Well drainage

Agricultural land is drained by pumped dry wells (vertical drainage) to improve the soils by controlling water table levels and soil salinity.

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.

[1] 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 (phreatic) aquifer with a hydraulic conductivity that is isotropic is:[1] 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,

Thus the basic equation can also be written as: With a well spacing equation one can calculate various design alternatives to arrive at the most attractive or economical solution for watertable control in agricultural land.

[3] The costs of the most attractive solution can be compared with the costs of a horizontal drainage system - for which the drain spacing can be calculated with a drainage equation - serving the same purpose, to decide which system deserves preference.

The well design proper is described in[1] An illustration of the parameters involved is shown in the figure.

The numerical computer program WellDrain[3] for well spacing calculations takes into account fully and partially penetrating wells, layered aquifers, anisotropy (different vertical and horizontal hydraulic conductivity or permeability) and entrance resistance.