Agricultural Drainage

Bulletin 871-98

Illinois

Highlights

• In the Embarras River watershed, nitrate-N in subsurface drainage waters and rivers has been closely linked to intensive agricultural production. Wetlands that intercept agricultural drainage are being evaluated as a method to reduce agrichemical inputs to rivers and reservoirs used as drinking water supplies, while maintaining agricultural production. Five wetlands were constructed 15 miles south of Urbana/Champaign along the Embarras River. Drainage water is diverted into the wetlands. Drainage water volume and agrichemical concentrations are measured entering and leaving the wetlands, with agrichemical removal rates then evaluated. Preliminary results indicate that most of the nitrate-N can be removed during spring and summer periods. Nitrogen removal is less during winter (too cold) and during extremely high flow events anytime of the year. The timing of precipitation events, soil and water temperatures, and the role of seepage from the wetlands must all be studied to determine overall wetland effectiveness over a multi-year period. In the future, the removal of nitrate-N, phosphorus and atrazine will be evaluated. In addition, detailed studies will be made of how the wetlands actually process chemicals in order to improve their design and function.

• In most areas where subsurface drainage is used, there are very few records of the actual locations of many drainage systems, especially those installed more than 75 years ago. The unavailability of accurate maps makes it difficult to locate nonfunctioning drains, or even to determine the position of functional systems in cases where additional drains are to be installed. Color infrared aerial photographs and geographic information system (GIS) analysis were used to map drains in Vermilion County, Illinois. The method appears to be a promising and cost effective tool as compared to conventional probe methods. The drain mapping procedure is based on the fact that the soil over subsurface drains dries faster than the soil at other locations in the field and has higher reflectance in the infrared region of the radiation spectrum. Color infrared aerial photographs for the study area, taken several days after a major spring rain event, were converted to digital format. A GIS package was used to overlay soil data, hydrological parameters, topography, and vegetation cover. The combination of these map layers made it possible to identify the layout of functional subsurface drainage systems.

• A method for using subsurface drain outflow rates to estimate drain spacing for incomplete drainage systems was developed and applied to four fields in the Little Vermilion River watershed. An optimization routine was used to determine the drain spacing that minimizes the difference between observed and DRAINMOD-simulated drain outflows. Under the prevailing conditions of the watershed, DRAINMOD performance was relatively insensitive to surface storage, the depth of the impermeable layer from the ground surface, diameter of the drains, and the lateral hydraulic conductivities in all the soil layers of the soil profile, with the exception of the layer in which the drains were located. The drain spacing information was used to estimate nitrate-N loading per unit area based on an appropriate region of influence for the irregular drainage systems. The region of influence of a single drain in a random subsurface drainage system on Drummer/Flanagan soils of east-central Illinois, estimated with the optimization routine and a graphical method, is 300 ft.

• Subsurface drainage systems are important, if often overlooked modifiers of flood peaks and flood-water quality in rivers, ditches and streams. Most existing watershed-scale flood models do not include the effects of subsurface drainage systems, while subsurface drainage models tend to focus on single fields. Research is being conducted to develop improved models for simulating flood flows and nutrient transport in the flat, subsurface drained watersheds that are typical in central Illinois. These models will provide planners and policy makers with a better tool to be used in decision making in floodplain areas. The models can also be used in the development of flood mitigation schemes and to evaluate the effects of changes in land use and urbanization on flood peaks. In the long run, the models can also be used to improve predictions of flood-water quality.

The top illustration is an aerial photograph taken in the spring of 1985
in the Embarras River watershed. The photograph shows variations in
soil moisture caused by subsurface drainage. The bottom illustration
was generated by processing the photograph in a GIS. The result is a
vector file of subsurface drain locations.