Lecture 25

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Chapter 17

Wetlands and Drainage


Wetlands were once quite universally considered a nuisance, to be drained and made farmable. Recently we have come to recognize the value of wetlands and the need to protect them. Not all drainage projects impinge on wetlands. Often, irrigated land benefits from subsurface drains that allow removal of salt. In certain other instances, drainage is a practical solution to a land management problem. This lecture is a brief introduction to the nature of wetlands and the technology of land drainage.


1. What are wetlands? 32% of America's cropland has been artificially drained. The present farm bill dictates no net loss of wetlands. However, land drained before 1985 can stay drained, and wetlands can be drained if a landowner creates an equal amount of wetlands on his or her property from land that is not presently wetlands. The U.S. Army corps of Engineers has jurisdiction over wetlands in the United States.

By definition, a wetland is an area of predominantly hydric soil that can support a prevalence of water-loving plants. Wetlands include swamps, marshes, bogs, etc. The hydric soil criterion is a problematic one. Scientists disagree on what should constitute a hydric soil. Presently the USDA publishes a national hydric soils list. The Federal register says: "Hydric soils are defined as soils that formed under conditions of saturation, flooding, or ponding long enough during the growing season to develop anaerobic conditions in the upper part." Note that whether or not the land is wet is not the criterion for designation of a legal or jurisdictional wetland.

Indicators of hydric soils include:

organic soil
histic epipedon
sulfidic material
gleyed horizon, chroma less than 2
aquic moisture regime
poorly drained soils
low redox potential

Wetland delineation is scientifically and politically difficult. One can assume that most landowners do not wish for their land to be designated jurisdictional wetlands because such a designation restricts the use of the land.


2. Beneficial functions of wetlands. Wetlands serve many purposes useful to man and to the environment as a whole. Some benefits are listed below:

Preserving biodiversity
Recharging groundwater
Regulating flow to control flooding
Providing pressure against saltwater intrusion
Buffering against storms and waves
Catching sediments
Retaining nutrients
Removing toxins
Producing medicinal plants
Providing wildlife habitat
Producing fuel products
Maintaining gene bank
Producing specialized crops such as rice and cranberries


3. To drain or not to drain. New drainage on undrained, jurisdictional wetland may be illegal by the Swampbuster Act of 1985. Landowners can maintain or redo existing drains. Landowners may drain one tract of wetland and create a wetland elsewhere. Just as the benefits from wetlands are undeniable, so are the benefits from drainage. Upland crops generally do very well on drained wetlands. Such lands are typically fertile, contain considerable clay, and have more uniform moisture characteristics than undrained lands. Also drained lands are better than wetlands for most engineering purposes. Many plots of land are not jurisdictional wetlands, but are wet enough to benefit from artificial drainage. Irrigated land is often drained to control salinity.


4. Drainage methods. The method one would chose to drain land depends on the needs of the landowner (see Table 17-4 in the textbook).

Subsoil drainage is accomplished using buried pipes or tubes. Many years ago, most such drains were made of tile. Some times the term "tile drains" is still used to designate subsurface drains. Subsurface drainage lowers a water table. Some soils will not drain, as explained below.

drainage capacity should be 10 or more
drainage capacity = % pore space (dry) - % qv (@ FC)
most sands and loams will drain
the Sharkey clay swells when wet; drainage capacity < 0

In some cases vertical drains connected to horizontal underground drain lines can be used to drain a wet spot. Subsurface drains require an outlet. In regions lacking the infrastructure for drainage, drainage may be impossible for lack of ditches in which to discharge the water.

Surface drainage is done to remove surface water. this can be accomplished by field smoothing, but construction of open ditches, or by using bedding or ridge-till farming systems.


Students are encouraged to look up the following vocabulary words in the textbook glossary or elsewhere and to browse the following web sites.



tile drains


Web sites

Drainage information web page from Ontario.
URL: www.drainage.org/planning.htm

The official USDA NRCS National Hydric Soils List.
URL: www.statlab.iastate.edu/soils/hydric/national.html

A wetlands web site with many valuable links to wetlands information.
URL: www.eih.uh.edu/syhabitats/resources/wetlands.htm


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