Short note: Design of drainage of irrigated land

Drainage of irrigated land is designed to keep the water table below the root zone and to remove excess surface water, so that water logging and salinity do not develop.

Design steps

1. Collect the data: rainfall, irrigation intensity and efficiency, soil permeability $K$, depth of the impervious layer, ground levels, the water table depth and the outlet level.
2. Find the drainage requirement (rate $q$): the part of the rainfall and irrigation water to be removed in a fixed time (for example 1–2 % of annual rainfall in 24 h), or a drainage coefficient in mm/day (or l/s/ha) for the paddy field from the crop tolerance.
3. Fix the water table level: the highest permissible water table is below the root zone plus capillary rise (about 1.5–1.8 m below ground).
4. Choose the system: surface drains (open field, collector and main drains) for excess rain water; subsurface (tile) drains for lowering the water table; tube wells where suitable.
5. Subsurface design: drain depth 1.5–2.5 m and spacing from Donnan's/Hooghoudt's formula $$S=\sqrt{\frac{4K(b^2-a^2)}{q}}$$ Size the lateral by Manning's formula for a pipe flowing full.
6. Surface drain design: discharge $Q=D\times A$; Manning's formula for the section; velocities within the non-silting and non-scouring limits; free board 0.15–0.3 m.
7. Layout and outlets: field, collector and main drains lead to a natural outlet higher than the high flood level, or provided with a flap gate; add structures and an operation and maintenance plan.