Typically, information on the depth and spacing of tile drains in Illinois is obtained from the Illinois Drainage Guide. For a specified soil, the Guide gives a combination of depth and spacing that will optimize yield. However, there is more than one combination that gives the same yield response. Currently, drains in Illinois are typically installed at a depth of 4 to 5 feet at spacings of 80 to 150 feet. Originally, tile systems were designed for the sole purpose of quickly removing excess water from the plant root zone to prevent wet stress and improve crop yields. These recommendations were made with no concern for water quality. According to computer models, shallower drains, spaced closer together, will perform just as well with regards to yield, but with reduced nitrate concentrations in the drain effluent.

With shallower drains, the water table is higher in winter and early spring, resulting in a larger anaerobic zone that promotes the denitrification of excess nitrate-nitrogen below the tile drains. Much of the nitrate that would normally be leached through deep tiles between growing seasons will be converted to a gas and released to the atmosphere. A practical advantage of shallow tile drains is that there is more plant available water in the root zone during the growing season. In dry years there will be less drought stress because of added soil water storage capacity while in wet years excess water above the tiles is removed as quickly and efficiently as it is with deeper drains.

Based on similarities in water table profiles, it has been hypothesized that shallow drainage systems could have a similar effect as controlled drainage on nitrate loss via drainage water. Various researchers have found that controlled drainage has led to reductions in chemical transport from agricultural fields. One source reported a decrease in aldicarb concentrations when controlled drainage was implemented on cultivated fields in North Carolina. In a three-year experiment in Iowa, one group examined the effect on crop yield and nitrogen concentration, of outlet level in a controlled drainage system. They observed a reduction in nitrate concentration for all outlet levels, and in increase in crop yield for most. Another source reported a 25% decrease in mean nitrate concentration, and a 49% decrease total annual nitrate load when controlled drainage was implemented on clay loam soil in Southwestern Ontario. They did not report the effect on yield. Working with two-year corn/soybean rotation on a silt loam soil in Quebec, measured nitrate concentration reductions of 76% and 69% were determined for two outlet levels in controlled drainage systems, as compared to conventional drainage. One other source reported increased yields ranging from 23 to 58% over three years from establishing a controlled drainage system in Ohio.

Though they might be similar in effect, the use of shallow drains offer some advantages over controlled drainage. Controlled drainage requires a higher level of management. Many fields in Illinois would require multiple control structures, some of which would have to be placed in the middle of fields. This is not an acceptable option to most farmers. There are, however, some disadvantages of using shallow drains as opposed to controlled drainage. Shallow systems do not provide the same flexibility of control of the water status in the soil. In addition, they cost more to install, even if the cost differential is decreased by using smaller tiles for the shallower systems.