Ingeotechnical engineering,watertable control is the practice of controlling the height of thewater table bydrainage. Its main applications are inagricultural land (to improve thecrop yield usingagricultural drainage systems) and in cities to manage the extensive undergroundinfrastructure that includes thefoundations of large buildings,underground transit systems, and extensive utilities (water supply networks,sewerage,storm drains, and undergroundelectrical grids).
Subsurface landdrainage[1] aims at controlling thewater table of thegroundwater in originallywaterlogged land at a depth acceptable for the purpose for which the land is used. The depth of the water table with drainage isgreater than without.
Inagricultural land drainage, the purpose of water table control is to establish a depth of the water table (Figure 1) that no longer interferes negatively with the necessary farm operations and crop yields (Figure 2, made with the SegReg model, see the page:segmented regression).
In addition, land drainage can help withsoil salinity control.
The soil'shydraulic conductivity plays an important role in drainage design.
The development ofagricultural drainage criteria[3] is required to give the designer and manager of the drainage system a target to achieve in terms of maintenance of an optimum depth of the water table.
Optimization of the depth of the water table is related to the benefits and costs of the drainage system (Figure 3). The shallower the permissible depth of the water table, the lower the cost of the drainage system to be installed to achieve this depth. However, the lowering of the originally too shallow depth by land drainage entailsside effects. These have also to be taken into account, including the costs of mitigation of negative side effects.[3]
Theoptimization of drainage design and the development ofdrainage criteria are discussed in the article ondrainage research.
Figure 4 shows an example of the effect of drain depth on soil salinity and various irrigation/drainage parameters as simulated by theSaltMod program.[4]
Historically, agricultural landdrainage started with the digging of relatively shallow open ditches that received both runoff from the land surface and outflow ofgroundwater. Hence the ditches had a surface as well as a subsurface drainage function.
By the end of the 19th century and early in the 20th century it was felt that the ditches were a hindrance for the farm operations and the ditches were replaced by buried lines of clay pipes (tiles), each tile about 30 cm long. Hence the term "tile drainage".
Since 1960, one started using long, flexible, corrugatedplastic (PVC orPE) pipes that could be installed efficiently in one go bytrenching machines. The pipes could be pre-wrapped with an envelope material, likesynthetic fibre andgeotextile, that would prevent the entry of soil particles into the drains.
Thus, land drainage became a powerful industry. At the same time agriculture was steering towards maximum productivity, so that the installation of drainage systems came in full swing.
As a result of large scale developments, many modern drainage projects wereover-designed,[5] while the negative environmental side effects were ignored. In circles with environmental concern, the profession of land drainage got a poor reputation, sometimes justly so, sometimes unjustified, notably when land drainage was confused with the more encompassing activity ofwetland reclamation.Nowadays, in some countries, the hardliner trend is reversed. Further,checked orcontrolled drainage systems were introduced, as shown in Figure 5 and discussed on the page:Drainage system (agriculture).
The design of subsurface drainage systems in terms layout, depth and spacing of the drains is often done using subsurfacedrainage equations with parameters like drain depth, depth of the water table, soil depth,hydraulic conductivity of the soil and drain discharge. The drain discharge is found from anagricultural water balance.
The computations can be done using computer models like EnDrain, which uses the hydraulic equivalent ofJoule's law in electricity.[6]
Subsurface drainage of groundwater can also be accomplished by pumped wells (vertical drainage, in contrast tohorizontal drainage). Drainage wells have been used extensively in the Salinity Control and Reclamation Program (SCARP) in the Indus valley ofPakistan. Although the experiences were not overly successful, the feasibility of this technique in areas with deep and permeable aquifers is not to be discarded. Thewell spacings in these areas can be so wide (more than 1000m) that the installation ofvertical drainage systems could be relatively cheap compared tohorizontal subsurface drainage (drainage by pipes, ditches, trenches, at a spacing of 100m or less). For the design of a well field for control of the water table, the WellDrain model[7] may be helpful.
A classification of drainage systems is found in the articleDrainage system (agriculture).
Most crops need a water table at a minimum depth.[10] For some important food and fiber crops a classification was made[11] because at shallower depths the crop suffers a yield decline.[12]
| Crop and location | Tolerance DWT(cm) | Classification | Explanation |
|---|---|---|---|
| Wheat, Nile Delta, Egypt | 45 | Very tolerant | Resists shallow water tables |
| Sugar cane, Australia | 60 | Tolerant | The water table should be deeper than 60 cm |
| Banana, Surinam | 70 | Slightly sensitive | Yield declines at water tables < than 70 cm deep |
| Cotton, Nile Delta | 90 | Sensitive | Cotton needs dry feet, water table should be deep |