrigated land is necessary. Describe the various methods (types of drainage systems) employed for such drainage.

Drainage is the removal of excess surface water and groundwater from land so that the crops have suitable soil moisture and aeration. Irrigation without drainage raises the water table and leads to water logging and salinity.

Why drainage of irrigated land is necessary

  • To prevent water logging: the water table must be kept below the root zone (at least 1.5 m below ground).
  • To maintain soil aeration, so that roots get oxygen and soil bacteria can work.
  • To prevent salinity and alkalinity by leaching and by removing the salts brought up by capillary action.
  • To remove excess rainfall (floods) quickly, especially in the Terai paddy areas.
  • To allow cultivation, machinery work and timely sowing, and to raise the soil temperature.
  • To control mosquito-borne diseases and to protect roads, canals and buildings.

Methods of drainage

 Drainage
 |-- Surface drainage  (open drains)
 |-- Subsurface drainage (tile, mole, vertical)
 '-- Interceptor / relief drains
  1. Surface drainage: removal of excess surface water through open drains. Field drains collect water from the fields, collector drains gather it from several field drains, and main (outfall) drains carry it to a natural stream. Land levelling and grading help. It suits flat fields and heavy rainfall, as in Terai paddy fields.
  2. Subsurface drainage: lowering of the water table by buried drains.
    • Tile (pipe) drains: perforated pipes or clay/concrete tiles laid at 1.5–2.5 m depth with laterals and collectors; the spacing is found from Donnan's/Hooghoudt's formula.
    • Mole drains: unlined cylindrical channels formed in clay soil by a mole plough, 0.5–1 m deep.
    • Open ditches (deep drains) to control the water table over large areas.
  3. Vertical drainage: pumping from tube wells to lower the water table where the aquifer is permeable. The pumped water can be used for irrigation.
  4. Interceptor drains: deep drains laid along the foot of a canal or hill slope to intercept seepage before it reaches the fields.
  5. Natural drainage improvement: cleaning and straightening natural streams.

The choice depends on the soil permeability, depth of the impervious layer, rainfall, slope and cost: surface drains in flat, low-permeability, high-rainfall areas; tile or vertical drainage where a high water table is the problem.

✓ Answer checked by Claude Opus 5.5 (high effort)

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