Water logging and drainage

A field is waterlogged when the water table rises so close to the surface that the root zone is saturated. Roots cannot breathe, crops yield poorly, and as water evaporates from the surface it leaves salts behind, turning the soil saline or alkaline. Large areas in canal-irrigated commands of north-west India and elsewhere have suffered from this.

The causes are an imbalance between recharge and drainage: seepage from unlined canals and distributaries, over-irrigation, poor natural drainage of flat land, obstruction of natural streams by roads and embankments, and rising rivers. The cure is to restore the balance: line canals where seepage is high, irrigate efficiently, use groundwater conjunctively with canal water, and provide drainage.

Surface drains remove excess rain and irrigation water. Subsurface drains, perforated pipes or tile lines laid at a depth of about 1 to 2 m, lower the water table between them. Their spacing depends on the soil's hydraulic conductivity, the drainage rate to be removed, the depth of the drains and the depth of an impermeable layer below. Simple steady-state equations, such as the ellipse equation, give a first estimate; Hooghoudt's method refines it for flow converging to the drains.

Once the water table is controlled, salts can be leached by applying extra water that percolates through the root zone and is carried away by the drains. Monitoring wells and soil salinity surveys show whether reclamation is working.