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Competency Area 3: Drainage and irrigation AEM

PO 19. Understand the potential impacts of the following factors affecting soil drainability and the installation of drainage systems:

  1. Location of Bedrock
  2. Topography
  3. Organic Soils
  4. Type of Crop
  5. Outlet

Location of Bedrock

Bedrock that is massive and unfractured creates an impermeable boundary which can restrict soil drainage. Soils that are shallow over this type of bedrock will saturate quickly, producing interflow and surface runoff. When this type of bedrock is within less than three feet of the surface, subsurface drains are ineffective and difficult to install.

Subsurface drains can be beneficial if this type of bedrock is deeper than three feet, and does not interfere with installation. Surface drainage can be used to facilitate the removal of ponding or shallow perched interflow water.


Slope or changes in topography tend to facilitate the overall drainability of soils as excess water has an opportunity to flow to lower elevations. Soils on convex type slopes are often better drained than those within concave slopes (particularly those at the toe of the slope) because rain and excess water tends to be dispersed rather than concentrated in the landscape.

The topography has a significant influence on the type of drainage systems and methods used because the installation of a drainage system by necessity also must have a place to dispose of or discharge the water that is collected. Since excess water collects at low points in the topography, the drainability of soils in this position is also made worse because outlets may be difficult to find for the water. Dikes and the use of pumping systems may be the only alternative remaining to enhance soil drainage in low lying areas. Thus, topography determines the layout of a drainage system (i.e., interceptor, random, pattern), and the outlet situation (i.e., natural, constructed open ditch, or pump).