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 discharge is found from a water balance.
The depth is selected in accordance to aquifer properties. The well filter must be placed in a permeable soil layer.
The spacing can be calculated with a well spacing equation using discharge, aquifer properties, well depth and optimal depth of the water table.
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:
where Q = safe well discharge - i.e.
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Soil salinity control refers to controlling the process and progress of soil salinity to prevent soil degradation by salination and reclamation of already salty (saline) soils. Soil reclamation is also called soil improvement, rehabilitation, remediation, recuperation, or amelioration. The primary man-made cause of salinization is irrigation. River water or groundwater used in irrigation contains salts, which remain in the soil after the water has evaporated.
Agricultural hydrology is the study of water balance components intervening in agricultural water management, especially in irrigation and drainage. The water balance components can be grouped into components corresponding to zones in a vertical cross-section in the soil forming reservoirs with inflow, outflow and storage of water: the surface reservoir (S) the root zone or unsaturated (vadose zone) (R) with mainly vertical flows the aquifer (Q) with mainly horizontal flows a transition zone (T) in which vertical and horizontal flows are converted The general water balance reads: inflow = outflow + change of storage and it is applicable to each of the reservoirs or a combination thereof.
Groundwater models are computer models of groundwater flow systems, and are used by hydrologists and hydrogeologists. Groundwater models are used to simulate and predict aquifer conditions. An unambiguous definition of "groundwater model" is difficult to give, but there are many common characteristics. A groundwater model may be a scale model or an electric model of a groundwater situation or aquifer. Groundwater models are used to represent the natural groundwater flow in the environment.
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