Drainage by wells

Well drainage means drainage of agricultural lands by wells. Agricultural land is drained by pumped wells (vertical drainage) to improve the soils by controlling water table levels and soil salinity.

Subsurface (groundwater) drainage for water table and soil salinity in agricultural land can be done by horizontal and vertical drainage systems.
Horizontal drainage systems are drainage systems using open ditches (trenches) or buried pipe drains.
Vertical drainage systems are drainage systems using pumped wells, either open dug wells or tube wells.

Both systems serve the same purposes, namely water table control and soil salinity control .
Both systems can facilitate the reuse of drainage water (e.g. for irrigation), but wells offer more flexibility.
Reuse is only feasible if the quality of the groundwater is acceptable and the salinity is low.

Although one well may be sufficient to solve groundwater and soil salinity problems in a few hectares, one usually needs a number of wells, because the problems may be widely spread.
The wells may be arranged in a triangular, square or rectangular pattern.
The design of the well field concerns depth, capacity, discharge, and spacing of the wells.

The determination of the optimum depth of the water table is the realm of drainage research .

The basic, steady state, equation for flow to fully penetrating wells (i.e. wells reaching the impermeable base) in a regularly spaced well field in a uniform unconfined (preactic) aquifer with an hydraulic conductivity that is isotropic is:

where Q = safe well discharge - i.e. the steady state discharge at which no overdraught or groundwater depletion occurs - (m³/day), K = uniform hydraulic conductivity of the soil (m/day), D = depth below soil surface, ${\displaystyle D_{b}}$ = depth of the bottom of the well equal to the depth of the impermeable base (m), ${\displaystyle D_{m}}$ = depth of the watertable midway between the wells (m), ${\displaystyle D_{w}}$ is the depth of the water level inside the well (m), ${\displaystyle R_{i}}$ = radius of influence of the well (m) and ${\displaystyle R_{w}}$ is the radius of the well (m).

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